"LFO" type effect on filter at high resonance

[chrismcg]

Hi there,

I've just finished building a x0xb0x from xoxshop.de and while the rest of the box seems fine I'm having trouble with the filter at high resonances. I'm not sure this qualifies as the filter warble mentioned in the troubleshooting faq. I've posted a recording of me playing some notes in Keyboard mode and moving the resonance and cutoff around (later on I move the envelope too) at https://dl.dropbox.com/u/781826/WeirdFilterProblem.mp3

I built some test projects and a Shruthi-1 before tackling the x0xb0x and kinda unfortunately they worked first time so I got no practise debugging! I have borrowed an old analogue oscilloscope which I can use for testing but I'm not sure what to check.

Anyone heard this problem before and/or can give me advice on what I should be checking? I've tried with another PSU and that didn't seem to make any difference.

TIA - Chris

[MMM]

Sounds to me like the "overshooting resonance" problem I've encountered. My guess is that the filter ladder is out of balance; a likely culprit would be Q22.

See also http://forums.adafruit.com/viewtopic.php?f=12&t=30490 including the audio example.

[chrismcg]

That certainly sounds like the same problem and measuring the voltages on Q22 they aren't the same. Time to try a new 2291 in Q22, unfortunately I don't have another x0xb0x to take one from like the guy in that post so I'll have to order one and be patient.

[chrismcg]

Replaced Q22 and everything sounds much better!