Pass Amps Class A operation

From my recollection, I believe once the meter needle starts to wiggle, that means the amp is not in Class A mode anymore and is in A/B mode. To be in Class A mode, the meter needle would be still. Hope that helps.

The sensitivity rating of the speaker is it's output with one watt input. To find out how loud 15 watts would be add 3 db for every doubling of output. IE, 2 watts would be 3 db higher. These are generally measured at one meter so room size and shape will determine your perceived level. 15 watts is usually quite loud. Under the conditions you describe I would think that virtually all of your listening would be in class A.

If you filter out the bass freequencies, than 10...15 watts will be pretty loud.
Otherwise it would mostly work in class B.
Typically the most of power goes towards bass freequencies and crossover point so even playing 1-st 2nd octave piano notes at moderate volume level(similar to natural loudness of the instrument) may cause switching to class B.

As an owner of a Pass 150.5, it transitions from Class A to A/B at 15 watts. It's just as Studio 68 said above -- the needle doesn't move in Class A, and moves up from rest when the amp goes to A/B.

So, the answer to your question, since you already own the 150.5 and the SF speakers, is simply to try play it and watch the meter. Question answered.

BTW, I'm always surprised at how much volume those 15 Class A watts can produce, even on my Magnepan 3.6's (85db sensitivity). For most material, it's reasonably loud.

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

1 watt =87db
2 watts = 90db
4 watts = 93db
8 watts = 96db
16 watts = 99db
THX is 110db, ultra THX is 112db

A couple of things to keep in mind:
1-there's no crossover in the amp, just over a certain load, it starts switching out.
2-you could be playing in class A all day long at 5 watts which is fairly loud. However, if there is a dynamic peak the amp will automatically fall into class b and then back again. So, it should be seamless when switching.

There are really only 2 ways to know: The amp will lose some of its warmness & the amp will run much cooler. In class A, the circuits are running at peak all of the time. They have to dissipate the power somehow if you don't need the volume so it's given off in heat.

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.

http://passlabs.com/pdf/articles/leaving_class_a.pdf

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....

bombaywalla - is your comment about the inefficiency of the Sonus Faber GP also applicable to the more recent Grand Piano Home model? My speakers actually are the GP Homes, different from the GPs, with the addition of a third driver for Bass. Do you have a sense of where they fall on the efficiency scale and what impact that has on performance with an amp like the Pass 150.5? Thanks.

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!