The Amplifier Classes

The amplifier class is a classification of the configuration and operation for output stages. The amplifier class is determined by way elements are connected (also known as topology) and by the electrical parameters (such as the bias value).

A-Class, single-ended

The single-ended A-class output stage is made of one tube (or more in parallel) which connects to an output transformer. This tube drives the transformer, the current flowing through it never cancels, and this is what defines the A-class. Distortions mainly depend on the tube type. They are mainly even for a triode, and odd for a pentode. The maximal theorical efficiency is about 25% with a triode and 50% with a pentode. But these are only achieved for the maximal output power.
Any noise on the high voltage supply is directly transferred to the output. The current taken from the power supply is not steady and varies accordingly to the amplified signal. These factors imply a way more clan and powerful power supply than with a AB class. This topology is adapted to low power amplifiers only.

  1. Vacuum tubeOne or more in parallel, handle the full signal.

  2. Output transformerSpecial transformer with only two bobbins : a primary and a secondary

  3. Loud speaker

  4. Tube's current

  5. Speaker's current

  6. Tube's current curveThe current never reached zero. The offset is the bias.

  7. Speaker's current curveThe speaker current is centered in zero, there is no bias in the speaker thanks to the transformer

  8. Tube bias valueThe bias value is the mean current when idle, and is also the offset current in A-class

A-Class, push-pull

Analogously, a push-pull topology where neither current of the two tubes cancels is working in A-class. The two tubes gives the output power and their currents never cancels. To achieve that, the mean current, which is the bias, is higher than that in an AB-class topology.
The currents flowing through the two tubes are analogs but in opposition. In the output transformer, the two primary windings (connected to the tubes) are identical and connected in such a way that alternating currents flowing through them gives analogous magnetic fields. Thus, the current induced in the secondary winding (connected to the speaker) is created by the sum of the two alternating currents in the primary windings.
Thanks to the symmetry in the transformer’s windings, even harmonics are greatly reduced, whereas odd harmonics adds each other.
Noise and hum from the power supply is not transferred through the output since the magnetic field they produce in the transformer cancels. This is a great advantage over the single-ended A-class leading to fewer constraints to the power supply.
The topology efficiency stays the same as for the A-class, 25 percent with a triode and 50% with a pentode.

  1. Right vacuum tubeThe push-pull needs two tubes.

  2. Left vacuum tube

  3. Left tube's currentEach tubes handle full parts of the signal

  4. Right tube's current

  5. Supply currentThe supply current is the sum of left and right tube's current. Without saturation it is almost constant.

  6. Output transformerSpecial transformer with three bobbins : a primary for each tube and a secondary

  7. Left tube's current curveWe can see the two currents are opposite and handle full part of the signal

  8. Right tube's current curve

  9. Speaker's current curve

  10. Tube bias valueThe bias value is the mean current when idle, and is also the offset current in A-class

B-Class, push-pull

In this push-pull configuration, there is no current in the tubes when there is no signal. When a signal is applied, the current flows through one tube only at a time, for a half period. The two currents in the tubes gives the full period.
Because the switching between the two tubes is not perfect, the outputs signal is not a perfect replica of the input signal near zero. This leads to a distortion named crossover distortions. This is the major drawback. Another drawback is that this topology is very sensitive to supply hum and noise, because they do not cancels in the transformer, thus passing to the output. The constraints imposed to the power supply are higher than that of A-Class, due to the high variations of the supply current, swinging between zero and maximal value. Theorical efficiency is identical for triodes and pentodes, and is 78%, which is quite high.
This topology is not adequate to high-fidelity audio.

  1. Left tube's currentEach tubes does not handle full parts of the signal. The current stays at null value for a certain amount of time

  2. Right tube's current

  3. Speaker's current curveThe speaker current is not perfect

  4. Cross-over distortionAt one point, the current in the two tubes is null, this creates a lack of output current.

AB-Class, push-pull

The difference between AB-Class and A-Class amplifiers is only the mean value of current in the tubes (bias). It is higher than that of B class, but lower than that of A-Class. Depending on the bias value, it is commonly classified with AB1-Class where current is relatively low, and AB2-Class where current is relatively high. With a low output power, it works as an A-Class amplifier because there is always a current in each tube. When output power is high enough, one of the tube’s current (depending on the polarity of the input signal) reaches a null value. Thanks to the global feedback, the current in the other tube increases more to compensate and the output signal is still clean. Then, the amplifier works in A-class, and switches through B-Class smoothly when the input signal is high enough. The distortion is by far lower in AB-Class than in B-Class, specifically for small signals.
The maximum efficiency is 78% which is high.

  1. Zero currentA part of the signal cannot be handled by the tube because the current reaches a null value.

  2. DistortionThe output only gets the power of one tube, because the other have reach zero current. This creates distortion in the output signal.

  3. Tube biasBecause the current in tubes reaches zero, the bias is not logner equal to the mean current value.

  4. With feedbackThe global feedback makes one tube to compensate for the zero current in the other tube, this gives good output signal.

  5. Small signalWhen signal is small, output power is low, no current reaches zero, this works as A-class, there is no distortions.

Final remarks

Single-Ended topologies are reserved to low power amplifiers and imposes severe constraints on the power supply, whereas Push-Pull is more tolerant and more powerful. The AB-Class combines multiple advantages, offering great power, good efficiency and sound quality. The A-Class is also appreciated for his musical qualities. Generally speaking, pentodes are more powerful than triodes, and doesn’t sound the same.
With the The Smart Bias technology Monange amplifier’s gives you the chance to switch between A and AB class, and tune the bias hottest or coldest.