A 4P voicecard with a low self-oscilation frequency

Hi there,

I finally found some time to finish 4 4P voicecards to put in my ambika rack. (Very bad) picture attached.
When tuning their VCF i realized that 1 of the 4 cards have a self oscillation frequency which is about half of the others.
It can be tuned though but sounds darker when i use the 4 cards in a same part.

I started checking R12 to R21 as they seem to be part of the self oscilation frequency formula. But they look OK.
Any idea what to check ?


The main capacitors are worth checking as well.

The oscillation frequency does not depend on the resonance part of the circuit but on the VCF frequency part. On the 4p cards there is no trimmer to tune the relative pitches of the cards. You can only tune them chromatically individual. So there will be always differences between the cards.
But one that’s about an octave off sounds extreme.

Hi , test the 4,7 v Z diode - maybe you have mix it with a 1N4148.

Just tested the 4.7 zener and the voltage between its 2 pins is around 4.2V for all cards. Not different for the weird one.

As a workaround, I turned the trimmer all way up to have a resonance that is close to the other ones.
It sounds OK in most cases. But the self oscillation is not tuned, the octave ratio is more than 2.
Anyway, that gives it a “random/analog” touch :wink:

Triple-check all the resistor values, maybe?

It’s gotta be a problem on the analogue side.