Originally Posted by commsysman
When you get your Master's degree in electrical engineering and have 40 years of teaching and circuit design experience, as I have, we can have a nice talk about this. Until then YOU have a very long way to go before you even understand the fundamentals of the subject at hand; your comments make that quite clear.
I've got similar educational credentials, and 52 years experience of designing and building audio gear, some of which has been marketed.
I think my following comments have useful weight in this discussion. ;-)
Here are the statements that you took exception to:
Originally Posted by SAM64
(1) We stopped impedance matching in pro audio over 40 years ago, consumer audio systems have never been impedance matched. A preamp doesn't do this, nor is it meant to.
(2) It doesn't provide isolation either, it actually connects your source to the amplifier, otherwise you wouldn't hear anything.
(3) A variable voltage divider, or volume control, presents a fixed impedance to the source.
(4) You don't have to consider impedance at all, there is no matching.
(5) You need to read up on modern audio equipment, you're hung up on something that doesn't exist.
(1) is true as far as it goes. However, tubed equipment is a different animal than SS equipment. You don't need to match impedances with it, but as its source impedance is generally high by modern standards, you have to take it into account. So while impedance matching is obsolete even for tubed equipment, considering the high source impedance is very relevant.
(2) Looks like hair splitting to me. A preamp does isolate the source from the load, even though it doesn't isolate the load from the source if the volume setting results in passing any signal at all.
(3) Is mostly true unless the load impedance is on the same order or smaller than the impedance of the attenuator. Unfortunately, with high source impedances, the load may have the same or lower impedance than the attenuator, in which case it can load down the source for the attenuator when the attenuation less than about 10 dB.
(4) may be wrong in this particular case because we are dealing with sources that are tubed equipment. Note that it is possible to build tubed equipment with decently low source impedances - often its as simple as one of the simplest circuits in the world - the cathode follower.
(5) Unfortunately, we're not dealing with modern equipment.
That all said, unless we find that the tubed equipment that is part of this system is designed to drive low impedances, which may or may not be true, it is best to be very wary of using modern attenuators that are designed to work with SS equipment.