Amp preamp impedance matching...can anyone explain?

Even with an ideal pre-amp to amp impedance match the B&K ST 140's sonic signature (as is the c-j's too) is a bit soft in deep bass, compounding it with a less than perfect impedance match might not be the best choice. I wouldn't recommend a low impedance speaker (<4 Ohms) with the B&K ST 140 either. Which is not to say that in an appropriate system the B&K ST 140 is not a good amp, as it certainly is, especially for the money. Keep in mind that the B&K ST 140 is getting a bit long in tooth and B&K is no longer in business. Though I would imagine that having the B&K ST 140 serviced by an outside source wouldn't be too difficult should the need arise.

I would like to add another factor to the discussion.

The ratio of 10/1, while ballpark, may not be the only factor if the solid state device uses operational amplifiers in the output stage. Many opamps have output impedances well below 1K, but if you try to drive a 10K load with these (particularly at voltages over 1 V RMS), you can often get flat, lifeless sound performance. Of course, the results also depend on the type of opamp used. Some are better than others.

For opamp driven output stages I suggest an imput impedance of 47K or higher, with output voltages below 1-2V if possible, irrespective of output impedance.

We have been talking ratios of pre-out to power-in as 1:10, 1:50, 1:75 or absolute numbers as 47,000 ohm.

Is there a limit to how high these ratios or the power-in should go, before it sounds bad?

I am a layman asking this... Thank you

Is there a limit to how high these ratios or the power-in should go, before it sounds bad?

No, not that I can envision.

Regards,
-- Al

As a simple rule of thumb most any transistor preamp can drive any tube amp.

Its not always the other way 'round as pointed out above. (FWIW we get around the problem by direct-coupling and using paralleled tube sections to obtain a low output impedance that can drive any amp around.)

But if there are coupling caps its a different ballgame. For example, ARC recommends no less than 30K as the input impedance of the amplifier with which the Ref 5 is used.

10:1 is a safe minimum value, but you can go 100:1 and its fine. So in the ARC example above, standard engineering practice suggests that the output impedance of the Ref 5 is 3K ohms. However they are fairly conservative and are showing 600 ohms on their website, although that might be at 1KHz. IMO the 20Hz output impedance tells the real story.

The lower the output impedance, the more the line section can control the interconnect- by that I mean the less sonic attributes the cable will have. So the lower you can get the output impedance, the better (all other things being equal...).