[sdiy] non-resonant low and highpass circuits?

[WeAreAs1 at aol.com]

In a message dated 12/24/04 11:51:36 AM, mmanard at columbus.rr.com writes:

<< Newb question.
I've been looking around today for some basic non-resonant low and high pass 
circuits and found a ton, but nothing with sweepable frequency. I want to add 
a simple low and high pass in and out to my noise module like the moog and the 
oakley had. Any suggestions, links or schematics? >>

You might consider building a couple of basic parametric equalizer circuits, 
but just preset the "Q" parameter to its widest bandwidth setting, and the 
cut/boost parameter to a fairly healthy "cut" setting.  This would allow you to 
sweep the high and low shelving frequencies.  There are a lot of parametric EQ 
circuits out there on the web.  You might try looking in one of the 
comprehensive guitar effect websites, such as Jack Orman's AMZ (<A 
HREF="http://www.muzique.com">http://www.muzique.com</A>), or R.G. Keen's Geofex site (<A 
HREF="http://www.geofex.com">http://www.geofex.com</A>).  One (or both) of those is 
likely to have info on building frequency-sweepable filter circuits.  Also look 
at the schematic libraries at the Rane, Symetrix, and Orban company websites -- 
they each make a bunch of studio-quality parametric EQ's.

On the other hand, why not build the parametric EQ's with all of the knobs on 
the panel?  This would be a total of six knobs instead two, but you'd have a 
LOT more fun with it, this I guarantee.

Yes, non-resonant filters can be quite useful with noise, but if you really 
want to get some of those classic Wendy Carlos/Isao Tomita Moog noise sounds, 
you need to use resonant filters (they often used bandpass filters, or a HP/LP 
pair configured as a pseudo-bandpass).  Carlos liked to run noise through a 
resonant filter which was set to track the keyboard CV, and then would play 
melodies with the resonant noise.  I'm pretty sure that Tomita used this effect, 
too.  It's a delicious sound.

Keep in mind that most parametric EQ circuits are not sweepable over a huge 
frequency range.  They are usually limited to four octaves or thereabouts.  
Many of them also have a "range" switch with at least three frequency ranges to 
set the center frequency (low/mid/high) of each band.  The ones that don't have 
a range switch simply operate within a limited fixed range, but are usually 
packaged in a group of multiple parametric filters, each of which handles a 
part of the total audible frequency range.  A voltage controlled filter would 
probably have a much wider sweep range.

Personally, I think the ideal noise filter would be the classic Steiner 
"Synthacon" filter.  Second to that I would choose the Korg MS-20 Sallen-Key 
filters (both HP and LP).  Both of these great filters are voltage-controllable and 
both have more character than the Moog or Oberheim filters.  Lots of examples 
and schematics are out there on the web; even ready-made circuit boards are 
available, if you're into that.

Michael Bacich