2. amps-preamps
  3. 1248105027

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
marakanetz's avatar
marakanetz
5,473 posts
 
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.
dne
141 posts
 
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.
almarg's avatar
Ag insider logo xs@2x
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
elevick
2,636 posts
 
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.
cycloneharry
11 posts
 
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.
More to discover