Modwright LS 36.5 or BAT VK-51SE

Roor, since the output impedance spec of the Dude is not provided, it seems to be a situation where you'd have to hear the preamp in your system to decide if it matches or not. You'd want to be listening for rolled-off highs. I don't know what a resistor pull-down XLR is.

The BAT VK-51SE was tested by Stereophile, and the output impedance was measured at 410 ohms (200 ohms spec'd by BAT), and rising to 4700 ohms at 20K. John Atkinson writes:

"This will not be an issue with components having high input impedances, such as BAT's own power amplifiers, but it does mean prematurely rolled-off low frequencies with low load impedances (fig.1), such as those offered by some solid-state amplifiers."

The measurements are here.

The Modwright preamp has a factory spec'd output impedance of 110 ohms, which is quite low. However, it has not been measured by Stereophile, so the question of how high the output impedance rises into high frequencies is unknown.

BTW...I once owned a Lamm L2 Reference preamp which I used with my XA-60.5 amps. I thought this match was very good. It took quite a while comparing it to the SMc Audio VRE-1 to decide to sell the Lamm. I imagine some listeners might even prefer the Lamm.

The L2 Reference runs about $6000 used. I think I bought (and sold) mine for less than that.

Tvad: Was there any hiss or noise with the L2 Reference with the Pass Labs XA-60.5's?

I've read several posts where the Lamm L2 reference hisses loudly with just about every other amp besides their own, and Lamm says it's "normal". It's driven people crazy that tried to fix the problem with no luck, so it sounds like a risky move. I'm leaning towards the Modwright LS 36.5 at this point or possibly that with seperate dual mono power supply.

Greg, I have no information specific to Lamm being noisy with other amps. However, you are correct to be concerned, and you should pay attention to this issue with a demo. Particularly with the Pass which are so quiet, adding a noisy preamp is one step forward, two steps back. If you can readily hear the difference in noise from your listening position in a quiet room with the preamp on vs. muted, then it may nag you in the end. I think noise destroys a soundstage.

In terms of mismatch, also consider that cabling is HUGE in defining bass and upper-end extension (PC, IC, SC). So many cables out there restrict both, or do one at the expense of the other.

Since the output impedance spec of the Dude is not provided, it seems to be a situation where you'd have to hear the preamp in your system to decide if it matches or not. You'd want to be listening for rolled-off highs. I don't know what a resistor pull-down XLR is.

The BAT VK-51SE was tested by Stereophile, and the output impedance was measured at 410 ohms (200 ohms spec'd by BAT), and rising to 4700 ohms at 20K.

Grant, I think you meant to say 20Hz, not 20K, and that what should be listened for is rolled off lows, not highs.

I don't know what a resistor pull-down xlr is either.

The posts by Atmasphere starting here provide some insight into the output impedance of the Dude. It sounds like a major contributor to its output impedance is the volume control and its setting (he indicates that the control is located at the output), with the output coupling capacitor perhaps being no more than a secondary factor.

Since the volume control is essentially resistive, rather than capacitive, its contribution to output impedance would be pretty much constant with frequency. So high output impedance would be much less of a concern than in the more common situation where it is due primarily to the output coupling capacitor, which would cause the output impedance to be much higher at very low frequencies than at higher frequencies. It is the DIFFERENCE in output impedance between those frequencies that causes low frequency rolloff when output impedance is too high relative to power amp input impedance.

Best regards,
-- Al