Altec valve compressors at EMI.

[Terry]

If you are referring to the STC R53 thermistor found in lots of Wein Bridge oscillators, I'm sure a quick Google will bring up lots of alternative circuits that use different forms of stabilisation.

Some US circuits use a small bulb in place of the thermistor but they always seem to refer to such bulbs by number - such as #40 - but I've never seen a look-up table to find out exactly what the bulb is. I suppose this demonstrates how long ago I came across such designs - these days I'd simply look on-line!

[Terry]

Having a circuit makes things so much easier!

Try reducing the value of C4 and see what happens ...

[Refugee]

Yes the R53. I have a signal generator that has a 2N3819 circuit.
I was thinking of a coil tester that can produce a sine wave that can be used to calculate the inductance value of various chokes.
I am also thinking of doing it with modern MOSFETs.
I am also planning to build an HT PSU with modern MOSFETs and this will be able to use both N-gate and P-gate devices. I am hoping to copy a transistor circuit with lots of resistor value changes.
MOSFETs are as common as muck and are worth some tinkering to get them to do what we want.

[Dick.1]

Having a circuit makes things so much easier!  

Try reducing the value of C4 and see what happens ...

That's what I've been suggesting to the owner (as stated in my post no1)

Currently using a 330 cap with a 1m r  in parallel has reduced the speed down to around 16ms but there is a slight low frequency rumble on the output that is interfering slightly with the music.

Using a 470 cap halves the speed to 25ms & works very well, but (as usual) my friend wants to get the speed down as far as possible.

The EMI version of this as used in Abbey Road studios to record the Beatles etc.....(RS124) uses what looks like a switchable resistive attenuator ( 6 way) with up to 6m loading available

The problem I have is that nobody seems to agree on what is the genuine cct mods used by the EMI guys................ 
People that have rebuilt these remain tight lipped about this cct 
There is a commercial operation that sells replicas understandably they do not release any details unless you buy from them (about £2,000)

As you say Terry, things would look a lot clearer if the cct diagram was available of this (now legendary) compressor


Dick.

[Terry]

You'll have to be more careful with your 'm's and 'M's, Dick!

I don't think one milliohm would work very well!  

Assuming, of course, that you mean't 1MΩ, what is it for?

If 1µF gives a 50mS attack, reducing it by 50% halves the attack time by 50%, as you found out.

Reducing it to a third should result in an attack of ~16mS as you have also discovered. 

Try just removing the resistor and see if it improves anything* or, as you demonstrated that 470nF works well,  reinstate it, and try reducing the value of R12 instead? Try 10kΩ to start with or, if you have a suitable value of pot or preset handy, you could try that to investigate the available range and fit the nearest preferred value afterwards.

* Of course, if removing the resistor stops the LF problem, leave the 330nF in situ and reduce R12 to 22kΩ.

[Mark Hennessy]

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.

[pwdrive]

I wouldn't have thought the schematic would turn up anytime soon:

http://www.preservationsound.com/?p=3761

Lawrence.

[Dick.1]

Hi Lawrence

I've seen that "schematic" in the link you posted....but a lot of people are saying it's incorrect

To be honest I have no idea if it is correct or just somebody's interpretation of the RS124

I must have seen 40 or so ccts & am still no wiser which one is to be believed

I had hoped a member here would have worked on one of these in the past & would have been able to confirm or supply a cct

Dick.

[pwdrive]

I don't know anything about them, I don't really know what a compressor does.

Lawrence.

[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.