Minimoog Filter Schematic Available (was Re: Design of VCFs)

[Sean Costello]

Hi all,

I just posted the Minimoog filter to my web site, at
http://www.seanet.com/~costello.  This is a 75K file, that should be saved
and resampled to print out at about 9 1/2 inches by whatever, at 300 dpi.  I
couldn't get it any smaller without losing all of the data.

This is the classic Moog ladder lowpass VCF.  It uses all discrete
transistors, several pairs of which need to be matched for Vbe by hand
(directions for this are also on my web page).  The advantage to this over
designs that use pre-matched transistor arrays (Moog Prodigy, Source,
Taurus, Rogue, MG-1 - hell, pretty much every Moog filter except the Modular
904A) is that all the parts for this filter can be found at Radio Shack, or
whatever your local low quality electronics store happens to be.  All of the
NPN transistor can be 2N3904, and the PNP can be replaced by the 2N3906. 

Gene Stopp has a posting in the archives about constructing this filter.  It
apparently sounds wonderful.  I haven't heard it (it is sitting on a
breadboard half-constructed), but I love the Moog filter sound in general.

Hope this is useful to someone out there.  If you have any questions about
accessing the file, or the legibility of the schematic, feel free to e-mail
me at costello at costello.seanet.com.  If you have any questions on
constructing this filter, ask someone else on the list, as I don't have a
clue (I just am not very good at electronics).

Later,

Sean Costello

P.S.  Does anyone out there have any opinions on the different resistor
values used in the Moog filter ladder, and how they impact the sound?  I
have a Realistic Moog MG-1, which uses 1K resistors for all of the resistors
in the transistor ladder (instead of the 220/150/150/150/150/200 ohm values
in the Minimoog).  The filter in this sounds GREAT - in my opinion, better
than the filter in my Multimoog - but I can't figure out why.  Would the
resistor values affect the cutoff range?  The amount of distortion?  Any
ideas, anybody?