Class A/B designs are class A up to some portion of their output, then class B afterwards.
You might have a 5W A/B amp that has say 500mW of class A, if you use more than 500mW it’s class B.
The advantage is if there is nothing playing it’ll draw maybe 2W’s, sitting idle, a 5W pure class A design would draw probably 20+W all the time and most of that is heat that has to be dissipated.
So you need bigger boxes better cooling etc etc.
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But what is the advantage of a straight Class A?
Something about having and sending all the power, all the time, without ever caring about power usage. No-compromise 100% sound quality, because 100% of the energy is always there for the loudest sound – even when it’s silence. Most things are Class D now, just never sending power when it’s not needed.
Technically, common class D amps are like “the variable bitrate mp3 of power” and Class A is 320kbps. Technically, every little detail can be heard “faster”, and every electronic music “kick” filling all the space cannot be “less” powerful or “slower” because “the necessary power wasn’t available to accurately produce these transients”.
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So all things being equal. is a class A better sound quality than an a class D?
Technically, yes.
In practice, the filter stages on modern class-D amps are good enough that the difference is not down to one being class A and the other being D.
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TL,DR: Smooth. Beautiful emphasis on cellos, vocals, trumpets, etc… but no “air” whatsoever.
Small review:
First, the on-off switch is behind. But there’s a tiny, tiny blue LED under the volume pot to show it’s powered on.
Oh, also, the volume pot… if you force it a bit you can make it turn around infinitely. Why? That’s the standard metal cylinder with a cut in it. And they put a perfect cylinder on it… and a perfecly round knob over it. (?)
Annnd on top of the unit, a yellow Hi-Res audio sticker. Not sure about that…
A/B-ing again…
Playstation 3 -> RCA output -> SAP-1 (or JDS Labs Atom) -> Sennheiser HD58X.
One word for this amp: Smooth.
Yup. If you believe the JDS Atom sounds “artificial” and “treble-y”, this is for you.
I’ll be really surprised if ASR or anyone else measures this and it measures “flat” (frequency response) like the Atom.
Another word: Analog.
If you believe the JDS Atom sounds “digital”, this is the “analog” version.
When have I heard an “analog” system? Hmm… I don’t remember. That’s the weird part. But this SAP-1 makes me remember it. Shows I went to. This sounds like that. The huge, overwhelming presence of the rumbling bass guitar or cellos. Vocal presence. And… sadly, lack of highs and details at 10khz and above. (Remember I got HD58Xs and I’m treble-sensitive… I know what “air” is.)
Is this Class A?
What I was expecting with the “all-the-power-all-the-time” theory was more detail and more “in-your-face” electronic music kicks or drum snares (etc). Nope. This is the Atom’s job. What I got was something different but as much enjoyable. Musical presence all the time. Dare I say “compression” ? Probably. Smoothness. Beautiful emphasis on cellos, vocals, trumpets, etc… but no “air” whatsoever. I’m tempted to say crap like “more detailed vocals” etc but, this would be worthless: This amp is boosting the bass and the mids. It might just be more volume.
Is this Hi-Res audio? (haha)
I don’t recommend this for electronic music (there’s a ton of high frequencies you won’t even hear). Therefore I definitely won’t create music using this either. And remember, the “hi-res audio” sticker is supposed to mean it goes to 20khz and above. I don’t think so. There’s a lot of harmonics I did not even hear in electronic music. Unfortunately, there’s a lot of information missing above 10khz. This is like, old-school Class A.
I don’t know, I got it yesterday, maybe it needs burn-in…
Also, apparently 270mA of Class A is enough for T50RPs.
This morning’s review was sponsored by coffee.
Edit: Forgot to mention something. Even with the volume at 100%, there’s no hiss at all.
Edit 2: Just to clarify, this amp also kills like half of the soundstage of your headphones. Obviously: Most of the left-right panning is in the treble and “air” (10khz+ treble). And this amp really lacks “air”.
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…in theory.
But in theory I should be better at basketball than Mugsy Bouges because I’m 16in taller.
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Wow Mugsy plays basketball too?
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love your impressions. since it’s so good for cello, I may have to get this ASAP! 
how well do you think it would do with a high impedance headphone? 250? 600?
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I got zero high impedance headphones, sorry. 
I don’t know how “risky” it would be to buy this if you only plan to use like 600 ohms headphones.
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A class A amp uses a single device (transistor/valve/whatever) to amplify the entire wave form, this requires the wave to be biased into the middle of the devices operating range, so it’s always on.
A class B amp uses 2 devices to amplify the wave, one for the top half and one for the bottom, this means when nothing is being amplified the devices are idle.
Because devices do not turn on a 0V you have to bias the devices by their threshold voltages.
So a class B amp can introduce additional noise at the crossover point, which a class A device can’t.
Since there are 2 devices in the amplification of a singe wave, you also need good matching of the devices in question for a linear response.
A class A amp requires at least 3x the output power of the amp, and in practice it can be MUCH more.
This impacts component selection, and drives cost up, so they aren’t usually cheaper than equivalent class B or class AB designs.
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Oh, and, FX Audio Tube-03 with Voskhod 6J1P-EV Tubes + this SAP-1 is the shit.
So, 50$ tube buffer, 20$ tubes, 90$ amp.
160$USD for tubes and Class A is a f-ing great deal.
I already reviewed this little tube buffer, and it makes everything “airy”. It’s like they made the SAP-1 for this.
You get a treble and bass knob on the Tube-03 if anything doesn’t sound right to you.
And you turn the volume knob to max on the Tube-03 for “more tubes”,
or to like 10% and you turn the Class A volume knob to max for more “analog” Class A “smoothness”.
Or 50-50, or 75-25, or 25-75, depending on how you feel and which one you prefer.
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Forgot this, added to the review.
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PS3 plugged via RCA in a tube buffer and a Class A headphone amp.
“When even fucking youtube ads sound classy”. 
Realy sounds like a great deal. Hng… I don’t have the money to do this…
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Dead honest, FX Audio Tube-03 is the only thing I got that makes music sound “normal” instead of “compressed” through this Class A amp. You need an “airy” source for this if your goal is “harman neutral to 20khz”. I don’t even know if a DAC with ESS chipsets is enough.
Also, 270mA of Class A power is more than enough T50RPs. That was unexpected!
A french website said the SAP-1 was enough for 300 ohms.
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Just got Monoprice (Monolith) M650s. Slight V-shape, a lot of information above 10khz. Poor man’s Sundaras or something lol. Lacking mids, but otherwise they’re a great pairing with this little amp (amp not doing much above 10khz, M650s compensate).
Added this to the review:
Nevertheless, I’m keeping it. I just got a great combo with this and my Tube-03 together for my tube and Class A needs.
Maybe it’s because I’m treble-sensitive, but I found the “PS3 -> JDS Atom” pairing to be a bit too treble-y for me (too much sibilance). But the SAP-1 with the PS3 makes a great CD player.
SAP-1 is not just good for tubes! 
Is SMSL SAP-1 the best (quality for price) headphone amplifier up to 90 $ ?