Moog filter fun

[Tony Allgood]

Hi,

Some of you may remember a question I asked a few weeks back, it
involved taking the output of a Moog ladder at different points up the
ladder. It turned out that there was indeed a circuit out on the net
that had done exactly this. However, it used one diff amp that was
switched across one of the four ladder caps. I decided to use the
existing circuit, one of my filter boards in fact, and add an additional
buffer and difference amp circuit onto the bottom cap. The output of
this and the other original diff amp was sent to mixer, controlled by a
pot. The pot was arranged so that the outputs could be cross faded like
on a DJ mixing desk. The output is amazing... it appears to be a slope
control for the filter, from 1 pole to four pole, which was what I hoped
it would do. Phase differences between the two diff amp outputs will
mean it is not a true slope control, but the sound is excellent. A
worthy mod, and one that will be squeezed into my monosynth when I get
round to it.

BUT the really cool bit was to come, if that wasn't great in itself.
Invert the bottom diff amp before summing with the top diff amp, and
with the right mix, your Moog low pass filter becomes a band pass. Oh
yes, it does. The sound is typical of the BP output of a SVF. I will
simulate this later on today to confirm this as my swept sig gen has
died a death... batteries ran out. More work is yet to be done... for a
start I want to get to grips with the maths of why its actually doing
this. Circuit will be forthcoming, but it can be done with just one
TL074 and a handful of passives, so long as you have a Moog ladder
somewhere.

Regards,

Tony Allgood, Cumbria, UK

e-mail: oakley at techrepairs.freeserve.co.uk

Rack mounted Moog VCF module. Details to be found at...

http://aupe.phys.andrews.edu/diy_archive/schematics/effects/filter.html