Thanks for correcting me. I haven't had good experience driving the Pass with tube preamps. I've tried the Cary SLP-05 and the Audio Valve Eclipse and the low bass is rolled off. It's one of the reasons I got the Bat. I'm currently auditioning a Ref 5 (with my Bat). I'll have to try it with the Pass and see how it fairs. |
The Stereophile measurements of the Cary SLP-05 state an output impedance of 3400 ohms at 20Hz from the unbalanced outputs. That's certainly on the hairy edge of acceptable with the XA-.5 series' 20K input unbalanced input impedance. Atkinson does not mention output impedance measurements from the balanced outputs that I could find. Atkinson also measured higher distortion at the frequency extremes from the balanced outputs. Both these facts taken together indicate that this preamp could very likely have caused the muddy bass you described. Taken as a whole, the SLP-05 is not a preamp I would match with a Pass Labs XA-.5 series amplifier. Atkinson measured the Audio Valve Eclipse from its unbalanced outputs, and again with this preamp he observed a rising output impedance to a very high 3090 ohms at 20Hz. He recommends using the Audio Valve Eclipse with amplifiers having a minimum input impedance of 30k ohms. The Pass Labs' XA-.5 series amps have an impedance of 20k ohms into their unbalanced inputs, so again this would suggest the Audio Valve Eclipse may not be a good match with the XA-.5 series. You do not mention whether you were running balanced or single ended, but the measurements of the single ended tests suggest that these preamps need to be matched with amplifiers having higher input impedances than the Pass Labs XA-.5 series offer. Thanks for your information. I presumed your bass muddiness problem might be due to a poor preamp/amp match, and it does appear this is the likely culprit. I suspect you will have much better luck with the ARC Ref 5 and the Pass Labs XA-.5 amps. |
Tvad- don't normal tube preamps have high output impedance, and as you said Pass tends to have very low impedance---therefore, tube preamps *in general* aren't a great match with Pass? Fully balanced preamps tend to be different--as I've noticed they have lower output impedance. Krell for instance on the other side has high input impedance making it pretty universally acceptable to tube preamps, so could be a better option for SS. |
