[sdiy] Filter Question

[Tom May]

Scott Gravenhorst <music.maker at gte.net> writes:

> Can a lowpass filter be built by using a hipass filter
> output fed back to a summing amp that sums the original
> signal and the hipass output to cancel high frequencies?  
> My guess is "no" because of phase issues.  Lunatic minds
> want to know.

In the frequency domain, a high pass filter is

   s
 -----
 s + 1

If you subtract this from the original signal (1), you get

        s      s + 1     s       1
 1 -  ----- =  ----- - ----- = -----
      s + 1    s + 1   s + 1   s + 1

which is a low pass filter.

fTom.