[sdiy] Hot Rod SynthDIY

[Michael Ruberto]

Why do us yanks always need to hot rod things? We have shows on TV about hot 
rodding cars, motorcycles, houses, lawn tractors, etc. etc.
I propose a show called Hot Rod Synth or eXtreme SDIY ;-)

Anyway, I had this synthacon VCF on the breadboard for months waiting for me 
to aquire the proper parts for it. Then I watched that Orange County 
Choppers show and applied their mentality to the VCF. Well I really needed 
to reclaim that breadboard space for another project. So here it is, the Hot 
Rod Synthacon VCF!!!

http://frankentron.freezope.org/source/REV4.gif

I am using the LM324 as a sort of half-assed power supply. The device can 
source 20MA and sink 8MA which is well above the requirements for powering 
the VCF circuit. An AD584 (not shown) is generating the 10V REF. The VCF is 
much more stable now that it has a balanced 10V supply instead of using 
dropping resistors from the 15V rails. The remaining 2 amps in the 324 are 
used as inverting CV scalers. The 2SC1583 seems to be a huge improvement 
over 2 separate trannys for the differential pair. The CV response is 
smoother and repeatable over varying temperatures. Sorry, no hard science to 
back that last statement up, I just compared the before and after 
performance with my ears.
I corrected the resonance droop problem caused by the FET booster amp. 
Increasing R12 from 1K to 1.6K restored the lost resonance. However, this 
resistor's value may need adjusting depending on the characteristics of the 
FET you use for Q3. Use the highest value you can before the VCF breaks into 
wild oscillation with resonance maxed.
The input resistors on the LP and BP inputs were lowered so all three inputs 
will have a similar resulting output level instead of the HP always being 
twice as loud. With resonance at zero the overall signal loss through the 
filter is only about 0.5V.
And the sound is great!!!
If you already have this VCF in your system - rebuild it. If you are 
planning to build this VCF I highly recommend this version.

I love this VCF design and now I have finally built a version that doesn't 
make me curse.

M. A. Ruberto