[sdiy] Ladder Filter Design - Help Needed

[Rob Keeble]

Hi
I need some help on how transistor ladder filters work....(again...)

I have built a clone of the E-mu Systems 24dB transistor ladder low pass
filter that Dave Rossum designed back in 1974. Whilst its not the best
"moog" like filter every made, it would be cool to recreate Daves "clean"
design.

The circuit is similar to many others. However when its built is way tooo
quiet. The audio signal is divided down 1/500 to go into the bottom of the
ladder and the differential Op Amp at the top of the ladder puts back a gain
of x20. The ladder provides no gain, so the signal output is way too low,
and of course resonance doesn't happen..Voltage control of frequency does
seems to work, but its a little odd - phasing like. I can reduce the gain
reduction at the bottom of the ladder down to 1/50, but its still not
rocking.

My question is: Is the ladder suppose to provide signal gain from bottom to
top? Whats gone wrong?

I have checked the schematics back to the blueprints, and the parts list.
All ok.
I have tried different transistor matched pairs at top and bottom, little
difference.
The ladder does work, inverted down one side, non-inverted down the other.
I have replaced most active parts...

Any ideas guys?

Many thanks in advance.
Regards
Rob Keeble
www.emulatorarchive.com

PS  The E-mu VCO clone works like a dream. Very clean sawtooth shape,
infinite frequency (well 100kHz), and drifts only slightly more than a
modern Oakley VCO with stablised VC inputs. And thats before the right LM318
fast slew Op Amp goes in.