Gaining ground

December 21, 2016
 by Paul McGowan

I had reminded us in yesterday’s post the three elements of a preamplifier:

  • Input selector
  • Volume/balance control
  • Gain stage

We know volume controls are intrusive to sound quality. Some, more than others.

I had come up with the idea of eliminating the volume control by designing a gain-variable amplifier stage. A great idea with only one problem. How to do it?

The gain of an amplifier is typically set by a pair of resistors. These can control a single stage or an entire gain block.

Here’s a typical application:

Non inverting

The gain of this simple amplifier is set by the two resistors I spoke of, Rg and Rf. Divide one resistor by the other to arrive at the gain. So, imagine Rf is a 10kΩ resistor and the other is 1kΩ. You get a gain of 10 (20dB).

So if that’s true, would it not be easy enough to replace one of the resistors with a pot? Then, turning the pot would vary the gain of the amplifier. Voila!

Unfortunately, that doesn’t work for a number of reasons—some very technical but let’s see what we can explain simply.

The first problem is the idea of adding a pot in this amplifier. Turns out it doesn’t matter where you place the pot, it will always add its sonic signature once in the circuit. So we’ve not managed to make an improvement if we implement it.

The second problem is even worse. This type of circuit uses feedback and it is the feedback ration the two resistors are changing. When you change negative feedback the sound of the amp changes too (as well as stability). Each different level would sound slightly different: lower levels would have greater amounts of negative feedback, louder would have less.

The third problem is perhaps the biggest. You can’t easily have much less than a gain of one. Which means we can’t have a volume control that goes from zero output to full output. At least not easily, practically in any useful way. Just ain’t gonna work.

There has to be a better way.

And, there is. One we use today in the BHK preamplifier.

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17 comments on “Gaining ground”

  1. Following my comment yesterday that a universal solution for perfect volume control would be an FPGA DAC/pre with ADC (internal or external) for phono if required, Bernd responded that he has indeed done this with his own FPGA phono/pre. (My apologies Bernd, did not read your earlier post, was away.)
    Now the discussion has taken the Roman route to the BHK pre, I searched that product and came across a review/infomercial/love-in by Karl Sigman who – surprise, surprise – has for a few years been using PSA’s ADC converter into a Directstream FPGA DAC/pre. (Bernd – Mr Sigman refers to his use of Merrill amps that I believe use Pudzey’s top-of-the-range NC1200 Class D units). That makes sense and yesterday I also searched ADCs, coming up only with the PSA unit and a similarly priced Benchmark unit.
    Mr Sigman’s article is not of much use other than to send Directstream DAC owners into a tail-spin of depression unless they have the cash for a BHK pre, which I assume many have and will. It’s just a waterfall of worn out audio-adjectives. It doesn’t tell you anything about the volume control over than a sentence or two that I assume comes from the manual/website (haven’t checked).
    So I would be interested to know:
    (1) Why the BHK volume control is better in audio terms than PSA’s FPGA volume control.
    (2) Whether, like FPGA, it can be implemented in a DAC/pre that can be used both as an audiophile system component and as a portable headphone DAC.

    1. The volume control in the BHK is purely analog as is the preamp, though it does use digital control of the analog circuitry. But its path is pure analog.

      The volume control is reasonably well described on our website. Basically, it is a two-part affair. The vacuum tube has zero feedback. The gain of it is determined by the plate and cathode resistors. So for the small 1/2dB steps of the control, the ratio of plate to cathode resistor is changed in steps. We can go ten steps, changing the gain of the tube. It’s tricky to do this quietly and without sonic problems, but we have good engineers.

      For the bigger jumps, we use a stepped attenuator pad activated by a series of relays. Together the two work well giving us the full range of level control in 1/2dB steps without leaving the analog domain.

      As to adding a DAC. Why? It’s an analog preamp.

