10 vca's in ssl comp

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Im doubthfull!

Gain linearity, perhaps, but is it actually "hearable" i im quite doubtful!

Decrease S/N by paralleling! Since all VCA's are operational transconductance amplifiers they are by design noisier, worst
at low Iabc, another design reason is to use input diodes to
increase S/N and distortion figures at hig levels so how can
a paralleisation decrease S/N figures i dont understand!

I did a quick test, i paralleled 4 Lm13700, noise went up,
BW response became flatter, ie, more linear contra Gm
variation, but im very doubtfull that parallelisation is actually
a improvement, it looked more like oscilloscope candy then ear
candy to me it looks more like "lets do something very expencive
because we are SSL" otherwise perhaps the DBX VCAs are crap!! :grin:

KK
 
I'm almost sure that it is for linearisation.
I remember i read a paper about the fact that VCAs by definition have low distortion only in gain range, when you are of that range distortions go up. Each VCA has a slightly diferent clean range because of the imperfect matching of the transistors from the chip. By paralleling chips distortion will go down.
PRR, please can you confirm or I'm totally wrong?

chrissugar
 
Wow Jakob that is a great article. Would you by any chance have the rest in the series to post?

Re paralleling to reduce noise: Sure it works, IF you assume you can ignore input current noise. The insight is that the signal goes up as N and the noise (since it is uncorrelated for each paralleled amp) goes up as root N, for a net root N improvement in S/N. This works for amplifiers in general. However, most of the time you get the same result with fewer parts by using larger geometry devices or paralleling input devices at least. Again, we assume that the source impedance is small, since o/w the current noise will eventually dominate.

Distortion, if it arises from a consistent mechanism (like "dead" [as PRR puts it] emitter resistance, hence a departure from good logging conformance) will clearly not be reduced by paralleling. However, if its effects are even-order distortion that at least can be reduced vastly by balanced topologies.

13700/NE5517 parts (the latter another casualty of the Philips Caen fire) are not very good but can have vastly improved distortion performance (even as singles) coaxed out of them by using the linearizing diodes and real differential drive (the app circuit in everybody's datasheet is a crude approximation to differential drive). Differential current drive is best but usually with just resistors running into the linearizing diodes, you get close to as good as it gets. Regarding noise, the sad thing, for these parts, is that a lot of the noise arises from the current mirrors and there is nothing to be done outside the chip to alleviate this. If they had provided pins coming out for the emitters of the Wilson cells you could add external resistors (which, in a current mirror reduce the effects of transistor voltage noise; unfortunately they reduce bandwidth too, and the lateral PNP's are already challenged itr). Of course that would be a bunch of pins and this is a cheap part.

I did a mid-fi product where I had to cheaply vary the gain from the satellite speaker with a single wire carrying a d.c. control voltage (commoned with the signal return for the speaker, so three wires in all in the cable). It was a four channel + subwoofer system and I used 5517s with a control current scheme that varied both the Iabc current and the linearizing diode current. It was complex but fairly cheap and worked way better for noise pickup and especially reliability, than running a million wires up and back from the satellite. S/N and distortion were not studio quality but not likely to generate user complaints. The product fairly bombed but not because of the gain control at least ;-).

Brad
 
They did it for noise reasons.

When you parallel the VCA's, the noise goes up, but the signal goes up by more. In the case of SSL, you need to remember that there are lots of VCA's in-line as a result of the automation system, so they probably just wanted a little more S/N when considering the full signal path.

Here's the quote from THAT corp:

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).

Cheers,

Kris
 
Oh, and I think it was not 10 VCA's that dbx used in the 202XT, but 8 VCAs and 2 SIP resistor networks (same package, resulting in the confusion).

Cheers,

Kris

PS: One of these days I'll finish my layout for a 202 substitute with 4 2181's. I think 4 is really the optimum # of VCAs. Having 8 ony buys you another 3 dB, and costs a bundle. For me, 4 is bearable cost (most would be happy with the performance of a single 2181)

Cheers,

Kris
 
We use (LM/NJM)13600 in several of our products at my day job for remote gain/level control. Its only used in lowpass subwoofer apps and then in most cases the LPF follows the gain cell, that way its much quieter. Basically that part family only has the distinct advantage of being cheap. I think its about 50 cents USD in quantity.

Analog Devices SSM2164 is a MUCH better part for not a lot of money. Its a quad and its about USD$2 each.

HTH!!
Charlie
 

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