Difference between Class a and a/b


I have been reading alot about class a and class a/b biased amps, but I am not really clear on what the difference is. I understand that it has to do with circuit providing power through the entire cycle. But I was hoping that someone may be able to explain it in simpler terms.

Any help is greatly appreciated.


Craig, In class a operation,the output devices are constantly on.In a/b,the output devices turn on and off {generally causing "switching distortion".The pure class a operation usually draws more power due to the devices not cycling on/off.In my opinion,it sounds better,closer to analog.Digital is an on/off medium and I'm sure you have heard the complaints there.Some amps will provide class a operation to a certain power output,then slide a/b {IE:Pass Labs X series}.I hope this helped,I know I over-simplified the explanation..But.
simple terms.

Class A - sounds much refine, good in resolution, focus on high and mid. Bass quality tends to develop vertically (deep deep deep)

Class A/B - sounds less bright (relatively), much bass and tends to develop horizontally. Punchy or boomy bass for extreme case.
I will try to explain it more technically, but maybe it will make more cents. Your amplifier is working to deliver some signal to the output. If you draw imaginary line through the center of that signal you will divide it into a positive half and a negative half. In Class A amplifier a single output device controls the flow of the entire signal. To do so it has to be biased in such a way that the lowest point of the negative signal still has to be above 0 level, at which point output device (transistor, tube, FET, etc...) shuts off and clips (distorts) the signal.
In class AB amplifier the output signal is controlled by a pair of output devices, where each one of the pair controls only the positive or the negative half (push-pull) of the signal. So each one is biased to be shut-off at zero crossing and stay off while the complimentary one is working on the opposite half of the signal (hence the crossover distortion).
This is basically how they work. I hope that my explanation was clear enough to follow.
Well... I tried...:)

your explaination is incorrect. Your description of class A is actually a single ended design(i.e. one output device handling the entire signal). class A can also use two output devices, one handling each phase. however both devices stay powered up at all times, even when not used to drive the speakers (i.e. each device stays powered up for the entire 360 degrees).

your description of class a/b is actually a description of class b (i.e. the out device powers down at zero crossing or at 180 degrees). In class a/b each device stays powered up beyond zero crossing, but not for the entire signal (say 200 to 300 degrees). when you hear manufacturers claim a "high class a bias", they are talking about how far past zero crossing the output device stays on.
if Audiogon didn't "expire" previouse and usefull archive information about classes of operation of amplifiers often appeared in discussions here than you'll find more detailed and deep explainations of classes ofoperation.

It's definitely something beyond sonic effects of one or another class of operation.

So far members explained to you differences between class A and class B but not realy class A/B.

Class A/B has a special diode placed between complementary pairs of output elements to bring the output devices from class A onto the B operation under demand of higher current or power. This actually may even be neccessary even on moderate volume levels depending on load i.e speaker. A complementary pair of transistors is defined to be opposite bias transistors(direct bias and reverse bias) with same input and output parameters. Such above described diode works as a gate between such complementary pairs and should stay "closed" when the input signal from the previous stage is less than peak and "open" once the peak input signal is reached by the previous stage transfering the output devices onto class B operation. Before the diode is "open" both groups of transistors direct and reverse bias are having their own zero-degree point of operation. After switching to class B both of output device groups transfer to one common zero degree point of operation.
Please note that previous stages in amplifier can also work as well in class A, B or AB operation.

The advantage of such A/B design is decreased distortions on higher volume levels and near-linear responce of class A on low volume levels.

Class B itself has higher distortions at low volume levels not only due to above said switching between complementary pairs but also due to some degree of parameter difference between such as well.
I have long thought that Soundstage reviewer Greg Weaver's concise explanation regarding this question, taken from his 1997 review of the Clayton S-70 amplifier, was one of the most easily digestible for the technically innocent that I've read.

This is the relevant passage taken from Soundstage's website archives (edited very slightly by me for transposition here):

"A brief explanation of amplifier types, and their operational modes: There are two basic types of power amplifiers today, the Push-Pull and the Single Ended designs. These terms apply to the methods of operation used with the power output devices in order to achieve the final signal amplification. With the Single Ended variety, one transistor (or a group of transistors working together) is used to reproduce the entire output waveform. In the Push-Pull method, two separate transistors (or groups of transistors working together) are used to reconstruct the signal. One of each of the devices (or group of devices) is then responsible for only one half of the resultant output waveform. Because the majority of today’s audio amplifiers [are of the push-pull variety], I will limit the rest of the discussion to the methods, or classes, of operation for that particular design.