      1. Thank you for explaining.

        Being relatively new to changing my hifi (about 5 years, system unchanged for many years before that), the idea of an analogue pre-amp that does nothing else is a bit novel/old-school.
        Maybe its my preference for a single box that takes a digital or analogue signal and can provide an attenuated output for a power amplifier. There are many such products available and there is demand for them, but the weakness is often the volume control. If the BHK volume control is excellent, why not package it with greater functionality? Surely that would also make it more marketable?

  2. Sorry for posting 1 and 2 (time difference). Outboards DACs with volume control used to be considered as a DAC with a pre-amp. As outboard DACs started to become redundant, they are now considered more as pre-amps with a DAC.
    Question: Was it considered, or would it not be sensible, to put a DAC into the BHK pre-amp? If not, why not? Heavens, even a phono input and I’d get one tomorrow. (These options would all use existing PSA tech.)

  3. Never realized the humble volume control is such a big issue in audio.
    I took it for granted. But I was wrong.
    Yes I know, ignorance is bliss, but from now on every time I turn up the volume, I’ll probably hear there is “something” wrong with the sound.
    Thanks for that.
    Maybe I’ll buy a canary. Nice sound an no volume control to mess with. 😀

    1. There is a lot of rubbish written about audio, including that a pre-amp can add to or improve the sound quality, but it can degrade it. I know this as I changed from using a PWD Mk2 volume control to a standard ALPS Blue potentiometer (purpose built into my phono amp) and the degradation was such that I eliminated the ALPS and got a dual mono stepped attenuator passive preamp to replace it. My first ever pre-amp after many years hoping I’d never need one.
      Hence I’m finding Paul’s posts on this issue most interesting.
      Whether it’s better than FPGA volume control requires a blind test, using a Directstream (or similar) set at 100% into a BHK pre, compared to a Directstream (or similar) using the internal volume control, level matched of course. Independent listeners need only apply!

      1. Okay joking aside.
        Rubbish or not, in an earlier comment I said that I tried the PWD Mk2 directly connected to my amp.
        The sound was worse compared to the sound via my pre amp. For me at least.
        Now I’ve got the DSJ with the same results.
        So for me an analog pre-amp still gives the best results and therefore is a must.
        I don’t like a volume control in the digital domain.
        But maybe in the future, when everything will be better…(?)

        1. Well, I got the exact reverse results in my systems.

          There were pre amps that added less coloration to the signal and the best til now was the PASS XP-30 but I found that I came closer to what was recorded when I connected my DACs directly to the power amp. I did the tests it with different music but all of it had in common, that I took part in the recording session. This included human voices to say it clear it were the King’s Singers, orchestral material with the Frankfurt RSO (Mahler, Berlioz and Bruckner) and the Sinfonieorchester des Westdeutschen Rundfunks (Mendelsohn ‘Lobgesang’ and Bernstein’s ‘Chichester Psalms’) , Keith Jarrett’s Köln Concert to mention a few.
          I did not test the PSA DirectStream so far, but DACs from, among others, Wadia, Krell, PASS, dCS, EXOGAL and Ed Meitner.

          You said that you do not like volume control in the digital domain – I wonder why. Did you ever test one of the DACs I mentioned.

