Transformerless Vari-Mu Compressor

[Heikki]

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]

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.


Attachment removed

 

[Heikki]

Here is a picture of the new compressor.

 

[abbey road d enfer]

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?

 

[Heikki]

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]

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?

 

[Heikki]

??? 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.

 

[Winston OBoogie]

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. 

 

[abbey road d enfer]

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.

 

[analag]

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.

 

[Heikki]

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.

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]

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]

...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.

 

[Heikki]

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

 

[Script]

Thanks for all the links. I remember now, I did see that first schematic in passing back in July 2017 (at a very busy time for me though).

[transformer in sidechain] ...backwards to get 12dB gain to the sidechain and it also forms -3dB at 50Hz high pass filter when driven with 600 ohm impedance...

Does this explain partially why there is less thump in your current version ??

 

[Heikki]

Does this explain partially why there is less thump in your current version ??

No because thump is caused by unequal change of the plate currents, but sidechain high pass filter might lessen other unwanted effects caused by the thump.


Here are some distortion and noise measurements. https://ghr.fi/varimu.html

 

[Script]

Not a tube guy, so forgive me asking: What other effects casued by the thump might a filter in the SC lessen ?

I understand (sort of) what causes thump in the first place. The lower the frequency (say around 30Hz to 20Hz, plus maybe residual dirt below that), the more voltage swing -- coupled with inevitably imperfect, and even worse unmatched, tubes and.. thump-di-dumpty.

[Veering off here, but also wondering cos in the PRR unit I built a long time ago I had changed the 1uf cap feeding the sidechain to 2uf, effectively lowering that high-pass filter even further. It sounded better to me back then around preferred setting range, which I have to add is not ultrafast attack on that piece, unless it be for sound-sculpting squash.]

 

[abbey road d enfer]

I understand (sort of) what causes thump in the first place. The lower the frequency (say around 30Hz to 20Hz, plus maybe residual dirt below that), the more voltage swing -- coupled with inevitably imperfect, and even worse unmatched, tubes and.. thump-di-dumpty.

No. The cause of thump is a sudden displacement of the anode voltage when the gain cell is dubmitted to the Control Voltage. It has nothing to do with the signal frequency. Thump frequently appears on transients.
There are two phenomenons at work:

• The two tubes do not match precisely, which results in different voltage variations between them. So there are both a common-mode signal and a differential-mode signal. the CM signal should be rejected by the subsequent stages, but the differential signal is impossible to remove.
• The subsequent stages may not have enough CMRR to completely ignore the CM signal.

In Heikki's build, these subsequent stages have a very large CMRR, so teh only remaining source of thump is tube unbalance. It's dynamic unbalance, so static balance adjustment has a limited action.

[Veering off here, but also wondering cos in the PRR unit I built a long time ago I had changed the 1uf cap feeding the sidechain to 2uf, effectively lowering that high-pass filter even further. It sounded better to me back then around preferred setting range, which I have to add is not ultrafast attack on that piece, unless it be for sound-sculpting squash.]

Indeed lowering the attack speed rejects the thump spectrum towards infrasonics, but the usual challenge is to get a fast attack and minimum thump.

 

[Heikki]

What other effects casued by the thump might a filter in the SC lessen ?

Lets say I have signal peak that would cause 1dB of gain reduction. Now I have badly balanced tubes and get a huge thump and the thump whacks the compressor in to 10dB gain reduction. If I have a high pass filter at the sidechain amp the huge thump perhaps only whacks the compressor into 9dB of gain reduction.

 

[abbey road d enfer]

Looks like we may have a different understanding of what thump means.
I described CV pass-through, when you describe pumping (punch-in).
As you describe, a high-pass filter in the side-chain may reduce pumping, when a low-pass would reduce CV pass-through.
I think we're both right in trying to cover two aspects of the same concern.
Maybe someone will cover another aspect...