[sdiy] moog high pass flter

[benno]

on 9/12/02 3:20 AM, harrybissell at harrybissell at prodigy.net wrote:

> Well I haven't built it but I have a BIG MOUTH so I'll chime right on.
> 
> The Moog HPF is the ugly twin of the LPF.  It uses the same variation
> of transistor impedance vs current... but has to use an NPN-PNP pair
> that must be closely matched to each other, and to three other pairs. That
> makes it hard to do

i find matching tranies not so hard, but time consuming. the type of thing
done drinking a sixpack and watching a movie (well thats what i was doing
when i built the lowpass)
 
> It also does not feature resonance as the LPF does.
> 
> and Hipass filters suck. oops did I say that outloud. Thats bad.. thats
> flamebait ;^P
> maybe blasphemy...
> 
> so let me make myself more clear (having JUST completed a dual MS-20 type
> filter with the control provisions of the Moog filter coupler - designed for
> the Moog
> LPF-HPF combo).  The LPF is a sort of natural filter... with the energy level
> going
> down with time. More entropy I'd call it. Grant Richter has done a lot of
> thinking
> along these lines as of late and maybe he'll say it better than me.
> 
> The HPF is more useful for removing the low frequency content on a static
> basis
> (this is all imho, btw).  I set my filter up so I can have dual LPF (in
> parallel) dual
> HPF (in parallel) one of each (in parallel) and series combos of each as
> well.
> 
> The series combos of LPF-LPF and HPF-HPF are sort of uninteresting.  If the
> cutoffs are the same... it becomes a 24dB filter (mine are S-K, 12dB).
> Stagger
> the cutoffs and it has a different slope... thats all.
> 
> With the HPF-LPF (parallel) it is a simple notch filter, in series a bandpass
> filter.
> I can make the CVs drive the frequencies in the same direction (the center
> frequency changes) and different directions (the bandwidth changes)
> 
> This is somewhat less useful than it seems, if you separate the frequencies
> by a couple of octaves, sweeping center frequency drives you out of the
> human hearing "sweet spot".
> 
> The BPF created does not have a very large "Q".  You can add resonance to
> both
> LPF and HPF in my case (not in the Moog HPF...) and get some interesting
> responses.
> 
> I find that the LPF-HPF is more usable as a means of achieving some formant
> tones, with slight resonances... than as a means of doing dramatic filter
> sweeps.

this was pretty much what i was thinking of doing. i wanted to get highpass,
bandpass and notch response, but as in the moog modular, but without
building the modular filter (and thus building another lowpass).

> There is/was a copy of a Japanese version of the Moog Ladder that included
> LPF, HPF, and BPF responses, by injecting the signal to the bottom, top, or
> both ends of the ladder, respectively.  I'd experiment with that one first,
> for sure.
> At least you can get around the really hard matching process, and you'd have
> resonance in all modes.  Osamu Hoshuyama had this on his site.  TomG also
> did a variation on this technique @ the EFM-Synth list.

this sounds interesting also... any links?

cheers
b