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Not only is this subjective in general, but varies by manufacturer, circuitry design, implementation and other aspects. You may want to go to Wikipedia or something to get a general (very general) idea on what the different amp classifications even mean.
I don't think it is truly possible to classify the sound of an amp strickly by the classification of the design used. If this were the case, all Class A amps would have the same general sound characteristics, and this certainly is not the case.
The simplest way to think about it is in terms of effencey; Class A draws full power all the time, usually about 4 watts for every watt of rated output. So they will be large and run very hot. As you progress down the alphabet the amps become more efficient and run cooler and can be made smaller. Does this affect sound quality? Many think so but others are happy with the sound they get; only you can judge.
In terms of heat:
class A => hottest but otherwise the most linear/lowest distortion
class A2 (tubes only) grid current may flow in the tubes during part of the waveform but the tube is conducting through the entire wave cycle. This allows for more power but has greater requirements of the driver circuit.
class AB => less heat, slightly more power but slightly more distortion
class AB2 => (tubes only) similar idea to A2, but the tubes stop conducting through part of the waveform. You get a lot of power and a lot less heat, but crossover distortion is more pronounced.
Class C => not applicable to audio
Class D => so far, for practical applications so far is transistor only. The devices are either fully on or fully off, avoiding the much greater power requirements of operating the transistors in the linear region. This makes the most power with the least amount of heat. Distortion can be very high, but this is a developing field, and is likely the area with the most potential for improvement in the next ten years.
Also class D had NOTHING to do wit digital. Many people presume wrongly that the D stands for digital. This is just not true.
There are amps being sold as class A but in truth some of these are Class AB with a high class A bias. But how much watt do you really need? I just spoke with someone who ones 1000watt mono amps. He never saw the needles move beyond 15 watt and normally not beyong 5 to 10 watt.
Mordante comments about class A marketed amps really falling to class A/B is important. IME there is a notable difference with many of these amps when they drop down into class A/B in terms of performance and sound characteristics. Personally, I am a believe in pure class A amps (ie. that remain in class A at all times), but I think this is a matter of opinion. Also, I am not suggesting that all class A amps are better than all Class A/B or D amps (for that matter).
How much power does one really need? That is always the question and there are so many factors that come into play. Obviously the speakers, the room, listening levels, even types of music and of course sound preferences.
I own a pair of speakers that are rated at a solid 8 ohms and the reviewer and printed/promoted mfg. recommended power is 60 wpc at 8 ohms.
However, based on direct communications with the designer of the speakers, he clearly suggests that the speakers operate in a range of 8-16 ohms and based on their design principles are relatively hard to drive. He suggests that for best performance, 200-300 wpc (rated at 8 ohms) will produce the best results. I would agree with his assessment as I have run the speakers with various amps that ranged from a low power SET design, mid power push pull design, mid power 100% class A design and now a higher power 100% class A design (rated at only 125 wpc into 8 ohms). Each power increase proved to deliver better performance in specific scenarios.
Can one have too much power? No, I have never experienced that problem. But I suggest that there needs to be a balance between just a lot of power and the quality of power. Is 1000 wpc monoblocks too much power? Depends on the speakers, room, etc. . . Just because the proverbial needle doesn't ever "seem" to go past 15 watts does not mean for a second that all that is needed is a 15 wpc amplifier or that the 1000 wpc amp is a waste.
I can pretty confidently say that if you have two identifcally designed amps (with the exception of power output) that in the referenced case (above here and by Mordante), the 15 wpc amp will suffer versus the 1000 wpc amp and very, very likely run out of headroom and produce notibly different sonic attributes.
Most transistor amplifiers do operate pretty close to class B; at low power levels most of the power comes from the driver transistors rather than the outputs. But in order to keep the amplifier from making significant distortion (notch and crossover) at the zero crossing point, they have to have a certain minimum amount of bias current to make that happen. As a result they are considered class AB.
Most transistor amplifiers do operate pretty close to class B; at low power levels most of the power comes from the driver transistors rather than the outputs. But in order to keep the amplifier from making significant distortion (notch and crossover) at the zero crossing point, they have to have a certain minimum amount of bias current to make that happen. As a result they are considered class ABMaybe you're meaning that most of the power dissapated is in the drivers at small signal levels? This is indeed true in some designs, notably emitter-follower output stages with one set of output transistors. But in a conventional solid-state amplifier, virtually all the loudspeaker current flows through the output transistors, NOT the drivers. In any case, I'd agree that what constitutes "Class A" and "Class AB" is indeed widely used imprecisely . . . even though in a solid-state amplifier, they are distinctly different operating points.
