Who is using passive preamps and why?

Can't argue with your math but I think there is a simpler way to look at it, something I should have brought up before. You need to consider 2 factors.

To make a match first look at ratio of source Zout to amp Zin. Convention says 1:10 minimum, I say closer to 1:100 is much better. In Pubul's case with 50 Zout and 100k Zin he is way above minimum at 1:2000.

Then pick a passive as low as you can get away with. 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.

For the reasons your math points out going with a passive much greater than required can cause problems just like going too low with too low being worse. That's why you see Placette building 9K passives. In Pubul's case many would argue a 50K pot is too high.

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Yes, that strikes me as a good way to look at it, Herman.

I notice that at this page of the Goldpoint site the following statement is made, which I believe is incorrect and misleading:

When choosing the stepped attenuator value for an in-line level control or "passive preamp" (such as the Goldpoint Level Control Boxes), the attenuator value is chosen to match the input impedance of whatever it will be controlling. example: If the amplified monitor speakers or power amplifier you will connect the output of your your passive preamp to has an input impedance of 20K, then order a 20K stepped attenuator for that application.

Although they then qualify that with this statement:

You can usually use a level control value which is LESS than the rated input impedance of the gear it will be controlling.... 25K is usually a good choice for both vacuum tube and solid-state equipment

Best regards,
-- Al

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.