Here's a "generic" and very simplified way to look at an amplifiers bias. Think of an amplifier as a rotating wheel.
Class A drives the wheel at all points of the rotation. This means that the wheel is always being directly driven and monitored. As such, it should offer the smoothest, most even rotation possible while always being under control. This means less distortion or irregularities in the rotation ( output ) of the wheel.
Class B drives the wheel for only the first half of the rotation and then is left to rotate on its' own. Due to the natural forward momentum of the wheel, the wheel can be left unattended for the second half of the cycle and remain relatively consistent. This is called "the flywheel effect" as it keeps rotating ( producing output ) regardless of whether or not something is energizing it. As the wheel ( output of the amp ) comes around to the beginning of the cycle, drive is re-applied again and then disengaged at a preset point. The benefit to all of this is that the wheel is only driven 50% of the time, so a lot less work and effort is put forth to keep it running on a relatively smooth basis.
The major problem with such a scheme is that there will be a point of "cogging" or "irregular motion" at the time that the wheel is re-engaged to direct drive or when it is released to "free wheel" for the second half of each rotation. Those slight "glitches" would be considered "crossover distortion". This can be somewhat minimized by use of various types of feedback, but that in itself is a controversial subject and a whole 'nuther ball of wax.
Class AB tries to combine the best of both worlds. The amp ( or "wheel" ) is always turned on and fully engaged ( Class A ) for the entire duty cycle of each signal for low level signals. That is, it operates in this fashion up to a given point. Above that point though, the amp switches over to Class B bias. This helps reduce the need for massive heatsinking and increased output devices to sustain such high output levels while increasing efficiency on the amp. From a manufacturer's point of view, it is "better" because it is both cheaper to produce and slightly more reliable due to less thermal problems.
In an AB amp, most of the signal would be reproduced while the amp is in Class A mode. This means that reproduction "should be" as good as possible in terms of linearity and low distortion. The exceptions to this would be if listening with a slightly higher than average spl, using in-efficient speakers or playing music at an average volume that has extremely dynamic peaks involved. All of these conditions require the amp to work harder and put out more power, pushing the amp closer to or well into the Class B range of operation.
Most "hi end" amps are biased to run in Class A for up to several watts, not just the first one or two ( or even just PART of one watt ) that is commonly found on most mass produced gear. Some will go as high as 30 - 50 watts per channel while still operating in Class A mode. My findings are that many of these amps run Class A up to appr 8 - 10 wpc and then switch over to Class B. This is just a generalization though, as there are always exceptions to "the rule". Obviously, the higher the bias, the "purer" the sound for most operating conditions. This also means more heat dissipation to deal with though and more power supply capacity being required. Since metal for larger chassis and heatsinking and higher power transformers and greater quantities of filter caps are not cheap, the manufacturer will typically compromise somewhere along the line. These are products that may work quite well under normal operation, but are built to a price point.
There are quite a few other classes of operation outside of A, B or AB. While Class C is not normally used in audio applications, there are amps that do make use of Class D, Class G, Class H, Class T, etc... Most of these are of high efficiency design, primarily trying to do the most in terms of output with the least amount of lost energy ( heat ) resulting in high efficiency. The different classes are simply variations of the same basic principle ( to a great extent ). All of these use some type of switched design, literally pulsing the "drive" to the "wheel" many times a cycle. Some even switch in auxillary amplification circuits as the earlier stages become overloaded. This can result in a LOT of "glitches" or "crossover distortion" due to the irregular or "twitchy" type of "steering" that is being applied at random intervals to the wheel.
Having said that, much has been learned in the last few years about amplifier technology, hence the arrival of "digital" amps, etc... Some of these are said to rival amplifiers of Class A or very "richly biased" Class AB ( stays in Class A for a longer time than most AB amps ) while offering drastically increased efficiency. While i have heard and actually own a few of these amps, my personal opinion and findings show that well thought out higher bias designs still have an advantage. Most of the advantages are in the areas of bass attack ( slam ), overall dynamics and spacial characteristics ( air, depth, separation of instruments and notes, blackness of background, etc ). Obviously, I can't lay claim to having tested or experienced everything that is currently available though. Others may have different experiences with various ( same or different ) products in different systems. That is why most of us stress the importance of in-home auditions. What you and i find "acceptable" or most / least importance may be quite different overall. Sean
>
