dbx 202XT 202R and 2002T Schematics

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Re: dbx 202XT 202R and 2002T Schematics

Post by mediatechnology » Tue Oct 09, 2007 12:39 pm

Gary Hebert of THAT Corporation provided this description of the Va adjust on the 2002T:
The gain cell core in the Blackmer VCA has PNP and NPN transistors in a complementary arrangement. As the gain control voltage moves either way away from 0V, the collector-base voltage on (at least) two of the four transistors changes. Due to Early effect, this induces a subtle change in the effective Is of these transistors. Essentially, this misadjusts the symmetry which has presumably has been nicely trimmed at 0 dB gain. If the NPNs and PNPs have matched Early voltages, this effect will cancel, but typically they do not. It turns out that the effect can be substantially eliminated by bleeding an appropriate amount of the gain control signal into the symmetry trim. This is what the Va adjust does.

One sets this by first trimming the symmetry adjustment for minimum THD (2nd harmonic) at 0 dB gain with a moderate level signal. Then the Va trim is set for minimum THD (again, 2nd harmonic) at gain or cut, typically at +15 dB or +20 dB gain with a moderate input level. It will usually improve the 2nd harmonic THD performance by several dB in a 2002 circuit.

All of this is described in a dbx patent, #4,454,433 by Dave Welland. It's now expired.
Thanks Gary!

Here's a link to that patent: https://www.google.com/patents?id=16M8A ... =4,454,433

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