Class A

What is class A?

In the BSC Audio Classic and 5 Series power amplifiers, the output stage is comprised of complimentary Bipolar Junction Transistors (BJT) utilized in a push-pull configuration biased in Class A. For the purposes of this over simplified explanation, picture a transistor as a valve. When a signal is applied to one part of the transistor the valve is turned On, allowing current to flow. When there is no signal, the valve is Off. When the transistor switches from Off to On, there are effects on the signal flowing through the valve. In a Class A amplifier, 100% of the input signal conducts or flows through the valve at all times. This minimizes and effectively eliminates the effects on the signal as it passes through the transistor. In comparison, a Class B amplifier allows 50% of the transistor to conduct or flow. A Class A/B amplifier is an intermediate class between A and B, so the transistor is more than 50% open but less than 100% open. Even in the case of a High Bias Class A/B design, there are still harmonic and distortion effects on the signal as it is not fully open, albeit these effects are minimized. Because these transitions between on and off states exist, the alternate methods to class A designs lose a certain degree of sonic performance albeit more efficient and consequently cooler operating. Class A operation is extremely inefficient requiring high power and casting off as much as 85% of its generated power in heat, but despite these shortcoming it stands as the standard for sonic excellence without peer. BSC Audio has gone to great lengths to combat the negative aspects of class A operation with clever but effective engineering.

Are sliding bias or time determinant sliding bias schemes the same as Class A?

At BSC we believe the answer is no. There are several extremely clever methods of ramping up the output stage bias current to meet the demands of the incoming audio signal, some of which were patented and are genuinely effective at reducing heat output. But are they Class A? It is our contention that the true benefit of Class A is the lack of current transition, or leaving the transistor in a more consistent state. When the transistor is turned on and off there is a settling time between states. During this settling time, in our opinion, the transistor is not at its best from a sonic stand point. In effect, any time you interrupt the constant or near constant current flow, the sound changes. So even if the output stage is technically biased in Class A during a specific period of time, the effects are minimally apparent because of the transition from off to on or from 50% on to 80% etc., and in fact probably perform better left without the bias scheme. That being said, if an amplifier doesn't have extremely large heat sinks mounted internally or externally, an over-sized power supply, and isn't basically hot all of the time, the current isn't constant and it doesn't benefit from Class A biasing.

Is class A more powerful?

The short answer is typically yes, but not in the way you might think and some qualification is required.

Class A amplifiers require a significantly more robust design when compared to their Class A/B or D counterparts. Example, a 50 Watt Class A/B amplifier might only require a 100 Watt power supply to provide the peak power required. In the case of a class A amplifier, that same 50 Watt amplifier would require a power supply of approximately 200-300 Watts to allow constant full power output. Not only would the power supply be larger, but the ability to dissipate heat must increase as does the power capacity of the internal components. Class A amplifiers often rival class A/B counterparts 2 to 3 times their 8-ohm power rating because, in essence, they are bigger amplifiers. Class A does not, however, create power magically above its 8-ohm rating, and in some cases, the higher wattage rating into an 8-ohm load from an A/B counterpart could be a better match in specific applications.

Why don't manufacturers use more class A designs?

Class A designs are historically recognized as top performers in their respective categories, however, Class A products present some distinct challenges - mostly in relation to size, continuous energy consumption, and heat.

Because the output circuitry in a class A amplifier is essentially on “full power” all of the time, the unit is hot to the touch and draws a considerable amount of continuous energy from the wall outlet. Sliding bias schemes as mentioned above help in heat reduction and constant current draw, but are not reaping all the benefits to Class A. Reducing the size means the product will get even hotter, or will require the use of noisy, unreliable fans to aid in adequate cooling. This, along with the additional power supply demands, make building Class A products an expensive proposition.

Perhaps more important to the design of any audio product is experience. There are no direct courses in audio design, it is a separate side deviation to traditional electrical engineering. Creating an audio circuit is actually relatively simple for an experienced engineer, but to build a circuit or product that contains the key elements in sound reproduction we covet as listeners is a very different prospect.