DBX 166 Modifications/Upgrades

These Mods clean up the DBX166 compressor making it high quality PRO Studio Gear.

PSU MoD: – Replace c3 ,c4, c7, and c8 with 1000uf-2200uf 35v caps
– Replace c5 and c9 with a .1uf film cap (preferably polypropelene)
– Bypass c6 and c10 with .1uf film caps
Channel 1:

IC MoD: – Replace chip OA1 with better ic (NE5532, OPA2132, opa2134, OPA2107, LT1358, or LME49860NA (My fav)) and install a gold plated ic socket.
– Repalce chip OA2 with better ic (LME49710NA, OPA134, OPA602, opa627, or LT1357) and install a gold plated ic socket.
– Replace the VCA IC1 with a THAT 2180AL08-U (lift pin 4, pin 4 should not connect to anything)
– Add a 47pf NPO ceramic cap from pin 3 of OA2 to ground (Suggested by Jim Williams)

Capacitor/resistor MoD:
– Replace c14 with a 22uf 35v bipolar cap
– Bypass that with a .22u film cap
– Note: c14 was originally a 3.3uf cap. You could fit in a film cap in there if you are carefull.
– Replace c25 with a 22uf 35v bipolar cap
– Bypass that with a .22u film cap

– Replace r17 and r18 with 22ohm metal film resistors
– Replace r1 and r4 with 1k metal film resistors
– Replace r3 and r6 with 7.5k metal film resistors
– Replace r2 and r5 with 10k metal film resistors
– Replace r7 and r13 with 2k metal film resistors

IC Control Voltage (Optional) MoD: Turns this compressor into a beast.
– Replace chips OA3, OA4, and OA5 with gold plated ic sockets and a better ic like a LT1124 or LME49860NA.

Peakstop switching MoD: Turns on and off the peakstop circuit.
– Drill a hole in the front plate about .25 inches diameter. Install and wire in a DPDT gold contact switch here. Lift one end of CR5 and CR6. Solder 2 wires. One to cr5 and one to cr6 and solder the other end of the wires to each center contact of the DPDT switch. Solder 2 more wires to the vacant holes of cr5 and cr6 and wire these to one side of the switch.

Channel 2:

IC MoD: – Replace chip OA6 with better ic (NE5532, OPA2132, opa2134, OPA2107, LT1358, or LME49860NA (My fav)) and install a gold plated ic socket.
– Repalce chip OA7 with better ic (LME49710NA, OPA134, OPA602, opa627, or LT1357) and install a gold plated ic socket.
– Replace the VCA IC8 with a THAT 2180AL08-U (lift pin 4, pin 4 should not connect to anything)
– Add a 47pf NPO ceramic cap from pin 3 of OA2 to ground (Suggested by Jim Williams)

Capacitor/resistor MoD:
– Replace c14 with a 22uf 35v bipolar cap
– Bypass that with a .22u film cap
– Note: c14 was originally a 3.3uf cap. You could fit in a film cap in there if you are carefull.
– Replace c55 with a 22uf 35v bipolar cap
– Bypass that with a .22u film cap

– Replace r117 and r118 with 22ohm metal film resistors
– Replace r101 and r104 with 1k metal film resistors
– Replace r103 and r106 with 7.5k metal film resistors
– Replace r102 and r105 with 10k metal film resistors
– Replace r107 and r113 with 2k metal film resistors

IC Control Voltage (Optional) MoD: Turns this compressor into a beast.
– Replace chips OA8, OA9 and OA10 with gold plated ic socket and a better ic like a LT1124 or LME49860NA.

Peakstop switching MoD: Turns on and off the peakstop circuit.
– Drill a hole in the front plate about .25 inches diameter. Install and wire in a DPDT gold contact switch here. Lift one end of CR5 and CR6. Solder 2 wires. One to cr5 and one to cr6 and solder the other end of the wires to each center contact of the DPDT switch. Solder 2 more wires to the vacant holes of cr5 and cr6 and wire these to one side of the switch.

DBX166 Service Manual

http://www.n01ze.com/synthwizards/ARKIVE/dbx_166.pdf

THAT 2180 Datasheet

http://thatcorp.com/datashts/THAT_2180-Series_Datasheet.pdf

THAT 2181 Datasheet

http://thatcorp.com/datashts/THAT_2181-Series_Datasheet.pdf

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TRIOXIN CloudMaker Granular Synthesizer

Started creating The V.S.R. “Trioxin” CloudMaker today
specifically for noise comp
Eye wanted drastically longer custom ranges with multipliers on grain size & rate parameters
up to 20 seconds on rate & 10 seconds on size
plus completely redesigned how the freeze/hold feature extracts 100ms samples from input
have some other functions that need adding
yet got the core engine constructed now
will make multiple revisions yet giving you first functional prototype

Revised…
Added 2x ADSR
tweaked times
recording some demo trax…
http://www.n01ze.com/synthwizards/ARKIV … RIOXIN.rar

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BOSS DR55 (Schematics/Manuals/Mods)

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BOSS DR55 (Schematics/Manuals/Mods)

DR55 does have an authentic sense of early 80s legendary nostalgia…
being BOSS’ first DR-machine and also quite a successful early programmable drum machine.
It has been used by New Order, The Cure, Chris Carter, Sisters of Mercy, Chris & Cosey, Soft Cell and Thomas Dolby.Boss DR-55: external trigger input mod

By default, the Boss DR-55 does not receive any kind of incoming clock. The ‘FS’ footswitch input takes a latching footswitch that starts and stops the existing clock, but that’s it. Although you can clock other equipment from the DR-55, it would be nice to be able to use an external clock to sync the Boss to, which would allow the Boss to trigger yet more devices with its CSQ and DBS outputs (active on Accented steps only and every step, respectively).

My mod as detailed here does exactly that. By replacing the existing FS jack socket, adding a small circuit, and replacing a jumper, we can safely trigger the DR-55 from an external trigger.

A quick internet search will turn up an existing clock input mod which is simpler to do and requires no extra parts; however, it puts the RAM at risk of damage from high triggers, and it does not sync the Boss’ own DBS output. It also requires ‘arming’ by hitting start before external triggering.

My own mod, though more complex, overcomes all these issues: the trigger input is protected, both the Boss’ trigger outputs maintain their correct functions, and triggering occurs without ‘arming’. The only two functional disadvantages of my mod are that you must set the Boss’ tempo to Fast, and to reset the pattern when stopped mid-way you need to remove the trigger plug. I’m going to blog another small mod which will overcome the latter inconvenience [EDIT: No I’m not! I sold both my 55s, thereby halting this particular project].
The Clock Modification in detail

Below is a diagram which shows everything you need to know about building this mod. Below that is a parts list. Key to this is the replacement FS jack socket; it needs to be TRS (ie. a stereo jack), with single pole changeover switches on the tip and ring contacts. I used a Lumberg KLBPSS3 (datasheet here, Farnell UK stock page here).

The additional circuit can be made very small indeed (3 rows * 8 holes on stripboard), and there is plenty of room for it inside the DR-55, particularly towards the right-hand end. The photos below illustrate my own placement.

There is one jumper to be removed, the one immediately to the right of the Variation switch. The replacement connections for the upper and lower point of this removed jumper are shown in the diagram, and you can see in the photos how I wired this up.

In brief: remove that jumper, solder the two points to two jack pins; build the extra circuit, and solder that to the jack and to the main PCB; replicate two of the pre-existing connections from the jack to the PCB. That’s it. I also stuck a small folded piece of card to the PCB to stop the extra circuit from shorting against components.
Boss DR-55 clock input mod

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Boss DR-55 clock input mod revised
Parts list:

1 * TRS 2-pole changeover jack socket – eg. Lumberg KLBPSS3
2 * 47k resistors – I used 1/8W for their smallness
1 * 10nF capacitor – I used a ceramic, again for smallness, but polyester film etc. would be usual
1 * 1N4148 signal diode or equivalent
1 * BC549C transistor or similar standard NPN

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Here’s the modified DR-55 (also incorporating my DC supply mod):
clock modded DR-55 overview

clock modded DR-55 overview

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And here’s a close-up of the clock mod:
clock modded DR-55 close up

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clock modded DR-55 close up

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How to use your new trigger input

The new trigger input will accept any positive pulse over a couple of volts. It’s edge triggered, so the pulse can be any length over a couple of milliseconds. The operating principle is to use the DR-55’s existing clock, but to gate it on for a very short duration; normally when the clock is gated off again, the pattern resets, but the new jack socket enables us to disable that by breaking the reset connection when a jack is inserted.

