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Old 5th May 2013, 10:20 AM   #1
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Default 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

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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!
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Old 5th May 2013, 07:33 PM   #2
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Peak control going into digital. As digital recording is inferior to analog tape recording.
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Old 8th May 2013, 04:22 AM   #3
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I have the same article..never built one...but would love to have some of the hardcore technical tube-circuit big brains here weigh in on this design...Eli, DF96, SY..et al?
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Old 8th May 2013, 11:30 AM   #4
DF96 is offline DF96  England
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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).
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Old 9th May 2013, 01:21 PM   #5
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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
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Old 9th May 2013, 01:26 PM   #6
DF96 is offline DF96  England
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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.
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Old 9th May 2013, 02:15 PM   #7
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by DavesNotHere View Post
Peak control going into digital. As digital recording is inferior to analog tape recording.
Some facts please? That's a bold blanket statement to make and not back up.
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Old 9th May 2013, 04:42 PM   #8
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Quote:
Originally Posted by DF96 View Post
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
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Old 9th May 2013, 04:53 PM   #9
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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.
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Old 9th May 2013, 06:33 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by ruffrecords
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.
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