[sdiy] Curious problem with VCA click

[Benjamin Tremblay]

Hello alls, 

I have a circuit I built twice and both times I have this issue with a VCA produces a loud click (with with a slow-ish envelope attack) but only when I use a specific sound input.
It uses a state variable filter I pulled almost verbatim from here: http://www.soundsemiconductor.com/downloads/AN701.pdf <http://www.soundsemiconductor.com/downloads/AN701.pdf>


I am using the ALFA AS2164.

From there I use the other 2 2164 stages to have a voltage-controlled mix of HP and LP. 
And from there I use the AS3360 VCA, one for a VCA and one for voltage-control of the filter resonance.
Again, the VCA circuit is close to the reference implementation, with a quad op amp handling buffering. 



So, what’s the problem? 

The problem is consistently I hear the click on the VCA when I use the high-pass filter, which is bad because it’s for “high-hat” percussion sounds. 
If I route the low-pass filer into the 3360 VCA, it sounds great, even when I apply aggressive cutoff and resonance and crank up the mixer. 

Would a buffer or inverter fix this? Would a decoupling capacitor fix this (bias voltage) ? Or, is it very-high-frequency noise coming off the high-pass line (would passive filtering help?). 

The fact that this circuit consistently causes this problem has me scratching my shapeless, lumpy head.

Ben


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