Amp preamp impedance matching...can anyone explain?

07-19-12: Dracule1
I have a tube DAC that uses a single 6SN7 tube with 1 microfarad coupling caps. I was told it has 8 kohm output impedance.

that is correct. impedance at 20Hz = 1/2*pi*20Hz*1e-6 = 7957.7Ohms, which could be rounded up to 8KOhms. The manuf wasn't lying! ;-)

I wanted to get this integrated SS amp but the input impedance is 10 kohm. So this is a horrible match, so I stayed away.

right again! a horrible match down the in bass region. Your system would have sounded tinny. So, good thing you stayed away.

But I really wanted that SS integrated.

better put a lid on that desire while you have this particular tube DAC! ;-)

Does the voltage gain play into this at all? The amplifier I am using is a McCormack DNA-125, which has 30db of gain. Its input impedance is 100 kOhms. I am using an Audio Research SP8 MkII preamplifier with it which has an output impedence of 1,000 ohms.

I had Audio Research lower the gain in my SP8 recently, but needless to say, the volume through my Vandersteen 3A Signatures still gets loud in a hurry.

I was wondering if someone could please recommend a low gain, solid state Audio Research amplifier that is rated between 100-200 watts per channel? I do not want a tube amplifier, and I do not want to get rid of the wonderful SP8 MkII.

07-24-12: Distortions
Does the voltage gain play into this at all?

No, gain and impedance compatibility are two separate issues, although if the amplifier's input impedance is too low in relation to the preamp's output impedance (which it is not in this case) there will be a very slight reduction in overall gain.

I see that in stock form, prior to the recent modifications, your preamp had a line stage gain of 26 db. That in combination with the amp's 30 db gain, as you realize, is way too high, especially if you are using a digital source having the typical full scale output of 2 volts or so.

I see that you have another thread in progress on amplifier replacement and other possible approaches, and you've already received an intriguing input from Steve McCormack himself!

Regards,
-- Al

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