Pure Class A

When Class-A amplification is mentioned, what is your understanding? Most likely you will associate the term with better sound. Many of the most musical amplifiers are Pure Class A, and with very good reason. Class A is a mode of operation wherein the amplifier conducts signal over the entire cycle of a musical waveform. It is smooth and continuous, perfectly analogue, and follows the music exactly. It is completely true to the original sound. It is the purest form of amplification. “Pure Class A” refers to an amplifier that runs in Class A right up to its rated maximum power.

So why don’t all amplifiers use Class A? Many amplifiers operate in Class B, or even Class D. Surely if Class A is better it would be universally used? Well the answer is generally either a marketing or economic one. Either the manufacturer wants to boast very high power, or the company accountants have insisted that the manufacturing cost must be as low as possible. Each of these options will likely count out Class A.

In order to realise its special benefit, Class A places additional demands on the product design, demands that also happen to be good for your music. To achieve the perfection that is possible, some additional heat will be generated. This requires more investment in the chassis structure, it has to be more substantially built, and has to have more efficient heatsinking. The higher build quality will usually result in a chassis structure that better supports a good sounding product. The power supply will see a constant higher level of current drain in Class A mode. This is good for your music too, but means the power supply design must be more thoroughly engineered with higher quality parts. Overall the design is more expensive to produce, but this additional expense contributes directly to long term reliability and the desired the end result: Your enjoyment of music.

Most amplifiers in our market operate in Class B. Class B is a mode of amplification that uses one device to amplify the positive part of the waveform, and another device to amplify the negative part. Because there is much less current flowing in these devices than in Class A there is a handover between the two halves at about zero volts. One device runs out of current and the other takes over. This creates a discontinuity in the waveform – distortion in other words. There are techniques used to overcome this but Class A does it better by running much higher current and completely avoiding the problem. The only advantage Class B has is that it runs cooler when not making music.

What about Class D? This is the mode some of the newer products operate in. The output is switched rapidly from a positive voltage to a negative one, with the time spent at each limit being proportional to the amplitude of the music at any given moment. This is highly efficient (very little heat is generated) and allows low cost manufacturing to product high powered products. However there are many pitfalls in the process of converting your original analogue music signal to a rapidly switching high power digital one. Substantial filtering is required to remove the switching residue. Achieving the very best musical result is very difficult.

What drives these choices in the marketplace? Generally the answer is marketing. Our industry’s marketing people love to find ways to make products more appealing to consumers, which is why we have seen a never-ending race for higher power. High power is better!! But Class A is a great buzz word too and so you also see this creep into the marketing material. High power and Class A, what could be better? The truth is that high power and Class A are pretty much mutually exclusive. It is reasonably easy to achieve Class A in single chassis designs up to about 100 watts but beyond that it becomes extremely difficult because of the heat.

A number of “tricks” have been used to claim both high power and Pure Class A. While it is perfectly OK to bias an amplifier into partial Class A while allowing additional power for short term transient headroom often this is done without declaring the true operating parameters. Commonly the amplifier is claimed to be “Pure” Class A, and yet only delivers Class A performance up to a percentage of its output power. Other schemes have been invented that cause the amplifier to vary its Class A operation depending on how loud the music is. The idea being that it only runs in Class A to as high a power as needed moment to moment, and thence avoids the heat. This causes the music to modulate the amplifier’s design parameters, and in turn introduces other forms of distortion.

How do you tell determine the truth? Its easy really, just ask what the amplifier’s Class A power consumption is when there is no music playing. Take the number you are given (Watts) and divide by two for a mono amplifier, or four for a stereo amplifier. The number you get is the maximum power that the amplifier could possibly deliver in Pure Class A without some trickery.

It is informative to consider the amount of heat generated by a Pure Class A amplifier. Typically the power consumption per channel in Pure Class A is at least twice the rated output power of the amplifier. A 200 Watt per channel stereo Class A power amplifier must get rid of 800 watts of heat just sitting idle. And it must do this safely. The temperature of any exposed heatsink surfaces should ideally be less than 55 degrees C. 800 Watts of heat is close to that generated by a 1kW heater. Think of an oil filled convector heater and how large that needs to be to safely dissipate this amount of heat. High power Pure Class A would mean extremely  large amplifiers.

It’s because of this heat that all Pure Class A amplifiers of reasonable domestic size have been and are of lower power. Typically 20-80 Watts output power per channel into 8 Ohms. This may seem to be too little power, but rest assured, despite what the marketing people have persuaded us to believe, high power is not needed in most homes.

One last note on Class A. Unless there is something very unusual about the design, all preamplifiers operate in Class A. At the low powers found in preamplifiers there is no heat to deal with, and no reason for the design to operate in any other mode than Class A. When you see a manufacturer proudly proclaim their preamplifier operates in Class A ask yourself why they feel the need to say so. Are they relying more on hype than honest truth to sell their products?

We encourage you to try our Reference monoblock power amplifiers. Experience the purity of sound a true Class A product can deliver. May the music rule!