Passive preamp question

Where do you get the idea that passive preamps have extremely low output impedance? Unless there is a buffer at the output, in which case it's no longer passive but a buffered volume control, any passive preamp has variable output impedance. If we are talking resistor or LDR passive preamps what it comes down to is the value of the attenuator. Some are 10k, others are 25k, 50k, or even 100k. In any of these instances the output impedance will be determined by the position of the attenuator. In some positions the output impedance will be on the low side, but in other cases it will be on the high side. Your best bet to ensure low output impedance is a 10k attenuator, in which case it would probably work best if it saw an input impedance on the amp side of at least 47k ohms, preferably higher. Then again with a 10k attenuator you may run into issues with the input impedance of the passive preamp. Transformer and autoformer volume controls operate similarly but in my experience are a bit more forgiving when it comes to output impedance variation.

@clio09 ,

That was my assumption which I now see is incorrect. Thank you for the explanation.

There are different types of passive preamps.  I'm building a transformer based passive pre that will pretty much work to impedance match any amp.