As I mentioned earlier, the Tempo must be set to Fast (ie. all the way clockwise) for correct function. This is because the DR-55’s clock, once triggered, finishes its pulse cycle. If this is longer than the incoming trigger cycle, it will ignore the new trigger; if we set the speed dial to its fastest, we can clock the DR-55 at any rate up to its natural maximum.

The pattern will cycle round as usual, but if you stop mid-pattern, new triggers will continue where they left off. To reset the pattern at this stage, you need to unplug the trigger jack and hit Stop. This is not ideal, I know, and I will be making an amendment to correct this later [EDIT: project halted, see above. I have no current plans to do any further work on the DR-55].

For now though, this mod works fine, as shown in the (slightly rubbish) video below:
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Boss DR-55: a 9V DC input modification

One of the drawbacks of the DR-55 as it comes unmodded is the power supply. In its original form, the DR-55 takes only batteries, and though this might be good for reducing cable clutter and having to find yet another wall-wart, it does mean you need to keep a regular stock of fresh AAs, and can guarantee that just when you want to use it, your DR-55’s batteries are too drained for the unit to function correctly.

Luckily, it is a relatively simple process to modify the DR-55 so that it takes a commonly-found 9V DC supply instead. I provide instructions for this below. It’s not the only way to do the job, but this is how I did it, and it works just fine. Modding the DR-55 in this way means it no longer accepts batteries, which means two things: 1) you will need access to a 9V adapter, and 2) pattern data will not be retained on power-off. Given that filling the memory of this humble machine can be done in less than five minutes, and I never use this outside my own home studio, I never found memory retention to be an issue. It would be possible to design a DC input that also catered for memory backup via battery, but I’m not going there.

There are two basic stages to this modification:

Making a 9V DC input: the basic voltage supply circuit
Installing the Mod: adapt some wire links on the output jack and PCB
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Because the DR-55’s RAM can be killed by voltages higher than around 7V, we take a 9V input and regulate it down to between 5V and 6V. I chose to use a 5V regulator propped up with a diode to give around 5.6V, but you could also use a 6V regulator and omit D2. The input jack I used is a 3.5mm mono minijack of the kind often used for audio and CV interconnects, mainly because I had lots of them and the holes are easier to drill than the larger ones needed for a plastic-bodied insulated barrel connector. Use whatever type you prefer, but note the polarity of your incoming DC, and don’t connect the +ve to the case… with a tip-positive 3.5mm jack, the sleeve of the input jack is connected to the shell of the socket, so it makes sense for that to be the ground. Some barrel connectors do likewise.

Here’s the schematic:
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Boss DR-55 DC input mod schematic

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Here’s the final circuit built onto stripboard. It will be panel-mounted using the socket:
DR-55 DC input build

DR-55 DC input build
Installing the Mod

Now we have a simple DC input, we could just solder the +ve and Gnd outputs to the corresponding solder points on the main board – that is, where the battery clip attaches. Black is ground, red is positive. This works, but you still need to insert an audio cable to turn the DR-55 on. I chose to remove that ‘feature’, as there are no longer any batteries to protect from accidental drain. It’s a simple mod that just means a couple of wiring changes.

The diagram below shows the required re-wiring. The audio output socket is wired by default to both ground and audio signal, as well as having two pins wired to act as a switch when a jack is insterted. We want to retain the audio and ground connections, but not the switch. We remove those wires and instead bridge the corresponding points on the PCB.

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Here’s a photograph of the full mod (note the wiring):
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DR-55 DC input wired and complete

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I damaged a track while desoldering the battery wires, which is why the red wire goes to the un-numbered hole next to point 9. They’re directly connected, happily.

Below are some photos of the hole I drilled for mounting the new DC input, and the final appearance when mounted and labelled with cheap Dymo (should have gone with black… oh well):
DR-55 DC input enclosure drilling
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So there you have it. My humble DR-55 now works from a regular 9V DC wall-wart supply, and switches on whether or not its audio is connected. The hardest part is putting the DR-55 back together again…

or you can just use old hacked (w/ 9v battery connector)phone charger from cheap Samsung 5v pay as you go track phone…
operating power range from the service manual for BOSS DR55 is 4.5v to 6v
Works for me!

next step external trigger inputs & outputs for each drum sound
http://manuals.fdiskc.com/flat/Boss%20D … Manual.pdf

https://synthnerd.wordpress.com/2016/05 … input-mod/
it’s obviously not a TR808 or TR909 yet picked it up ages ago for $20 in the original box :P
replaced bad cd4011UB chip…
for getting the clock working on this old machine
was going to buy the CR55 DIY eurorack then remembered had this old gem

808 style BASS Kick Sustain mod:

Well, this weekend I met up with a friend of mine who also has a BOSS DR-55 Dr.
Rhythm (groovy, groovy, groovy).

So I say to him, ‘Hey, have you modified the bass drum decay?’, and he hadn’t so
we flipped the top off and had a look inside…

On my DR-55, in the bottom left (near the tone pot) there was a little trim-pot
which varied the kick decay, but on my friends there was only a resistor. Later
study of the service manual shows that only early serial number DR-55s have the
trim pot, so if you’ve a later rev. DR-55 here’s the extremely simple way to
modify the bass drum decay..

The resistor is in the lower left of the circuit board (looking from above),
just below the tone control. ‘BD’ is stencilled on the PCB next to it, and the
resistor is over a little stencilled trim-pot picture. The following ASCII
diagram should help you locate the right resistor.

/-\
l l Tone Pot.
\-/
o
o
o ^
^==/ <<– Resistor to be replaced.

Simply snip out the resistor, and replace with a horizontal trim-pot of several
K ohm. The trim pot goes between the three holes in the silly ASCII diagram
above, with the wiper terminal attached the middle/rightmost hole. I used a 2.2K
that I had floating around (what? your parts don’t levitate like mine? freaky),
and that gave a reasonable control range…

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Vox Treble Booster

The Vox Treble Booster was a simple silicon treble boost effect that plugged straight into an amp’s input jack. It allowed the user a boost in high frequencies for lead work. I laid this one out to use as a built in effect on a guitar (hence the lack of a power filtering cap and polarity protection). Note that the 10uF cap is on its side to reduce the height of the populated board. It should easily fit in a 1590A. If you want a volume control use a 100k pot with the Out pad going to lug 3. Lug 2 becomes the Out and lug 1 goes to ground. I found that I really didn’t need a volume pot though, as the effect level was right around unity.

Awesome perf/strip/pcb layout resource site:
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Replacing CR3032 Battery in BOSS SE-50

https://medias.audiofanzine.com/files/se-50-manual-476630.pdf

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Replacing Dead Battery in Digitech DSP 256XL

When the internal alkaline battery of the Digitech DSP 256XL fails, “Battery Failure, All Data Lost” will appear on the unit‘s LCD screen. Unlike other audio hardware, the 256 only wipes out saved patches; it doesn’t wipe its entire memory contents (like Korg’s M1 synth, for example).

 

.The internal battery died in my DSP 256 XL.The battery in the unit is an Energizer 4.5 volt #523 that is obsolete.The length is about 1 and 7/8″ long X 16.8 mm in diameter.

same battery/process as Digitech GSP21 as in the above video^

DigiTech-DSP-256XL

This month’s Test Drive puts us in the driver’s seat of yet another effects box. The DigiTech DSP-256XL falls into that category of the affordable, yet powerful effects boxes that, like most under $500, are designed for the musician. Even so, as many of you know, these little musician’s boxes are jam packed with high quality effects perfectly suited for radio production with price tags that don’t put your GM into cardiac arrest.

The primary features of the 256XL include 128 fixed factory presets, 128 programmable slots, up to four effects at once, twenty-six different effect configurations, and full MIDI capability. You get reverb effects, chorusing/flanging, parametric EQ, graphic EQ, and a not too often seen 4-tap delay.

Aside from its clean sound and numerous useable presets, perhaps the most impressive aspect of the 256XL is its ease of operation. When a program is selected, the 2-line, 16-character per line LCD display shows the program title on the top line and the effect configuration in use on the bottom line. PROGRAM UP/DOWN buttons scroll through the 256 memory slots in the unit, and if you hold the UP or DOWN button in, the unit shifts into a “fast scroll” mode for quick selection of any program (or adjustment of any parameter). In radio production, probably the most used parameter of an effects box is the wet to dry MIX parameter. If you’re the type that doesn’t care to get into the parameters of an effect and shop around for the MIX parameter before you make an adjustment, you’ll appreciate the MIX control knob on the right side of the front panel. If you’re looking for an effect for a voice track, let’s say, just set the MIX level to 12 o’clock (or 50%) and start moving through the different programs as you input the voice track. When you find one you like, it’s immediately on line and you don’t have to enter any “edit” mode to adjust the mix. The other two knobs next to the MIX control are INPUT and OUTPUT level controls.

