AW: [sdiy] DBX VCA

[jhaible at debitel.net]

>you will find some old DBX documents here....
>http://www.n-tosch.com/dbx/
>
>these are data sheets for the old DBX VCA modules, including schematics
>for the 200 and 202/203.  You will notice that the schematic is very
>similar to the gain cell independantly invented by Jürgen Haible for the
>VC HADSR and the gain cell used in the Serge VCF and Dual Universal
>Slope Generator (and maybe their current VCA's, I can't varify that though).


Yes, it's very similar to "my" VC ENV core. But the VC ENV core - derived from
SSM and CEM chip descriptions and an idea of Joachim Verghese - does
*not* work as an audio VCA, because it's "class B" operation which is deadly
for audio, but very pleasant for VC ENV. But when I saw a DBX VCA circuit
for the first time, I was quite sure that they _must_ have started with the
class B circuit and then added the "variable zener" transistor to get class A
or AB operation. 

Now I'm curious about the Serge VCF and DSG circuits. I would not expect
the _need_ for such a circuit in the DSG (it has linear ramps, hasn't it?).
I would expect exponential current steering with two complementary pairs
without the opamp / feedback loop would do for this function. But I really
don't know; I've never seen this Serge circuit.

As for the dbx VCAs: They need a lot of trimming and matching to get
a symmetrical response for both half waves (4 transistors involved ...)
Are these the VCAs that are infamous for their rather "harsh" sound?

JH.