dbx 200, 202, 202C, 2001 Internal Schematics, 2002, 202X Module Schematics

[mediatechnology]

Gary Hebert of THAT Corporation was kind enough to send me schematics of the open-frame 202 replacement modules which they no longer make.

Datasheets:

https://www.thatcorp.com/datashts/202data.pdf
https://www.thatcorp.com/datashts/2002data.pdf

VCA History:

https://thatcorp.com/a-brief-history-of-vcas/

Schematics:

https://proaudiodesignforum.com/images/ ... _202XT.pdf

https://proaudiodesignforum.com/images/ ... A_202R.pdf

https://proaudiodesignforum.com/images/ ... _2002T.pdf

Replacing Modular VCAs:

https://www.thatcorp.com/datashts/dn127.pdf

[mediatechnology]

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

[mediatechnology]

I just obtained copies of dbx internal schematics for the legacy 200, 202, 202C and 2001 VCAs.

This pdf also contains schematics for the module replacements models 2002T, 202XT and datasheets for the earlier canned modules.

Legacy dbx VCAs: https://proaudiodesignforum.com/images/ ... Merged.pdf

GKH's matching notes for the 2001 open better in a non-browser-based pdf reader.

I also extracted a copy of the article "A Brief History of VCAs" posted on THAT's website.

https://proaudiodesignforum.com/images/ ... f_VCAs.pdf


Early dBx 200 VCA Internal Schematic Circa 1975

[emrr]

Fantastic, thanks