[sdiy] moog high pass flter

[Osamu (Sam) HOSHUYAMA]

Hello Benno and List,

I came back this morning from one week business trip.

benno wrote:
> 
> i am enjoying the response to my questions very much. once again this list
> is cheerfully insightful. but i'm still learning so i'm gonna pester
> everyone a bit more yet ;)
> 
> on 9/12/02 3:20 AM, harrybissell at harrybissell at prodigy.net wrote:
> 
> > 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.
> 
> the highpass version i saw on osamu hoshuyama's site obtained the highpass
> by mixing the output with the inverted input (well thats how i think he did
> it if i'm looking at the right one).

That is not what Harry mentioned.  I removed the schematic scanned from
a 
magazine because I received a warning of copyright violation.

> i have seen tom g's version. this looks like a much easier solution, but
> makes me wonder how driving/loading the top pole of the filter affects the
> response of the top pole of the filter.
>
> has anyone tried this? does injecting a signal from a low impedance source
> into the top of the filter distort or effectively remove the top pole from
> the filter?

TomG's transistor ladder version is based on the schematic I had on my 
site, which uses a diode ladder.  The part to inject the HPF input
signal
is almost the same as the schematic I had.  (I myself have never tried
them.)

To minimize the influence on the pole of the top stage, the capacitors
to inject 
the HPF input should be twice as much as those of the other stages, and
the 
amplifiers (attenuators) to generate balanced signals should have very
low 
output impedance compared with the impedance of the capacitors.

Actually, the attenuators can be implemented with capacitors not
resistors.
By using this technique, we'll be able to build transistor-ladder
Sallen-Key 
filters.  If I have time in the coming holidays, I'd draw some
schematics 
to explain the ideas.

Please note that the slope of these HPF's is not so steep, though the
filters 
have 4 poles.


Sam Hoshuyama
-- - ---  - -- ------ - -------- -- - ---- -
Osamu (Sam) HOSHUYAMA      houshu at rogers.com
                              Ottawa, CANADA
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