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Pass Amps Class A operation

I just started using a Pass Labs X150.0 power amp. Love it so far. I know that it operates in Class A mode up to a certain point - I think 10 or 15 watts. How does that translate into approximate volume level? Is 15 watts coming out of this amp (into Sonus Faber Grand Piano speakers) generally enough to drown out all conversation in a 20 x 15 room? Or is 15 watts a volume level that is easily and typically surpassed, except when listening at midnight in an apartment with thin walls. In other words, I am trying to gauge how much of what I am listening to is Class A mode and to get a general sense of where the Class A/B transition point is. Is there a way to tell? The meter on the amp never seems to get more than approx 25% towards pegged (or move much at all for that matter). I also have read that this amp actually puts out much more than 150 watts (200-250?), even though 150 is the published rating. Is the Class A crossover similiarly conservatively rated, or is that a more precise number?
gipp

Showing 2 responses by almarg

almarg's avatar
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almarg
9,670 posts
 
I would add to the previous comments that perhaps the most significant factor is the dynamic range of the music you are listening to. The fact that much popular music is highly compressed dynamically (i.e., at a fairly constant volume) will make relatively few watts sound subjectively quite loud. Jazz and some classical chamber music will also often have relatively limited dynamic range. The worst case would be well recorded (i.e., relatively uncompressed) classical symphonic music. That would cause you to turn the volume control up to get reasonable volume on the soft passages, which would result in very large instantaneous power demand on orchestral peaks, bass drum whacks, etc.

Btw, it looks like your speakers have a rated sensitivity of 87db/1W/1m (representing 2.83 volts into their 8 ohm nominal impedance), just a bit greater than that of the Maggies which Dne referred to (85db/2.83V/1m, representing 2 watts into their 4 ohm impedance).

Regards,
-- Al
almarg's avatar
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almarg
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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
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