Yes, preamp output impedance should be much less than power amp input impedance.

The commonly cited rule of thumb is a 10x ratio. However, a 10x ratio is truly comfortable (in assuring that frequency response flatness will not be significantly affected by the interaction of the two impedances) only if the worst case (maximum) output impedance of the preamp across the 20Hz to 20kHz frequency range is known. Manufacturer's specs commonly just indicate a nominal output impedance, presumably at 1kHz or some other mid-range frequency.

If the preamp has been reviewed by Stereophile, John Atkinson's measurements usually indicate the worst case output impedance. If only a nominal output impedance is known, IMO a minimum ratio of at least 50x, and ideally 75x, should be used, especially if the preamp may have a coupling capacitor at its output (as most tube preamps and some solid state preamps do). An output coupling capacitor will typically cause a large rise in output impedance at deep bass frequencies.

Regards,

-- Al

The commonly cited rule of thumb is a 10x ratio. However, a 10x ratio is truly comfortable (in assuring that frequency response flatness will not be significantly affected by the interaction of the two impedances) only if the worst case (maximum) output impedance of the preamp across the 20Hz to 20kHz frequency range is known. Manufacturer's specs commonly just indicate a nominal output impedance, presumably at 1kHz or some other mid-range frequency.

If the preamp has been reviewed by Stereophile, John Atkinson's measurements usually indicate the worst case output impedance. If only a nominal output impedance is known, IMO a minimum ratio of at least 50x, and ideally 75x, should be used, especially if the preamp may have a coupling capacitor at its output (as most tube preamps and some solid state preamps do). An output coupling capacitor will typically cause a large rise in output impedance at deep bass frequencies.

Regards,

-- Al