          Regards

          1. What I heard was :
            DSJ directly connected to power amp gave a little bit more the impression the artist was in my room. A more “direct” sound.
            DSJ via pre-amp to power amp more the impression that I was with the singer in the concerthall.
            I prefer the latter. My pre-amp ads more soundstage, depth.
            A matter of personal taste what you prefer, I guess. Not necessarily that one is better than the other.
            And using the volume control of a DAC always has one problem : to avoid that the music sounds WAY too loud in my room (and I’m not afraid of loud !) and even could damage the speakers, I need to turn the volume down to about 65% %; 70% max.
            With the result of less resolution (obviously every manufacturer claims his DAC does not do that)
            Use my pre-amp and I can turn the DAC-volume up to 100% if I wanted to; my setting is mostly 95%. Sounds (a tiny bit) more “analog” (I agree with what Mark-d said about this).
            BTW : my pre-amp is the excellent Levinson 326S.
            With 1 db steps from 0-23 db and 0.1 db steps from 23 to 80.
            So I’ve got 593 volume steps. Works wonderful.
            dCS sounds very good, but I never heard that one in my system. I had to rob a bank before even thinking of buying one.
            As far as Wadia is concerned, I don’t know if you mean the old (for some the one and only) Wadia or the new one(s).
            I think the new ones have a more natural sound instead of the (in)famous Wadia sauce of the older series.
            For me not better than what I’ve got now.
            I never liked Krell very much. Much power, not a lot of refinement.
            Pass makes excellent amps, but no DAC.
            And my Levinson pre-power combi still does the trick for me.
            And I never heard Exogal.
            But in our hobby there is always the “law of uncertainty”.
            You can never be certain that, within your budget, you have the best sounding system.
            There are just too many brands (amps, dacs, cables, players, speakers). You cannot hear everyting, let alone all the the different combinations. Impossible.
            And as I said before, I always buy what I like best, not what reviewers, dealers or whoever say.
            For me that is one of the beauties of our hobby. Everone has a different taste, so a different system. If we all had the same one, that would be boring. Nothing to talk about anymore.

            the possibikl

            And loike I said, I had the same experience with the PWD (Mk2).

            1. @jb4

              I see that you experience quite the opposite to what I hear.

              You like the new Wadia stuff, I do not like them any more. The last excellent sounding DAC was the series 9 and I’ve had Wadia at home since the Wadia 2000 combi which was the first digital system that could really compete with vinyl in my audio chain.

              You’ve written “I need to turn the volume down to about 65% %; 70% max. With the result of less resolution (obviously every manufacturer claims his DAC does not do that)” That a digital volume control has loss of resolution in modern well designed DACs is simply not true. There is more than one manufacturer who could tell you how it could be done without loss of resolution. The dCS was a loan from the distributor – far to expensive for me.

              You are totally wrong regarding DACs from PASS. PASS LABS as well as its predecessor Threshold have built a DAC and I still have both at home (I am a nostalgic person) . BTW Wayne Colburn has done the work and Nelson did put in some ideas for the analog stage. I do not remember whether Desmond Harrington was involved too.

              Although I admit the build quality of ML products is excellent I did not like them since the time when Tom Colangelo left ML.

              Regards

              1. We agree on a lot of points here.
                Yes, we do come from different planets, maybe even from parallel universes considering our taste (e.g. the artificial Wadia sauce from the past.you seem to like so much).
                And yes, I agree with you that “There is more than one manufacturer who could tell me how it could be done without loss of resolution”.
                My question : So why don’t they do it ?
                But let me tell you there is also more
                than one manufacturer who could tell you why their amp is better than all the rest.
                And there is also more than one manufacturer who could tell you why they make the best sounding speaker.
                As always, I believe my ears. But if you wanna believe what manufacturers say, good luck with that.
                And please read my comments carefully before you react.
                I wrote “Pass makes excellent amps, but no DAC”.
                If you think otherwise, you are totally wrong. Just try to order one. Not possible.
                And if you don’t like ML “since Tom Colangelo left ML”, then maybe (just maybe) you are one of the nolstalgic people who don’t like fine products from the moment one of the original members leaves the company.
                Reminds me of the guy I saw on you tube raging against the NEW speakers of Thiel. They were rubbish, garbage etc.etc.
                After about half an hour he said “oh BTW, I haven’t heard them yet…..”
                I wouldn’t be surprised if you never heard most of the new ML amps, maybe not even one. But yeah, they’re new, not from “the good old days” (when everything was better), “so” they can’t be good…..

