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
Comments
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.
"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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Well yes, there are a few forward looking amp designers that "do different"!
Dave.
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.
Nil Satis Nisi Optimum
"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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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.
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).
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.
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.
Nil Satis Nisi Optimum