Class warfare

Join Our Community Subscribe to Paul's Posts

First off, we’ve published a new videohttps://vimeo.com/55572974 which is a behind the scenes look at what goes into building a PerfectWave product. It’s pretty interesting if you’ve never seen what it takes to make a modern piece of audio equipment.

It’s probably not politically correct to write about different classes of people, but I trust it’s ok to broach the subject when it comes to power amplifiers.

You’ve no doubt read about amplifiers classes such as A, B, AB, C , D, G and H. What’s a little confusing about these classes is they’re not all referring to the same thing. Classes A through C refer to how the output stage of the amplifier is biased and D, G and H refer to different schemes of producing audio than what’s found in A through C.

So today let’s start with the first, class A. The simplest test for a true class A amplifier is to measure the amount of power it draws from the wall: that power never varying regardless of how loud the amp plays. There are numerous class A wannabe’s that have varying power draws from the wall, but if they cannot pass this basic test then they simply cannot be called class A.

In a “normal” power amplifier, the input power to the amp from the wall socket goes up when the power amp delivers more power to the speaker. This is sort of logical but that logic cannot be applied to class A designs.

Here’s another interesting difference: class A power amplifiers produce the lowest amount of heat when they are playing at the loudest level. This is exactly the opposite behavior of any other power amplifier I can think of.

So here’s what’s happening. In simple terms a 100 watt class A power amplifier draws a constant 200 watts out of the wall socket. The wattage is used in the power amp’s output stage to keep the output devices on constantly. All that power is serving only one purpose: heating up the heat sinks of the power amp.

Indeed when a 100 watt class A power amplifier is sitting on your shelf idling without any music playing through it, it is nothing more than an expensive heater – the output transistors spending all their energy on heating up the heat sinks – so you’re generating 200 watts of heat and nothing else.

When you put a signal into the amp’s input, as much as half of the 200 watts of energy is sent to the loudspeaker instead of being converted to heat. So just imagine you put in a steady tone that was the maximum loudness the amp could play at – the result would be 100 watts of energy is being delivered to the loudspeaker and 100 watts of energy is being used to heat the heat sinks of the amp. This is why when a class A amplifier plays loudly, its penchant for generating heat goes down.

Why, you might ask, is this a good thing? It certainly isn’t to improve efficiency and without question it’s a power hog.

The answer lies in the way transistors and devices operate. Without getting too technical, devices such as transistors like being on – and if they never turn off – they provide lower distortion outputs and require less help from mechanisms like feedback to correct for distortion and non-linearities.

In general they sound very open, sweet and free of compression and change in character with dynamics: traits we all prize in our systems. But their costs are high and we can achieve very similar results in more efficient manners.

Tomorrow we’ll look at class B.