                1. @jb4

                  I do not know what you’ve eaten or drunk today.
                  Your agressive and supercilious intonation makes me think. It seems that you know it all better but …
                  Your statement on Wadia is simply ridiculous so is your statement on DACs by PASS – I wrote in capital letters. If you cannot order a PASS LABS DAC today does not mean that they never have built one – go on and order a pair of IRS Vs from Infinity – good luck!
                  I continued liking products designed by Tom Colangelo namely Cello and Viola Labs. I do not like the changes in sound the ML products had since TC left the company. When Tom died in 2007 Paul Jayson continued to do the work for Viola – btw he was in the MLAS team and in the Cello team too. Well I did not intend to do a lecture in HiFi history at this place.

                  Let me end the ‘alien’s answer’ by sending you season greetings
                  Merry Xmas
                  Bernd

                  1. My dear Bernd,
                    First of all I did not make any statements about Wadia or any other brand.
                    Just my opinion, I never say what sound is better, just what I like better. It’s a free country, at least where I live. If you don’t like people who don’t have the same opinion as you have, then you are dangerous.
                    And then you tell me my “statement ” (as you call it) IS simply ridiculous. If you would have said “I FIND you’re statement is simply ridiculous” then you would have been much more sympathetic.
                    Everybody is entitled to his own opinion (even when it’s wrong). By using the word “IS” you present your opinion as the absolute truth.
                    And that is wrong Bernd, so wrong. I even know a better word for it : supercilious.
                    Secondly, for the last time : Pass does NOT MAKE a DAC. I NEVER said they never MADE a dac.
                    Read, BERND. Read, think and absorp before you write (in lower case or in capital).
                    But that’s probably the difference between you and me : you live in the past, I’m in the present.
                    Oh and just a friendly warning from one audio buddy to the other in case you’re thinking of ordering a pair of INFINITY IRS speakers : Infinity DOES not make the IRS speakers. They DID make them.
                    But now you understand I hope. I’m not gonna explain it again Bernd.
                    BTW., I think you were in the army, used to drilling shaky soldiers. Or a guard in some prison camp. Or a teacher with no contradiction from the students. That’s what you like.
                    Let me know if I am right, my grumpy old man. 😀
                    Oh and develop a sense of humour, life is too short for being always so damn serious as you are.
                    Now I stop and gonna listen to a cd via my Pass Audio dac, erm my PS Audio dac of course.
                    See, your fault, you got me confused.

                    1. @jb4

                      Thanx for the English lesson!
                      The grumpy old man is going to tell you that you’re wrong with your assessment of me.
                      Indeed I am a “Kriegsdienstverweigerer” – I do not know the right English word for it. Instead of going into the army I worked as a nursing assistants in a hospital.
                      So please accept my apologize!
                      You’re right, I’m wrong.

                      Regards

                  2. Bernd,
                    Thanks for your nice reaction. I appreciate that.
                    And no need to apologize.
                    If there is one thing I learned in the past 40 years in this wonderful hobby you and I share, it’s that it’s all about taste. No right or wrong here.
                    So I felt a bit “attacked” by you when you said I was “wrong” while I was only talking about my personal taste.
                    So my response to you was a bit harsh.
                    And I think the sound we like to hear from our speakers has a lot to do with the kind of music we like. Classical, jazz, rock or whatever. Beethoven, Coltrane or Coldplay.
                    I listen occasionally to classical music, I think you much more.
                    And you being a “Kriegsdienstverweigerer” (“conscientious objector” in my dictionary) is good. Everybody should do that, no more war. But that’s Utopia, alas…
                    Und für jetzt : frohe Weihnachten.

          2. I think you misunderstood. I like digital volume controls in DACs. I use them all the time and our own sounds excellent. No issues at all.

            What i don’t appreciate is the same in an analog preamp.

            Analog preamps should be pure analog. There a digital volume control makes no sense since you have to first quantize the audio before controlling the volume, a far more intrusive process than merely adding an attenuator.

            1. Paul,

              I understand your point of view quite well.
              Although quantization of audio is done every single day now in allmost every recording session it might be difficult to declare why digital volume control is used in an pre amp to those who still prefer analog media and even more to sell these kind of pre amps to the targeted users.

              Regards

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