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Balanced Volume Controller / Line Stage
Balanced Volume Controller / Line Stage
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Old 21st July 2018, 02:16 AM   #1
alexcp is offline alexcp  United States
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Balanced Volume Controller / Line Stage
Default Balanced Volume Controller / Line Stage

This is a high performance balanced active volume control / line stage, using a linear potentiometer or a digital volume control in the Baxandall configuration, coupled with a differential input stage and a ground-sensing output stage.
  • The differential input stage provides a balanced, differential input that works very well with single ended sources.
  • The output stage can be configured as a balanced output, for when your downstream stage has a differential input.
  • For downstream stages with single-ended inputs referenced to their local grounds, the output stage can be ground sensing (a.k.a. ground cancelling), eliminating e.g. the problem of cross-channel ground loops.
  • The ground sensing feature is primarily intended to work inside an integrated amplifier, particularly with a previously made (e.g. AmpsLab Lm60) or ready made (e.g. TPA3255 EVM) power amplifier blocks with either balanced or single ended inputs. It is great with LM3886 or similar chipamp based power stages.
  • The volume control can be used as an output stage of a DAC to form a "digital preamp".
  • If an input selector is added, will become a high performance balanced line stage / minimalist analog preamp.
I made versions with mechanical (pot) and digital (WM8816, PGA2310 and MUSES72320) control, with similar performance.

The schematic is very much a re-implementation of Bruno Putzeys' demo project from his article "The G Word, or How to Get Your Audio off the Ground", originally published in Linear Audio Vol.5. In designing the PCBs, I followed as much as I could the design methodology described by Mr. Putzeys in his article. There are some differences compared to Mr. Putzey's demo project, made to accommodate the use case above:
  • There is neither an input selector nor any connectors on the PCB. EMI protection is not included either as it need be by the input connectors.
  • No output muting. You probably have a protection relay between the power amps and the speakers anyway. Even without such relay, no thumping was heard during testing with an LM3886 power stage.
  • An output stage is added that can be configured as either a balanced output or as a ground sensing (a.k.a. ground cancelling) output. This should help solve the so called "cross channel ground loop" problem, which is simply two power amplifiers having inputs with different references ("signal grounds").
  • Power supply is designed to work directly from the power amplifier's supply rails. Rails much higher than +/-35V may require heatsinking for the two transistors, see the schematic.
  • No surface mounted components (except for the WM8816 and the MUSES72320) - all the other parts are through hole. One can install IC sockets and roll opamps - which I did a little, swapping LM4562 with NE5532 and even TL082.
I attach the schematic, a couple of PCB photos and some THD+N plots for the mechanical pot version. The digital control versions are posted below - see posts #2 (WM8816), #3 (PGA2310) and #16 (MUSES72320). The PCB in each case is 2.9x2.6in (74x66mm), two layers.

My soundcard (E-MU 0204) is not good enough to measure the distortion of this thing at 0.0005% at 1kHz - please look at the baseline plots of the soundcard with its input connect directly to its output. Also, note the difference in PSRR between NE5532 and LM4562.

The last plot is one ground referenced channel of a stereo LM3886 amplifier (with one power transformer and one power supply shared by both channels and the line stage) driven by this line stage in its ground sensing configuration. I used my own LM3886 PCBs discussed here.
Attached Images
File Type: png avc+s sch left ch.png (24.0 KB, 1435 views)
File Type: png SC loopback.png (37.2 KB, 1420 views)
File Type: png AVC balanced.png (37.2 KB, 1358 views)
File Type: png thd+n 1kHz avc LM4562.png (37.4 KB, 1344 views)
File Type: png thd+n sc only.png (23.5 KB, 1335 views)
File Type: png thd+n avc.png (22.9 KB, 161 views)
File Type: png thd+n avc lm4562.png (23.1 KB, 135 views)
File Type: png AVC+LM3886 ground sensing.png (39.1 KB, 155 views)
File Type: jpg IMG_0732.JPG (102.5 KB, 507 views)
File Type: jpg IMG_0737.JPG (86.9 KB, 507 views)

Last edited by alexcp; 10th August 2018 at 07:19 PM. Reason: Added MUSES72320 version
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Old 22nd July 2018, 02:06 AM   #2
alexcp is offline alexcp  United States
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Balanced Volume Controller / Line Stage
Default WM8816 Based Balanced Volume Controller / Line Stage

The same line stage with the pot replaced by a WM8816, controlled here by a rotary encoder and an Arduino. The measured performance (with NE5532's) is very similar to that with the pot.
Attached Images
File Type: png avc+s_8816 sch left.png (24.7 KB, 496 views)
File Type: jpg IMG_0739.JPG (96.4 KB, 468 views)
File Type: jpg IMG_0741.JPG (94.1 KB, 291 views)
File Type: png WM8816+NE5532 1kHz THD+N.png (38.0 KB, 245 views)
File Type: png avc+s_8816 bottom copper.png (18.7 KB, 245 views)
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Old 22nd July 2018, 08:05 PM   #3
alexcp is offline alexcp  United States
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Balanced Volume Controller / Line Stage
The same line stage with the pot replaced by a PGA2310, controlled here by a rotary encoder and an Arduino (with a different sketch, of course). The measured performance (with NE5532's) is even better than with the pot.
Attached Images
File Type: jpg IMG_0742.JPG (80.3 KB, 290 views)
File Type: png PGA2310+NE5532 1kHz THD+N.png (37.9 KB, 285 views)
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Old 23rd July 2018, 10:23 AM   #4
NATDBERG is offline NATDBERG  United Kingdom
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Quote:
Originally Posted by alexcp View Post

An output stage is added that can be configured as either a balanced output or as a ground sensing (a.k.a. ground cancelling) output. This should help solve the so called "cross channel ground loop" problem, which is simply two power amplifiers having inputs with different references ("signal grounds").
I'm only of basic electronics knowledge here (so forgive me..). Is this part kind of ignoring the concepts of "The G-word" when applied to the following power amplifier stage? Following those concepts, cross-channel grounds loops would be avoided wouldn't they? Or is the ground sensing confiuration just there for compatability?

