Vortex Sound Research

Hi everyone

Just got this weird piece of gear and I’m kind of a synth/vocoder noob.

I can put my SM57 mic in and select formant input as “Mic” - but it will only change pitch when I turn knobs.

I tried plugging in a synth, and later a bass, into the formant input (and source input), but nothing happens, and none of the other LINE formant inputs light up when I cycle through them. What am I doing wrong? Please help ??