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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 series of high performance balanced active volume controls with differential input, ground-sensing output (goodbye cross-channel ground loops!), and a choice of analog or digital volume controls (sample Arduino sketches for the latter are included):All versions feature:
  • Balanced differential input with excellent common mode rejection - works very well with single ended sources, too!
  • Output stage that can be configured as balanced (best for downstream stages with differential input) or ground sensing / ground cancelling (best if followed by a single-ended input referenced to its local ground - the vast majority of audio power amps are configured this way).
  • Compact (2.9x2.6in or 74x66mm) two layer PCB, all though hole parts (except WM8816/MAS6116 and MUSES72320, which only come in surface mount packages)
  • Can be part of an integrated amplifier (great with LM3886 or similar chipamp based power stages); an output stage of a DAC to form a "digital preamp"; with an input selector, a high performance balanced line stage / minimalist analog preamp.
The schematic is based on Bruno Putzey's Purist Balanced Preamp (aka BPPBP) from his article "The G Word, or How to Get Your Audio off the Ground", originally published in Linear Audio Vol.5. PCB design also follows Mr. Putzeys' method.

Attached are the schematic and measurements for the analog pot version. 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.

UPDATE: boards are available at HiFiOcean.com.

UPDATE2: a single-ended volume control based on DS1882 is added - it includes a Firstwatt B1-style JFET buffer and a low-noise regulated power supply, all on 1.1x1.1in (28x28mm) board.
Attached Images
File Type: png avc+s sch left ch.png (24.0 KB, 2359 views)
File Type: png SC loopback.png (37.2 KB, 2345 views)
File Type: png AVC balanced.png (37.2 KB, 2261 views)
File Type: png thd+n 1kHz avc LM4562.png (37.4 KB, 2237 views)
File Type: png thd+n sc only.png (23.5 KB, 2222 views)
File Type: png thd+n avc.png (22.9 KB, 243 views)
File Type: png thd+n avc lm4562.png (23.1 KB, 205 views)
File Type: png AVC+LM3886 ground sensing.png (39.1 KB, 234 views)
File Type: jpg IMG_0732.JPG (102.5 KB, 804 views)
File Type: jpg IMG_0737.JPG (86.9 KB, 805 views)

Last edited by alexcp; 23rd April 2019 at 02:31 AM.
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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, 781 views)
File Type: jpg IMG_0739.JPG (96.4 KB, 734 views)
File Type: jpg IMG_0741.JPG (94.1 KB, 438 views)
File Type: png WM8816+NE5532 1kHz THD+N.png (38.0 KB, 372 views)
File Type: png avc+s_8816 bottom copper.png (18.7 KB, 384 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, 457 views)
File Type: png PGA2310+NE5532 1kHz THD+N.png (37.9 KB, 458 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, 648 views)
File Type: png ground sensing sim results.png (21.8 KB, 605 views)
Attached Files
File Type: asc ground sensing.asc (16.5 KB, 22 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 offline 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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