A preamp has four functions:
1) control gain
2) provide any needed gain
3) provide input switching
4) control the interconnect cable
Of these functions, most digital sources provide one, which is to control gain. Unfortunately, Redbook specs call for the digital device to put out 4 volts, which is far too much for any power amplifier, so right away the signal has to get knocked down (and on the way, degraded) in order to be useful.
The problem is that many volume controls degrade the sound, and if they are not buffered (as in passive controls) the math often works against you, causing loss of bass if the control is anything less than full on, and often a lack of dynamic punch. This is caused by the resulting output impedance of the control in series with the source being so high that the interconnect becomes of sort of tone control that is interjected.
Now you could throw a lot of money at that problem by looking for just the right cable and many audiophiles do. But the simple fact is that the passive control is a big compromise, not based on the quality of the control so much as the approach: the control and interconnect effectively interact with the source impedance to behave as a filter.
One way around this is to have a PVC with a lower resistance value overall, but this can be hard for some sources to drive.
A buffered volume control does not have this problem. You will find that most preamps are far less susceptible to the cable than passive controls are as a result.
FWIW, the industry would be better off if the output of digital devices was limited to about 2 volts. One really does have to wonder what the authors of the Redbook were thinking!
Due to the variability of competence in line sections of preamps, you will find that the issue of passive (and TVC) volume controls VS active line stages will be an on-going debate. There are line stages that can so completely control the interconnect cable that you can no longer hear the differences between them. IMO/IME this is when you really start getting to transparency, as the filter I mentioned earlier is removed.
1) control gain
2) provide any needed gain
3) provide input switching
4) control the interconnect cable
Of these functions, most digital sources provide one, which is to control gain. Unfortunately, Redbook specs call for the digital device to put out 4 volts, which is far too much for any power amplifier, so right away the signal has to get knocked down (and on the way, degraded) in order to be useful.
The problem is that many volume controls degrade the sound, and if they are not buffered (as in passive controls) the math often works against you, causing loss of bass if the control is anything less than full on, and often a lack of dynamic punch. This is caused by the resulting output impedance of the control in series with the source being so high that the interconnect becomes of sort of tone control that is interjected.
Now you could throw a lot of money at that problem by looking for just the right cable and many audiophiles do. But the simple fact is that the passive control is a big compromise, not based on the quality of the control so much as the approach: the control and interconnect effectively interact with the source impedance to behave as a filter.
One way around this is to have a PVC with a lower resistance value overall, but this can be hard for some sources to drive.
A buffered volume control does not have this problem. You will find that most preamps are far less susceptible to the cable than passive controls are as a result.
FWIW, the industry would be better off if the output of digital devices was limited to about 2 volts. One really does have to wonder what the authors of the Redbook were thinking!
Due to the variability of competence in line sections of preamps, you will find that the issue of passive (and TVC) volume controls VS active line stages will be an on-going debate. There are line stages that can so completely control the interconnect cable that you can no longer hear the differences between them. IMO/IME this is when you really start getting to transparency, as the filter I mentioned earlier is removed.
