Amp preamp impedance matching...can anyone explain?

Ideally the input impedance of the amp should be 10 or more times greater than the output impedance of the preamp, at the frequency for which preamp output impedance is highest. Otherwise audible frequency response irregularities MIGHT result.

If, as is often the case, the highest output impedance of the preamp across the audible frequency range is not known, and only a nominal output impedance is specified (perhaps based on a frequency of 1 kHz), I suggest using a ratio of 50 or more, and preferably 75. Many tube preamps, and some solid state preamps, use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies.

Impedance incompatibilities are most likely to be encountered when using a tube preamp with a solid state power amp.

If the power amp has an input impedance of around 47K or more, it is unlikely that there will be an issue, even with a tube preamp.

Regards,
-- Al

I too have heard the 10x ratio, but are not there some companies that preach a transmission line approach? I have seen references to a 600 ohm standard in older studio equipment.

Onhwy61, that would apply to situations where the component providing the signal is specifically designed to be able to drive 600 ohms, without a significant increase in distortion or a significant impact on bandwidth or frequency response.

Most consumer gear will not meet those requirements. I believe that Atmasphere is a notable exception. According to posts Ralph (Atmasphere) has made in the past, a benefit that would result from loading the balanced outputs of his preamps with a low impedance like 600 ohms is minimization or elimination of cable effects and cable differences.

Best regards,
-- Al