The inside of the small Koss E-90 energizer is fully stuffed with two circuit boards. It's easy to get the above view simply by removing the back cover. From there, however, disassembly gets tougher, you have to remove the feet (which would presumably need to be glued back on), the volume control, and the front headphone jack to get the two circuit boards out. I didn't feel sufficiently motivated to do that.
The bottom board appears to be mainly power supply, and ends in a row of electrolytics that couldn't be much bigger and still fit. The upper board appears to be the amplifier, and seems stuffed with small transistors, resistors, and other parts. Notably toward the front there are a few mylar caps which might be easily replaced with polypropylene.
Everything I've seen continues to convince me that the power supply is almost certainly a switching design (down to the choke in the back similar those seen in all switching supplies), and most likely the amplifier is a kind of switching type (actually Pulse Width Modulation) too. One interesting feature is the ribbon cable that connects bottom board to top board. This is independent of all the signal connections, which are quite obvious. Is this because the top board needs many different independent power supplies? That may be part of it, but it could also be that the top board ultimately synchronizes with the bottom board through multiple phases of a high speed clock.
Now PWM amplifiers are VERY efficient, and usually quite capable in delivering full power at any frequency, attributes that appear in the E-90. But where they fall down is that there is not as much resolution in the upper frequencies as in the lower frequencies, because as the audio frequency approaches the switching frequency, it can only be constituted from a smaller number of up and down choices. This is a problem with all 1-bit-like systems, including the DSD system used with SACD, despite the 2.88 Mhz sampling rate of DSD, a huge amount of digital processing called "noise shaping" is used to shift the noise (i.e. low resolution) from upper audio frequencies into the supersonic. Earlier PWM systems attempted to use frequencies as low as 500kHz, which is obviously way inadequate.
Although Infinity and Sony made PWM amplifiers in the golden 1970's, the idea became unpopular either for real or rumored reasons. Bob Carver has always wanted to make the highest power highest value and highest efficiency amplifiers, and said he tried to make a good sounding PWM design, but gave up, and instead went with the rail voltage switching designs he dubbed Magnetic Field amplifiers with characteristic flair (i.e BS). The rail voltage is switched to whatever the current signal level requires, allowing the rails to stay as low as possible, therefore requiring the amplifier to dissipate as little power as possible (most of the energy going into the load instead of into amplifier heatsinks), even less than an Class AB amplifier, though otherwise it is made just like a Class AB amplifier. Actually the idea seems a bit rube goldberg, but Carver got it to work by using chokes to store energy sufficient to kick the rails up when needed, and that's where the Magnetic Field name truthfully comes from, but reading Carver's ads you would think of something entirely different. Since the late 1990's, however, electronic technology has advanced to the point where very high speed PCM digital systems could drive a PWM amplifier much more accurately than previous analog systems could. So PWM amplifiers are back on the scene in a big way, even made by high end manufacturers such as Tact and Rowland Research (the latter formerly known mainly for building massive high bias amplifiers).
But now that I've also mentioned the rail switching design, it is entirely possible that the E-90 is a rail switching design also; it would provide the needed efficiency and eliminate the need for significant heat sinking just as in Carver's Magnetic Field amps. Koss is rather mum about how their E-90 actually operates, giving only the barest of specifications to a direct inquiry.
Stax, in contrast, has gone for Class A electrostatic headphone amplifiers. Class A amplification is the most linear and needs the least corrective feedback to work decently well. It is also the most inefficient, consumes the most power, and needs the most heat sinking. But many think it's the way to go for audio because it is the highest quality. Stax is the commercial leader in making electrostatic headphone amplifiers, it's a veritable giant compared with the tiny perfectionist operations that make amps like the Blue Heaven. That being said, even Stax is fairly small as electronic companies go, and even smaller since the 2000 bankruptcy in which all other business lines were dropped but headphones, their most well known product, were continued (thank goodness).
Stax tube amplifiers may be the most famous and popular among it's top line products, and many feel give the right harmonic balance to the otherwise slightly bright sounding phones, Stax has made transistor Class A amplifiers for headphones since the 1970's, and many feel the transistor amps offer the greater ultimate resolution and clarity. My view is that the Stax tube amps aren't worth the money. The tubes used do not seem to me like the right choices. From reading many reviews (never tried one myself) it seems the tube amps seem deliberately down rez'd to soften the music. If you want tubes, get a true perfectionist tube design like the $4995 Blue Heaven I described in an earlier post. But there is little such complaint to be made about the Stax transistor amps. They seem honest designs, if not as far out as some made by elves.
I have now bought a Stax SRM-1 MkII Pro for $325 (the low price coming from the fact that the seller can't actually test, but he is allowing me a 7 day trial and examination). That is a Class A solid state amplifier similar in concept and execution to Stax's latest top-of-the-line amplifiers (which cost up to $2400), but made from 1982-1995. It's said to be better than the current lower end Stax amplifiers, if not quite as good as the current top-of-the-line. Picture below is from article by Ken Rockwell which you can read at this link.