Now, should you want to edit the program, that too is quick and simple. The two PROGRAM UP/DOWN buttons mentioned above are dedicated to program selection. Similarly, separate PARAMETER buttons are dedicated to program editing. There are two UP/DOWN PARAMETER buttons and two LEFT/RIGHT PARAMETER buttons. The LEFT/RIGHT buttons are used to select the parameter you wish to edit. The UP/DOWN PARAMETER buttons increase or decrease that parameter’s value accordingly. As you step through the various parameters, the entire top line of the display is dedicated to describing the parameter; so there is limited use of abbreviations which tend to confuse the novice effects editor. The bottom line of the display is reserved for the value of the parameter, and again, limited use of abbreviations helps to shorten the learning curve.

Once changes have been made to a program, the COMPARE button can be pressed to compare the edited program with the original. If you like what you hear, press the STORE button. You are given the option to store the edited program in the current position or to any of the other 128 user slots. The STORE button is also used to simply copy an existing program to another place in user memory. Once again, storing and moving programs is very easy, and the LCD display kindly guides you along the way with prompts such as “Storing,” and “Copying.” When you’re just playing around with a program’s parameters and then decide to go to another program, the unit will remind you that you’ve changed the program by giving you the option to save the changes before you go to the next program.

DigiTech’s 128 factory programs are stored in locations 129-256. User locations 1-128 come from the factory with copies of programs 129-256 in them, and these programs can be edited or completely removed. We thought it was nice of DigiTech to put the user locations at 1-128 instead of at the other end. This way, all your favorite programs, whether they be factory programs or ones you create, can be at locations 1 through 10, for example, making the program number easier to remember than, let’s say, 129 or 233!

ftp://ftp.digitech.com/pub/PDFs/Discontinued/Manuals/dsp256xl.pdf

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The Technology of Wah Pedals

 

The stock “wah” pedal has been around since at least the early 60’s. While there is a persistent rumor that an early version of a wah mechanism was found in the wreckage at the Roswell crash site, I can categorically state that the government says that this is nonsense, and that no such thing happened. It was only a weather balloon. Made of … uh… magnesium… and uh… nylon, which was top secret then. That’s my story and I’m sticking to it.

In any case, this thing produces a distinctive tone that is well loved by the expressive guitarist. The history is a bit obscure, but it probably came about as a follow on embellishment of the first Vox mid range booster effects. The nasal tone of the MRB became a vowel-like “aaaahh” with the arrival of a way to sweep the center of the effect.

What a wah does is clear – it is either a bandpass filter or an overcoupled lowpass filter that exhibits a resonant peak just at its lowpass rolloff frequency. The resonant peak can be moved up and down in frequency by the player, and this makes for a striking emulation of the human voice making a “waaaah” tone, or its tonal inverse, “aaaooow”. There are several means to this end, and the circuits are well understood for the classical implementations of such filters with opamps, state variable filters and the like.

Human Voices and the Wah Pedal

-or-

Why a wah pedal sounds something like a voice, and why it doesn’t sound more like one

Copyright 1999 R.G. Keen. All rights reserved.

The thing that makes a wah pedal so immediately catchy is that it has a real “vocal” quality – it sounds like a a human voice making that noise. Everyone has done a quiet little “waaah” with their mouth to imitate it.

On the other hand, that’s all it does. There’s clearly something missing because it won’t do anything but that “wah” sound. What’s going on?

People who study the human voice have provided a number of answers. If you take common voice sounds and calculate the frequency spectrum – that is, how loud is it at each separate frequency – you find some interesting features. There is a basic frequency, which is kind of like the basic note you hit with your voice when singing. Above that, there are at least three and often more resonant peaks. In voice research and musicology, such resonant peaks are often called “formants”. A saxphone sounds different from a clarinet at least in part because the resonances associated with its different physical form cause different formants to be audible.

To shorten a long story, us humans listen for and assign meaning to the relative spacing of the first three formants of the human vocal tract. We hear and notice the fundamental frequency, but that seems to be almost inconsequential in assigning meaning to vocal sounds. At most, we notice relative shifts of the fundamental frequency as denoting emotional states – as in someone’s voice going up in pitch when they’re under stress. The relative positioning of formants, and largely the first two formants, forms a kind of code that we interpret as vowel sounds. Here’s one version of the decoding key, based on information from Bell Labs research into humal voice sounds early in the 1900’s:

While this chart implies that there are discrete regions where a sound is more like an “ee” than an “i” sound, that is an oversimplification. There are no real boundaries, only a continuous shading from one vowel sound to another. This is one source of regional accents – everybody in a “local area” kind of learns from everyone else what they all agree will be an “ee” and an “ah”, and they all use that “code” to talk to their friends. It’s only when someone from somewhere else that uses a slightly different mix of formants comes in that they notice the differences.

We can immediately pick up some things about wah pedals from the chart. The “ah” sound has a first formant resonance from about 700 to 1200 Hz. This matches almost exactly the typical range of the resonant peak in the semi-standard wah circuit. We can also see what’s missing – the second formant. A wah pedal has only one resonant peak.

However, it also is a lowpass filter with a resonant peak, not single frequency peaks like human vocal resonances. The presence of the frequencies below the resonance peak is an almost, kind of, maybe reminder of a “peak” lower than the resonance; at least it does not drop off much. As a result, we get a sound like the first and second formants are very close together – very much like the “ah” region.

Here’s another way of looking at wah frequency response:

This shows the relative ranges of the first two formants along with the range of a typical wah pedal. The wah pedal range covers most of the frequencies where the first two formants overlap. Not surprisingly, a wah sounds something like a human voice, but not close enough to be really mistaken for a vowel sound.

This does point the way to even more “vocal” sounds from a wah. A second resonance, even a fixed one, should make for more vowel-ey sounds. To really make it “talk” you could do a second wah circuit that has a resonance that moves around in a different manner than the first. This is exactly the trick employed in the Electro-Harmonix Talking Pedal.

WAH CIRCUITS

There are several kinds of wah circuits That have been used through the years. The original was the Vox style inductor based wah circuit, followed by others, most notably the twin-T circuit, the multiple feedback opamp active filter circuit, and most elaborate of all, the “state variable” style circuit used in the Mutron 3. Everyone’s favorite still seems to be the inductor based Vox style, although the others have their supporters. You’ll find more information on the twin-T circuit, the multiple feedback circuit, and the state variable circuit in this article as well.

The Vox Mystery

The Vox circuit is pretty simple, only a couple of transisors and an inductor. The really pertinent question that had puzzled many people – including especially me – is how do you get a moving resonant frequency out of a fixed inductor and a fixed capacitor? How does that silly two-transistor wah circuit get a moving bandpass out of a circuit that changes neither the inductor value or the capacitor, but only what amounts to a volume pot?

It took me a while, but the trick is – the wah pot, the second transistor and the fixed capacitor implement an electronically variable capacitor. The inductor is and remains fixed, and the capacitor is electronically varied and so the circuit has a variable tuning LC filter to cause the effect.

To get down to how this works we’ll disassemble a two-transistor wah of the classic Vox style and learn how everything in there works, and have a good time with the circuits on the way.

Let’s dive right in. The first transistor is a straightforward feedback amplifier. Ignore for the moment the parts separated by the dotted lines. These are separated from the first transistor by capacitors, and so cannot participate in DC biasing. The transistor is biased into linear amplification by the voltage on its own collector which feeds current to the 470K resistor, some of which is shunted to ground by the 82K resistor. The rest of the current through the 470K goes to the base through the inductor and the 33K resistor which parallels it and the 1500 ohm resistor leading to the base. The inductor’s DC resistance is quite low compared to any of the other resistors (typically 40-75 ohms), so the base current is determined primarily by the 470K and 82K resistors and the 1500 ohm resistor. In fact, the 1500 ohm resistor is small compared to the 470K resistor, so we’ll ignore it for a moment; we can ignore the inductor, 33K resistor and 1.5K resistor. This is one form of the classical voltage feedback biasing arrangement, and the values are chosen to give a reasonable linear range of swing on the collector.

