Who is using passive preamps and why?

...if the impedances and cable capacitance are right, so that a transformer doesn't really solve a problem, wouldn't a resistor based passive, like my Goldpoint, perform better than a TVC, especially in terms of bandwidth?

Theoretically it should and that is what I found in my comparison of the Slagle autoformers and Lightspeed attenuator. However, something about the sound of a transformer or autoformer type passive is still very appealing.

At full volume (worst case) the combination of amp Zin and passive should be at least 10X source Zout but let's go with 100X. Pubul's 50 ohm source should see at least 5K so a 10K passive looks to be a great match. A 10K passive in parallel with the 100K amp is 9K.

My source is 50 ohms Zout and the amp is 100k ohms Zin. The passive I just built uses transformers to step up the signal from the source. These transformers are wound for sources that are ideally 100 ohms Zout or less (but could go as high as 300 ohms). Also, the sources must be opamp driven, as many CD/DACs are, not transformer driven. The passive also uses 10k Alps pots.

So basically, the 9k number listed above would apply giving me about a 200:1 ratio. Also, and I think that I have read this above, just want to make sure I understand, with a 10k pot the output impedance rises as the knob is rotated clockwise until it reaches 10k. Did I get this right?

I too wish like AA pictures can be posted here. If you would like to email me offline with one that would be fine.

Looking at this from Roger Modjeski's pot-in-the-box concept, which he recommends for the RM-10 MkII, and I have seen him use with the RM-200, the specifications are as follows:

Input Impedance 50K Ohms
Output Impedance 0 - 25K Ohms
Frequency Response DC - 100K Hz

Do I assume correctly that the Noble pots are the 25k version? What does this do in my case where Zout of source is 50 ohms and Zin of amp is 100k ohms.

I think I answered my own question. Using the link below, a 25k pot and 100k amp Zin equals 20k.

Parallel Resistor Calculator

Also, and I think that I have read this above, just want to make sure I understand, with a 10k pot the output impedance rises as the knob is rotated clockwise until it reaches 10k. Did I get this right?

No, Herman's relevant statement was:

In my example they were both 10K so it is 1/2 of the 10K. With the volume close to zero it rises to about 10K. and falls as the volume goes up.

In your case, with a 10K pot and a 100K amp Zin, at minimum volume control setting the input impedance (not output impedance) looking into the input side of the pot would be 10K. With the control turned all the way up, the impedance would be 10K in parallel with 100K, which is 9.1K.

The source component would see a load impedance equal to that value (dependent on the volume control setting) factored by square of the ratio of transformer primary turns to secondary turns (assuming the transformer is "ahead" of the volume control).

As I indicated a couple of posts ago, the OUTPUT impedance of the passive, which must be kept low in relation to the amp input impedance, will be at its worst case maximum when the volume control is set to the mid-point of its resistance range (assuming the source component's output impedance is small). Apart from the presence of the transformer, with a 10K pot and a 50 ohm source impedance, that output impedance would be 2.5K. The step-up transformer will raise that value a little, by stepping up the (very small) contribution of the 50 ohms. But for any reasonable turns ratio the resulting output impedance is likely to still be well under 5K, and therefore fine in relation to the amp's 100K Zin.

BTW, calculating the resistance of a parallel combination of two resistances is easy. It's just the product (multiplication) of the two numbers, divided by their sum.
If more than two resistances are in parallel, it's a little more difficult, the result being equal to the reciprocal of the sum of the reciprocals of the individual resistance numbers. (Reciprocal = the number divided into 1).

Best regards,
-- Al