Altec valve compressors at EMI.

[Dick.1]

Can I ask a question to clear a point up?

More than once you've said there is no schematic. But then you post a link to one.

So, the schematic in post #10 is the original Altec model. But what you don't have is a schematic that shows EMI's modifications to the Altec unit?

It seems to me that if you reduce C4, you'll shorten the release time by a similar proportion. So when you say that 0.22uF made things go wrong, it is possible that the owner forgot to adjust the release time control? Perhaps P2 and R9 need increasing?

At short attack times, you run the risk of the audio being modulated into distortion - that's just what happens. Adding another time constant (making a second-order filter) might help matters. Or, adding a "hold" facility - whereby the control voltage stays constant for a brief time before sinking according to the release time constant - is another trick (also used in PPM meters).

In short, what you're trying to achieve is quite complex, and it's no wonder if the people who have figured it out wish to keep it to themselves.

That's right Mark

I have the Altec 436c  cct  & the missing part is the mods by EMI.

Various stories go from....minimal mods,  just a couple of components all the way to complete rebuilds

The problem to solve is the initial delay (attack time 50ms) that needs to be brought down to ideally 10ms but around 15ms is OK.

With the Altec there is only a limited adjustment of release time & threshold & the RS124 has much more adjustment control available including the "hold" facility on the attack 

As you say, it looks to be complex & if I could see how somebody else has done it life would be simpler for me

The sad fact (for me) is that even 10 years ago I would not be asking questions like this, sadly time marches on & takes it's toll as it does


Dick.