If we just guess that the collector is sitting at 4.5V, that gives a collector current of about 200uA, an emitter voltage of about 0.1V, and a voltage at the base of 0.6V roughly. The voltage across the 470K is then 4.5V – 0.6V or 3.9V, and the current through it is about 8uA. If the gain of the transistor is around 200, then Ib is 1uA, and the voltage across the 82K resistor is 0.6V for a current of about 7uA. Hah! An approximate match!

The biasing seems to work for high gain transistors at least. We could re-refine the estimate, but for our purposes of understanding the operation, it’s enough to be able to be sure that the transistor is in its linear region, and not going to be clipping for small signals at least. In fact, measurements on real circuits show the collector of Q1 normally sits between 3.0V and 5.0V, so the guesses we made were OK, if a bit rough and ready.

In terms of gain, the gain of just the transistor itself, from base to collector is about equal to the load resistor divided by the effective emitter resistor. This is the 470 ohm external emitter resistor plus the internal base emitter junction resistance, about 25mV/Ic or about 125 ohms. So the small signal voltage gain of the transistor is about 22K/595 = 36. On the other hand, we have that input resistor we ignored that will drop some signal. The input voltage is divided down by the voltage divider composed of the 68K input resistor and the effective transistor input impedance. The input impedance is that same 595 ohms times the (ill defined) Hfe which we guessed at 200, or about 119K. The input voltage is divided by 119K/(68K + 119K) or about 0.636. So the overall stage gain is 0.636*36 or about 22, give or take some. Once again, we can’t know this exactly because it involves the transistor gain, but we can get a good enough ballpark number to understand the operation.

Leaving aside the first stage for a moment, let’s look at the second transistor. The second transistor is biased into the linear region by a 470K resistor from the collector of the first transistor, and if its Hfe is high, the voltage at the base will be only slightly smaller than Q1’s collector voltage. The base current needed to bias Q2 is only the current to cause its emitter to pull a 10K resistor up to about 4.5V. This is (4.5V/10K) divided by the Hfe of the transistor, or about 2.25uA. The voltage dropped across the 470K is then about 470K*2.2uA=1.1V. The base of Q2 should sit about 3.4V – not quite as high as we had guessed, but we now know it will be greater than this voltage and less than 4.5V, which is close enough for our purposes. Q2 is a linear emitter follower, and small signals will not run it into saturation or cutoff.

Nota Bene – I did go measure a real Wah pedal’s voltages. The collector voltage on Q1 was actually 4.14V and the base voltage on Q2 was actually 3.64V . Not bad for some back of the envelope scratching, huh?

This emitter follower buffers the signal from the wiper of the wah pot, which is simply a volume-control-style voltage divider to ground. The emitter of the transistor connects through a 0.01uF capacitor to the junction of the inductor and the 1.5K resistor to the base of Q1. How the devil do you get a moveable frequency resonance out of that?

The secret is this. The inductor looks to the second transistor like its far side is grounded, through the 4.7uF capacitor. To the inductor, the capacitor kind of looks like it’s grounded because its far side is connected to the emitter of Q2. Q2’s emitter has a low output impedance and therefore looks like “ground” if you ignore the signal coming out of the emitter. At the junction of the inductor, capacitor, and 1.5K resistor, the voltage looks like the voltage that would happen across a parallel L/C circuit. However – the current through the capacitor is NOT determined by the voltage across the inductor/capacitor, it is also determined by the voltage driving its “ground” side, and that voltage is increased or decreased by the position of the wah pot. If the wah pot setting increases, the capacitor will let more signal current through because the voltage driving it at Q2’s emitter is bigger, so the capacitor has to let in more signal current. If the wah pot setting decreases, the capacitor will let in less signal current. A “capacitor” may be thought of as a special instance of ohm’s law by the amount of signal current it lets through. The change in the effective current through the capacitor makes the capacitor look bigger to the inductor and rest of the circuit than it really is! We have a variable capacitor!

That’s why the frequency of resonance changes. The capacitor looks bigger than it really is for resonance purposes, and the amount it looks bigger is controlled by the wah pot. The first transistor is a block of gain to give you an active resonance, the wah pot and second transistor modulate the effective capacitance in a resonant circuit composed of the inductor and the variable capacitor.

As another way of looking at it, the capacitor is being fed by a buffered replica of the signal from the collector of Q1 and is fed back to the base of Q1, so it looks and acts like a Miller Effect capacitor. The difference between this and a real Miller effect capacitor is that the wah pot has the ability to vary the amount of signal that drives the capacitor. Since the Miller Effect multiplies the actual value of a feedback capacitor by the gain of the stage, the wah pot can vary the apparent value of the capacitor from its normal value up to Q1’s voltage gain times the actual value.

Because of the way the feedback is connected, the actual overall response is that of a lowpass filter with a resonant peak, the peak being the LC peak. In the stock circuit, the gain through the circuit is overall slightly less than one, peaks at resonance, and falls off above the resonance.

There are more implications of this than are immediately apparent, as well as some exciting effects that can be made from it.

It turns out that except for the subtle distortions they generate, there isn’t any magic about the transistors. You can use opamps to do the same job. They won’t give you the subtle tone shadings of a vintage transistor wah pedal, but the wah will work the same way.

Although opamps are not the be-all of the signal world, they simplify some things. In particular, the basic wah circuit can be made to do some outstanding tricks with an opamp implementation – more flexibility, more controls, Vox-style inductor saturation, multiple wahs, and much more – coming in the next installments.

 

Inductor Magic

There’s a legend in the music world that the sound of the old, original Vox Wahs with “Fasel” inductors is superior to what can be had from modern wahs. It turns out that there is some fact behind this legend. While it’s clear that the other parts in a Vox Wah have something to do with the tone, the inductors have long been the subject of speculation. The wahs with Clyde McCoy’s picture on the bottom plate and inductors marked “Fasel” are especially prized. I have a longstanding mistrust of any legendary mystical goodness that is not explainable by technical analysis, so I always wanted to test the magic inductor.

I was entrusted with one of the magic versions by a friend, and spent some time in an EE lab with this wah and a garden variety Crybaby. I took both inductors out and measured their inductance, resistance, self-capacitance, and came to no good conclusions on why there should be any difference in the sound. It wasn’t until I put a sine wave generator through the inductor and looked at the current through the inductor on a spectrum analyzer that the differences showed up.

I saw no differences at first with tiny sine wave drives. It wasn’t until I turned the generator up that differences appeared. The Crybaby inductor performed exactly as I would have expected it to. That is, it had an output that was essentially a pure sine wave right up until the sine was big enough or lowe enough in frequency to start it into the first touches of saturation. When that started, I got precisely what theory predicts: appearance of the third harmonic of the dirve waveform, followed by fifth, and finally a touch of seventh when I really pushed it. However, when I did the same to the Fasel inductor, the onset of saturation-generated harmonics happened a bit sooner, and a second harmonic appeared with the third! As I turned the drive up, the fourth rose with the fifth, and I never got a seventh harmonic. The inductor, all by itself was clipping asymmetrically.

I queried some older and wiser EE’s who have spent a career on magnetics. We came to the conclusion that the only way this could happen was if the inductor core had some kind of magnetic offset in it, so one polarity of the waveform saturated earlier than the other. However, none of them had ever seen this in a signal inductor like the ones I was testing. The only good explanation was that the inductor core itself was carrying a magnetic offset, a whiff of permanent magnetism. This was mildly astonishing because that is something that linear ferrite cores are explicitly designed NOT to do.

The best explanation I could come up with is this. The inductor in the classic wah setups carries the DC bias current for the first transistor. While this is only microamps, long exposure to this unidirectional bias could result in a remanent magnetization of the inductor core if the core material was not very good in the classical, linear EE sense. It’s possible that Vox merely specified the circuit, the maker (Jen, I think, in Italy) made the early wahs from as inexpensive a material as they could, and the slight deviation from linearity resulted in a sound that the folks at Vox liked. That is – it was a happy accident resulting from being cheap. I’ve never heard another explanation that accounts for the differences. There are differences, and measurable ones, and ones that square with reasonable explanations for how the thing works and sounds. This legend’s true.

I have not seen or heard any of the supposed “next generation” Fasel style inductors, so I can’t say whether they are true to the originals.

