Class A vs A/B Amps?

I believe voltage is the key factor & not watts.
Speculation tells me quality of components & voltage would dictate the answer.

 

Impossible to answer the question as asked - there are great Class A amps and crummy ones. The same is true for Class A/B. Without knowing the amps in question and the speakers you plan to use, there is no way to really say which would be better.

 

Hi Jeff,

A few things that might be important to note, would be the sensitivity and impedance of your speakers-these are generally on the speaker or in the manual. Also the way in which you intend to use the speaker; near-field (within 6 ft. of your ears) or a more common distance, the size of the room including the ceiling height. Even you listening tastes can play a role; type of music most often listened to...recorded live on in a studio. do you prefer listening at higher or more moderate volumes.

What's equally as important as the issue of power, is the issue of the circuit involved, and as the previous writer eluded to, the voltage-which relates to the damping power needed to control the driver(s). The circuit is the largest determining factor of sound. Whether class A, class A/B, class D, etc, it may be good to know how many amplification stages are in each design.

Any one of the above can produce excellent results, it's more getting what you want from the speaker. Given you allusion to the wattages at play, it wouldn't surprise me if you were choosing between two Nelson Pass designs. In any event, if you have more information I'm happy to throw in a suggestion.

Fred

 

Pass Labs was the first manufacturer that came to mind when the OP mentioned 2 amps (one class A, one A/B) at the same price. Without meaning disrespect to the OP, my second thought was that anyone considering a Pass product would likely already have a pretty strong preference and notions towards/for Class A or Class A/B. I'd bet dollars to donuts that each of us agree that the quality of any Pass product, no matter what its class of operation would be hard to disparage - just a matter of matching with the right speaker & listening needs.

 

According to the Pass Labs website, the X150.8 leaves class A at 15 watts, so 0 to 15 watts is all class A. The difference between 15 watts and 30 watts is 3 dB, which is a difference in power level that is *just* audibly detectable. On the flip side, if the signal content is such that the XA30.5 amp starts clipping, then you will introduce distortion products much greater than whatever overlap distortion exists when the X150.8 is operating in AB mode. In the meantime, the X150.8 has 7 dB of headroom over the XA30.5.

0-15 Watts: X150.8 and XA30.5 similar
15-30 Watts (3 dB range): XA30.5 slightly better (slightly less distortion)
30-150 Watts (7 dB range): X150.8 much better (XA30.5 won't cover this range or will clip)

If you need an explanation of the difference between class A and class AB or the significance of dB, just ask.

Given that you like to try different speakers, some of these will likely need more drive, so the X150.8 would give you much more flexibility at a small sacrifice between the range of 15-30 watts. This is assuming well-designed amps of each type and based on the published specs, as I've never listened to these amps myself.

 

I don't post often, but thought I would chime in.

* If you haven't gone to Audiogon.com and explored the amp forums there, that would be a good start.

* Nelson Pass is active at diyaudio.com; see pass labs on home page; left NAV. If your handy, there are a lot of Pass amp circuits you can DIY - including the power sizes your considering.

* While watts is important and understanding the context that you like to change amps/speakers frequently, you should consider the size of your room and the type of music your playing. I like rock, some classical, and more recently more jazz (specifically solo instrumentals - piano only, guitar only, etc.); I began playing piano again after a long hiatus and listening to music, for me, has now taken a technical turn to help me play better, and near field listening is paramount.

I found my class A/B amp at 30 tube watts /channel was musically beautiful with my Janzen hybrid electrostats; room is relatively small; speakers >90 efficient; my general listening is usually near field listening. Listening to Bill Evans, Oscar Peterson, George Winston, (or Bach by London orchestra) etc. near field with 30 watts/channel is lovely, intimate warm experience. By contrast, if I want to listen to Van Halen, Foo Fighters, etc. 30 / watts per channel is loud enough, for that room with those speakers, that even my kids would tell me to turn it down; hence watts/current isn't everything. New speakers are generally more efficient unless your thinking of adding Wilson Watt/Puppy to your speaker repertoire.

Evolving, I wanted to return to Magnaplanars (long time maggie fan and arguably one of the best for reproducing piano / string instruments) leading me to purchase mag 1.7s - not large and overwhelming for my small listening room. Mags require a LOT of watts/current.

To get the same near field listening tone (assuming same volume) as my older 30 / tube watt per channel amps with the Janszen's, I heard from many other audiogoners you really need at least 150 watts / high current to adequately drive mags. The audiogoners were right, and the solo piano note decays, for example, sound closer to my piano (holding volume level constant) relative to the 30 watt amp / speakers. I also like listening to my wife playing her violin and then listening to classical violin chambers...same volume but vibrato sounds more natural and more intimate with Mag 1.7s and new tube amp.

I purchased a pair of used MFA 200Bs (mono block tube 200 watt / channel) to complement the mag 1.7. Not played any louder than the 30's

my two cents.

 

I'd say most gung ho audio hobbyists keep themselves informed about the offerings of at least the major producers and the works of the true standout designers and/or engineers. Nelson Pass has been at it for a LONG time, and has an incredible (fully deserved) reputation.

Plus - fairly few manufacturers offer some amps that run full TRUE Class-A, and others that run Class A/B. A few have amps that can be switched to run Class A or Class A/B (say, Plinius). Very few have one amp that is Class A priced the same as another amp that is Class A/B in their lineup - so Pass Labs wasn't that much of a reach.

I've not heard Pass's most recent offerings, so I can't really say which version would work best for you. I WOULD say that his older Aleph, single ended Class A amps used some of the simplest and best amplification circuits ever produced (you could purchase one used with zero loss if you decided it was not for you) - you may want to write or call the fine folks at Pass and discuss your needs.

 

Back in the day when I competed in car audio sound-offs I had a few Class A amps made by Soundstream (6.0 and 3.0 models) and I also had Class D amps that were part of Infinity's Kappa lineup. (DPA-250 & DPA-275 models). Comparatively speaking I preferred them over AB's made by Orion, Hi-Fonics and several others at the time.

The Soundstreams were some of the cleanest amps I've ever heard. Yeah the power rating is deceptively low on paper but the class A amps have a 100% duty cycle and the most linear. They also run very warm and because of the current draw larger gauge wire is needed. I ran 0 AWG in my power distribution.

ABs are more mainsteam as they are much more energy efficient while producing great sound. There are 2 sets of output circuits that operate in a push-pull fashion with each 'side' having just over a 50% duty cycle. The better blending of the op-amp circuitry due to better and cleaner circuitry = greater cost.