Pass Amps Class A operation

07-23-09: Gipp
bombaywalla - is your comment about the inefficiency of the Sonus Faber GP also applicable to the more recent Grand Piano Home model?

No! I would not extrapolate my comment w.r.t. the SF Grand Piano to your particular speaker. Almarg was kind enough to provide the efficiency number that i keyed off.
This is my opinion:
The SF GP Home specification is 90dB/2.83V/1m into 6 Ohms.
From what Almarg wrote & what I found in Stereophile, the SF Grand Piano is spec'd at 90dB/1W/1m into 8 Ohms.
Needless to say, since one is 6 Ohms & the other 8 Ohms, we cannot compare the efficiency #s directly.
For the SF GP Home, the 2.83V into 6 Ohms yields 1.3348W. So, of course, you end up with a more efficient # since you are inputting more power compared to 2.83V into 8 Ohms. According to my calc, 1.3348W into 6 Ohms is 1.25dB more power than 2.83V into 8 Ohms, so subtract 1.25dB from the SF GP Home efficiency #, which makes it 88.75dB normalized to 8 Ohms. (did I calc this correctly, Almarg??)
Thus, the SF GP Home is marginally more efficient than the original SF GP. IMO, then, if a X150.5 is good for the SF GP then it should be OK for the SF GP Home as well.

Yes, I agree with those calculations, Bombaywalla, aside from what amounts to a typo where you listed the older, non-Home version as 90db instead of 87db.

Here is a link to a review of the Home version, which confirms the 90db/2.83v/1m/6 ohm numbers you quoted for it:

http://www.enjoythemusic.com/magazine/equipment/1001/sonusfaber.htm

But I think that a better way to look at it (which results in essentially the same conclusion as you reached) would be to make the comparison in terms of output current requirements. I say that because the original question relates to the threshold at which Class A is left, which in turn is basically dependent on relations between output current and bias currents.

Analyzed in that way, we have:

GP (old version): 87db is produced at 1 watt into 8 ohms, which corresponds to a current of 0.354 amps.

GP Home: 87db is produced at 3db less than 2.83 volts, which is 2.00 volts, which corresponds to a current of 0.333 amps into its 6 ohms.

0.333 amps vs. 0.354 amps is a difference of 0.53db. So I would expect the "leaving Class A" threshold to occur at a volume level which is 0.53db higher for the Home version than for the old version. Which is an insignificant difference, as you essentially concluded.

Regards,
-- Al

Gipp, if this is more than a curiosity question, or if it can lead to spending money, I would suggest you contact Pass Labs directly and ask this question. Simply ask them what power amp is needed to drive whatever speakers you want. This kind of theoretical talk happens on this board before, and people mean well also. In reality, that's just isn't how it works. Bombayawalla already hinted at problem when the base driver is present. The last time I check, practically all speakers have some kind of a woofer or midrange-woofer combo. Then also there was some suggestion months ago that perhaps a diffent amp could drive the woofer of some speakers, with associated problems as the discussion goes on. Well on top of all that mess, I am not even convinced that all tweeter and the midrange combos could be driven by tiny amp. It depends on cross-over design, and not all midrange are really "midrange" either.

silly question: if you can't "tell" by listening then why bother obsessing about when it "leaves" class A? It's not like your speakers are suddenly fed a distored signal the magnitude of which swamps out low level signals-- 15 watts rms (in any class of operation) in a 20x15 room with your speakers would be quite loud.
forget all the numbers nonsense: does the combination sound good to you? I bet it sounds excellent!