Passive preamp

A few of the replies are saying no added gain, or no gain stage, but more precisely a passive preamp should use no active components in the audio signal path (e.g. it would be fine to use active components for a display or to support a remote control). Active components aren't only used to add gain, but can also be used as an input or output buffer, or implement feedback, etc. Roughly anything that includes a tube or a transistor is an active component.

The downside to a passive preamp are the downsides resulting from the lack of those active components. Impedance mismatches, voltage/current limitations, etc. can all suddenly start to play a role and affect the sound quality, accuracy, or precision.

You can see how this can make selling or buying a preamp much more of a hit-or-miss situation, where one person's experience does not at all translate into another person's experience.

The advantage of a passive preamp is that you minimize any potential non-linearity and noise introduced by those components that you've instead chosen to leave out.

You use and connect a passive preamp the same way you would an active one. However there may be some situations where it is inadvisable to connect more than one input or output at the same time, depending on the internal design.

nikoaudio,

Any active device in the signal path means that the preamp is no longer passive.

MrD, and Neko both said it well.  A major problem, which I learned the hard way is: Impedance Mismatches between passive pres and amp

I loved everything about the looks and function of a Hattor passive, alas, it was a mismatch for the amp I had at the time. I wish I could have kept it, but the cost to add a buffer was prohibitive, AND the sound of Buffers is a whole nother level of discussion.

Nord offers several Buffer options for their designs

@georgehifi
How does one avoid impedance mismatches?

This is very important to me as I am putting together a used system from scratch starting with Dali Euphonia MS4 speakers. Thinking about the Classe CA-M600/CT-M600 monoblock amplifier. Would love 3 recommendations for passive tube preamps in the $1k, $2k, & $3k price ranges (used of course).

Thanks,
Zeus

How does one avoid impedance mismatches?

Using the accepted impedance matching ratio of 1:10 or higher.
A 10khom series/shunt passive preamp is the best most versatile value to go for as it has:
10kohm input impedance
2.5kohm max output impedance.

The source (dac ect) should have an output impedance
10 x lower than the input of 10kohm passive pre (which most are) so sources with 1kohm or lower output impedance are a match.

The power amp/s should have an input impedance 10 x higher than the output impedance of the passive pre 2.5kohm (which most are) so amps that are 25kohm or higher are a match.

The above is good for "nearly" 100% of systems out there

Good practice also is to use low capacitance interconnects (100pf per ft or lower), which most good quality ones are.

Tube sources should be questioned as they have output coupling caps that "may" not be large enough value, and easy fix though.

So your Classe with 27kohm input will be fine for a 10kohm passive preamp like mine.
https://forum.audiogon.com/discussions/lightspeed-attenuator-best-preamp-ever

Cheers George