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