The "cutoff" region in a tube or transistor is defined as the area where its transconductance drops sharply as the current through the device is reduced. In true Class B operation, the bias point is chosen so that the cutoff region of both halves of the output stage occur inversely at the same time . . . that is, the upper half is turning on at the same time, and to the same degree, that the lower half is turning off. In Class AB, one half of the output stage remains turned on before the other is turned off, thus giving a region (in the "middle") where both devices are conducting at full transconductance together. This is of course its Class A region.
The problem with Class B is well known -- it's due to the fact that pairs of output devices operating this way are non-conjugate, and the turning-on of one half never lines up exactly with the turning-off of the other. But in (solid-state) Class AB, the transition-region of the output device is left completely "exposed", without the other half compensating for its change in transconductance at all. Thus, the consequences are more severe - once a Class AB output stage leaves its Class A region, it produces more distortion than a pure Class B design.
On the test bench, a solid-state Class B design produces its best distortion at a specific bias level, and increases in distortion are evident both when the bias is too low AND when it's too high. Maintaining it at the proper point with regards to temperature is a very complex task, which is dependent not only on circuit design, but physical layout, assembly, and calibration procedure. Class AB on the other hand simply needs to have sufficient bias control to avoid overheating. For those who hear differences when their solid-state amps are left on all the time, variations in output stage bias current are the obvious reason why.
However, in a transformer-coupled push-pull tube output stage, the crossover behavior is fundamentally different for two reasons. First, it's a power amplifer (rather than just a current amplifier) which means the linearity is dependent on both the voltage and current across the device, as opposed to just the current - the latter more determining the cutoff characteristics. Second, this is accentuated by the fact that the primary inductance in the output transformer "forces" the plate of the non-conducting tube through its transition region, which in turn has less influence (the Rp increases) the more "turned-off" it's driven.
The consequent is that in a traditional tube amp, there is much more "grey area" between Class B and AB, because the presence/absence of crossover distortion is determined principally by the quality of coupling in the two halves of the output transformer's primary. The Class AB bias current serves mainly to keep some magnetization in the transformer core to compensate for leakage reactances. And on the test bench, P-P tube amps tend to show their most pronounced crossover distortion at the high-frequency, high-power end of the scale, where the output transformer core becomes most saturated.
On a person note if I was looking for a power amp. I would look van a SS class A/B amp with a high biased Class A. So that normally you would play in pure class A but when you need lots of power for a second or two it would switch. So a 250watt amp with first 50watt or so in pure class A. Don't know if such a thing does exist.
Mordante, the Monarchy SE-250 meets your requirements. 250 Watts into 8, 500 into 4, biased Class A to about 50 Watts. It also uses a 6922/6DJ8 or 12AT7 tube frontend, has NO negative FB whatever, and uses MOSFET output transistors. I've had its 'little' brother, the SE-160, for several weeks; it sounds VERY good.
Yes Stanwal, I did mean the power amp - my dyslexia! Still, what was (is) meant by 'current-dumping'?"Current-dumping" is Peter Walker's terminology, it refers to the power side of what is essentially a feedforward amplifier. In this topology, there are actually two amplifiers summed together in the output, one being a low-power Class A amp, and the other being a high power push-pull amp with virtually no bias (even less than Class B, sort of a symmetrical Class C). Here the high-power "current dumpers" send the vast majority of the current to the loudspeakers, and the Class A amp supplies current opposite to the inherent non-linearity in the current dumpers.
It's actually quite a brilliant circuit, typical of Peter Walker's elegant engineering approach . . . and it definately made sense with the power semiconductors of the time, which had linearity problems all across their range of operation. Modern power transistors however can be extremely linear in Class B right up to the crossover region, so it may be that the power-dissipation spent in a separate Class A amplifier in a feedforward design, may be better spent simply applying bias to the output stage itself, maybe in combination with lower-value emitter resistors.
Another "hybrid" Class A approach that holds promise is Class G (aka Class H), with the inside (main) amp biased into Class A. This might be thought of as a symmetrical Class C amp wrapped around a Class A amp (inside a dream within a dream, within a Taco Bell, inside a KFC!) Seriously, though, the problem with glitches in Class G/H amps in Walker's era are solved today with better switching diodes, which may also make the feed-forward approach moot in modern times.