A volume control is a voltage divider. To achieve this the sensitive signal must pass through either resistors or transformer windings. There will be two voltage dividers, one for each channel. It’s essential that these dividers achieve the same result for each channel, as the volume is varied, in order to maintain a good centred image. It is also essential that they don’t affect the signal in any way as they’re varied, ie, the sound quality is exactly the same at really low levels as at higher levels.
If the voltage divider is a potentiometer, then the material the resistive track is made from will effect the sound quality and the two resistive tracks must match exactly. If the divider is switched resistors, this gives the opportunity to use more precise channel matching, and ‘better sounding’ resistors. If the divider is windings on a transformer, you avoid resistors all together and can achieve perfect channel matching. The only other approach, and the best I’ve yet heard, is how Lyngdorf do it, which is not to vary the music signal itself, but to vary the PSU voltage to the output driver stage.
In order of sound quality from poor to best, its Potentiometer, Switched attenuators, transformer and finally Lyngdorf’s approach.
As the volume control quality improves, the sound stage becomes more stable, sound quality more consistent from low volume to high and transparency improves.
