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
On my X250's meter, with significant movement indicates the amp is moving out of class A. It’s somewhere about 12 o’clock that the amp is into A-B operation and the first 10% is class A watts.
I was thinking almost exactly along the same lines at Elevick. Continuing on those lines then:
I'd like to make a few mods to the values posted by Elevick - you need to reduce the SPL at the listening chair by atleast 9dB.
The listening chair is not going to be at 1m away from the speaker; it's probably more like 3m (9.9ft). The SPL drops 6dB for every doubling of distance. From my *assumption* that your listening chair is at 3m, you need to subtract 6dB+3dB = 9dB. Additionally, there'll be absorption from the rug/carpet, drapes, furniture, etc. So, subtract another 2-3dB.
That brings us to 87dB/1W/1m - 9dB - 3dB = 75dB SPL at your listening chair when the 150.5 is outputting 1W.
2W = 78dB
4W = 81dB
8W = 84dB
16W = 87dB
Now, since the music is in stereo, you need to add 3dB to the above numbers. So, if the 150.5 is outputting 4W, you'll be getting 84dB SPL at your listening chair. That's pretty loud - you'll need to raise you voice to talk. At 8W, you'll be getting 87dB SPL. At this point you cannot have a normal conversation in the room.
Of course, this does not include dynamics peaks for which the amp has no class-A capacity (probably by design) & it'll need to go into class-AB & then switch back (just like Elevick wrote).
So, overall you have most of your listening in class-A barring dynamic peaks but w/ this amp you do not have any class-A wattage to spare for dynamics. All your dynamics will be class-AB.
I think Pass' X350 has 75W in class-A - in this amp, you could have some reserve wattage for dynamics in class-A mode.
The power to SPL calculation is pretty much aligned with engineering understanding. Yet reality does not seem to support this kind of calculation. For example if we can all agree that 2W is all needed to produce 78 dB at 3m away, then adding extra cushion by a factor of 5 even (to bring the power requirement to 10W) is not bad an engineering cushion, really, to account for dynamic peak or other Murphy's law situations. Yet in reality, would we buy a 10W/channel amplifier to drive anything? Even a factor of 10, bringing the wattage to 20W/channel is a no-no.

I like the idea of using the needle movement on Pass equipment to tell when the amp. moves out of class A. My speakers has a 4-ohm impedance, meaning 120W/channel drawn from the XA60.5. The needle vibrates ever so slightly around 100 dB when my ears were about to burst because I accidentally set the volume too high. The vibration is very light, as the needle likes to lean back to its static position around 12 o-clock when the dynamic peak is gone.
07-21-09: Spatine
The power to SPL calculation is pretty much aligned with engineering understanding. Yet reality does not seem to support this kind of calculation. For example if we can all agree that 2W is all needed to produce 78 dB at 3m away, then adding extra cushion by a factor of 5 even (to bring the power requirement to 10W) is not bad an engineering cushion, really, to account for dynamic peak or other Murphy's law situations. Yet in reality, would we buy a 10W/channel amplifier to drive anything? Even a factor of 10, bringing the wattage to 20W/channel is a no-no.
Spatine,
my calc were meant to be a mere guidance to give the author of this thread an idea of the ballpark SPLs that he can expect in his room. it was not supposed to be 100% accurate. You can see that I've taken a swag at the attenuation factor for distance & furniture. So, take it as a guidance.

then adding extra cushion by a factor of 5 even (to bring the power requirement to 10W) is not bad an engineering cushion, really, to account for dynamic peak or other Murphy's law situations.
hmmmmmm...... I'm not sure that I'd agree. A factor of 5 in dBV terms is 14dB (we assume that the impedance the amp is working into remained the same, for argument's sake). Are you saying that allocating 14dB for dynamic peaks is "not bad an engineering cushion"?? If you look @ the spectral content of music you'll find that peaks can be much higher.

Yet in reality, would we buy a 10W/channel amplifier to drive anything? Even a factor of 10, bringing the wattage to 20W/channel is a no-no.
sure! the horn & single-driver speaker guys do it all the time. I've personally heard several very, very dynamic systems in Atlanta that were all Jordan & Lowther driver based speakers that were powered by tube power amps that were in the 5W/ch to 8W/ch range. Nothing higher! In fact, flea-watt power amps is the paradigm that horn, single-driver folks stick to.
The reason this won't work for a Sonus Faber Grand Piano is 'cuz that speaker is so damn inefficient mostly in the bass region. You couldn't get a 14W or 20W amp to provide enough current into a low impedance that the Sonus Faber Grand Piano presents. If you remove the bass driver, I bet that a 14W or 20W amp could drive only the mid & tweeter drivers with high enough SPL to run you out of the room!

lastly, it seems that you might not have read my post properly as I wrote & I've cut & paste:
Of course, this does not include dynamics peaks for which the amp has no class-A capacity (probably by design) & it'll need to go into class-AB & then switch back (just like Elevick wrote).
you might want to re-read my orig post before you take up any issues....