Tweaking the Doepfer A-124 Wasp Filter self-oscillation mod

cementimentalCementimental wrote 03/28/2020 at 13:58 • 2 min read • Like

I modified my Eurorack Wasp filter module a while back, adding a 10k resistor to allow it to self oscillate as suggested on the Doepfer site. However I found it didn't work well, it would only really get oscillating in a limited range and above or below would drop out again and stop. To try and improve this I added a 20k trimmer pot instead, and also tweaked the two existing trimmers, which are for the offset (left trimmer) and range (right trimmer) of the frequency control, I believe. Now the oscillation has a pretty full range and the pot's range is generally more playable; previously quite a large part of the pot's turn cut out the sound entirely. It still won't start oscillating at the extremes of the range but works well for my purposes. Haven't measured the resistance which the trimmer I added is set to, but it presumably ended up lower than 10k.

The original mod, 10k resistor added in parallel to R13:
10k removed and replaced with 20k trimmer (it's not very clear in the photo but the top leg is cut off and not soldered to or touching anything):
My original on-board trimmer settings (marked with a sharpie in case I needed to go back to them) :

My new offset and range trimmer settings: