[sdiy] Rick Jansen Moog filter revision -> problem!

[Rutger Vlek]

Hi everyone,

recently I've been working on a revision of Rick Jansen's Moog filter schematic with parts that are (for me) easily available. I added CV control for the resonance by means of an LM13700. I've created a PCB with 8 of these filters and DAC's to control frequency and resonance. Everything seems to be working fine except that there's no sound and I can't pinpoint what's causing the silence.

Maybe you guys have some good idea's. I built the filter according to this schematic:

http://www.euronet.nl/~rja/Emusic/Moog/

Changes I have made to this schematic are:
- I've replaced all opamps with TL-series opamps
- I've added an LM13700 to the resonance feedback path for CV controlled resonance
- I left out temperature compensation 
- I've replaced all transistors (even the ones in the matched array) with BC549C's (which I plan to match when everything is working good).

The problem is that no sound comes out of the filter (even when sound IS going in, hehe ;) ). With a scope I confirmed that the signal reaches Q14. However, I can't see the signal being forwarded through the 'ladder' and it certainly does not reach the output buffer (IC5). I'm running the circuit on +12 and -12 V instead of 15. What could cause my problem?

A thought I had is that perhaps the cutoff frequency far below audible, due to changes in transistor type or PSU voltage. Is there a way to calculate the cutoff frequency with this setup? Or should I simply raise the freq control voltages to see if I can get the cutoff frequency up? The frequency and V/oct trimmers have no effect by the way!

Kind regards,

Rutger

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