> HI there I want to mix some signals in my matrix,
>
> I have a frostwave resonator pedal that outputs 15V
>
> I was wondering if I can put it in the a-138m and what kind of voltage would I expect out in that case

Voltages beyond -12V and +12V may damage the module ! Please do not apply 15V voltages. A passive attenuator (e.g. A-183-1) may
solve the problem but you have to pay attention not to increase the attenuator control by mistake.

Best wishes
Dieter Doepfer