Wednesday, November 28, 2012

A-136 Pre-amp Modification

I discovered another nice light-weight DIY-project on the Doepfer DIY-page.

Locating the R2 resistor
It seems that the A-136 Distortion/Waveshaper module contains an internal pre-amplifier that is used to amplify the input signal by about 3.5 before it is processed by the A-136.
Especially for low level audio signals (e.g. output from a VCF) this is useful.

But for all signals with a level beyond ~ 7V this causes clipping before the internal processing takes place. 
Especially for the processing of LFOs or unfiltered VCOs this may cause a problem.

The wiring, additional pictures
on the PatchPierre Facebook page
To totally bypass the internal pre-amplifier, resistor R2 can be removed (e.g. by pinching off).

R2 is the resistor in the upper third of the pcb which is very close to the rear edge.
When R2 is removed the amplification of the internal pre-amplifier becomes "1" (which means that it does no longer amplify).

Finished modified A-136 

Because i love to keep myself busy (I'm slightly ADD-ish), i decided to add a switch to the module, so i can switch between pre-amplifier on and off modes.
It looked like an easy project, so... what could go wrong? ;-)

Besides the front-panel it was a pretty easy job;
I desoldered one side of the R2 resistor, bent it over towards the switch, and wired it with black wire.
After that i soldered another wire (red) from the switch to the hole where R2 used to be. Easy as that.

Now i can manually switch the internal amplifier on and off when required.
The clipping did annoy me at times and now i prevent that and try the un-amplified signal with alternate settings*.
it's not a groundbreaking project, but a nice extra.

* In addition one has to pay attention that the knob positions may vary a bit from the front panel printing because of mechanical tolerances of the potentiometers and knobs (i.e. "0" is not always exactly the neutral position for "A", "+A" or "-A")

The modifications described on this site will most likely void any warranty and, if not done carefully, can damage the circuit board, IC chips, and faceplates.
PatchPierre can not be held responsible for any damage.