Doepfer

I have looked at the Doepfer DIY page and did some web searching but have found no posted schematics or mod diagrams. Basically I want to mod a A-138 mixer to give gain if that is easily done. I would assume it is a simple resistor change in the feedback loop of the last stage op-amp but without a schematic or assembly drawing I don't feel like tracing the circuit out to figure which resistor to change, I am looking for modest gain, enough to boost a typical line/headphone out of a MP3 player to levels that work nicely with a synth rig. I have a pair of A-119s already but they seem wasted on some applications where I don't need the other bells and whistles they offer, sometimes all I need is gain - no gate detection or envelope followers. I know it is a dead simple circuit to make too but since I have six A-138s in my rig, I thought it would be easy to just mod one. All my A-138s are the older versions (without the DC offset) if that matters. Thanks!

-James

> I have looked at the Doepfer DIY page and did some web searching
> but have found no posted schematics or mod diagrams. Basically I
> want to mod a A-138 mixer to give gain if that is easily done. I
> would assume it is a simple resistor change in the feedback loop
> of the last stage op-amp but without a schematic or assembly
> drawing I don't feel like tracing the circuit out to figure which
> resistor to change, I am looking for modest gain, enough to boost
> a typical line/headphone out of a MP3 player to levels that work
> nicely with a synth rig. I have a pair of A-119s already but they
> seem wasted on some applications where I don't need the other
> bells and whistles they offer, sometimes all I need is gain - no
> gate detection or envelope followers. I know it is a dead simple
> circuit to make too but since I have six A-138s in my rig, I
> thought it would be easy to just mod one. All my A-138s are the
> older versions (without the DC offset) if that matters. Thanks!
>
> -James

As suggested you simply have to increase the feedback resistor. For the old
version of A-138a/b that's R7 (100k). R7 is located at the edge of the pcb
next to the integrated circuit (TL062 or TL082).

Best wishes
Dieter Doepfer