Climbing the Moog ladder

[Tony Allgood]

Well in my search for the perfect filter, I've given up on the noisy
13700, I had to try the famous Moog ladder. Well I've built it before,
but I thought I would try to improve on it. I used a stereo pot for the
Q ( Emphasis!! ), one of the gangs controlled the feedback as in the
original design and the other gang controlled the input level to the
filter. The input levels are dropped to about 25% at low Q settings to
compensate for the drop in output at higher Q settings. Using 10k log
pot and 3k3 resistor, this works really well. The output level stays
pretty much constant over the pot's travels. I used the 10k pot in
potential divider mode in the feedback path. I think this gives a
better law than the traditional 47k/50k pot in variable resistor mode.
Plus I can't get miniature pots at 47k in stereo anyway.

It sounds great, especially when you overdrive it. Problem. It still
has unacceptable CV breakthrough for my post-VCA processing. I might
try the suggested SSM parts, but I do like the sound of this filter.
Now, I think my CV problem could be avoided by using matched transistor
pairs up the ladder. At the moment I'm using CA3046 for the top and
bottom pairs. But I might try matching the others as well. There is a
5mV differential at the top of the chain when modulating the filters
cut-off from 100Hz to about 20KHz. This can only be down to mis-matches
in the chain. Oddly enough it is worse at low Q settings.

So who makes the cheapest matched transistor? 

Tony Allgood.

Cumbria, UK

'I may live in the middle of nowhere, but the views are nice'