What is the difference between class A and class A/B?

I am writing this article to shed some light on the difference between Class A and Class AB amplifiers. While Class A does an outstanding job of providing superior performance, we have found that in some situations Class AB fits best. At Summit Audio, years of experience guide us to employ both classes depending upon which works best given the scenario.

Class A amplifiers were the first to be developed with the vacuum tube. Class AB tube circuits were later developed for higher output. These first AB tube circuit designs could perform well, however fidelity was sometimes limited by the input and output transformers required. The slightest imbalance in transformer windings meant distortion. Early attempts at push-pull designs not only used those transformers, but were also shackled by the lack of bias and thermal stability with germanium transistors.

The early push-pull transistor amplifiers were poor and caused audio professionals to look down upon all transistor electronics until the advent of stable silicon transistors used in class A signal amplifiers. This is when good fidelity solid-state designs made their first appearance.
Transistors then became accepted in preamp and other signal circuit design. Class A still had advantages over Class AB amplifiers because the lack of complimentary polarity transistors meant the use of a driver transformer or phase inverter. With both tubes and transistors, Class A still had the advantage.
Over time, improvements in transistors continued such as true complimentary pairs, low noise, high gain, and wide bandwidth. These improvements were stronger for discrete transistors than for integrated circuits. Well-designed class AB solid-state can now not only meet the performance of class A, it can excel in some specifications. When it comes to tubes, class A still results in the simplest circuits with the lowest distortion, since no practical way has been devised to do away with the need for phase inversion needed for push-pull operation.

At Summit Audio we employ Class AB discrete solid-state signal amplifiers where the advantages do the most good, in low noise balanced input stages and robust, high slew rate output stages. Careful selection of state of the art transistors and attention to detail are used for gain stages to provide that classic tube warmth. Since the tube stages in Summit designs only see constant high impedance loads, the slew rate of these stages tends to be symmetrical. Some Summit products have both solid state and tube gain stage paths, allowing the user to choose between an all solid-state path or passing through tubes, or to use both at the same time.

In conclusion, Class A operation is not the only way to get superior performance from transistor signal amplifiers, in fact; some specifications are better with class AB. With tubes, class A is still the most practical way to get tube warmth without excessive distortion. With Summit products, we use the right topology for the right job.