Sallen-Key question at end...Re: [sdiy] lowpass gate etc (apologies to Kurt Cobain)

[Michael Ruberto]

> > Sallen-Key just made me remember my question about the ones that use 
>diodes:
> > Does it help to match the diodes?
>
>In filters like the Steiner-Parker Synthracon, it is just as inportant as 
>good
>cap
>tolerance, in fact even more so. Matching of the tuning elements usually 
>results
>
>in greater CV rejection (less thump with fast changes of CV).   Try the old
>PAiA 2720 bandpass and lowpass filters for a great example of just how BAD
>CV thump can be  :^P
>

In the Synthacon I serviced a few years back I found the diodes were not 
matched at all. Also of particular interest, all the filter caps were junky 
disks and the blocking caps were all tantalum. I'm guessing that's why the 
VCF had a slightly different biasing scheme than you find in "modern" 
schematics for the circuit, to minimize the thump. The VCA suffered very 
badly from thump in the Synthacon and I had to trim the heck out of the EGs 
to eliminate it. I did not reverse engineer the VCA but it looked like the 
same topology as the VCF but without the filter caps.
I personally have never had a thumping prob with my clones of the VCF and I 
did not match anything. I did use better caps though, the old box types.

M. A. Ruberto