I can direct you to articles by professional amp designers, but everyone knows what wikipedia is.
Take a look at the link to the test of the Sansa Clip: it's amp is built into a chip along with a DAC and a 4-band equalizer. Bought in bulk, the chip costs about $2. And it's output is "essentially distortionless"! Building a class A to get distortion free sound with a reasonable component cost stopped being necessary at least 20 years ago.
That's quite OK. It's 20-odd years since I used to share a lab with 'professional amp designers', but we still keep in touch. Everyone knows what Linkedin is.
Without wanting to get into a flame war over something as unimportant as hifi, as only a keen amateur these days, I do not wish to design with an all in one chip (assuming one could buy the chip from CPC or RS) with an amp, a DAC and a 4 band EQ in there as anyone can do that given the data sheet. Where's the fun?
Also, a 5 minute google doesn't elicit it's noise or distortion specs other than those by NWAVguy - audible hiss? Jesus... I absolutely guarantee you, for example, it's basic measured amplifier noise spec will not meet what I could come up with in 10 minutes - the Clip will be hobbled by inherent noise of passive smd parts for starters, a switched mode power supply for seconds, and a switched mode amplifier for thirds. Being able to measure THD+n down to around 0.05% isn't wildly impressive either - and it may or may not be audible, but why not aim higher? Most current op-amps suitable for audio can achieve a couple of orders of magnitude better than that. <edit> Interesting to note that the Sansa's distortion is primarily odd order harmonics and the iPod is even order. Some might say the iPod is better then for this reason alone. It's a bit simplistic to state that low impedance headphones are more efficient than high too. Depends on the characteristics of the amp feeding them, the output voltage swing, current drive etc, though 1 ohm isn't too bad for a portable. I'm guessing the Sansa has been designed like this as most portable phones are low impedance? I don't know, not too familiar with portable type phones.
Paralleling NE5532's as OP devices will drop output impedances far far lower - and reduce noise into the bargain. You could just use conventional OP devices though and end up with as close to zero as sensible. It wouldn't fit with portable sensibilities, but that's not really what I'm interested in.<end edit>
Coming back to class A, I didn't realise this Clip was a cheap portable suffering poor battery life as it is. Obviously, class A isn't suitable for portable (and component cost certainly isn't 'reasonable'), though you may get away with it in an enthusiast headphone amp but is it worth it? Who knows, not tried it, but the *measured* distortion (assuming you've not made a mess of the input stage and VAS) WILL be lower distortion than the 'Clip' - which will definitely measure worse than most domestic amps as, as previously mentioned its amplifier will be so called class T (really class D but with a hint of marketing) type in order to make decent battery life from a tiny Lithium cell. Class D, T or whatever it's called has it's own set of problems. For home audio (connected to the mains) there is certainly no reason to use that topology - other than price (or if you're one of those odd sort who think that a £10 eBay 'T-amp' sounds better than a well designed conventional amp). From memory, because of the inductive filters on the output to take out the couple of hundred kHz switching frequency, they only really work properly with a very narrow loading range - but this wont matter for 99% of the MP3 using public. I believe that some computer speaker amps etc don't bother with a filter at all and just use the fact that the speaker wont reproduce the switching frequency. That's properly inelegant though...
There's lots of design issue references to Class D out there, so important it is to the portable market. Though it's not necessary to dig too deep to realise that it's not a 'hifi' topography. Having said that, there's a lot of web space devoted to saying it's incredible. That's the web for you. For sensible views, there's several JAES papers out there. I have references to the JAES papers buried deep in books but it's too sunny outside to spend time looking for them - if you're seriously interested, I can have a look this evening for you?
Don't get me wrong, current portable technology is mind blowing. The storage capability, the battery life, the user friendliness etc, but it's been built for a particular demand. Cheap and cheerful audio on the go for a market that values being able to store a million albums, and enough eBooks to fill the British library more than it values absolute quality. I don't believe (and nor does industry) that the ultimate quality rests with an iPod just yet. It doesn't need to.
I'm not sure now where your original point was going. As far as sound quality goes, any competent designer could come up with an amplifier that measures better (and therefore potentially
sounds better) than that in a mass market portable
device. On the flip side, a portable device as smart and cheap as a Clip is way beyond the realms of DIY and will always be so. The proper hifi, home based market is a different ball game - very few physical or economical restraints.
Look at that! The 'hifi chat' group really is!
"Two very small men cutting steps in the roof of the world"