Picture a sine wave. Got it? Good. Now, envision a horizontal line running directly through that sine wave at exactly the vertical mid-point, now referred to as our null point. We now have the top one half of the waveform, or the crests, residing above the horizontal line. The bottom one half of the waveform, the troughs, reside below our imaginary null point, the horizontal line. In the Push-Pull design, one of the two transistors (or groups of transistors) is responsible for reproducing that top half of the waveform and the other, its complement, is responsible for reproducing the lower half of the waveform. In other words, one complement recreates the crests and the other the troughs. Which complement is on and which is off at any given time is determined by where the drive signal is, in relationship to that null point. Above the null point (our imaginary line running horizontally through the full sine wave), and the Push complement is in its duty cycle. Below our null point, and the Pull compliment is in its duty cycle. Now we can look at how the power for each half of the waveform is managed, which will describe its method, or class, of operation.

Class B amplifiers are typified by having one of the complements, responsible for only one half of the full output, turn on only during its duty cycle. It then turns completely off when it is done reproducing its half of the output and rests while the other complement runs through its duty cycle. When that second complement is finished, and the drive signal swings back past the null point into the first complement’s half of the waveform, it then switches back on again. In other words, each complement is only on when the audio drive signal requires it to be. It then switches completely off as soon as it is done with its half of the signal.

In class A amplifiers, both halves of the complement are on all the time. This means neither half of the Push-Pull complement ever shuts off nor throttles back from full current draw, even during the half of the cycle that it is at rest. This is achieved by the application of a forward bias current applied to both complements so that, even with no drive signal, each complement remains fully on. Because they have the same amount of current running through them at all times, under duty and at rest, they draw as much power at idle as they do at full volume!

Class A/B is simply a combination of the two previous methods. Each complement draws current during the entire cycle, just drawing slightly less during its rest half of the cycle. This prevents it from ever switching completely off, thereby providing a much faster turn on response than a class B device when called upon to deliver its half of the output.

If you were to take a close look at the resultant output signal from each class of operation, you could see why no one uses pure class B amplifiers for audio applications. There is a marked distortion, right in the middle (vertically) of the reproduced audio signal, as the complement called upon for duty takes a fraction of a second to 'switch' on. The brief period of time required for turn on causes a gross distortion in the waveform. The result is very amusical. Class A/B is a much better method, as each transistor is always slightly on. This prevents 'hard' on and off switching, smoothing out the 'notch' distortion from class B operation quite considerably. Most of today’s audio amplifiers operate in Class A/B. Finally, with class A operation, the devices are completely on all the time yielding the lowest switching distortion available from a Push-Pull design."

For completenesses sake, I'll add that single-ended amplifier designs (non-push-pull in other words, with only one - or one paralleled set - of output devices handling the entire waveform cycle full-time) are by definition always running in class-A mode of operation, the corollary of which is that class-A/B (and class-B) modes of operation apply only to non-single-ended (push-push or shared-duty-cycle) designs. Also that class-A/B designs, when running at low output powers, essentially operate as full class-A, the extent of which is determined by how much bias is applied to their output devices: the more bias employed, the higher in power the amp operates as class-A before merging into class-A/B as power increases (typically from just a watt or two up to several tens of watts in higher-biased designs). And a rule: the higher the bias setting (the more an amp is biased toward class-A), the more power the amp draws from the wall and dissipates as heat for equivalent rated output power (in other words the lower the amp's efficiency), so the greater the need for sufficient heat-sinking (in transistor amps) in order to keep the output devices operating within their safe temperature range (class-A runs hottest, being maximally efficient only at full output).

Hope this helps (and that I didn't f*** up my addendum!) - there's more on this subject in the archives as well.
Here is one archive description of amplifier classes:

What is Amplifier Class A? What is Class B? What is Class AB?
What is Class C? What is Class D?

All of these terms refer to the operating characteristics
of the output stages of amplifiers.

Briefly, Class A amps sound the best, cost the most, and are the
least practical. They waste power and return very clean signals.
Class AB amps dominate the market and rival the best Class A
amps in sound quality. They use less power than Class A,
and can be cheaper, smaller, cooler, and lighter. Class D amps
are only used for special applications like bass-guitar amps and
subwoofer amps. They are even smaller than Class AB amps and
more efficient, yet are often limited to under 10kHz (less than
full-range audio). Class B & Class C amps aren't used in audio.

In the following discussion, we will assume transistor output
stages, with one transistor per function. In some amplifiers,
the output devices are tubes. Most amps use more than one
transistor or tube per function in the output stage to increase
the power.

Class A refers to an output stage with bias current greater than
the maximum output current, so that all output transistors are
always conducting current. The biggest advantage of Class A
is that it is most linear, ie: has the lowest distortion.

The biggest disadvantage of Class A is that it is inefficient,
ie: it takes a very large Class A amplifier to deliver 50 watts,
and that amplifier uses lots of electricity and gets very hot.

Some high-end amplifiers are Class A, but true Class A only
accounts for perhaps 10% of the small high-end market and none
of the middle or lower-end market.

