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

[harrybissell]

Actual results may vary. You may have gotten a bunch of
diodes from the same die and they were 'luck of the draw' matched.

You'd know if you made a bunch of them. It would also depend on if
you CARED if there was CV feedthrough.  Someone might hear the
little 'thump' and think that it added 'hit record sound' to the mix :^P

I made a little constant current source and feed 100uA to the diodes...
then measure mV of drop across them.

H^) harry

Michael Ruberto wrote:

> > > 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