Pre amp for Pass Labs XA30.5 – Tube or SS

Directly from the mouth of Nelson Pass.

"Our experience is no problems at all with the ARC or CJ products with regard to interface. The amp doesn't care about source impedance. It's actual input impedance is close to 0 ohms, and the input impedance spec comes from the resistors we put in series with that input. Adding a little more from the source doesn't create issues."

Tls49 I agree and the one I would go with if it were my decision.

Chuck

RO817,
If Mr Pass himself says the ARC and the CJ will be fine, thats good
information. Based on the criteria you list as desirable I'd select the CJ line
stage or the Leben(from your stated choices) as most likely to provide the
full body/tone you want(as well as excellent vocals and
soundstaging).Tube rolling will fine tune to your preference. I believe these
two will be more musically engaging and less analytical and dry.
Good Luck,

I would guess the Pass preamp should be the best match, although there should be no problems with the others.

It might come down to what kind of sound you prefer. I would think the Pass will be accurate and clear, while the tube preamps would lend a more tube-like sound. Given that the XA 30.5 may already have a bit of this, you might not want to overdo it. Can you try them out in your system before deciding?

Given the somewhat low input impedance of the Pass amp, specified as 20K single-ended and 30K balanced (those numbers having been essentially confirmed in John Atkinson's measurements), I took a look at the output impedances of the tube preamps listed in the OP for potential incompatibilities.

It should be kept in mind that it is very common for tube preamps to have output impedances at deep bass frequencies that are much higher than their specified output impedances (which are typically at mid-range frequencies). That low frequency rise will occur due to the output coupling capacitor that is used in most (but not all) tube preamps, and will reach a maximum (within the audible range) at 20 Hz.

Chances are that with nearly all tube preamps that will not be of significance to you with your present speakers, because of their somewhat limited deep bass extension. However, it may become a significant factor if you later change speakers, or add a sub that is driven from the preamp's outputs.

Based on my research, I would rule out the Premier 17. From JA's measurements:

The output impedance was below a lowish 800 ohms in the midband and above, but rose to 1.6k ohms at 20Hz, due to the presence of a finite-sized output coupling capacitor. The C-J should therefore be used with a power amplifier having an input impedance of at least 22k ohms if the bass is not to sound a little lean.

Also, the Leben preamp models shown at their website (RS38CX, RS-100, and RS-100U) all have specified output impedances in the vicinity of 600 ohms. I couldn't find any data indicating their 20 Hz output impedance, but the 600 ohm figure is high enough to suggest caution. If you want to consider those models, I would ask the manufacturer what their output impedance is at 20 Hz.

Also, the ARC datasheets for all of their models which are listed in the OP recommend a minimum load of 20K (although without indicating whether that applies to their unbalanced outputs, their balanced outputs, or both). So you should be ok with any of those models, but without a great deal of margin.

Regarding the statement by Mr. Pass that is quoted above, while I certainly would not question any statement he might make, when he says "the amp doesn't care about source impedance" I would point out that that does not necessarily mean that the preamp doesn't care about load impedance.

Regards,
-- Al

Again it was a direct quote form the designer (Nelson
Pass)concerning tubed pre-amp choices and their ability to
work well with his XA series of amps. I am sure he knows a
fair amount about the amps he designs and their ability to
mate up with other manufacturers tubed pre-amps.

And as far as system synergy goes the best match with a Pass
amp is a Pass Pre-Amp. I suspect any designer is going to
use his own products to optimize their synergies together.

Chuck