Class B amps have output stages which have zero idle bias
current. Typically, a Class B audio amplifier has zero bias
current in a very small part of the power cycle, to avoid
nonlinearities. Class B amplifiers have a significant advantage
over Class A in efficiency because they use almost no
electricity with small signals.

Class B amplifiers have a major disadvantage: very audible
distortion with small signals. This distortion can be so bad
that it is objectionable even with large signals. This
distortion is called crossover distortion, because it occurs at
the point when the output stage crosses between sourcing and
sinking current. There are almost no Class B amplifiers on the
market today.

Class C amplifiers are similar to Class B in that the output
stage has zero idle bias current. However, Class C amplifiers
have a region of zero idle current which is more than 50% of
the total supply voltage. The disadvantages of Class B
amplifiers are even more evident in Class C amplifiers, so
Class C is likewise not practical for audio amps.

Class A amplifiers often consist of a driven transistor
connected from output to positive power supply and a constant
current transistor connected from output to negative power
supply. The signal to the driven transistor modulates the
output voltage and the output current. With no input signal,
the constant bias current flows directly from the positive
supply to the negative supply, resulting in no output current,
yet lots of power consumed. More sophisticated Class A amps
have both transistors driven (in a push-pull fashion).

Class B amplifiers consist of a driven transistor connected
from output to positive power supply and another driven
transistor connected from output to negative power supply.
The signal drives one transistor on while the other is off,
so in a Class B amp, no power is wasted going from the
positive supply straight to the negative supply.

Class AB amplifiers are almost the same as Class B amplifiers
in that they have two driven transistors. However, Class
AB amplifiers differ from Class B amplifiers in that they
have a small idle current flowing from positive supply to
negative supply even when there is no input signal. This idle
current slightly increases power consumption, but does not
increase it anywhere near as much as Class A. This idle current
also corrects almost all of the nonlinearity associated with
crossover distortion. These amplifiers are called Class AB
rather than Class A because with large signals, they behave like
Class B amplifiers, but with small signals, they behave like
Class A amplifiers. Most amplifiers on the market are Class AB.

Some good amplifiers today use variations on the above themes.
For example, some "Class A" amplifiers have both transistors
driven, yet also have both transistors always on. A specific
example of this kind of amplifier is the "Stasis" (TM) amplifier
topology promoted by Threshold, and used in a few different
high-end amplifiers. Stasis (TM) amplifiers are indeed
Class A, but are not the same as a classic Class A amplifier.

Class D amplifiers use pulse modulation techniques to achieve
even higher efficiency than Class B amplifiers. As Class B
amplifiers used linear regulating transistors to modulate output
current and voltage, they could never be more efficient than
71%. Class D amplifiers use transistors that are either on or
off, and almost never in-between, so they waste the least amount
of power.

Obviously, then, Class D amplifiers are more efficient than
Class A, Class AB, or Class B. Some Class D amplifiers have
>80% efficiency at full power. Class D amplifiers can also have
low distortion, although not as good as Class AB or Class A.

Class D amplifiers are great for efficiency. However they are
awful for other reasons. It is essential that any Class D amp
be followed by a passive low-pass filter to remove switching
noise. This filter adds phase shift and distortion. It also
limits the high frequency performance of the amplifier, such
that Class D amplifiers rarely have good treble. The best
application today for Class D amplifiers is subwoofers.

To make a very good full range Class D amplifier, the switching
frequency must be well above 40kHz. Also, the amplifier must be
followed by a very good low-pass filter that will remove all of
the switching noise without causing power loss, phase-shift, or
distortion. Unfortunately, high switching frequency also means
significant switching power dissipation. It also means that the
chances of radiated noise (which might get into a tuner or phono
cartridge) is much higher.

Some people refer to Class E, G, and H. These are not as well
standardized as class A and B. However, Class E refers to an
amplifier with pulsed inputs and a tuned circuit output. This
is commonly used in radio transmitters where the output is at
a single or narrow band of frequencies. Class E is not used
for audio.

Class G refers to "rail switched" amplifiers which have two
different power supply voltages. The supply to the amplifier
is connected to the lower voltage for soft signals and the
higher voltage for loud signals. This gives more efficiency
without requiring switching output stages, so can sound better
than Class D amplifiers.

Class H refers to using a Class D or switching power supply
to drive the rails of a class AB or class A amplifier, so that
the amplifier has excellent efficiency yet has the sound of a
good class AB amplifier. Class H is very common in professional
audio power amplifiers.
Richingoth: I think that covers everthing except class-T (a catagory describing a kind of digital switching amp, and discretion being the better part of valor, I won't pretend to go there...) But please clarify for us, who authored that post originally? Yourself?
Thanks to you all for taking the time to educate me on this issue. I appreciate all of the informative feedback.