Friday, June 24, 2011

Modifying the A-124 Wasp Filter for self-oscillation

Out of the box the A-124 Wasp filter can not go into self oscillation, in contrast to most of the other filters in the A-100 system.
Lucky for us modifying the module for self-oscillation is quite easy;
Doepfer's DIY page tells us that;

"Soldering a 10k resistor in parallel to R13 (27k) leads to self-oscillation of the filter at the max. resonance setting of the resonance control."

Locating the R13 resistor was perhaps the most difficult part of the job. Because the resistors are all soldered on the PCB very tightly it is hard to read what the printing on the board says about the mounted resistors... but i found it!
Locating the R13 resistor
R13 is located in the gap between IC1 (CD4069) and IC2 (i.e. the upper CA3080, close to C4/100pF). See picture above.

I did have a few spare parts lying around, including a 10k resistor. Soldering in parallel in this case was very simple. I did this on the backside of the PCB.
After locating the correct resistor you can simply solder the new resistor on the other side of it. Not much can go wrong.
Just be aware that your soldering-iron doesn't overheat or burn your resistor.

I read mixed reviews of this modification, but you have to remind yourself that the A-124 itself acts kinda unpredictable in the first place, due to the design that "abuses" digital inverters as analog operational amplifiers leading to distortions and other "dirty" effects.

A post-modification audio demo by Madrayken ( Dene Carter ) can be found HERE


  1. Thanks for this! FYI, it doesn't matter which direction resistors go.

  2. Thanks for the feedback @recompas. I changed it in the post.

  3. Cool. I just did the mod on my filter and I got self-oscillation in band pass mode only. is the the case for you?

  4. Hehe... i'm not sure what i hear, but in band-pass mode it is clearly self-oscillating indeed.
    At the LP/HP output, with the mix knob completely to the right, the resonance completely up, and the Frq knob at around 2 or 3 it does sound like some self-oscillation to me. I'm not sure how to check though... But thanks for the feedback!
    ( FYI: pictures are verified by Dieter Doepfer himself, so no worries )

  5. Thanks for this info, I did this mod a while back and it worked OK but the range in which it would actually self-oscillate seemed limited. I just today replaced the 10k resistor with a 20k trimmer to tweak it, and got much better results - didn't measure the resistance I tweaked it to yet but a little under 10k I presume. Also fiddled with the two existing trimmers on the board, which control the offset and range of the CV control. Pics and full details of my mod here: