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Well, I think things are a bit more complicated than that. For speaker amps, you generally need a lot of power unless you have very efficient speakers, e.g. horns, which is another topic altogether. We're talking about 50-100 watts per channel or more. So the amp needs to produce 40 volts peak for a 100 watt amp, and on the order of 10 amps if we have a 4 ohm speaker. Now, a class A push-pull amp has a maximum theoretical efficiency of 50% (single ended class A has a max efficiency of 25%), so that means it runs at 100-200 watts/channel all the time. This means lots of tubes (old days) or lots of heatsink for solid state. All this means lots of heat and lots of money (and that doesn't even include the cost of running the damn things). So basically, until Mark Levinson and Krell came along, pretty much all amplifiers were class AB because nobody thought that there was a big enough market to support a class A commercial amp. Class AB was considered the best compromise between cost, efficiency and sound quality. It was never considered the best, just that Class A was considered too expensive. The first Mark Levinson Class A amp was rated at 25 watts (but was reportedly class A down to 2 ohms, at which point it was a 100 watt amp), was mono, and was big, heavy and cost a few thousand dollars - I think you could have bought over 100 NAD3020 integrated amps for the same money, and the 3020 was stereo, had roughly the same power rating, and included a phono section. But, it turned out that there was a market for it, and Krell came out with stereo class A amps that had more power and generated more heat. And it's gone on from there. The limiting factors for speaker amps is basically how much power do you need and how much money (and size, and heat) do you want to spend getting it. For non-electrostatic headphone amps it is significantly different. Here, you only need a few watts at most, into higher impedances than speakers have. So low to moderate voltages (say 15 volts at most)and relatively low currents (an amp or so at most). Based on specs, the least efficient non-electrostatic headphone is the HFM Susvara, and 5 watts into that should produce 120 dB, which is enough to give you hearing damage in minutes if sustained. So in theory, a class A headphone rated at 5 watts into the Susvara's impedance should do the trick. Which means that an amp that draws 25-30 watts from the wall should be able get you close to the threshold of pain In that case, there isn't a lot to be gained by dropping back to class AB. That said, some cheap headphone amps ARE run in class AB because the manufacturer doesn't want to lay out money for heatsinks. Every penny counts. For electrostatic headphones, it is different again, stat headphones have very high impedances, so you need very high voltages but relatively minuscule currents - a few mA. because you need to have high voltage - the Stax amps generally have +/-350V supplies, the KG amps generally run +/-350V to +/-450 volts, the Stax and DIY T2 run +/-500V supplies. Now the Stax amps run around 5-7 mA/output device, the KG amps up to 20 mA/output device, and there are 2 output devices per channel. So given that, the Stax amps draw about 40-55 watts from the wall, the KG amps run about 140 watts to more than 200 watts (the latter from a DIY T2, which also has 8 tube heaters using energy).

Yup, you set the desired current and then it goes out of that range. Odds are though that most of the time...people never will.

I was mentioning the rega speakers story. I'll tell you another: I bought a lyngdorf audio amp and speakers about one year ago at the portuguese dealer, who gave me a 15% discount on a brand that has a fixed price list. the prices online are all the same everywhere, so I got a very nice deal. There are good dealers and bad ones everywhere!

I have a Marantz PM-64 here which is Class A up to 25W I think but no switch. I need to check...

One other detail is how the push-pull output stage is biased. This is basically what determines whether an amp is Class B, AB or A. For class B, the output stage is biased at zero, meaning with no signal there is no current running through the output devices. This is basically only used in radio frequency transmitters. In class AB, there is some current running through the output devices with no signal, but when a signal is sent through the amp, each of the two output devices only reproduce half plus a bit more of the output, and the combination of both devices is needed to produce the whole signal. In reality the output devices are turning on and off during the output, which can produce crossover distortion, which is a transient that occurs when the device turns on or off. The problem is that this distortion actually gets worse as a proportion of the output was you go to lower power, until you get down to the point where the signal is small enough that the turn on and turn off does not occur. Now with most music other than the compressed crap that some modern producers are putting out (i.e. loud all the time), the fact is that much of music is at a volume that is 10-20 dB below maximum, so that an amp is usually running a 1-10% of max power. For example, take a typical speaker with a sensitivity of 86 dB/watt/1 meter. Now, most people listen a few meters from the speakers, so the sound level there is several dB lower, but this gives you a rough idea. Typical conversation is about 60 dB, so 86 dB level is like standing a few feet apart and yelling at each other. What you can see is that with most music, the amount of power an amp is putting out is usually on the order of a watt or less. In early transistor amps crossover distortion was a problem, because they were run at such a low bias so that there was lots of crossover distortion being produced a lot of the time. The solution is to increase the bias so that most of the time the amp is running in class A (ie. both output devices reproducing the whole music cycle), but that means more current and bigger heatsinks. Tube amps, OTOH, generally ran with enough bias current that they were basically class A up to a couple watts, which for most music meant they were actually class A most of the time. . Now for true class A, there is enough current running that both devices output some signal current at all times. In theory this should only matter for music peaks. In reality, since active devices (tubes, transistors) are more linear in the middle of their operating range rather compared to their limits, this decrease distortion at all levels.