SVF troubleshooting

I’m getting no sound from filter2. No matter what settings I try. Switching the mode sometimes results in an audible change in level, but I think that’s just because of the routing alterations. I have never heard any change in sound by tweaking its cutoff or resonance.

Filter 1 seems to be fine. Where should I probe the board to find out what’s wrong?

Looking at the schematic, filter 2 outputs via C22 and R33 (33k).

Check on pins 12, 13, and 1 of IC12 that you hear the LP / HP / BP signals.

If yes > something wrong near IC12, C22, R33 the filter core is OK but the signal switching/mixing is wrong.

If no -> SVFs are hard to troubleshoot because they can’t be decomposed into smaller pieces (if there’s something wrong somewhere, the whole thing crashes on one supply rail). One very simple thing to check, though: voltage on pin 7 of IC4 should go from 2.5V to 0V as you sweep cutoff.

I have only sound on the HP output, but there’s no amplitude envelope on it / constant sound. BP and LP are quiet.

IC4 pin 7 is a constant 2.12V and doesn’t respond to the cutoff. I also tried IC4 pin 1 and that one works as you described.

One thing I noticed when tweaking the v/oct of filter 1 is that I had to keep it all the way to one side (counter clockwise) to get it to tune, and even then it wasn’t completely scaled correctly (few cents off). So there was no way to correct it since the pot was already at its minimum.

I will continue to stare at the resistor values now. Hope this provides some clues.

Thanks for helping out.

Checked all the resistors carefully. Nothing wrong there.

I wasn’t clear about this: since you are troubleshooting the second filter ; you have to sweep cutoff2 to see the voltage on pin7 of IC4 go from 2.5V to 0V.

Yes I understood that. I did sweep cutoff2 and pin7 keep at 2.12V Sweeping cutoff1 moves pin1 voltage correctly it seems.

Does that make any sense? I will try to replace the IC tonight I guess.

Alright. Then you have spot the source of the problem since this means filter2 is always at minimum cutoff. This also explains why you have sound only on the HP pole (a HP with a cutoff of 10 Hz doesn’t change the sound much, a BP/LP with a cutoff of 10 Hz kills everything audible).

Things to check:

  • CV2> pin of the MCU should vary from 0 to 5V as you sweep cutoff2.
  • With the two boards connected, resistance between CV2> pin of the MCU and pin 6 of IC4 should be in the 32k … 39k range.

Ahaaa! I’m such a muppet sometimes. I didn’t have cv1/2 connected in my test setup. I used the same wire configuration as I did for the other filter boards, not realizing that now cv1 and cv2 are actually in use. I had connected the output expansion pins instead :s

The filter is perfectly fine. Sorry for the noise and thanks for helping me out!