VCA design rules have changed?

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audiox

Well-known member
Joined
Feb 25, 2007
Messages
610
I am working with an interesting limiter project...

Until now I have interfaced VCA control ports like Symetrix does in the following schematic:

http://www.symetrixaudio.com/kb/565E_sch.pdf

The small resistor at the control port causes some unbalance and therefore distortion, but part of it can be eliminated by adding another resistor at the opposite port. If the resistances are small enough this circuit works fine and provides low distortion. A huge amount of limiters and compressors are made this way.

VCA manufacturer THAT Corporation however recommends in all their application notes another solution. It connects output of an op-amp directly to the control port and grounds another port. You can find an example on page 1.

http://www.thatcorp.com/datashts/dn122.pdf

The THAT circuit has two bad sides:

1. The inductive impedance of the op-amp output at high frequencies is poison to the VCA (can cause oscillation). It can be cancelled by a capacitor, but still.
2. The benefit of attenuating the op-amp output noise by resistor divider is lost.

Plus connecting an op-amp output directly to the control port feels like quite a harsh solution...

Which to believe?
 
Ask Wayne over at the Pico forum; he's the THAT app development guy we all know. 
 
Indeed there is a thread over there on Wayne's list where we explored VCA drive in connection with one of my pet projects (current source sum bus using VCAs as the current sources).

I even exchanged a few emails with Gary Hebert from THAT corp on the subject.

The short version is when trying to milk the last drops of performance out of those VCAs, the control port drive matters. Not only does termination impedance introduce a distortion component, but noise at those nodes causes a modulation noise on the signal. It is also useful to drive the control ports differentially.

These newer generation VCAs are quite good but as the VCAs themselves get better, the other error sources around them become more apparent.

JR




 
audiox said:
I am working with an interesting limiter project...

Until now I have interfaced VCA control ports like Symetrix does in the following schematic:

http://www.symetrixaudio.com/kb/565E_sch.pdf

The small resistor at the control port causes some unbalance and therefore distortion, but part of it can be eliminated by adding another resistor at the opposite port. If the resistances are small enough this circuit works fine and provides low distortion. A huge amount of limiters and compressors are made this way.

VCA manufacturer THAT Corporation however recommends in all their application notes another solution. It connects output of an op-amp directly to the control port and grounds another port. You can find an example on page 1.

http://www.thatcorp.com/datashts/dn122.pdf

The THAT circuit has two bad sides:

1. The inductive impedance of the op-amp output at high frequencies is poison to the VCA (can cause oscillation). It can be cancelled by a capacitor, but still.
2. The benefit of attenuating the op-amp output noise by resistor divider is lost.

Plus connecting an op-amp output directly to the control port feels like quite a harsh solution...

Which to believe?
David Blackmer invented the log/antilog VCA, and he founded a company named dbx. Check out the dbx schematics. They always drive the control port directly from an opamp's output. Yes, they have to make sure there's no oscillation (most often a cap across the opamp's output will make it unstable and start an oscillation - one must use a Zobel network there), and also that their control voltage (including the opamp driving the control port) is as clean as possible. But they have proven it works...
 
I guess the important question is are you using old school dbx VCAs, or the latest generation THAT corp VCAs? Not only are they integrated circuits, but they are on a different process IC technology than the early IC VCAs.

There was an AES paper published around the time the newer generation VCAs were released that documents many of these performance interactions. If you are using the modern VCAs follow the modern advice.

FWIW, the THAT VCA paper used very low noise, high performance IC opamps to drive the control ports, obviously to show their parts in the best light. Many of their practical application circuits used cheaper parts and deliver somewhat relaxed performance.

Some of these errors like modulation noise are not very audible in use so ignored for less than tweaky design applications. 

JR
 
Thanks for the comments. I think I will follow the THAT suggestions. I guess they know better since they have designed the chip.
 

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