See bellow the answer of Mr. Pass:
The F7 has a lower input impedance than most, at 10K, and a little less gain than most. Preamps that would have difficulty would be ones where the output impedance is greater than 1000 ohms or so, or tube preamps that cannot driver 10 Kohm, or which have no voltage gain. As it is, I routinely use the the F7 with a B1 which has no voltage gain, but my sources have 2V or more.
I am starting questioning do I really need a dedicated preamp to improve the sound. My PS Audio DS has gain of 1.414 VRMS for Single Ended outputs (F7 has SE only inputs) and
Output Impedance 100Ω/200Ω .
Looks like reasonable preamp for F7!
I use DS as my only source ( DS CD transport connected to it), so I started looking for preamp because I've been told that both F7 and DS will benefit from preamp .
I guess I'm opening the can of warm "Do you and when you need a preamp"
