CLASS A AMPLIFIERS

Charlot

This web site goes into a bit more detail concerning the differenct classes of amplifier circuit designs http://www.knowledgeisfun.com/E/El/Electronic-amplifier.php

In general class A is much less effecient than either class B or A/B(most common design) but procduces less distortion (e.g.cross over distortion in class AB) than either class B or A/B. With the inefficency comes heat,caused by bias current, which must be dealt with by utilizing large heat sinks.This generally makes pure clas A unsuitable for high powered amplifiers. An example that comes to mind is Nelson Pass his X seriers is class A/B and ranges in power from 150 (X150) watts to 1000 (X1000)watts per channel. The XA amps are pure class A and range in power from 150 (XA150) watts to 200 (XA200) watts per channel.

Chuck

Chuck

There are very high powered pure class A amps. Needless to say they are typicaly very expenisve, very big, very heavy, run very hot and are costly to operate.

The details are below; the bottom line is that in a class A amp, the output stage is on and passing current for both halves of the signal (+ and -). Consequently, during the time there is no signal, the devices dissipate power as heat. Class AB amps are designed so that there are devices dedicated to each half of the waveform so that less power is dissipated as heat and they're more efficient. The down side is that during the time the signal "crosses over" from + to - there may be a small delay in the device turn on that handles it and this cause a type of distortion known as cross over distortion. Many modern designs today work in class A up to a certain power level then switch to AB. Sorry for the verbose answer, but this isn't a simple topic.

Class A Amplifiers
Class A amplifiers operate over a relatively small portion of a tube’s plate-current or a transistor’s collector-current range and have continuous plate- or collector-current flow throughout each RF cycle. Their efficiency in converting DC-source-power to RF-output-power is poor. DC source power that is not converted to radio frequency output power is dissipated as heat. However, in compensation, Class A amplifiers have greater input-to-output waveform linearity (lower output-signal distortion) than any other amplifier class. They are most commonly used in small-signal applications where linearity is more important than power efficiency, but also are sometimes used in large-signal applications where the need for extraordinarily high linearity outweighs cost and heat disadvantages associated with poor power efficiency.

Class AB Amplifiers
As the designation suggests, Class AB amplifiers are compromises between Class A and Class B operation. They are biased so plate- or collector-current flows less than 360 degrees, but more than 180 degrees, of each RF cycle. Any bias-point between those limits can be used, which provides a continuous selection-range extending from low-distortion, low-efficiency on one end to higher-distortion, higher-efficiency on the other.

Class AB amplifiers are widely used in SSB linear amplifier applications where low-distortion and high power-efficiency tend to both be very important. Push-pull Class AB amplifiers are especially attractive in SSB linear amplifier applications, because the greater linearity resulting from having one amplifier or the other always conducting makes it possible to bias push-pull Class AB amplifiers closer to the Class B end of the AB scale where the power-efficiency is higher. Alternatively, push-pull Class AB amplifiers can be biased far enough toward the highly-linear Class A end of the scale to make broadband operation without resonant tank circuits possible in applications where broadband operation or freedom from tuning is more important than power-efficiency.

Class A operation is the transistors are fully biased (turned on) meaning there is no crossover distortion (notch??) when the signal swings from + to - and vice versa.

A watt is a watt, so Class A watts are not more powerful than say Class AB watts.

Of course, the drawback of Class A is heat and energy consumption. The amp must dissipate all of it's power whenever it is turned on whether playing music or not. Either the energy goes out the heatsinks as heat, or to the speakers as electrical energy, or a combination of both in most cases.

Much of the newer A/B designs will run up to nearly 25% of the output in pure class A and switch to AB for cooler operation and lower power draw.. and many of the newer A/B designs today rival and beat many of the older Straight Class A in my opinion, newer technology capacitors, transformers, transistors can make this possible. Older Thresholds, CODA, and Levinson's I owned are good, but my newer odysseys and many mcintosh amps sound better sometimes and do not run hot at all... so your taste after audition is going to determine if they are better for you or not, I hated the rediculous Heat and draw of electrical with low efficiency in comparison, so even if not sonically better I find some reliablity, ease of use, and cost benifits to the newer Class A-A/B designs.