THAT 202XL recreation

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
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mediatechnology
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THAT 202XL VA Trim Adjustment

Post by mediatechnology » Sun Jun 24, 2018 4:15 pm

Found it. Hit paydirt.

Gary gave me permission back in 2007 to quote him so here we go:
I promised the rundown on the Va adjust, so here it is. 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.

Have fun.

Regards,
Gary
So I was right in my initial guess.

SEO-optimized: THAT 202XL VA adjustment is an "Early Effect" THD trim at elevated (or reduced) gain.

ilya
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Re: THAT 202XL recreation

Post by ilya » Sun Jun 24, 2018 4:40 pm

Hmmmm.... That's interesting. So what are the possible ways of trimming the feedthrough then? Applying step impulses and monitoring the output?

EDIT: just saw your last post with the quote from Gary. That's what I heard somewhere as well. So this leaves the first question that I asked: I get the lowering of the THD with very low values of VA. Is that normal?

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mediatechnology
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Re: THAT 202XL recreation

Post by mediatechnology » Sun Jun 24, 2018 5:06 pm

I just calculated at 20 dB gain (6.1mV/dB) that the current with VR2 at minimum is 12µA. I think that works out to be about 75 uV added offset between pins 2 and 4 in addition to that provided by SYM.

I suppose it depends on your application but I wonder if -20 dB attenuation with an elevated signal in would provide a different adjustment point.

There is a bullet point on the schematic that the 2181s are selected "2002" grade.
It looks like they only used that design for about a year.
I wonder if the strategy of having three of the four VCAs not trimmed and the fourth one trimmed with a fixed VA would be more predictable. (Like the 8 VCA models.)

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JR.
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Re: THAT 202XL recreation

Post by JR. » Mon Jun 25, 2018 9:59 am

I'll bet you didn't know that Gary owns and plays a set of Ludwig drums. 8-)

If this application is for general purpose compression/limiting, most of the time the VCA is spent at modest attenuation 0 to -12dB. ASSuming that the distortion mechanism is symmetrical with distance above/below unity gain, one could scale the nominal 0dB to actually be +6dB through the VCA, so relative -12dB is only -6dB actual through the VCA.

JR

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mediatechnology
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Re: THAT 202XL recreation

Post by mediatechnology » Mon Jun 25, 2018 10:20 am

JR. wrote:
Mon Jun 25, 2018 9:59 am
I'll bet you didn't know that Gary owns and plays a set of Ludwig drums. 8-)
I do. I've seen them in the break room. He also owns one of your drum tuners.

Les plays keybaords, Tom and Rosalfonso play guitar. Quite a few musicians there and history. There's a rehearsal space near the wafer-grinder downstairs in the old motor factory/jazz club now THAT HQ.

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JR.
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Re: THAT 202XL recreation

Post by JR. » Mon Jun 25, 2018 11:04 am

mediatechnology wrote:
Mon Jun 25, 2018 10:20 am
JR. wrote:
Mon Jun 25, 2018 9:59 am
I'll bet you didn't know that Gary owns and plays a set of Ludwig drums. 8-)
I do. I've seen them in the break room. He also owns one of your drum tuners.
I sent him one on loan, but did use his positive quote on my website. Kind of an insider brag as I don't expect my typical customers to recognize Gary's name.
Les plays keybaords, Tom and Rosalfonso play guitar. Quite a few musicians there and history. There's a rehearsal space near the wafer-grinder downstairs in the old motor factory/jazz club now THAT HQ.
I didn't know that... cool.
JR

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mediatechnology
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Re: THAT 202XL recreation

Post by mediatechnology » Mon Jun 25, 2018 1:10 pm

The walls of THAT HQ are lined with photos of the Jazz greats that played there when it was a club.

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