[sdiy] HP from LP ?

[Donald Tillman]

> On Jul 31, 2017, at 11:10 AM, Michael Zacherl <sdiy-mz01 at blauwurf.info> wrote:
> 
> Is there a ‘cheap’ way of turning a low pass filter into a high pass?
> How’s that possibly approached? 
> Just something like subtracting the LP’s output from its input signal?
> If so, what about resonance?

In general, no.

In the case of a single pole filter, subtracting works fine.

In the case of higher order filters, it depends on the specific filter, but subtracting usually results in a 6dB/octave high pass response with a little weirdness near the tuned frequency or with various Q's.

Which means that if you call it a "high pass", you'll want to provide a certain amount of explanation to describe the specifics.

> My point about doing that cheaply is the (upcoming) release of two synths with Moog ladder filter featuring a switchable high pass mode.

Adding resonance to a Moog ladder filter changes the low frequency gain, so subtraction will no longer cancel out the lows.

  -- Don
--
Donald Tillman, Palo Alto, California
http://www.till.com