[sdiy] Making a HPF using a LPF...??

[David G Dixon]

> > Actually, this sort of filter-stage summing is done all the 
> time, and it works quite well.  This is, after all, how the 
> highpass response is derived in a state variable filter.
> 
> I'm not sure I would describe it exactly like that, since 
> it's part of the entire feedback loop that makes the SVF 
> tick... not that I'm sure how I would describe it. ;)

OK, but if the input resistors are not well matched, doesn't the input bleed
through into the SVF highpass output, or does the level of the feedback
(lowpass) signal automatically adjust itself to cancel it out?

In OTA-buffer filter stages, highpass can be achieved by bringing the signal
in through the filter cap (where ground would normally be).  However, in
VCA-integrator filter stages, the cap is in the feedback loop of the
integrator, and always tied to virtual ground.  Hence, the only way to make
a highpass is by summing, and the resistors must be tightly matched to avoid
bleedthrough.