Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #20 on: November 24, 2020, 11:53:00 PM »
Last weekend I got back to working on this project. I changed the tubes to PCC189 and made some distortion measurements. Now I just need to order front plate for it so I can get it out there for people to try it.



Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #21 on: March 29, 2021, 01:39:53 AM »
Since the last one turned out well I built another transformerless compressor. This time I used INA2137 and INA2134 differential line receivers so I don't need 0.1% resistors. Sidechain is not drawn on the schematic but it is the same that the previous compressor has.

This topology might have some advantages when it comes to "thump". With more traditional tube compressor I have had to sift trough quite a few tubes before finding ones that are well enough matched. Now I'm having hard time finding tubes that audibly "thump". In the previous compressor I only tried few 12AU7 and then later few PCC189 tubes and all worked pretty much perfectly. I only have few PCC189 tubes but I have 100 or so soviet version of 6SK7. Just picking at random all the 6SK7 that I have tried don't audibly "thump".

Does anyone have any PCC189 or ECC189 tubes that have been found out to be unusable in a compressor and wants to send me some? That would be easiest way for me to confirm if I'm just lucky. I'll of course pay for the tubes and shipping.

Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #22 on: March 29, 2021, 01:42:02 AM »
Here is a picture of the new compressor.

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #23 on: March 29, 2021, 02:33:38 AM »
This topology might have some advantages when it comes to "thump".
If I understand well, you attribute the thump cancellation to the two cascaded differential stages. I may be blind but I don't see any other explanation for the second diff driver...?
Am I correct in thinking that the signal at the output of the first diff driver is correct except for residual thumps?
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 #24 on: March 29, 2021, 04:50:54 AM »
I should have been clearer. I think that bringing the control voltage to cathode is what makes it thump less. I'm too lazy to set up a test where I could compare if there is any actual difference with CV to grids or CV to cathode. I'm hoping someone has and is willing to part with a PCC189 that is known to be not well enough matched for use in a compressor. I could quickly see if there's any validity to what I'm saying.

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #25 on: March 29, 2021, 10:36:09 AM »
I should have been clearer. I think that bringing the control voltage to cathode is what makes it thump less.
??? What would be the explanation?
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 #26 on: March 29, 2021, 11:48:39 PM »
??? What would be the explanation?

I don't know. It shouldn't matter how the grid to cathode voltage is changed. Maybe I'm just lucky getting tubes where plate currents change withing 5uA of each other.

Edit: but there is a slight difference between these circuits.
« Last Edit: March 30, 2021, 01:07:31 AM by Heikki »

Re: Transformerless Vari-Mu Compressor
« Reply #27 on: March 30, 2021, 02:58:52 AM »
Is there a difference in CMMR between cathode input, and grid input?

I didn't think there was but, if so, it'd be a valid enough reason for less 'thumps' in the "transformerless, control voltage via the cathode" scheme. 



D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

abbey road d enfer

Re: Transformerless Vari-Mu Compressor
« Reply #28 on: March 30, 2021, 05:49:57 AM »
Edit: but there is a slight difference between these circuits.
Yes, very slight variation of Vka, but it amounts to an insignificant fraction compared to the swing.
Mystery unsolved yet.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

analag

Re: Transformerless Vari-Mu Compressor
« Reply #29 on: March 30, 2021, 07:48:30 PM »
Biggest flaw in transformers is that the good ones cost too much money.

Transformer saturation sounds better than IC clipping to my ears, nor does it need the extra PS with it's noise and harmonics, electrolytics in the signal path etc. What you get is galvanic isolation, better 3D soundscape, higher CMRR, simplicity, free gain depending on ratio. Keep the ratio low if you want that solid state sound but bigger, if you know what I mean. The novelty of the hybrid circuit is fun at the very least. Anyway the schemo looks good. I like to triode connected remote cutoff pentodes since they are easy enough to find.
Audio engineering suffers from misinformation, disinformation, and downright lying more than most fields of endeavour.


Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #30 on: March 31, 2021, 03:53:35 AM »
Transformer saturation sounds better than IC clipping to my ears, nor does it need the extra PS with it's noise and harmonics, electrolytics in the signal path etc.
It's easy enough not to clip IC's. In the design I posted it's not easy to get in to a situation where the user could be clipping any of the INA stages. Most likely whatever equipment is connected after the compressor will clip first. Easy enough not to have the power supply cause any noise or harmonics, especially since the current draw is so small. INA2137 outputs have 3.5uV noise voltage (f = 20Hz to 20kHz), so no need to worry about that noise either. Tubes will probably cause 100 times more distortion at all times compared to the line receiver chips. I didn't use electrolytics in the signal path but there's nothing wrong with using them. If distortion is a worry large enough capacitance is needed not to have any signal across the caps. Also if small amounts of distortion are a worry it's best not to use tubes.

Quote
What you get is galvanic isolation, better 3D soundscape, higher CMRR, simplicity, free gain depending on ratio.
I'm not sure if galvanic isolation is needed at home or professional recording studio but transformers do give that. I don't know what 3D soundscape is but maybe line receiver chips give better 4D soundscape or at least that's what I will claim. INA2134 has 90dB CMRR at low frequencies, 85dB at 20 kHz, 50dB at 1MHz. The cross coupled connection I used might have slightly better CMRR. Transformers probably have even better CMRR at low frequencies. If better than 90dB CMRR at low frequencies and ability to handle very large common mode voltages are needed, then transformer are good way to go.

Gain is cheap with solid state electronics.

Script

Re: Transformerless Vari-Mu Compressor
« Reply #31 on: April 10, 2021, 07:08:54 AM »
Following this with great interest. Question: Why an (expensive/cheap) transformer in the sidechain ? Could get rid of it ? Question 2: Entire side chain looks strangely familiar, maybe not,  but I can't pinpoint it...

PermO

Re: Transformerless Vari-Mu Compressor
« Reply #32 on: April 10, 2021, 08:21:17 AM »
...I'm hoping someone has and is willing to part with a PCC189 that is known to be not well enough matched for use in a compressor. I could quickly see if there's any validity to what I'm saying.

This shop has 40 pieces NOS Siemens, 9 pieces Philips in stock, at a fair price;

https://www.stockclear.nl/a-54969579/buizen/siemens-pcc-189-buis-nos/#description

It's a Dutch webshop, but I could help you out if you need that.
"It's very important that you run trough the door, not trough the wall" - Sadhguru

Heikki

Re: Transformerless Vari-Mu Compressor
« Reply #33 on: April 10, 2021, 01:59:31 PM »
Question: Why an (expensive/cheap) transformer in the sidechain ? Could get rid of it ? Question 2: Entire side chain looks strangely familiar, maybe not,  but I can't pinpoint it...
Sidechain transformer is not needed. I used cheap Hammond 107N backwards to get 12dB gain to the sidechain and it also forms -3dB at 50Hz high pass filter when driven with 600 ohm impedance. Same could be achieved more cheaply with op amps.

I have used the sidechain design previously.
https://groupdiy.com/index.php?topic=66412.0
But I originally stole it from PRR.
https://groupdiy.com/index.php?topic=11469.0

BBC tube limiter uses similar sidechain, except it uses tubes.
https://groupdiy.com/index.php?topic=77232.msg984191#msg984191

This shop has 40 pieces NOS Siemens, 9 pieces Philips in stock, at a fair price;
I'm only interested in PCC189 tubes that are know to be bad thumpers. When I bought my PCC189 tubes I thought they were too expensive and they were 3 times cheaper than the ones in that Dutch webshop.


For anyone interested here's a circuit to replace input transformer in a more traditional vari-mu arrangement.
https://groupdiy.com/index.php?topic=77400.msg985750#msg985750


 

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