Vari-mu caps, ripple and oscillation.

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Heikki

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I have a bunch of 5752, 6AS6 and bunch of the Russian equivalents and all of them are too microphonic. 6J7 and 6SJ7 work well, they just need about 10 times more control voltage than 6AS6. It's very easy to overload suppressor controlled vari-gain stage. If I were playing with suppressor controlled vari-gain stage again I would use 6SK7 and take the full control voltage to G3 and from there with a voltage divider to G1. Then again single ended 6SJ7 G3 controlled vari-gain stage can be a nice distortion generator.
 

PRR

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> I'm looking into alternative ways of reducing gain for compressors other than the usual CV on the IPT.

You are not the first.

> The 6AS6 tube for example, was designed for suppressor grid 3 modulation whilst keeping, current constant.  I would have thought that would eliminate thump.

Reduce, not eliminate. This IS what someone did given the rise of tape recorders and access to a tube-bending factory. Plenty good enough for $99 home recorders. Not quite good enough for Pro use. Maybe fun as an "effect limiter", especially in high-level pop music.

There is also a trick (AH! Heikki has it) of using two 6J7 cross-coupled. Variation of G3 diverts current to plate or screen. The cross-couple gives fairly constant DC current but variable signal current. If you want large GR, you have to keep output level low, which probably reduces S/N. This was done with BJTs in many TV Audio chips, as a Volume control for fixed input max-to-zero output, which suits it well.

I've fooled with many limiters, not comprehensively. With the right tubes and heavy idle current (more=better), the classic push-pull limiter can be too-good for FM/Phono tho not quite to CD standards. Before photo-resistors became illegal, a good photo-R job could equal it. FETs come a little short but will all the frills are really good enough unless you use all 16 bits (most tunes don't). dBx/THAT VCAs leave them all in the dust: low hiss, low THD, no thump.

IMHO, most "good" limiting should start with a slow average gain-ride. This may be a place for motorized pots.
 

Heikki

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emrr

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Collins may have used that in some communications units, I don't think it's in any of the broadcast products.  Thanks for spotting that. 
 

DaveP

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Well done Heikki,

It looks like a kind of twin triode the way its configured, one down to earth and one to the plate.

Best
DaveP
 

abbey road d enfer

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Heikki said:
DaveP said:
I'm looking into alternative ways of reducing gain for compressors other than the usual CV on the IPT.

Here's one I haven't seen before. The input signal is connected to the screen grid and CV to control and suppressor grid.
That's an interesting alternative, although it remains to be seen if there are any advantages in terms of noise, THD and/or control feedthrough.
There are possible alternatives such as using two complementary LTP's, but just producing the correct opposite control voltages is a PITN (unless you cheat and go SS for the side-chain). You'd better start a DIY nuclear plant...
 

ComodoComplex

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I'm working on a client's Magna-Tech Type 32 compressor that's popping (oscillating at VLF) and stumbled across this thread in which the absolutest legends of legends discuss, among other things, feedback loop stability!

goosebumps...
 

abbey road d enfer

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ComodoComplex said:
I'm working on a client's Magna-Tech Type 32 compressor that's popping (oscillating at VLF) and stumbled across this thread in which the absolutest legends of legends discuss, among other things, feedback loop stability!

goosebumps...
Isn't there a link missing?
 

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