Doepfer

Hi Mick,

> Can someone tell me if my A102 filter is acting correctly - The resonance seems to be set really high.  The filter will self oscillate with the resonance set to just 4 (when the cut off is set between 3 and 6).
> 
> Is this normal?

Yes, both mine behave similarly - basically there is way too much gain around the feedback loop.

> Is there any way to adjust the resonance to a more usable range?

I tried a couple of things very quickly (the first one was spectacularly ineffective!). However the following seemed OKish: between C10 and Q7 there are 4 resistors - the 2k2 amongst these is R24 (the 'bottom one' if you hold the faceplate nearest you); swap this for a 5k6 say, which will cut the gain to a third, so it won't resonate until the pot is further round. A 4k7 may not be enough of a difference, and a 6k8 (I temporarily added a 4k7 to the 2k2) may restrict the range of frequencies at which it will oscillate. Unfortunately it will also reduce the output amplitude somewhat.

To do the job 'properly' would mean re-vamping the entire 'gain regime' around the loop, which would require considerably more thought, and replacing maybe half-a-dozen or so resistors. My initial idea was to *double* R5 (the 10k in the middle of the only group of 5 resistors, central to the board), but this seemed to have little impact, mainly I think because the resonance pot is a 'law C' ('rev log'), and so naturally 'all the action' is in the first part of its rotation.

Let us know if you have any success with this!

Tim