Amp preamp impedance matching...can anyone explain?

I have a Pass X250.5 amp whose input impedance is 22 kOhms. I've been reading about tube preamps, specifically the ARC Ref 3 (output impedance 600 ohms balanced) and Cary slp-05 (output impedance 500 balanced), both of which seem to draw comments that they wouldn't be good matches with Pass amps because of the low input impedance of the Pass amps.

So I went on the Pass website and see that the XP10 and XP20 preamps have output Impedances (for balanced) of 1,000 ohms! Presumably, these two preamps are ideal matches with my X250.5 amp, but if I do the math, how is an XP10/XP20 with 1000 ohms output impedance a good match for the X250.5 when an ARC Ref 3 with 600 ohms output impedance is considered by some to be a bad match? 22000/600=37 for ARC and 22000/1000=22 for Pass XP10/XP20

What am I missing or not understanding?

Lpw

Lpw, as I indicated earlier what is important is the preamp's output impedance at the (audible) frequency for which that output impedance is highest. That figure will often occur at 20 Hz, especially in the case of a tube preamp (due to the coupling capacitor that most but not all tube preamps have at their outputs), and in the case of a preamp having an output coupling capacitor will very often be much higher than the specified value. The specified value can be assumed to be based on midrange frequencies, unless explicitly indicated otherwise.

According to Stereophile's measurements:

Ref 3:

The output impedance was also to spec., at 635 ohms balanced and 326 ohms unbalanced in the treble and midrange, but rose to 1437 ohms and 625 ohms, respectively, at 20Hz.

SLP-05:

The SLP 05's output impedance is specified as a usefully low 400 ohms. However, I got significantly higher values for the unbalanced output of 1500–1600 ohms in the midrange and treble, rising to 3400 ohms at 20Hz, with similar if slightly lower figures for the balanced output.

The specs for your X250.5 indicate an input impedance of 30K balanced and 20K unbalanced, which are the same as the input impedance specs for many of the other Pass amps. At some point in the past, though, their unbalanced input impedance may have been spec'd at 15K, lower than it really was and is. See the specs and measurements in Stereophile's review of the XA30.5. That discrepancy perhaps contributed to some misconceptions.

The XP-10 and XP-20 are spec'd as having output impedances of 1K balanced, 150 ohms unbalanced. Presumably balanced connections would be used between all of these particular components.

Based on the foregoing numbers for balanced connection, the Cary's output impedance rise at deep bass frequencies results in it not meeting the ideal minimum ratio of 10 that is applicable if the preamp's maximum output impedance across the audible frequency range is known (30K/approx. 3.4K = 8.8). Although it comes close enough to probably be marginally acceptable in some systems (depending on the deep bass extension of the speakers, for one thing).

The Ref 3 should be no problem, in terms of impedance matching. 30K/1.437K = 21.

Although I couldn't find measurements, it can be presumed that the specified 1K output impedance of the solid state Pass preamps does not rise significantly at 20 Hz (or other) frequencies, so it too should have no problem working into a 30K load.

Regards,
-- Al

Al, isn't it the case that ARC does not recommend less than a 30K load with their preamps? I know that is true of at least one of the recent Ref series.

Hi Ralph,

Yes, I seem to recall that being true of some of their later preamps and/or phono stages. For the Ref 3, though, the specs shown here, and also in the Stereophile review I linked to earlier, indicate 20K. Although the wording leaves it unclear whether that applies to the balanced or unbalanced outputs, or both.

In any case, given the 30K impedance of the balanced inputs on the Pass, and the measured worst case (20 Hz) output impedance of 1437 ohms for the Ref 3 (which it should be noted is only a bit more than a factor of 2 greater than the midrange output impedance), it would appear that there shouldn't be any frequency or phase response issues. And I'd be surprised if there were any audible distortion issues either, although you can speak to that possibility more knowledgeably than I can.

Best regards,
-- Al

Al and Ralph, both the ARC Ref 3 and 5 literature recommends that the minimum combined load on the Main outputs is 20K Ohms. My amp is the ARC VS-115 which has an input impedance of 300K Ohms (balanced) -- so no problem there. If you pull my threads, you'll see where I got into a problem was when I asymetrically loaded the 2nd Main output with a 20K Ohm load. Main 1 -- VS 115 (balanced) 300K Ohms; Main 2 --self powered subwoofer (SE) 20K Ohms.

As I mentioned in my old threads, the solution was that Tom Tutay designed and built an impedance buffer device that summed the left and right channels without shorting the Ref 3/5 output Mains and enabled me to load Main 2 (self powered woofer) with a balanced input having 330K Ohms impedance. So my overall preamp output impedance load is 157K Ohms -- well above the recommended minimum.

Result -- tighter and more extended bass. Possibly more open midrange. Per ARC, symetrically loading the Ref 3/5 will also extend tube life -- but what do I know???? FWIW.