• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

An Electronic Loss Compressor

Hello!

I wanted to discuss building/experience with tube compressors.
Especialy the one mentioned on this site:
Service Call | Preservation Sound

[IMGDEAD]http://imageshack.us/a/img194/928/onetubecomp719x1024.jpg[/IMGDEAD]

[IMGDEAD]http://imageshack.us/a/img341/848/onetubecompp22261x1024.jpg[/IMGDEAD]

Where in the audio chain should I use this compressor?
I imagine, for example a Line output from my computer through this compressor to lets say a tape deck (cassete) line input.

Would I need to use impedance matching transformers on both input and output?

What do you think of it?

Thank you!
 
The article mentions compression starting at about 3V signal and working up to around 15V so not line level. You would need an amplifier in front of it and (possibly) an attenuator after it.

You could use a 5751 instead of 6SL7, or a pair of 6AT6. A 12AX7 might work, but could require some component changes. You would need to readjust it when you change the valve, as it appears to rely on grid current (which varies widely from sample to sample).
 
No, it doesn't amplify. The first valve is a rectifier, which changes the bias on the second valve. The anode impedance of the second valve acts as the attenuator. Note that the signal is sent to the second valve anode.

Yes, but the anode of the second tube does have a plate resistor connected to HT so it will amplify. That's what puzzles me.

Cheers

Ian
 
This is the same tube compressor that keeps getting posted over and over again in new threads over the years; go to the definitive source: US patent 2,679,626 by the same author as the article in the OP. The patent description does suggest that the 1st triode both rectifies and amplifies the input signal over the cathode resistor.
 
ruffrecords said:
Yes, but the anode of the second tube does have a plate resistor connected to HT so it will amplify. That's what puzzles me.
To act as an amplifier a valve needs bias and an input signal to the grid or cathode. This one has bias, but no signal - except to the anode. As I said, it uses the anode impedance as part of an attenuator. The grid/cathode just gets a DC control signal.

With no HT supply the valve anode would be an open circuit so incapable of controlling anything. Note that the analogous circuit using a JFET can work with no DC bias as a JFET is essentially a gate-controlled resistance.

I can't find anywhere in the text where it says that the valve has no HT supply. All it says is that it does not act as an amplifier, which is true.
 

FoMoCo

Member
2012-12-04 10:04 pm
What I don't understand is it says the tube is used just as a variable variable resistance with no HT supply but all the circuits have the plate connected to HT so it will amplify. If it amplifies then there will almost certainly be thump.

Cheers

Ian
It does not amplify the original input signal, as DF96 stated. It's just a load for the first amplifier that decreases the gain as needed, by loading it more heavily. It needs a DC bias to operate properly.

However, it does effectively amplify the rectified control signal. As it modulates the output voltage it also modulates the DC level at the output. And, that could pass through the coupling capacitor as thump if the components are not chosen well. I'd also guess that distortion was atrocious by today's standards. But, probably acceptable for non-hifi applications.

This circuit is a crude, but cheap, feed-forward compressor. There's a lot of compromises to be made to achieve such a low parts count. There's plenty of better ways to do it now, but it's an innovative, cheap, way to do it. Especially considering that this was made before I was born...
 
It does not amplify the original input signal, as DF96 stated. It's just a load for the first amplifier that decreases the gain as needed, by loading it more heavily. It needs a DC bias to operate properly.

Are you talking about grid bias here or a bias voltage on the plate?

However, it does effectively amplify the rectified control signal. As it modulates the output voltage it also modulates the DC level at the output.

That was exactly my point. So why does the article say no HT is required?

Cheers

Ian
 

FoMoCo

Member
2012-12-04 10:04 pm
Are you talking about grid bias here or a bias voltage on the plate?
In this case, the plate. That second section is acting as the bottom leg of a voltage divider. However, it can't conduct in both directions. It needs a DC bias to keep the voltage from getting near zero.

That was exactly my point. So why does the article say no HT is required?
I don't see that claim being made, either.
 
OK, it does not specifically say no HT is required but it does say the tube 'does not amplify' which I admit I read as 'and therefore does not have an HT supply and thus cannot produce a thump in the output' which is the reason why all vari-mu compressors are balanced. That would have been a significant advance had it been true.

However, the circuit quite clearly has HT applied and the gain controlling tube does amplify (it just does not amplify the signal). In fact, as far as I can see , this circuit will impart a significant thump to the signal which probably explains why its frequency response starts at 200Hz.

Cheers

Ian