Amp preamp impedance matching...can anyone explain?

07-11-12: River251
Thanks Almarg. What sort of frequency response irregularities?

If I may be allowed to chime in - like Almarg wrote "Many tube 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". So, the preamp output impedance is usually not a constant. It's often lower in the mid/hi freq & higher at bass freq. When higher in the bass freq, you *might* see a loss of signal power from pre to power + the loss of control by the power amp of the bass region. The listener usually perceives this as flabby/bloated bass.
I know that a friend of mine experienced this with a highly regarded preamp that had 600 Ohms output impedance while his power amp had 10K input impedance. It was a bad match & the preamp was sold off immediately.

I have a CJ PV-5 (only thing I found was a post saying 200 ohms output impedence) and Marantz 8B (and Mac MC240 and MC30s but listening to the 8B). I am considering trying a SS amp and considering B&K ST-140 (24Kohm input impedence), Aragon 2004 MkII (22Kohm), and McCormack DNA .5 or 1 (100Kohm).

from my personal experience this should not be an issue. My preamp has 200 Ohms output impedance & my power amp has 10K input impedance. The 2 units mate up very nicely across the entire audio freq range.
FWIW.

Thanks for chiming in, Bombaywalla, and saving me some time :-)

I found some information specific to the CJ PV-5, though. The schematic and parts list can be found here. It can be seen that the output coupling capacitor is two capacitors in parallel, totalling 2.35 uF. At 20 Hz, that corresponds to an impedance of 3388 ohms, based on 1/(2piFC). (That figure will be vastly lower at mid-range and treble frequencies, consistent with the 200 ohm nominal output impedance). It is most likely safe to assume that the overall output impedance at 20 Hz will be just a little higher than the impedance presented by the capacitors at that frequency, so let's call it 3500 ohms.

22K/3.5K and 24K/3.5K are ratios that are between 6 and 7, lower than the ideal of at least 10. So there may be a slight impact on the deepest bass frequencies, which may or may not be perceptible depending on the deep bass extension of your speakers, and on room acoustics. If your speakers have good deep bass extension, my instinct would be to play it safe and avoid the B&K and the Aragon, although it's a close enough call to be debatable.

Regards,
-- Al

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