Jtinn, I know that the -120db number is a real tough one to achieve. If I had been reading that post myself, I would also have been skeptical.
Since this is a custom prototype, I don't have a spec sheet with that number on it, and I don't think that David would quote that number anyway, because he is so conservative in his specs. However Harvey Rosenberg had quite a bit of experience with these amps before he died, and went to triode heaven, and he quotes a -120db figure on his Triode Guild website. David actually only specs -92db figure on the Siegfried amp, but this is very conservative.
However, my amp is not a Siegfried,and has some significant differences which go a long way to minimize noise further than the Siegfried model. No AC power enters the amp, as it is totally DC battery powered. So there is no step-down power supply transformer, nor any conventional rectification circuits. The power supply is as "stiff" as it gets. Pure battery with 650 cold cranking amps. There is no hum. The only noise generated could only come from the passive parts and the tubes themselves. The mesh plate 45s from Emission Labs are very quiet tubes, and the only other tube in each channel is a single Sylvania 6SN7. 2 tubes on each channel. That's all. Audio signal path was specified by me to be very short and hand wired. If you are concerned about noise from the ZOTL, anything possibly coming from that is well(10x) out of the audio range.
So, that is why I hedged a little when I stated earlier "about -120db quiet". I don't have a measurement figure on it, to back that up. Harvey did those things, and he is the one originating those claims, and I think he's right. I can place my ear right in front of my 100db Lowthers, and hear only the faintest noise at all. If my ear is more than about 4" to 6" away from contact with the driver, I don't hear anything at all. Any hiss in my system is coming primarily from my preamp, not my amp. Any way you slice it, this amp is virtually dead quiet. If I had "normal" speakers of about 88db efficiency, I would hear no hint of noise at all coming from them. You must remember that I am relating this very small sound from 100db/1watt speakers.
So, can I back up my statment with papers? No. Do I feel confident that I made an accurate statement? Yes.
I don't think I've been lied to, nor do I feel I am doing any lying. It is in that neighborhood (within a few db), I am certain.
But I also agree with the posters who mention that the quietness is not the main function of an amp. The musicality is much more important. But I simply responded to the question at hand. BTW, the musicality of this amp is quite good also.
