[sdiy] Built my first VCF (Moog Transistor Ladder) - Works a treat (CA3086)

[Christian]

You know those days when things just seem to go right.

I had one today -  have spent the last two days building a Moog style
transistor ladder filter. This is my second project after building my
first VCO (remember the thread, Newbie needs help with VCO).

Well as luck would have it, the VCF worked first time - I was
surprised at this as 1) Nothing ever works first time for me and 2) I
had made some modifications to the circuit diagram, as the Expo
circuit and input signal summer were not to my liking.

Anyway, I am getting some phenomenal sounds from it, and all I have
done is link in that first VCO I built into the filter. Superb -
Lovely sweeps, self-oscillation etc. I am using CA3086 for the
transistors in both the Expo and the ladder circuits and although
people have mentioned that it is not stated that the trannys are
matched in this IC, it works fine.

A couple of questions.

1) The VCF Balance trim pot (applies between -15 and +15 to the
feedback line). How should I set this, please? The feedback line has a
capacitor in it, so DC will be blocked, and the feedback wave will
'sit on top' of this applied voltage as it goes into the base of the
transistor at the bottom of the ladder. (Minimum CV breakthrough???)

2) I have a trim pot (1K) for Maximum Emphasis (Feedback, Q etc) in
series on the feedback line. The circuit self oscillates, but at what
point would people suggest that I should set this trimmer to.

Thanks for your help
Christian.