[sdiy] Moog VCF output noise

[Kenneth Martinez]

OK, I've played with this on my own for a while...time to consult the
experts...

I've constructed a Moog VCF and VCA on one board and two EGs on another
using the schematics in the Minimoog service manual.  I put all controls
on a rackmount panel and added lots of extra jacks so I can use the
VCF's output directly, or defeat the normalled connection from the VCF
to the VCA and feed a different signal into the VCA, or use the EG
output separately, etc.  I checked my Minimoog and it does have the same
part values as the schematic.  My circuit is working and sounds very
similar to the Mini except that I can hear noise from the VCF when I'm
playing low notes, while my Mini doesn't sound like that.

At first I thought my circuit was producing more noise than the Mini,
but my scope tells me there's about 3mV of noise coming from my VCF with
no input signal, and my Mini has 6mV of noise.  The level and sound of
the noise is constant regardless of the filter cutoff setting (the scope
shows it, and my ears verify it).  The difference must be that the noise
from mine has more high-frequency content, because it makes more of a
hissing sound which is noticeable against low notes (VCO set to 16') but
is masked by higher notes, while the Mini's noise isn't noticeable when
playing those low notes.  Granted, the output signal is at least 3V p-p,
so 3mV (or 6mV) of noise is a lot quieter...it wouldn't bother me if it
wasn't audible on low notes, and well, I did hope to use my VCF for bass
sounds.

Has anyone experienced this with their Moog ladder filters?

For mine I used mostly 1% metal film resistors and the rest 5% carbon,
depending on what I had on hand.  I substituted 2N3904s for TIS92s and
2N3906s for TIS93s as the schematic suggests...I also guessed and
substituted 2N3904s for all the TIS97s, as I couldn't find a
cross-reference for those.  I matched the NPN pairs (well, I tested for
matching - as it happens, all that I checked were already within 2mV of
each other).  The parts list specified polyester and I chose AVX
metallized polyester film caps for C3,C7,C11,C16 in the ladder and C1,C5
connecting to the differential amp (I chose these caps because they were
available and were "polyester").  Is it possible that using another type
of cap and/or another type of tranny in the ladder would affect the
frequency content of the noise?


p.s. schematics are at  http://www.nmrailrunners.org/temp/Minimoog.html