My apologies for the digression Nate, but I wanted to comment a little bit on the following:
Really never got the passive pre-amp thing, as it's really nothing more than an attenuator box that places larger impeadance load's on the power amp input section. Inserting extra isolation transformer based pot's in the low level stream, certainly can't be helping thing's.
There's really not such thing as a passive pre-amp, just an attenuator box placing more of a impeadance load than need be on the power amp's input section. Having to use an external phono section, seem's to throw out the purist intention a bit. Already know what better than a passive attenuator box sound's like, as I've experimented connecting 500mv equipment directly to power amp input's. Sound's just fine, but little flexibilty. If you're running a power amp with level control's, then best to skip the extra box all together.
It is true that the resistive potentiometer is nothing more than an attenuator. However, if properly designed, it
will provide low enough output impedance to properly drive the power amplifier connected to it. For example, if one were using a 10Kohm pot (wired as a voltage divider), the maximum output impedance that this pot will exhibit is 2.5Kohm (give or take a few ohms depending on the output impedance of the source driving it). This pot would do quite well driving an amplifier whose input impedance is say 30KOhms or higher.
But the 10Kohm pot might not be high enough of a load for the source driving it. So, some care is needed in designing a passive resistive volume control. Of course, if the power amplifier is an integrated, with input selection and attenuation, I would probably use it alone.
A transformer volume control works somewhat differently. As we know, it transforms voltage on its input (primary) to output (secondary) in direct proportion to the ratio of the turns of copper wire on the primary and secondary. That is, if there are Np turns of copper wire on the primary, and Ns turns of copper wire on the secondary, and a voltage Vp is impressed on the primary, the secondary voltage Vs = Vp x Ns/Np. In other words, Vp/Vs = Np/Ns.
So, as the knob on a TVC is turned up, Ns increases and therefore Vs increases.
But where the TVC excels is in the behavior of impedances that are connected on the primary and secondary. While voltage is transformed in the ratio of Np : Ns, impedance is transformed by the square of the turns ratio, viz. (Np / Ns) ^2.
So, if we connected an amplifier with an input impedance of 50Kohms on the secondary, it would appear to the source (on the primary) as an impedance whose value = 50Kohms x (Np/Ns)^2.
By the same token, the output impedance of the source (say ZS) will appear to the amplifier as ZS x (Ns/Np)^2.
Take the case when Np/Ns = 2.
The 50Kohm amplifier impedance will appear to the source as a 200Kohm impedance, making the amplifier a much easier load to drive.
If ZS, the source impedance = 1Kohm, it will appear to the amplifier as a 250 ohm source.
I guess the point is that theoretically at least, the TVC is a lot less of an impedance problem than a resistive volume control. But it should be noted that as Ns gets closer and closer to Np (volume is turned higher and higher), Np/Ns, and therefore, the square of Np/Ns goes closer and closer to 1. Which in turn means that the reflected impedance is not that much different from the actual impedance, as we go towards zero attenuation.
I would imagine that at normal listening levels, Np/Ns is greater enough than 1.0 to let the beneficial effect of impedance reflection to be achieved.
Further, it is not possible to build an ideal transformer. So, the resistance/inductance of the copper wire and the iron core of the transformer will affect the analysis, but the beneficial effect of the impedance transformation from primary to secondary is still valid.