Did you find through-hole to be any less accurate than the original SMD in terms of matching in the balanced circuits? I guess the answer is no, looking at the distortion graphs...

Good work by the way..
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Old 23rd July 2018, 10:31 AM   #5
maxw is offline maxw  United Kingdom
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Balanced Volume Controller / Line Stage
Very nice!
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Old 24th July 2018, 12:17 AM   #6
alexcp is offline alexcp  United States
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Balanced Volume Controller / Line Stage
Thank you for positive replies!

Quote:
Originally Posted by NATDBERG View Post
Is this part kind of ignoring the concepts of "The G-word" when applied to the following power amplifier stage? Following those concepts, cross-channel grounds loops would be avoided wouldn't they? Or is the ground sensing confiuration just there for compatability?
Yes - cross channel ground loops would be significantly reduced.

My thinking was that not every power amplifier out there has a differential input allowing a balanced connection. There may be good reasons for this - for example, a power amp may be unstable with its "negative" or "cold" input floating, and it is safer to ground it. This, however, presents two problems:
  • Connecting a power amp with a ground referenced output to a line stage is not trivial, because the reference point for the single ended intput is somewhere inside the power amp board, not inside the line stage. Ground loops may add noise and hum, but the problem is deeper. Simply connecting the two reference points with a wire sort of works at DC, but leads to higher distortion levels at high frequencies and may lead to unpleasant, irritating sound. This is how cables affect the sound!
  • In a stereo or multichannel setup, the same effects creates to the "cross channel ground loop" effect. I once tried to power two channels of Zen V4 from one power supply - it was humming until I installed input transformers, making inputs differential. Another way of dealing with this one is a "dual mono" setup with separate, isolated from each other power supplies for each channels.
Attached is a little simulation of four different ways of dealing with the difference in the signal references ("signal grounds") between a line stage and a power amp. The balanced connection is the best, but a ground cancelling/sensing approach works well, better than the often used "ground loop breaking resistor".
Attached Images
File Type: png ground sensing simulation.png (33.5 KB, 380 views)
File Type: png ground sensing sim results.png (21.8 KB, 362 views)
Attached Files
File Type: asc ground sensing.asc (16.5 KB, 15 views)

Last edited by alexcp; 24th July 2018 at 12:20 AM.
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Old 24th July 2018, 12:39 AM   #7
PRR is offline PRR  United States
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Quote:
Originally Posted by NATDBERG View Post
...kind of ignoring the concepts of "The G-word"....
You should probably re-read Putzeys' paper cited in the first post.

Ground over here is not ground over there!!

Let's instead say "the G pin". And also the H pin, center of an RCA interconnect.

What matters is not the voltage on the H pin, but the voltage *difference* between the H and G pins.

Say the G pin is not your ideal "ground". It never is.

If the source can sense *both* the H and the G pins at the destination, and adjust its output so the H-G difference IS the desired voltage, then signal delivery is perfect.

Ground-cancelling schemes don't just nail the RCA shell to the preamp chassis. They leave it somewhat floating, detect any stray voltage on the G, and adjust the H voltage so the H-G result is perfect.

I first saw this on dBx studio gear, aimed for a transformer-balanced world, but the builders wished to avoid transformers. The implementation served me well in a variety of situations, unbalanced, balanced, and floating. (You did have to wire it correctly when driving grounded unbalanced links.)
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Old 24th July 2018, 01:11 AM   #8
alexcp is offline alexcp  United States
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Balanced Volume Controller / Line Stage
Quote:
Originally Posted by PRR View Post
If the source can sense *both* the H and the G pins at the destination, and adjust its output so the H-G difference IS the desired voltage, then signal delivery is perfect.
Thank you - I have though about this one, too. I agree that conceptually, differential sensing would be closer to perfection. In my simulations, though, differential sensing didn't massively improve the results versus single ended. Intuitively, you cannot (or it is difficult) to get perfect cancellation across audio spectrum, esp. when longish runs of cable are involved.
Quote:
Originally Posted by PRR View Post
Ground-cancelling schemes don't just nail the RCA shell to the preamp chassis. They leave it somewhat floating, detect any stray voltage on the G, and adjust the H voltage so the H-G result is perfect.
This floating arrangement looked hairy to me. The signal and sense are two single ended connections, but what are the reference points for them? This is where differential sensing comes to mind. In the end, however, I made a decision to keep it single ended for a specific situation where I have a power amp with its input hard referenced to its local ground, and there is no alternative. Seems to work well on my bench. Should I need to run a cable from one box to another, I'd use a balanced pair.
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Old 24th July 2018, 07:24 AM   #9
00940 is online now 00940  Belgium
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Interesting stuff, thx. I was just wondering: why not go quasi-floating for the output ? Then you can have balanced and ground cancelling with just a change of cable (xlr-xlr, xlr-rca).

In the sims, shouldn't R32 be connected to the top of R33 ? Btw, what did you plot in the .asc exactly ? This look like an ac analysis but I see no source for it.
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Last edited by 00940; 24th July 2018 at 07:26 AM.
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Old 24th July 2018, 10:00 AM   #10
NATDBERG is offline NATDBERG  United Kingdom
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Thanks for the replies to my post, people - I'm learning..
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