Ralph, thanks for chiming in. I had brought this issue up in
another of the OP's current threads. The preamp in question is a Hovland HP-100 with the moving coil option. Its output impedance, per JA's measurements in Stereophile, is 2.4K (presumably at 1 kHz), and 4.3K at 20 Hz.
The power amp he is considering is a Bryston 28BSST2, which is specified as having an input impedance of either 15K or 16.5K, depending on what document is looked at. He is using Maggie 20.7 speakers.
The opinion I expressed is that while that combo may work reasonably well with many recordings, especially those having limited deep bass content, I see no point in introducing what is at best a known marginality into the system, when other alternatives are available that would avoid the issue.
The output impedance at 20Hz will tell you. If it is the same value then you have no worries. If it is a lot higher at 20Hz they you might have a problem.
This is an excellent point that I think warrants emphasizing. What matters most is not the relation between amp input impedance and the nominal output impedance of the preamp, but the relation between amp input impedance and the range of VARIATION of preamp output impedance as a function of frequency. That is why, for example, some people report that resistance-based passive preamps, having VERY high output impedances, still work fairly well into amps having input impedances such that the 10x guideline does not come close to being met. The output impedance of that kind of preamp will have very little variation as a function of frequency.
Regards,
-- Al