One thing that became obvious is that you could artificially get a more linear core material to have an offset, and in the easiest way. If we’re always pumping current through the inductor, we can get any offset we like by just pumping more. If we were to put a second winding on a wah inductor, we could force DC through it from a current source circuit, which would force the “center” of the magnetic operation toward one or the other saturation points. Of course, this is not possible with a pre-wound and potted wah inductor, but is emminently feasible if you happen to wind your own. It’s even more feasible if the inductor you use happens to have a second winding, like the Radio Shack transformer that is mentioned later. This secondary can just be hooked up and current fed through it. I intend to do this as as soon as I get some bench time. Note that I’ve been saying “current source”. You can’t just use a resistor, because transformer action would reflect this resistance into the inductance winding as a load and damp the resonant action of the inductor. The minimum you need is a transistor connected as a current source to keep from doing this.

Pot Secrets

The potentiometer (pot) that gets rotated to make the wah do its thing is also a source of myth and legend. While the original Italian Vox pots were almost certainly off-the-shelf things, they were obtained at a time before the MBA’s convinced everyone to keep far fewer things on the shelves than they do today. As a result, “off the shelf” might well have had a far richer meaning than it does today. The questions surrounding the pot are (a) what value of pot resistance do you use and (b) what taper is the resistance in the pot?

As to value, the earliest wahs are supposed to have used 470K, 500K or 1M pots here. All modern pots use 100K. There is still more work to do to find out how this affects tone.

Taper is a big item. It’s pretty certain that the pot taper in commercial pedals is not linear. An audio taper comes close, but wah afficionados say that it’s not quite right. The best candidate seems to be a semi-logarithmic taper (like audio, but not as extreme) or a semi-log taper. Part of the search for perfect taper is a result of the fact that the full mechanical travel of the wah rocker pedal will not turn a normal potentiomenter through its full 300 degrees of mechanical rotation, so there is some pot travel that remains that is not used because the rocker can’t turn it far enough.

The Teese Wah Pot is reputed to have “dead zones” at each end of it’s travel, possibly a linear taper between the extremes. The Fulltone pot is also reputed to have a modified taper. Whether the HotPot, Rock Potz, and others have standard tapers or not is not well known.

Now that we know that the pot is doing, we can do some things about making special tapers for pots. More on those later.

Tropical Fish Capacitors

“Tropical fish” caps are named for their multicolored outer surface. Don’t know the material. The other parts make so much difference that I would recommend using current production Mylar or polypro and twiddling all the rest of the stuff before counting on the cap material to be a big deal. There may be something here – I’ll do more digging.

What Affects What? – or – What Do I Change to Make It Do (whatever)?

The basic wah circuit itself can be modded to do a number of things that might be useful. To determine what to change to get what effect, I threw the circuit into my circuit simulator and looked at what happened to the responses. Using the following schematic, we’ll look at what happens. In the schematic I have “genericized” the naming of the parts so we can talk about them by something other than their values.

From our earlier tear-down report, we know that Q1 is a voltage gain amplifier, and that Q2 is an emitter follower that just buffers the signal voltage from the wiper of the Wah pot, Rw. We know that the inductor is fairly passive, and only participates by setting the L side of the LC filtering. Let’s take a look at every part, with some comments on what the parts do and what happens when they change.

Part What it does
Q1 Voltage gain transistor. Open loop voltage gain is partly determined by this. Has some effect on the overall sound. Its distortion, if any, contributes to the tone of the pedal. There is some tone change to be had by substituting for this transistor as a result. This is the one to substitute if you can only find one “5117” transistor.
Q2 Emitter follower transistor. Very little effect on tone as long as the Hfe is large enough. Gains of 200 or greater will all sound pretty much the same.
Rin Affects the overall signal level out. Lowering this will increase the output signal level. Somewhere between 33K and 47K gives you unity apparent gain with true bypass switching. However, lowering it also lowers the input impedance and therefore increases tone sucking if you don’t either use true bypass or buffering at the input.
Rfb — still working this one —
Rc1 Directly affects biasing point and gain of Q1. Open loop gain goes up as this goes up, and at the same time, the bias point on Q1’s collector goes down, moving it closer to saturation. May affect tone if it moves Q1 into a nonlinear region for part of the signal. Large values (47K to 100K) will almost certainly cause distortion.
Re1 Directly affects biasing point and gain of Q1. Open loop gain goes up as this resistance goes down, and at the same time, the bias point on Q1’s collector goes down, moving it closer to saturation. May affect tone if it moves Q1 into a nonlinear region for part of the signal. Small values (0 to 200 ohms) may make good changes to the wah’s tone by moving the transistor into a soft saturation region of its biasing. The wah range also moves down as Re1’s value decreases.
Rb1 Primary bias resistor for Q1. This resistor largely determines the operating point for Q1. As it increases, the voltage at Q1’s collector goes up and vice versa. No effect on Q1 gain as long as it’s reasonably big because of the AC bypassing effect of Cbp.
Rb2 Secondary bias resistor for Q1. This resistor is the second major determinant of the bias point for Q1. As it goes up, the collector of Q2 goes down, closer to saturation. No effect on Q1 gain because of the AC bypassing effect of Cbp.
Rb3 Main biasing resistor for Q2. The value of this resistor is not very critical as long as it (a) is not so small that it offers a significant amount of signal leakage around Rw and (b) it is not so big that the tiny base current of Q2 drops a lot of voltage and lowers the DC voltage at Q2’s emitter and causes distortion. Probably anything between 220K and 2.2M works, although I haven’t checked those values closely.
Rq This resistor is the primary determiner of the Q, or sharpness of the bandpass/resonance effect of the filter Values lower than 33K make the filter less sharp, reducing the quality of the wah effect. Values up to 100K contribute to sharper, peakier, more resonant tones. If it gets too sharp, the wah effect can be lost because it may not hit harmonics to emphasize.
Rc2 Provides noise isolation for Q2. Can probably be omitted with some degradation in noise, or maybe no ill effects at all.
Re2 Not too critical. Probably OK between 4.7K and 18K. Not much effect on tone unless Q2 is biased into an area where it clips.
Cin making it bigger can allow more lows in and add fatness. If you want this, change it to about 0.1uF to 0.22uF.
Cbp Important that it be large enough to bypass all signal at its (+) terminal to ground. From 4.7uF on up, little effect on sound. As it gets smaller, the sound becomes more of a loudness variation and less of a wah. If this cap is defective, wah pedals sound like volume pedals.
Cout — still working this one —
Cef — still working this one —
Cf Primary determiner of the center frequency of the wah effect. Changing its value moves the whole wah sweep range. Bigger values move it down towards bass, smaller values move it up.
Lf The inductor. Just make sure it’s in the range of 400mH to 600mH, then tune with Cf.; “magic” inductors have properties all their own, and can add a sweet tone by virtue of their saturation charactersistics
Rw Sweeps the Wah. Usually 100K. The exact value may not be too important as long as Rb3 and the gain of Q2 are large enough. Can be modified with tapering resistors to get a specific sweep, and the sweep can be narrowed by putting fixed resistors in series with the outside ends of it. Usually people want the opposite, a sweep across more of the range in a smaller foot pedal travel.

 

MODS

Switched capacitors for Cf

Install a switch to select between different values of Cf. Smaller values move the sweep range up, bigger values move the sweep range down. You can use any kind of switch. A SPDT will give you two choices, a 1P6T switch as shown will give you six choices.

Add caps in parallel with the inductor
better to switch the Cf value

Vary Re1 up or down
A popular mod is to temporarily replace Re1 (stock value 470 ohms) with a 1K linear pot. As the resistance is decreased through the original value towards zero, the sound starts getting richer, as a consequence of the first transistor’s gain going up. The increase in gain is accompanied by a modest increase in distortion, accounting for the fatness. When the resistance gets near zero ohms the wah will be at or near self resonance, and will self oscillate at the low end of its range, then wah into notes as you rock forward on the pedal. Touchy, but cool! As the resistance of the pot increases above the nominal value the sound starts to get less “wah-ey” as the gain of the first stage drops and the feedback can’t make as peaky a resonance. You can either find a value you like and put in a fixed resistor with that value, or mount a pot somewhere you can twiddle it.

Change the Rq to change the “sharpness” of the bandpass.
If you change the value of Rq, the nominally 33K resistor parallel to the inductor, you change the sharpness of the resonance. Larger value resistors narrow the resonance band. Smaller values damp the resonance more and spread out the resonance band but make it less peaky, so the effect thins out. Some people swear by values like 51K as being incredibly good.

