I think I've made up my mind

Pubul57,

Some (like ARC in their SS amps) and others do provide that.

Some do not.

Hopefully someone knowledgeable in SS amp design can chirp in and clarify what the tradeoffs are.

I guessing it has something to do with what is required to deal with distortion cost effectively then in the amp itself (negative feedback , etc.) in conjunction with driving lower impedance speakers effectively as well.

And/Or it might be that certain amps go for a certain type of sound that can be achieved more consistently with lower input impedance and a SS pre-amp.

I suspect part of the bottom line is just that no two amps work exactly the same way and synergy among components (pre-amp on one side and speakers on the other) then is always key as a result.

Pubul57, Perhaps the better question is why; don't high end manufacturers comply with the highly compatible pro standards?

Re the sub-thread about the Ref3 sounding dull and bland, and whether or not that can be attributed to impedance mismatches between preamp and power amp, I agree with Tvad, at least in the context of his XA60.5's.

A balanced output impedance that, per John Atkinson's measurements, is 635 ohms in the treble and mid-range, and rises to 1437 ohms at 20Hz, will not have any audibly significant consequences working into a 30K balanced input impedance. Especially given that the 30K is "constant across the audio band."

Putting it quantitatively, the effect of the rise to 1437 ohms will be an attenuation at 20Hz of less than 0.2db relative to the impedance-related attenuation at mid and high frequencies. The attenuation at higher bass frequencies, relative to mid and high frequencies, would rapidly fall off to considerably less than 0.2db.

That is simply not audible, and even if it were audible it would not correlate with a subjective perception of dullness and blandness (which would relate to effects at much higher frequencies than 20Hz).

The 635 ohm output impedance at high frequencies might be marginally significant, however, if cable capacitance is extremely high. A long length of very high capacitance cable (say 25 feet at 100 pf/foot) would produce a very slight but perhaps audible amount of upper treble rolloff. That would have nothing to do with the power amplifier, though.

Mapman, your LS16 preamp appears to have an output impedance which is considerably lower even than the REF3's. And your Bel Canto's appear to have little similarity to your prior Musical Fidelity amp beyond the fact that they are both solid state. I would not attribute any of the sonic differences you perceived between the two amps to their differing input impedances. To the extent that those sonic differences relate to interface characteristics (as opposed to the undoubtedly considerable internal differences), their output characteristics (damping factor, current and power capability) are probably much more significant.

Pubul57, re your question about why solid state amps are often not designed to have higher input impedances, I can't offer too much in the way of specifics. But a general answer would be that transistors are inherently low impedance devices that require significant current drive, compared to tubes which are inherently high impedance devices that are voltage driven and require minimal input current. It is certainly possible to design a solid state amplifier with very high input impedance, but the choices of device types and circuit topologies would figure to be significantly limited by that constraint. And in most situations a 30K or 50K input impedance is good enough.

Tvad, thanks for the nice words.

Best regards,
-- Al

Al,

Do you think the reason companies like ARC and some others (like Bel Canto with the Ref10000 mkiis specifically) design SS amps with 100K input impedance or higher is to help assure that they work well also with the largest # of tube pre-amps possible?

Other tube pre-amps may have significantly higher output impedances at certain frequencies than either the ref 3 or sp 16 specifically and the high input impedance could help there, correct?

Plus also, who's to say how accurate published specs are or that specific units operate to spec? The higher input impedance might be regarded as something of an insurance policy perhaps even in cases where on paper everything looks good, don't you think?