[sdiy] moog high pass flter

[harrybissell]

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

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.

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.

So after playing with this filter for about a week... I'm slightly
dissappointed that
it was not more spectacular than it was.  For science-fiction soundtracks I'd
be
using it all the time... both filters in self resonance are way over the edge
!

Any other comments ???

H^) harry


benno wrote:

> on 9/12/02 12:43 AM, spr at spridley.freeserve.co.uk at
> spr at spridley.freeserve.co.uk wrote:
>
> >
> >> Can't seem to find much on it besides the 904b and the Moog patent.
> >
> > There's also an AES Preprint covering the Moog 904A/B/C theory.  See:
> >
> > http://www.till.com/articles/moog/#articles
>
> thats where i got the link for the patent from.
>
> > I've never used a Moog HPF so I can't comment on it.
>
> neither have i, thats why i was asking. it seems to me the lowpass is the
> one everyone fell in love with, which makes me wonder if the highpass got
> forgotten, or it was the dany devito of the twins.
>
> b