Cure for Pot Scratch, or Mod for Remote Wah (from Anderton)

This trick will let you either cure a scratchy pot (which is what Anderton originally proposed it for) or put the rocker pedal arbitrarily far away from the wah circuit. To do that, just remove the circuit board from the inside of the wah, being very careful to note what connects to what so you can undo this if you want. Neat diagrams help! Then make up a cable with shield and two wires in it. One supplies +9V to the top of the pot, the other carries the wiper voltage back to wherever the wah circuit physically resides. The shield carries ground. I did this with a Vox Reissue, and it worked great. There is still this little problem of how do you bypass it then, but I have to save some secrets.

Two Voltage Controllable Wah Schemes

Here are two ways to make a voltage controllable wah. In both cases, the actual circuit relies on a transistor to take the place of the LDR in Anderton’s wah retrofit. The first uses a P-channel JFET, which is on (low resistance) when its gate and source are at DC ground, and goes progressively higher resistance as the gate is taken more positive than the source.

The second way uses the collector-emitter resistance of an NPN transistor in the same way the LDR was used. In this case the resistance is highest when the control voltage is low, and gets lower as the control voltage feeds a trickle of current into the base of the NPN.

Both of these suffer from distortion as the signal level gets high. The LDR has no such problem.

 

Common Problems

Tone Sucking (loss of treble in the bypassed position)
The bypass switching in wahs up through the mid 90’s used an SPDT switch. The switch does not provide for true bypass switching, so the input of the effect is connected to the input jack at all times. This means that the wah pedal input loads down the guitar signal, and worse, loads it more at treble frequencies than at bass frequencies. The sound gets duller and less lively. There are two cures, and they work about equally well. First, you can put in a DPDT true bypass switch. Second, you can add a buffer in front of the wah to keep it from loading down the guitar signal. There is an article on how to build the buffer onto the internal wah printed circuit board at GEO.

Pot wear
With all the rocking back and forth that a wah pot gets, it gets more wear than any panel mounted control ever does. The mechanical slider that moves over the resistor element inside literally wears some of the material loose. This material can collect in ways that can cause the slider to lose contact with the resistor material, and when that happens, it makes a “scrackle” sound as the pot is rotated. There are two cures: either (a) clean the pot or (b) replace the pot.

Cleaning the pot should be regarded as a stopgap measure. It will help for a while. Go to a Radio Shack or an electonics supply house and get a spray can of “tuner cleaner” or “electronic contact cleaner”. Spray this into the pot while rotating the pot shaft. The scratchiness should be much better. Opinions vary as to whether you should buy tuner cleaner with a lubricant in it and/or lubricate the pot after cleaning it. Geoffrey Teese has advised people to spray inWD-40 after using tuner cleaner on the theory that the cleaner dries out the factory applied lubricant and WD-40 is a reasonable replacement. There are tuner cleaners that say they leave a lubricant on the surfaces.

Some people swear that any lubricant will accelerate the deterioration of the pot. The right thing to do is to consider cleaning as a temporary measure and replace the pot.

Replacement pots are available from several sources. Analog Mike, Fulltone, and Mojo sell a heavy duty 100K 2W pot for about $30. Dunlop sells the “ECB24 Hot Potz” which comes pre-assembled with nylon gear, nut and washer and is available in quantity from New Sensor Corp for $20.25 in a $100 minimum order. Analog Mike and Mojo both sell the Teese Roc Pot. Geoffrey Teese custom builds these pots to original taper of the original Vox Clyde McCoy pots. Fulltone sells a similar pot. In any case, count on spending $15 to $30 to keep your wah wahing. Some people think the replacements sound better than the originals.

I suppose since I’ve already put the schematic in, I should mention that you can use Craig Anderton’s LED/LDR trick from his GP column to fix a scratchy pot. The schematic has already been shown under “Mods”. This replaces the wah pot with a fixed resistor and LDR, and uses the original pot only to change the current on the LED. It works, and it’s very smooth indeed. The original pot will then last until it develops completely open spots.

Loss of signal level
After putting in a true bypass switch, people often find that they lose a bit of volume when the wah is kicked in. This comes from one of two places; either the forward gain of the wah circuit is a bit below one, or the loading of the wah circuit cuts the guitar signal down a bit. You can correct for this by lowering the value of the 68K input resistor somewhat (to 33K-47K maybe) to increase the gain. Notice that this also lowers the input impedance and may change the tone of the wah in the effect setting. The bypass setting will be unaffected because of the true bypass switching.

No “wah” sound, only volume change
Cbp is failing. Replace it with a new 4.7uF to 22uF aluminum electrolytic capacitor.

No “wah” sound, only treble change
Inductor Lf is probably open. Get a replacement.

“Wah” range noticeably decreased when certain effects used AFTER the wah.

The input impedance of the effect is loading down the output of the wah, as the input impedance of the next effect appears effectively in parallel with the collector resistor of Q1. This directly cuts the gain, which we’ve seen is responsible for the variable-capacitance effect that gives the wah its variable-frequency sound. The solution is pretty simple – buffer either the input of the following effect or the output of the wah.

Building a Wah from Scratch

Let me start this with advice – don’t do this, at least not the classic foot operated rocker pedal. It’s not because the electronics are hard. They’re not, they’re almost trivial. You don’t even have to come up with any super special parts, excepting the inductor, which we’ll cover.

Rather, it’s because making a reliable foot-rocker pedal to turn a pot is HARD. If you’re a good machinist or tinkerer and also play a guitar, OK, go for it. Otherwise, buy a dead Crybaby and refurbish it. The rocker pedal mechanics is not something to attempt lightly if you don’t have the tools to do the metal work. I often find repair shops have a pile of dead Crybaby shells in various states of cannibalization that they will part with fairly cheaply. You should consider making the whole mechanical setup only if you have no other good options.

The electronics isn’t too hard. Perfboard works well, and PCB’s aren’t too difficult to make for this one. I recommend getting your mechanical packaging settled first, then making sure that the board you’re going to build it on fits in side the mechanical packaging properly. Do this before putting parts on the board.

Beyond mechanical packaging of the pot rotation setup, the critical issue for wah builders is finding a 500mH inductor. There are several ways:

  • Get a dead Crybaby and cannibalize one out of there
  • Use some off-the-shelf inductors in series, as in Mouser’s 434-03-154J 150mH chokes ($1.36 each). Three of them in series works well.
  • Use a small audio transformer primary, ignoring the secondaries. Radio Shack’s 275-1380 transformer ($2.40 each) has been found to work OK. I’ve had email and seen postings that modem coupling transformers may work as well. Probably some of Mouser’s audio transformers would work, too.
  • Buy an after-market Wah inductor. Geoffrey Teese and Fulltone sell inductors that are designed to be equal to the original Vox inductors for great tone, or at least they did at one time. These tend to be expensive, maybe up to $30-40.
  • Roll your own. Not recommended unless you have a personal calling from some deity or other. It’s lots of work, and hard to find the very specialized parts and pieces.

Here are some pictures of the easiest solution, the Radio Shack transformer.

 

 

 

Twin-T Circuits

A much less mysterious circuit is the twin-T style wah. The naming of the twin-T is reasonably obvious from looking at the circuit. There is one “T” composed of two resistors in series and a capacitor to ground. The second T is made of two capacitors in series with a resistor to ground. The T’s are hooked in parallel. They’re not exactly twins, but close enough for naming. The twin T network all by itself is a notch filter. As a sidelight, it’s about the only circuit composed of only resistors and caps that can have an infinitely deep notch – that is, if you tune the values of the R’s and C’s perfectly, the frequency at the notch of the filter will be completely removed. Most applications don’t need this perfect tuning, and in particular wahs don’t. To get a bandpass (humped) response, we use the twin-T network as negative feedback around an amplifier. The Twin T passes all frequencies outside its notch, but attenuates the frequencies near its notch. Since this is negative feedback, the frequencies we pass through most easily reduce the gain most, and the frequencies that are not passed through the feedback network are not reduced by feedback, so they have a great deal of gain. The negative feedback connection therefore inverts a frequency notch to a frequency hump, which is exactly what we need for a wah.

This circuit is taken from the 1970 Popular Electronics article “The Waa-waa” by Simonton. I’ve deleted the switching so we can see how it operates. The first transistor is a high gain amplifier, with the actual gain settable by the 1K “Q” trimmer. The second transistor buffers the collector of the first so that output loading will not affect the gain. The twin T is clearly discernable. The control resistor is just a variable resistor to ground.

Multiple Feedback Opamp Circuits

Another bandpass filter that can be wah-ed back and forth with a variable resistor is the multiple-feedback active filter. These things use RC networks around opamps to simulate the second order response of an LC network. As you can seen in the simplified schematic, once again only one variable resistor to ground can vary the center frequency of the bandpass over the useful range of frequencies for wah pedals.

