High Current SS amps with input impedance = 200K

Newbee, the sonic differences between a 10k Ohm and a 20k Ohm amp when used with most tube preamps would be applicable to a real live person in a real room provided the person was an audiophile with an audiophile quality system. No question about it. Not just theory. Read Atkinson. He lays it out in plain English. No technical knowledge required.

Although you just tossed out the 10k Ohm impedance spec, it is not an unrealistic number for solid state amps. McCormack amps have an input impedance of 10k Ohms, and in my experience using them (DNA-2, DNA-500), they don't work well with most tube preamps due to the input impedance issue.

Here are Stereophile's output impedance measurements for the SP16L line-stage preamp, for which the ARC specs are identical to what Mapman quoted:

http://www.stereophile.com/tubepoweramps/901/index6.html

The measurements indicate 262 ohms at mid and hi frequencies, and 549 ohms at 20Hz.

Into a 600 ohm load (sic), they measured a loss at 20Hz of only 2.5db (see Figure 1). By my calculations, into a 10K load the 549 ohm output impedance would result in a loss at 20Hz of less than 0.4db.

So the output circuit, and particularly the output coupling capacitor, of the SP16 appears to be designed more conservatively than in the case of most tube preamps. And the recommendation of a 20K minimum load appears to be conservative as well.

Regards,
-- Al

Al,
c
Thanks for the details.

I can say with confidence that the sp16 sounds very balanced, natural and lively with the MF a3CR I'm using (31K input impedance spec).

There is a larger (>200 w/ch)MF A308CR amp up for sale by a local Agoner. I think the impedance specs are the same as the A3CR but need to confirm. Since I like the current sound an would be happy with more of it, that might be something for me to consider again now that I have a handle on the impedance issues.

Glad you found that, Almarg. I spent a bit of time Googling it myself without success.

Atkinson states the SP16 should not be used with amplifiers having input impedance "much below 10k Ohm".

So, yes the SP16 appears to work well with most SS amps.

If that's the case, Mapman, you might consider one of the McCormack amps...like a DNA-2 Deluxe or a DNA-500. Check with Foster_9 about his experience using a DNA-500 with his Ohm loudspeakers. I believe he's using one with a VTL linestage.

Looks like a good call on the A308CR, Mapman.