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

[David G Dixon]

> > Jean-Pierre Desrochers wrote:
> >> Can I make a High pass filter using a Low pass filter and 
> a summing 
> >> amp using both positive and negative inputs?
> >
> > In theory, yes.
> >
> > In practice, try it.

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.  It is also how highpass is derived in my
SSM2164-based MS-20 clone, the Korgasmatron.  The only problem with it is
that the impedances in the summed stages must be very tightly matched or
there may be audible input signal bleedthrough in highpass mode at low
cutoff frequencies (in other words, the signals which are summed must be
more or less perfectly cancelled out for highpass cutoff to be complete).
This can be achieved with 0.1% resistors and 2164s, or (even better) with a
small trimmer in series with one of the summing resistors.