Both of these circuits are what I call resistor-to-ground wahs – their center frequency varies with only a single variable resistor to ground. This is much simpler to automate electronically than the inductor style wah, where the wah frequency is controlled by a three terminal voltage divider. The disadvantage of these two circuits is that they don’t have the subtle distortions that the inductor circuit does, and so they don’t have as interesting a tone – on their own at least. As usual, that merely means that they have other uses for which they’re best suited.

 

Messing around with Wahs – What can we do?

 

The resistor-to-ground wah circuit gives us a simple way to mess with the center frequency. Anything that makes for a variable resistor to ground will make them wah, so we can play tricks. The resistor can be any style – an LED/LDR module, a JFET, a diode string, even the collector-emitter resistance of a bipolar transistor, as well as a plain old pot.

 

 

 

One of the simplest things we can do is to just drive the variable resistor with a Low Frequency Oscillator (LFO) like this:

This was another application that was published by Simonton and originally used the Twin T circuit, but it works equally well for the multiple feedback circuit or the inductor wah as converted to resistor-to-ground, as we’ll see later. Although this is not a very good simulation of a real leslie cabinet, it does have its own unique tone, and is a lot of fun. It has a tone reminiscent of the organ in a county-fair merry-go-round.

 

 

You can also convert this into an auto wah by just deriving the control voltage from the loudness of the incoming signal, and using that to modify the center frequency.

 

This very arrangement is the essence of several commercial pedals. The prototypical one is the Dr.Q follower from EH.

 

 

 

 

 

 

 

And here’s how to convert the inductor style wah to a single variable resistor to ground.

 

 

 

 

 

 

 

 

 

 

 

More than one wah at a time

There’s no law that says only one wah at a time sounds good. In fact, it will sound more like human speech if we use two. Let’s look at some ways to do that.

With two resistor-to-ground wahs, we can make an adaptation that uses a single pot to move both of the frequencies around in opposite directions at the same time

 

 

To do the same thing with inductor style wahs, we can either first convert them to the single-resistor-to-ground circuit first, or we can use a dual pot. This is a pain because we’d have to get a log/antilog pot. Tapering resistor as noted in “The Secret Life of Pots” come to the rescue, as we can make a dual log/antilog pot from a dual linear one.

Notice that in this sketch, I’ve shown the input buffer to prevent tone sucking by the input loading, and a resistive mixer to mix the two outputs that was not shown in the first wah/antiwah sketch.

 

 

 

 

 

 

As noted in the blurb on human speech and formants, for some really interesting vocal effects, we’d like to have the two wahs move not just opposite one another. We can do this with another pot trick. If we have a wah that only needs a variable resistance to ground, nothing says how we have to use the end lugs. We can ground both of them if we like. That means that the resistance from the wiper to ground is maximum in the center of the pot’s rotation, and gets smaller whenever we turn it away from center in either direction. The wah’s center frequency will be lowest in the middle, get higher toward either end.

If we combine a stock wah with a wah set up for center-lowest by using a dual linear pot, we can get a much more vocal quality. EH did much the same thing in the “Talking Pedal”. It used two multiple feedback wahs in this circuit. The exception was that it used a special dual pot where one section had a tap near the center of the pot rotation that was grounded. The wiper then was closest to ground in the center, but had the highest resistance to ground at either end. It’s possible to disassemble a pot and paint on a tap with conductive paint if you’re desperate – see”The Secret Life of Pots”.

 

 

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Yamaha SPX50D

https://usa.yamaha.com/files/download/other_assets/9/317169/SPX50DE.pdf

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Yamaha FX500


YAMAHA HAVE ALWAYS been at the forefront of the development of effects processors. From fully professional reverb units like the REV 1, down to the popular multi-effects unit, the SPX90, their presence has always been felt – or heard. The only possible hole in their operation is at the budget end of the market. True, the REX50 added overdrive characteristics to their range but it was a budget sound at a budget price. The current vogue is for units which offer four or five effects at the same time, complete with some degree of sound quality. Well, we need wait no longer for Yamaha to respond because the FX500 is now ready to take its place in the music shops.

DESCRIPTION

AND A SMALL place that will be, as Yamaha have decided to go for a 1U-high half-width rack unit, plastic encased with an external power supply.

The basis of the FX500 is that it has five “modules” available, namely; Compressor, Distortion, Equaliser, Modulation and Reverb. The last of these encompasses both reverb and delay and so has various options for these effects including the order in which they occur. More of this later. Quality? 20Hz-20kHz quoted bandwidth with 16-bit resolution and a sampling rate of 44.1kHz is now becoming the norm, but let’s not get blasé about it. At this price such characteristics are impressive. Memory-wise, there are 60 presets and 30 for you to program.

The front panel is pretty self-explanatory. The rear has a single input (the front panel input takes precedence) and two outputs with a flick switch for level, -10 or -20dB. A rather odd choice this, because it makes the unit a little difficult to match up with any equipment intended for use with the professional level of +4dB. Headphones socket and level control (nice touch), MIDI In and two footswitch sockets for bypass and memory change/trigger complete the line-up.

INDIVIDUAL EFFECTS

EACH EFFECT HAS various programmable parameters pertaining to its nature, and an output level to set the overall gain for each stage. As there is only a single input, the first three effects are mono.

The Compressor has parameters for Threshold (-6OdB-OdB), Compression Ratio (1:2, 4, 8 and limit) and Attack Time (1 to 20 milliseconds).

The Distortion has variable Amount which also increases the volume, Trigger – an expander noise gate – (threshold between -80 and -30dB), and LowPass Filter (thru or 400Hz to 16kHz).

 

“The FX500 boasts four types of basic reverb with reverb times of up to 40 seconds, High Frequency roll off and a pre-delay of up to 335 milliseconds.”

The Equaliser operates in three bands. The High and Low are of the shelving type with variable cut/boost of -15/+15dB while the Mid band sweeps between 400 and 6300Hz with gain of -40/+15dB.

The Modulation comes in four types: Flange, Symphonic, Tremolo and Chorus. Each has variable oscillator Speed (0.1-20Hz), Depth and Mix between incoming signal and effect. Parameters specifically associated with each effect are also included. For instance, the Flanger has Feedback, while the Chorus has both Pitch and Amplitude Modulation. Everything you would expect from a complete multi-effect unit. The Reverb section is most impressive. There are seven choices, the first of which is Reverb(!). Four types of basic reverb (Hall, Room, Vocal and Plate) with reverb time up to 40 seconds, High Frequency roll off and a pre-delay of up to 335 milliseconds. Other manufacturers should pay attention to this because pre-delays are imperative for the accurate setting up of that first reflection and the times existing in many units are nothing short of useless. Next follows Early Reflection with Hall, Random, Reverse and Plate being the options, again with up to 400 milliseconds for the predelay. Delay (up to 740 milliseconds per side) and Echo (up to 370 milliseconds) each have Feedback and Left/Right balance while Reverb + Delay combines the best of both worlds. Finally, the option of whether the Reverb feeds into the Delay or vice versa. Comprehensive, or what?

A quick glance at the front panel shows that there are LED’s above each of the effects and that the Modulation and Reverb modules can be changed around in order. This is particularly helpful as they are dealing with stereo effect.

EDITING PARAMETERS

WITH ANY MULTI-EFFECTS unit, ease of programming is important. Starting from a preset close to the required effect, how easy is it to achieve the necessary result? Judge for yourself. After selecting one of the current memories, the LED’s above each effect will either light up to show that effect is in use or not. The buttons below each effect’s name are toggle switches and by holding down the Parameter button at the same time as pressing one of these, edit mode is entered for this effect. Further pressing of this effect button at this stage effectively causes the effect to be played solo or in line as part of the overall setting. Once in edit mode, the Memory and Parameter buttons double as cursor keys to allow for movement across each page with the effect button moving through the various pages of parameters for each effect (three maximum) and the up/down arrow keys change the value of the selected parameter. Then press another effect, either leaving the previous one in line or “muting” it, and continue on your way. The result can be stored at any point. A piece of cake.

MIDI FUNCTIONS

OVER THE LAST couple of years, MIDI in the context of effects units has evolved through simple patch changing to the situation where MIDI controllers can alter the values of parameters. The FX500 has a program change table which allows you to set up which memory location is called from a MIDI patch change command and also has the capability of allowing any two of 28 listed parameters to have their values changed by MIDI information. MIDI controllers 0 to 31 (continuous), 64 to 95 (switches) and 102 to 120 (undefined) can all be used, as can the note on/off velocity and channel pressure (aftertouch). For instance, the distortion amount could be controlled via a foot pedal sending out portamento time (MIDI controller No. 5) and to this end the Anatek Pocket Pedal (reviewed last month) would be invaluable as it can inject two extra MIDI controllers into the MIDI system. The range of the pedal can also be set so that full movement will only result in the chosen value changing within specified limits.

