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In Reply to: RE: Tube preamp with Bel Canto S300?? posted by rebbi on November 05, 2009 at 08:00:41
..so I guess I can comment.
1/ 10k is a silly low value for a poweramp input impedance and will limit your choice of preamps. I see no advantage of such a low value - none.
2/ A preamp should have an output impedance less than 1/10 the load of the following poweramp, which means it should be 1k or less, and most present day preaamps will be in that area. Our preamps have figures closer to 100 ohms, but the Audible Illusions range are around or even above 1k. CJ and ARC would all be comfortably below 1k.
3/ 99.9% of tube preamps have an output coupling cap, and in conjuction with a low input impedance poweramp this can lead to bass end roll off. Designer's chioces vary, but you'd need to listen to the expected match to be sure of compatibility.
This is the only real problem in matching a tube pre to a 10k input impedance poweramp, and the simple cure is to increase the size of the output coupling cap x5 or so.
Regards, Allen
Allen,I'm honored to hear from you, sir, and thank you for the input!
I noticed a Blue Circle BC-21 preamplifier at a good price on Audiogon. Out of curiosity, I called Blue Circle and spoke with the designer, whose name escapes me right now. Anyway, he said that the BC 21 has an output of something like 2.2K ohms and would mate just fine with the Bel Canto amp.
Thanks for mentioning also that Conrad Johnson preamps would also tend to have the right values. I've seen a fair number of those, used, on Audiogon as well.
Edits: 11/05/09
His name is Gilbert.I suspect you misheard what he said about the output impedance (that's the spec you would be interested in) as the Stereophile measurements were considerablyless:
The output impedance was quite low at 453 ohms at 1kHz and 20kHz, this increasing slightly to 727 ohms at 20Hz. So no problem with the Bel Canto amp.
Everything matters, don't forget to tweak your placebos!
Edits: 11/05/09 11/05/09
Nt
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