I am trying to build an audio-frequency state-variable resonant filter.

Power supply is ±15V. The circuit contains three TL072 op-amps and three LM13700 OTAs. It receives audio input from -2.5V to +2.5V and outputs a filtered signal depending on its cutoff frequency control voltage. This part works perfectly.

Decreasing the voltage on REZ from 5V to 0V should introduce resonance.

Here's the problem:

When I try to decrease the resonance control voltage (REZ), it works for a while, then stops outputting sound, starts drawing way too much current, and IC13 heats up.

I would sincerely appreciate if someone could help me debug this or offer any suggestions.

Thanks in advance.


Schematic is located here: http://stingrayapps.com/SchematicVCF.png

 

Have you checked it with a scope? Is there a DC level or high-frequency iscillation at the output of the hot IC? Resonant filters are by nature prone to oscillations, both the ones you want and some you don't! I don't see much in the way if bypassing or compensation shown...

 

If it makes any difference, the circuit's full schematic is located here: stingrayapps.com/SchematicVoxV6.pdf
I have removed R91, R105, R113, and R133 from the PCB in order to isolate this filter from the lower filter. To clarify, the lower filter has been isolated from the signal path, and the upper filter is exhibiting the problem.

 

Next step's still the same...what's at the output of IC13A when it stops sounding? Is there a DC voltage or an oscillation, there? Second, if you lift R91, does anything change?