MIDI aside, one of the footswitch sockets on the rear panel has a dual identity. It can either act as an increment/decrement switch for memory numbers or can set up the delay time by tapping in via a footswitch. I couldn’t test this but would assume that a non-latching pedal would do the job.

 

“The FX500 is destined to be a best seller, make no mistake – excellent audio sound, easy to use, low noise… the list is practically endless.”

So it would appear that the FX500 has a lot going for it both in the way of effects and facilities. Now, how does it perform?

IN USE

THE ACID TEST for a multi-effect unit has to be the quality of its reverb. Most of the other effects are quite easy to implement, but a good quality reverb usually equates to dosh – loads of it. In this respect the FX500 scores well. There’s a slight flutter when long reverb times are selected, but otherwise it’s pretty well grain free. Another useful test is to check how long delay repeats remain faithful to the original. Again, the FX500 presents no problem. Even when the feedback is increased to the point where the repeats are of the same volume as the original there is still little difference.

How about a guitar on the input? After all, many of you are going to be using this as a replacement for guitar FX pedals. For clean sounds, the FX500 is excellent. That typical sparkle which we have come to expect of Yamaha since the SPX1000 was introduced is certainly evident here and the various modulation effects really made my faithful Strat sing. The compressor tightens up funk chords nicely and is reasonably quiet. Unfortunately the fly in the ointment has to be the distortion which shows characteristics typical of digital fuzz – harsh even when only slightly in evidence, although no glitches as used to occur with the REX50. The low-pass filter is useful for removing the buzz and the expander noise gate keeps things quiet when you stop playing, but overall I felt that this was a letdown.

Distortion apart, I have to admit to being impressed with the FX500. The sound practically glistens as though it’s being passed through an aural exciter, which is most pleasant and cuts through on mixes. The parameters have been well selected and care has been taken with most of the little things – like incorporating the type of reverb with the name of each memory location.) The stereo image created by the modulation effects is also worthy of special mention.

Battery:CR2032

– Unsolder the pins beneath the PCB, you have to unscrew almost everything to accomplish this…

(optional) – Get a battery holder (http://www.ebay.com/itm/Lot-5-Pcs-N…772?pt=LH_DefaultDomain_0&hash=item2a1ee765bc)
Make sure you bend one of the pins in the holder because it will not fit properly in the PCB.

– Solder.

– Place battery.

– Plug the unit to AC and wait a few minutes before turning the unit ON.

– Factory Reset (Hold Comp and Mod + press power button).

– Voilá

http://www.muzines.co.uk/articles/yamaha-fx500-multi-effects/142

https://usa.yamaha.com/files/download/other_assets/4/320544/FX500E.pdf

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VOX847 Axis Wah Modifications

Axis Wah Mods

A Great Sounding Wah…On A Budget
Over the years, the wah pedal has become one of the most popular and influential of all guitar effects. Many famous guitar players have used this piece of equipment in their songs…who can forget the opening of “VooDoo Chile (Slight Return)” or the awesome wah pedal parts of “Machine Gun” both by Jimi Hendrix. All of these famous guitarists have created a huge demand for the wah pedal…but unfortuntely all is not well in the world of mass produced wah pedals. The sound of these pedals tends to be…not so great. But fear not, ye tone seekers, for there is a remedy to your wah pedal ailments…and it won’t break your bank account, either!

A couple of years ago I exposed the guts of the famous Vox Clyde McCoy wah pedal for all to see, and even built a clone of the wonderful pedal, but the problem is that it required the purchase of a replica Halo inductor and a replica of the ICAR-taper wah pot, which can end up being quite pricey. As a result, I’ve been working on modifying a Vox V847, that I just received a few days ago, into an excellent sounding pedal…minus the new inductor and wah pot! That’s right! For this project we’re using the original Vox inductor and the original Vox wah pot! You can even use the original wah pedal circuit board for this project too…but I etched a new one just so I would have a fresh start, and a fresh look. I’ll include the PCB and Layout files for the replacement board if you decide to take that route.

There are really only two major changes being made to the pedal…the addition of true bypass switching and the addition of an output buffer circuit board, which allows the wah pedal to work in series before a fuzz pedal. The other changes are all quick and simple resistor and transistor changes, which can be made quickly and easily by using a desoldering braid.

True Bypass Switching
Okay, so the first thing we need to do is to install the DPDT switch for true bypass. If you have a Vox V847 wah, then follow this diagram, and if you have a new Dunlop Crybaby, then you will need to follow the directions on this page since the operation for the Crybaby is a little more complicated with the PCB-mounted jacks. For the Crybaby, I would recommend that you follow the second set of intructions…”Eliminating Input Buffer.” The input buffer will no longer be needed with true bypass switching. Both of those great true bypass conversions are located on Stuart Castledine’s website, so be sure and check it out! Make a note that if you wire the bypass switches as shown in those great diagrams, then you may omit the 1M resistor in the input of the circuit that’s shown in the schematic that’s shown down the page a little.

Adding an Output Buffer
The second operation that we’ll perform is adding the output buffer circuit to the wah pedal. This step is optional, but I would certainly add it if you plan to use a fuzz pedal (like a Fuzz Face) in series after the wah pedal. A common problem is that the wah pedal simply won’t wah when put in series before a fuzz pedal. Unfortunately, this is the way it sounds the best to most people. This problem can be solved by adding an output buffer to the Axis Wah, which won’t alter the tone of the pedal. This is a simple JFET buffer that’s based on the Wah Wah project at Tonepad, and can be added to any wah pedal that doesn’t have an output buffer, namely the Vox V847s and the Dunlop Crybabys. The input impedance of this buffer is set by the 1M resistor from the Gate of the JFET to ground. Since this buffer can give a slight vme boost, so a 100K trimpot is on the output of the buffer to act as a volume pot so you can keep the volume of the wah pedal at the same level as when it’s off. The PCB and Layout for the output buffer are in the “Project Files” section at the bottom of this page.

Resistor Changeups

  • First, if you’re using the true bypass wiring instructions on Stuart’s website, then you can just forget about the 1M input pulldown resistor, but if you’re wiring the switch like this, then you’ll need to solder the 1M resistor to the solder side of the circuit board, from the input end of the input resistor to ground. This resistor will prevent a loud “pop” when the pedal is switched on and off.
  • Next we’ll replace the 68K input resistor with a 47K resistor. This will give a little volume boost to go along with the true bypass switching.
  • To enhance the mids of the pedal, and to smooth out the bass to treble transition, we’ll replace the 1K5 resistor on the base of Q1 with a 2K2 resistor.
  • The next resistor to change is the 470 (or 390 or 510) resistor on the emitter of Q1. Using a lower value will increase the gain of the pedal, and it will also enhance bass response, which is sometimes a problem with some pickup configurations. For this change I chose a 330 resistor, which isn’t too big and it isn’t too small. Using one that’s too small could result in distortion, which we don’t want.
  • The final resistor change that we’ll make to the circuit is the “Q” resistor…the 33K resistor that parallels the inductor. To help give the wah pedal a more vocal quality, I’ve chosen a 47K resistor, which isn’t quite as drastic as the 68K that some people use. It’s very nice sounding and certainly an improvement over the original 33K.
Changing the Transistors
The transistors that come in the Vox V847 and Dunlop Crybaby pedals are the high gain, general purpose MPSA18. I personally think that these transistors are way too high gain to make a good sounding wah pedal. The toe-down position is quite ear-piercing and unpleasent. To help make a more mellow wah pedal, I’ve chosen to use a pair of nice BC109C transistors that I ordered from Steve Daniels of Small Bear Electronics a while back. For Q1 I chose a unit with a gain of 400, and for Q2 I chose one with a gain of 388. The end result was a very nice, a bit more mellow sounding wah that’s not as ear-piercing when toe-down, and the heel-down is still nice and fat…the bass response actually made the springs in my Twin Amp’s tube shields rattle!Below is a schematic of the Axis Wah with all modifications mentioned above.
Axis Wah Schematic

Vox V847 Wah Schematic Blocks

http://fuzzcentral.ssguitar.com/axiswah.php

https://www.electrosmash.com/vox-v847-analysis

 

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