[sdiy] ARP FIlter Problem

[Rob Keeble]

Hi guys
Thanks for the advice...very much appreciated....the board is laided out
much larger than a potted module, I have checked back to 4012 skis, I did
have -13V on the output at one stage, but the offset trims this out nicely,
the filter refuses to oscillate (high resonance is causing signal
cancellation rather than + feedback). A CV op amp summer would be a good
improvement, once I get the ladder to be frequency controlled. A 4035
replica is on the drawing board...but I am determined to get this 4012
working first!

Can anyone point me to an article that explains how a ladder transistor
filter works? The current sink for the filter seems to be the problem, as it
is static at 8mA with varying CV's. The ladder seems okay as it has a signal
output on both sides at the top (5mV), + and - phase.

The expo converter around T14/15 could be a problem. Anyone know what
voltage the base of T3 should be at?

http://www.emulatorarchive.com/assets/4012.jpg

Regards
Rob
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