DIY 2002 multi-vca

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matthias

Well-known member
Joined
Jun 4, 2004
Messages
768
Location
germany / frankfurt
can anybody tell me what's the real advantage of outting more that 2181 in parallel ??

the noise and distortion specs of one 2181 are great themselves
... aren't they ??
 
the reason for doing this is simply lower noise. The noise gets lower with every extra VCA you put in parallel. Since the 2181 is very quiet, I think it is not realy worthwhile. Especially since hand wiring probably picks up more RF than the VCA noise anyway.

Mark
 
I'm on this one... I have a schematic, but am waiting for an AES paper written by one of the folks at THAT which explains why an AD797 as a CV buffer might serve to reduce distortion and modulation noise.

oh, and the reason for paralleling is to increase dynamic range. The noise does go down slightly, but the signal increases more. But it's diminishing returns.

Cheers,

Kris
 
Hi all,

I mailed That Corp about the 2002; more info can be found at

http://users.pandora.be/Rogy/Paralleling%20VCAs%20notes%20Rogy.pdf


This 4 x 2181 arrangement is used in SSL's latest Xlogic buss compressor, which is an evolution of the fx384 buss comp after which the Gyraf SSL is modeled.

Enjoy!

Greetz,


Rogy
 
To quote THAT Corp's DN127:
When paralleled, the signal currents rise linearly with the VCA count, but noise current rises only as the square root of the number of devices. Thus, for four paralleled devices. the maximum signal current goes up by four, but the noise current only goes up by two, resulting in a 6dB improvement in SNR (signal-to-noise).
 
damn, I deleted the pdf by accident...now the link is dead...

Rogy, or someone who saved the file, can you email me the file please?

s.mueller at macnews.de

thanx very much

steff
 
[quote author="verbos"]the reason for doing this is simply lower noise. The noise gets lower with every extra VCA you put in parallel.
Mark[/quote]
You can omit input tansformer if you use paralel actives,
you must use transformer ratio powered to two to determine
number of actives.

Once I made mic amp with four
5534 in paralel. I solder power and input leads together.
(it looks like tower from op amps and resistors)
and used feedback and output summing resistor(200 R)
for each op amp.
It was asymetric mic amp and sounds good.

It is hard to do it on printed circuit without towering.

xvlk
 
Since this is an exercise in being totally anally retentive anyway, wouldn't it make sense (from an anal retention point of view) to have a separate symmetry control for each VCA? One control for four VCA's assumes that they are all identical, which we all know from experience is rarely the case.

Joe
 
joe-electro said:
Since this is an exercise in being totally anally retentive anyway, wouldn't it make sense (from an anal retention point of view) to have a separate symmetry control for each VCA? One control for four VCA's assumes that they are all identical, which we all know from experience is rarely the case.

I thought the shared symmetry control a bit odd as well (from a purely technical and anally retentive point of view). You will at best have an average of those parallel chips.

I left that as a nice thought exercise, skipped the inpracticalities, and used 2181A's.
 
Ok, maybe I'm a tool, but there's a few things I don't get while looking at doing this...

According to that corp's 2002 data sheet http://www.thatcorp.com/datashts/2002data.pdf the control signal is hooked to Ec+, but on the schematic they sent to Rogy, it's hooked to Ec- and Ec+ is grounded...what's the deal there?

 
Ptownkid said:
Ok, maybe I'm a tool, but there's a few things I don't get while looking at doing this...

According to that corp's 2002 data sheet http://www.thatcorp.com/datashts/2002data.pdf the control signal is hooked to Ec+, but on the schematic they sent to Rogy, it's hooked to Ec- and Ec+ is grounded...what's the deal there?

I'm not sure what schematic you're talking about but the old 202 grounds one of the EC pins internally. For modest amounts of gain control it's ok to use either one (when available), noting the proper polarity for boost or cut. For larger control voltages, like when commanding large amounts of attenuation (for fader automation or VCA grouping) it is preferable to drive both control ports with symmetrical DC signals equalling half of the total control voltage needed at each.  Clearly not possible with old 202.

JR

this seems like an old thread, is this the same subject?

 
Rogy said:
Hi all,

I mailed That Corp about the 2002; more info can be found at

http://users.pandora.be/Rogy/Paralleling%20VCAs%20notes%20Rogy.pdf


This 4 x 2181 arrangement is used in SSL's latest Xlogic buss compressor, which is an evolution of the fx384 buss comp after which the Gyraf SSL is modeled.

Enjoy!

Greetz,


Rogy

It's the schematic located above.
 
OK, that schematic shows newer parts with both + and - EC ports available.

As I posted you can use either or both control ports when two available. When using one, the polarity of the control voltage needs to agree with what you are trying to accomplish boost or cut.

JR
 
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