An idea for a different (?) volume control for a SS amp

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RockerRocker Frets: 4945
while staring into space today, I started thinking about amplifier volume controls. As I understand it, the gain of an amp is fixed, the volume control dumps most of the signal, only allowing a user selectable amount through to the power stage.  In hi-fi amps the pre, actually the volume control pot itself, is the weak link in most if not all systems. Is it feasible to somehow vary the gain rather than limit the input to obtain control over the volume level? Would varying the PSU DC voltage have the same effect? Needless to say I am always in search of a 'better' preamp but so far my search has been in vain. So Techs, is this workable?
Insanity: doing the same thing over and over again and expecting different results. [Albert Einstein]

Nil Satis Nisi Optimum

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Comments

  • Maybe I'm dumb but isn't this how the output power control on, say, a Vox valvetronix would work?  Probably not by varying the supply voltage directly but by altering the voltage on the required pin on an output IC.  
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  • ecc83ecc83 Frets: 1589
    If you are talking hi end audio then yes, the conventional volume pot has limitations.
    Where you put it in the signal chain is always a compromise twixt noise and headroom. Duggy Self goes into this quite deeply in his book  Small Signal Amplifiers, required reading IMHO for anyone interested in sound amplification. In the book he shows that THE best solution is an "active" volume control built around a couple of op amps.

    However for the vast majority of us, even hi fi buffs, the basic "10k pot" works well enough. For guitar amp use anything else would be an overkill. N.B. .Almost all guitar amps, valved and sstate that have "gain" controls and a MV have in fact interstage  VOLUME pots. Some designs put a resistor in the bottom of the track to prevent total shut off. Gain control proper is done by varying NFB but rarely in gitamps AFAIK.

    Varying the supply volts will not change the gain around a sstate circuit (op amp) it will change the gain of a valve stage but not by much. Low HT will of course mean lower drive and more distortion.

    Dave.
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  • ICBMICBM Frets: 71957
    ecc83 said:

    Gain control proper is done by varying NFB but rarely in gitamps AFAIK.
    Apart from in amps which use the "Tube Screamer type" NFB-loop IC gain stage, of course :).

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • ecc83ecc83 Frets: 1589
    ICBM said:
    ecc83 said:

    Gain control proper is done by varying NFB but rarely in gitamps AFAIK.
    Apart from in amps which use the "Tube Screamer type" NFB-loop IC gain stage, of course :).

    Well yes, there are a few forward looking amp designers that "do different"!

    Dave.
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  • jpfampsjpfamps Frets: 2723


    Hi-fi pre-amps are designed with variable gain.

    Varying the voltage to an amp has limited effect on the gain the amp; all you would be doing is reducing headroom. This is useful in a guitar amp as it allows clipping of the amp at lower volumes, but I can see any application in a hi-fi amp.

    A well-design pre-amp should not introduce significant distortion into the audio.

    The main issue with volume controls is keeping maintaining balance on each side of the stereo image.

    There is no reason in a properly design amp for this to be a "weak-link"; modern resistors don't introduce distortion, and if the surrounding circuitry is well designed then the values in the volume control can be low enough to reduce Johnson noise to below the amp's noise floor.

    The notion that the volume control is a week link probably originates with manufacturers of (expensive) stepped attenuators.
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  • RockerRocker Frets: 4945
    The 'best' hi-fi pre amp/volume control I ever experienced was a Music First Silver transformer job. This had a great many taps on the traffo to a multi way selector switch, and it really opened up the soundstage. It was nothing short of remarkable, so simple yet so 'right'. Sadly I could not come up with the cash and it went instead to a buyer in Holland. The usual volume control pot is an Alps. This is bettered by a Dact stepped attenuator. The traffo box is in a different league. None of this makes any difference on a guitar or guitar amp as a guitar amp is designed for a different purpose. My initial post was my wondering if there is another way to control the volume levels (by varying the gain for example). It might be possible, I do not have the electronic knowledge or experience to follow this concept up. Assuming it is possible that is....
    Insanity: doing the same thing over and over again and expecting different results. [Albert Einstein]

    Nil Satis Nisi Optimum

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  • ICBMICBM Frets: 71957
    One reason it might not be commonly done is that variable gain, in a normal feedback-loop type configuration, will only give a minimum setting of unity. That isn't low enough to turn down an amp to very quiet. At the end of the day a simple potential divider volume control is very effective, easy to implement and doesn't introduce many other issues.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • ecc83ecc83 Frets: 1589
    If the Silver m'tapped traff sounded "better" than a properly implemented decent quality pot then the traff was wrong!

    As JPFA said, modern MF resistors do not cause distortion (well, not any signal voltage we are likely to use!) Transformers do, especially if driven from a non-zero source Z, as most will be.

    But of course, like fitting the "wrong" valves. If you like the result...You like it!

    Dave.
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  • jpfampsjpfamps Frets: 2723
    ICBM said:
    One reason it might not be commonly done is that variable gain, in a normal feedback-loop type configuration, will only give a minimum setting of unity. That isn't low enough to turn down an amp to very quiet. At the end of the day a simple potential divider volume control is very effective, easy to implement and doesn't introduce many other issues.

    You can have a variable feedback gain stage going to zero signal, but it needs to be an inverting shunt feedback stage (which may or may not be helpful).

    A series feedback stage will only go down to unity (but is non inverting).


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  • jpfampsjpfamps Frets: 2723
    Rocker said:
    The 'best' hi-fi pre amp/volume control I ever experienced was a Music First Silver transformer job. This had a great many taps on the traffo to a multi way selector switch, and it really opened up the soundstage. It was nothing short of remarkable, so simple yet so 'right'. Sadly I could not come up with the cash and it went instead to a buyer in Holland. The usual volume control pot is an Alps. This is bettered by a Dact stepped attenuator. The traffo box is in a different league. None of this makes any difference on a guitar or guitar amp as a guitar amp is designed for a different purpose. My initial post was my wondering if there is another way to control the volume levels (by varying the gain for example). It might be possible, I do not have the electronic knowledge or experience to follow this concept up. Assuming it is possible that is....

    Transformers have questionable linearity at the best of times, especially at the low end of the frequency spectrum. Transformers can also introduce unwanted phase shifts, and ringing in the audio band.

    Transformers are generally avoided in audio applications, except in circumstances where the non-linearity is part of the appeal and low noise is important (eg microphone pre-amps), and in situation where rejection of external noise is important (eg ground lifting, and broadcast applications).

    In contrast a properly designed pre-amp will have a distortion performance below a level at which anyone could argue it's audible.

    Was this an (oxymoron) "passive" pre-amp? In which case it's likely that the transformer will be being driven by an undetermined source impedance which is far from ideal.

    Furthermore, for best performance, a transformer needs to have a relatively complex pattern of interleaving, which would hard to maintain on all the transformer taps.

    One thing you can guarantee with a stepped transformer attenuator is that it will be very expensive.


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  • RockerRocker Frets: 4945
    @jpfamps, @ecc83, you guys know more about the operation of these things than I do but having heard the result myself, I am very seriously impressed.  Impressed enough to consider selling a couple of guitars to fund the purchase.  In the end the cost was too high, even if the performance matched the cost [which is something not often said about hi-fi products].

    This is the pre-amp section from the MF website:

    https://mfaudio.co.uk/preamplifiers/

    The pre-amp I tried was similar to the Reference model but it had silver wires in the transformer.  Not sure if that model is available any more though....

    Whatever the theory, the results spoke for themselves.
    Insanity: doing the same thing over and over again and expecting different results. [Albert Einstein]

    Nil Satis Nisi Optimum

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