CLASS A AMPLIFIERS

Class A amplifiers nearly always exhibit more authority, but this has nothing to do with power.

I should point out that a Class A amplifier can be either push-pull, single-ended, tube or transistor. What is important is that the amplifying device or devices never goes into what is known as 'cutoff'. IOW the device or devices each amplify the entire waveform all the way up to full output regardless if single-ended or push pull.

Distortion is generated when an output device goes into cutoff. In a push-pull circuit, when the device is cutting off while at the same time passing off the signal to its complement that is just barely turning on, the resulting distortion obscures low-level detail (as in the case of Class AB2 and Class B push-pull amplifiers). This can be complicated by the presence of an output transformer in the circuit as magnetics tend to react badly to this sort of thing!

So ultimately, Class A is the lowest distortion form of amplification, regardless of the circuit topology or amplifying device.

The price paid is Class A amplifiers are less efficient as they are 'on' all the time and therefore they make a lot more heat. They also require fairly beefier power supplies for the same reason. Such is the price of increased performance!

Unsound, just so you know, Class A has nothing to do with push-pull vs single-ended. Also, a Class A amplifier's peak power will always be the same as its continuous power, IOW the 'headroom' rating will be 0db. An amp that has more that 0db headroom is likely an AB amp.

Krell's sliding class A approach (a similar form of that being first used by Brooks in the 50s if I recall right), allows the amp to run cool at low power. On account of its power supply regulation it should exhibit 0db headroom.

Changing the load impedance changes everything! Contary to popular belief it is not always a good thing that the amplifier power changes as the load impedance changes. There are speakers that are of course designed for this, and there are other speakers to which this behaviour is unacceptable (i.e. electrostatics, horns and full-range high efficinecy drivers to name a few).

This ability of transistor amplifiers has to do with feedback just as much as the fact that they are solid state. The effects of Negative Feedback in an amplifier runs counter to the way our ears have evolved over millions of years. As a result it is one of those things that we should always question. Once you do so *honestly*, a lot of other things get questioned too, but ultimately the right answers also yield better sound. BTW these comments have nothing to do with transistor vs tubes! -although many will think that they do. The underpinning issue is more fundamental.

Calanctus, Your speakers are a good example of what I was talking about. ESLs in general, regardless of their particular impedance curve, want to see the same power for a given sound pressure regardless of what their impedance is at a particular frequency.

In effect, they are driven by power.

If an amplifier driving speakers like this puts out a different amount of power into different impedances, the effect is a shift in tonality. This is why transistor amplifiers (in general) tend to be very bright on ESLs, as their 'constant voltage' characteristic causes them to put out a lot of power into the low impedance of the ESL at high frequencies. Some speakers (for example B&Ws) are designed to expect a 'constant voltage' amplifier, and so will have flat response in the room. This is much trickier with ESLs!

In essense, there are two paradigms competing in high end audio today- the 'voltage paradigm' and the 'power paradigm'. No-one talks about this!- but we see it all around us:

The Voltage Paradigm is the reigning test and measurement paradigm. It is only concerned about voltage, and so when measuring an amp or speaker, voltage is the only thing considered. 'Constant voltage' output refers to an amplifier that puts out constant voltage regardless of load (contrary to morbious' comments, this does not require power supply regulation to accomplish this- merely enough feedback). Such an amplifier is regarded by these rules to be 'load impervious'.

The problem lies in the fact that negative feedback runs counter to how our ears detect volume (higher odd orders being the key- these are enhanced by negative feedback). Additionally there is the question of whether or not a speaker is driven by voltage or power, and of course the answer is power. So the 'power paradigm' says that the amplifier will have little or no feedback to reduce the odd ordered harmonic that the human ear dislikes, and that the amplifer will produce (or attempt to produce) constant power into all loads. This, BTW, is very nice for ESLs, horns and magnetic planars. Amps that fall into this category are SETs, some OTLs, transformer coupled push-pull tube amps and a very small number of transistor amps with zero feedback. Such amplifiers typically have higher output impedances, and nearly all will be unsuccesful at the goal of constant power (in fact many designers of such gear may not even acknowledge that they even *have* such a goal).

A parallel controversy is the subjectivist/objectivist debate. Roughly, the power paradigm is more subjectivist and vice-versa, but this is *not* by any means cast in concrete!

The power paradigm has its roots much further back than the voltage paradigm (1920s), and only seemed to resurface in the last decade or two. The obvious proliferation of advanced SETs (and tubes in general), horns, single driver full range speakers and the like are an indication that no-one has all the answers.