[sdiy] SVF High Pass Mode not Cutting Off

[David G Dixon]

> I have recently built a couple of 2-pole State Variable 
> Filters (OTA designs either using BA6110 IC or transistor 
> current mirror OTA)  and both have the same problem that in 
> Low Pass mode the frequency cutoff is perfect and does its 
> job, but in High Pass mode the filter will not cutoff and go 
> into silence. Freq pot does modify the cutoff in HP, but not 
> to full cutoff. Any ideas on whats wrong would be most 
> gratefully received.

I find this to be a common problem in filters generally, where the high-pass
mode is derived by summing inverted lowpass and input.  Having precision
resistors in the core is helpful, but doesn't completely eliminate
high-frequency bleedthrough in highpass.  Try using hand-matched 1%
resistors in the core for a start, particularly on the input summer.