Pubul57, there are 4 functions of a line stage:
1) add any needed gain- not all sources are able to drive an amplifier to full output.
2) provide for input and volume control- this function is shared by most passive and TVC systems.
3) buffer the volume control from the output- this prevents the load from interacting with the volume control setting, and prevents the control from exercising a tonality.
4) control the interconnect cable- which is done by having a low output impedance which swamps (makes negligible) the capacitive, inductive, resistive and other aspects of the cable.
Of these four, the latter is least understood, even by the industry that makes active line stages. This issue though is so profound that it is arguably the most important beyond actual volume control.
Its been my experience that if the line section controls the interconnect cable, then it has a good chance of outperforming a passive control or TVC. The reason is that the interconnect cable will cease to be an important part of the system sound. I'm pretty sure just about anyone who has set up a system using single-ended cables is aware of how much difference the cables themselves can make.
With any passive volume control, the cables are paramount and must be kept short for best performance. OTOH if the line stage is designed properly then you can run cables of nearly any length and the difference between the most expensive and the least expensive will be hard to hear. This latter fact is one that most cable manufacturers would rather you not know.
IOW if you can hear differences in the cable between the line section/passive and the power amp, then the cable is not being controlled.
A barrier to performance in many preamps, particularly tube preamps, is the output coupling capacitor. It must be made large enough so that phase shift is not evident in the lower frequencies (no loss of bass, IOW) and it has to do this with a transistor amplifier since the manufacturer has no way of knowing what amp the preamp will be connected to. Since transistor amps have a lower input impedance (usually 1/10th that of tube amps) this forces the output coupling cap to be a rather large value.
There is no way you can make large coupling caps sound right- they introduce coloration out of inductance and other well-known artifacts of larger capacitors. IMO, direct-coupling is the way to go. This allows you to bypass a primary concern of most tube preamps. Once this is done, the circuit has only to be merely competent and it will outperform any passive or TVC made.
It comes as no surprise to me that passives and TVCs are as popular as they are. What this tells me is how poorly active line stages are at the functions I outlined above. But just because *some* are bad at it, does not mean that *all* are.
1) add any needed gain- not all sources are able to drive an amplifier to full output.
2) provide for input and volume control- this function is shared by most passive and TVC systems.
3) buffer the volume control from the output- this prevents the load from interacting with the volume control setting, and prevents the control from exercising a tonality.
4) control the interconnect cable- which is done by having a low output impedance which swamps (makes negligible) the capacitive, inductive, resistive and other aspects of the cable.
Of these four, the latter is least understood, even by the industry that makes active line stages. This issue though is so profound that it is arguably the most important beyond actual volume control.
Its been my experience that if the line section controls the interconnect cable, then it has a good chance of outperforming a passive control or TVC. The reason is that the interconnect cable will cease to be an important part of the system sound. I'm pretty sure just about anyone who has set up a system using single-ended cables is aware of how much difference the cables themselves can make.
With any passive volume control, the cables are paramount and must be kept short for best performance. OTOH if the line stage is designed properly then you can run cables of nearly any length and the difference between the most expensive and the least expensive will be hard to hear. This latter fact is one that most cable manufacturers would rather you not know.
IOW if you can hear differences in the cable between the line section/passive and the power amp, then the cable is not being controlled.
A barrier to performance in many preamps, particularly tube preamps, is the output coupling capacitor. It must be made large enough so that phase shift is not evident in the lower frequencies (no loss of bass, IOW) and it has to do this with a transistor amplifier since the manufacturer has no way of knowing what amp the preamp will be connected to. Since transistor amps have a lower input impedance (usually 1/10th that of tube amps) this forces the output coupling cap to be a rather large value.
There is no way you can make large coupling caps sound right- they introduce coloration out of inductance and other well-known artifacts of larger capacitors. IMO, direct-coupling is the way to go. This allows you to bypass a primary concern of most tube preamps. Once this is done, the circuit has only to be merely competent and it will outperform any passive or TVC made.
It comes as no surprise to me that passives and TVCs are as popular as they are. What this tells me is how poorly active line stages are at the functions I outlined above. But just because *some* are bad at it, does not mean that *all* are.