Heikki

Transformerless Vari-Mu Compressor
« on: July 24, 2020, 06:44:53 AM »
Here's what I'm thinking about building next.  There is one cheap audio transformer in the sidechain but it would not be necessary to use one there.  Control voltage is brought to tube cathodes so no need for input transformer.  The stage after the tubes should be able to handle approximately 25V of common mode voltage and using 0.1% resistors should guarantee good common-mode rejection. If each triode has quiescent current of 5 mA, plate resistors up to 4.7k can be used and 25V common-mode voltage won't be exceeded when going to deep gain reduction.  Since the audio signal will be much smaller than the common-mode voltage, the combination of differential and common-mode voltage won't be much larger than the common-mode voltage alone.

This is probably the cheapest way to build high quality tube compressor since there are no expensive audio transformers needed.


gyraf

Re: Transformerless Vari-Mu Compressor
« Reply #1 on: July 24, 2020, 09:33:26 AM »
Try it and let us know?

I wouldn't get my hopes up too high on how it sounds though, having tried many many variations over this theme. But who knows - maybe this is the ticket there?

If you build it, I'd suggest that you also try the ECC86 tube (at properly reduced plate voltages) - that is the most interesting direction Ive seen/heard so far for this sort of idea [Behaves tubish within reach of 5534 inputs].

Make sure to report back!

/Jakob E.

« Last Edit: July 24, 2020, 09:40:14 AM by gyraf »
..note to self: don't let Harman run your company..

Re: Transformerless Vari-Mu Compressor
« Reply #2 on: July 24, 2020, 10:54:15 AM »
That looks very interesting! how high is B+? How is the power supply implemented? A special transformer, or two separate units? Voltage multiplier or charge pump?

ECC86 seems to be a good idea, if the gain is high enough for this application?

New design ideas are always welcome! 👍

Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #3 on: July 24, 2020, 12:20:37 PM »
B+ is going to be 100V. I'll most likely order custom power transformer from Toroidy.

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #4 on: July 24, 2020, 02:42:28 PM »
Here's what I'm thinking about building next.  There is one cheap audio transformer in the sidechain but it would not be necessary to use one there.  Control voltage is brought to tube cathodes so no need for input transformer.  The stage after the tubes should be able to handle approximately 25V of common mode voltage and using 0.1% resistors should guarantee good common-mode rejection. If each triode has quiescent current of 5 mA, plate resistors up to 4.7k can be used and 25V common-mode voltage won't be exceeded when going to deep gain reduction.  Since the audio signal will be much smaller than the common-mode voltage, the combination of differential and common-mode voltage won't be much larger than the common-mode voltage alone.

This is probably the cheapest way to build high quality tube compressor since there are no expensive audio transformers needed.
Since you already have a lot of solid-state things, I suggest you use a synthesized common-mode inductor to reduce thumps. See attachment.
I'm dubious about 1k as plate resistors; indeed it reduces thumps, but it also reduces the level of signal.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #5 on: July 24, 2020, 11:38:35 PM »
Since you already have a lot of solid-state things, I suggest you use a synthesized common-mode inductor to reduce thumps. See attachment.

I can't see how those two gyrators would emulate a center tapped choke, I probably fail to understand the circuit. I need to simulate it  feeding it some common mode voltage and see if it cancels any of it out.

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #6 on: July 25, 2020, 06:05:05 AM »
I can't see how those two gyrators would emulate a center tapped choke, I probably fail to understand the circuit. I need to simulate it  feeding it some common mode voltage and see if it cancels any of it out.
Please do it. You will see that the input impedance for common-mode signals is about 1 kohm with the actual values and the impedance to diff-mode signals is very high, several Megohms.
If you feed it it with a perfect source (zero-ohm impedance), you won't see a change, but you must consider the source impedance of the tube/plate resistor. In your case it's dominated by teh plater resistor.
Since you're considering using only 1kohm for the plate resistors, some of the proposed circuit's values must be scaled down considerably. By making R1=10ohms, you get 30dB attenuation of all thumps below 10 Hz.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #7 on: July 26, 2020, 02:51:54 AM »
Spice must be lying to someone. I don't understand how that Bob Pease floating synthetic inductor (https://www.youtube.com/watch?v=AEJtajaRj_s&feature=youtu.be&t=285) could ever work like a common-mode choke and even spice seems to agree with me. It's just a floating synthetic inductor.

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #8 on: July 26, 2020, 04:35:08 PM »
Spice must be lying to someone. I don't understand how that Bob Pease floating synthetic inductor (https://www.youtube.com/watch?v=AEJtajaRj_s&feature=youtu.be&t=285) could ever work like a common-mode choke and even spice seems to agree with me. It's just a floating synthetic inductor.
Agreed. Now, what do you think about this?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Transformerless Vari-Mu Compressor
« Reply #9 on: July 26, 2020, 06:32:24 PM »
Hasn't Ian 'Bluebird' been tinkering with this idea a long time ?
Might be worth a look back to previous posts for inspiration .


Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #10 on: July 28, 2020, 12:17:41 AM »
Agreed. Now, what do you think about this?

That looks like something that should work. It should greatly reduce common-mode voltages but I am worried how it will handle high audio frequency differential signals.
« Last Edit: July 28, 2020, 12:47:18 AM by Heikki »

gyraf

Re: Transformerless Vari-Mu Compressor
« Reply #11 on: July 28, 2020, 02:56:09 AM »
Dosen't it need some time-constant capacitors to get chokis'h?
..note to self: don't let Harman run your company..

Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #12 on: July 28, 2020, 03:14:13 AM »
Dosen't it need some time-constant capacitors to get chokis'h?

No because it's not a gyrator circuit.  Common-mode voltages appear in opposite phase at the other ends of R5 which will cause them to cancel and same happens at R10. For differential signals no current flows through R5 or R10.  But I think that at high frequencies opamps will cause some phase shift which will cause current to flow through R5 and R10. Because the resistor are only 10 ohm even small phase shift will cause significant current to flow. We could increase the resistors but that would make the circuit less effective canceling common-mode signals.
« Last Edit: July 28, 2020, 03:25:56 AM by Heikki »

gyraf

Re: Transformerless Vari-Mu Compressor
« Reply #13 on: July 28, 2020, 03:26:40 AM »
aah, it's a common-mode short, not a choke.. :-)

I'd like to try this, have a hard time wrapping my head around how it would behave in real life

/Jakob E.
..note to self: don't let Harman run your company..

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #14 on: July 28, 2020, 03:43:17 AM »
aah, it's a common-mode short, not a choke.. :-)
Choke is synonym to strangle.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #15 on: July 28, 2020, 03:52:59 AM »
No because it's not a gyrator circuit.  Common-mode voltages appear in opposite phase at the other ends of R5 which will cause them to cancel and same happens at R10. For differential signals no current flows through R5 or R10.  But I think that at high frequencies opamps will cause some phase shift which will cause current to flow through R5 and R10. Because the resistor are only 10 ohm even small phase shift will cause significant current to flow. We could increase the resistors but that would make the circuit less effective canceling common-mode signals.
Simulation shows that even with pedestrian opamps (GBW=1MHz), the -3dB HF point would be about 50kHz.
I know that simulation is not real life, but I thinks it justifies breadboarding.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #16 on: July 28, 2020, 06:30:13 AM »
I know that simulation is not real life, but I thinks it justifies breadboarding.

I'm going to try it out when I get to building my compressor.


 

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