Pellucid Compressor: an ultra low noise compressor for guitar and bass

[jonny.reckless]

Sorry for the delay I've been out of the country for a while.

A full noise analysis is quite complex due to the feedback nature of the way the variable gain stage works. The NE5532 is pretty quiet, you have to consider both current and voltage noise. Typically for a 5532 the point at which they are equal is around 22k. This is where you theoretically get optimum noise figure. Treat the guitar pickup (grossly simplified) as a 10k resistor in series with 1H inductor. The 10k (R20) on the input is a "grid stopper" and there to prevent parasitic input oscillations due to pickup inductance and cable capacitance. So you get a source impedance around 20k at the input of the opamp. Not perfect, but pretty close to as good as you can get with such a cheap and simple circuit. The feedback noise gain is higher, and varies with gain reduction so you have to consider the effect of that at high compression settings and no gain reduction. Although R14 (10k) dominates that node from a noise perspective. There's not much I could do about that without making all the impedances so low that the currents would be too high to use an LM13700 in the feedback path.

[puretube]

Oh - I wudda thought R20 were there to soften the D1/2 clipping knee of "too hot" transient attacks.

[Eb7+9]

Thing is, Jonny certainly does know what he's doing and when he claims that this thing is not noisy I'll take his word for it. But then there is  ...

... when the unit is idling and you crank the SUSTAIN/COMP/RATIO control (1Meg) what happens to background noise ?!

[jonny.reckless]

... when the unit is idling and you crank the SUSTAIN/COMP/RATIO control (1Meg) what happens to background noise ?!

It's at its maximum, due to the gain being at full, as with all compressors. However in this case you can consider the LM13700 to be out of circuit and not contributing to noise, and compute the output noise based on the NE5532 and surrounding electronics. It's important to consider the impedance of the source when doing this.

[Eb7+9]

It's at its maximum ... as with all compressors ...

well, not really ...

only the two-birds-for-a-stone designs such as the Dynacomp and, similar to yours, the THUMBS compressor
notice the NE570/571 current-feedback circuits (similar to yours and THUMBS) do not use variable gain around op-amp

theirs is fixed, for a good reason

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I get the simplicity of your overall design // definitely an elegant solution

but if you decouple signal path from side-chain
then you can operate your signal path at minumum noise at all settings instead ...

for example, this is how the LA-2A and 1176 do it ... noise floor is not affected by side-chain settings

maybe worth a try!

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I made use of the idea in tacking on an optical limiter stage to my Acoustic 360 preamp clone project

https://viva-analog.com/360-kit-ver2-0-add-on-limiter-board/


which was derived from my "Envelope Lock" design ...


consider the noise-less/transparency aspect of this one-stage opto design mentioned in this review:

http://www.ovnilab.com/reviews/envelope.shtml


you can go an analogous way with yours, and not change dynamic specs any
simply by adding an extra op-amp gain stage to the works ...

eg., opa134 for the signal path and TL072 for sidechain

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However in this case you can consider the LM13700 to be out of circuit and not contributing to noise

yes, I see that - and really like your side-chain mixed current-voltage drive circuit

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two components in your circuit play a big role in making this potential transition happen

seeing that R9/R10 current dividers are also clamping OTA's at around 800uA each (1.6mA total)
I think you can drop those safely to 4k7 if you needed to ... or even a little further

and then be able to live with a fixed 10k resistor in the feedback path of the signal amplifier
(yielding a total max gain of x2 which is acceptable in a limiter)

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again, I very much like your design Jonny ...
just saying that wrecking that gestalt by one op-amp should give you pro-level noise specs without changing anything else

... an idea to try out if ye be interested

[POTL]

is this circuit a worthy alternative to VCA compressors. It is known that studio professional compressors use VCA chips (of course we are not talking about Tube and Fet compressors now), but I have never seen OTA compressors among studio models. OTAs are only found in vintage pedals like the dynacomp and its clones. This circuit is different from the dynacomp and clearly better, but can it be as good as real VCAs?

[jonny.reckless]

is this circuit a worthy alternative to VCA compressors. It is known that studio professional compressors use VCA chips (of course we are not talking about Tube and Fet compressors now), but I have never seen OTA compressors among studio models. OTAs are only found in vintage pedals like the dynacomp and its clones. This circuit is different from the dynacomp and clearly better, but can it be as good as real VCAs?

No it's not as quiet or as low distortion as a THAT corp VCA, but it doesn't cost as much either. It's OK as a guitar compressor, which is its intended purpose, and much quieter than a dynacomp or similar, but would need a much more sophisticated circuit to make it suitable for studio use, at which point you'd be better off using a more conventional circuit from one of the excellent THAT application notes.

[Eb7+9]

point is that you "can" make it nearly as quiet as a THAT VCA, for those who would like a somewhat cleaner effect ... not to crap on the holy Dynacomp, but I did re-designed it the same way - by separating side-chain from signal-path, and get the OTA to operate in its better S/N zone (ie., with a much lower output resistor than the stock 150k) ... same thing happens, noise stays low'ish when cranking the COMP control ... that's with the side-chain simply preceded by a variable gain stage that goes down to no output for a minus infinity RATIO, real simple step to make any coupled comp design a little more noise friendly without changing dynamic characteristics (timing, etc ...)