Class Dismissed

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Class Dismissed

When I was attending school there were no sweeter words than the headline of today’s post. Free at last.

Let me start today’s topic with an apology. Because I disliked school so much my math skills suck. Badly. I tend to get confused around percentages especially when they work in reverse.

Armed with the correct info, let’s review: if a 100-watt amplifier is 50% efficient, it draws 200 watts from the wall and delivers 100 watts to the load. Half of its energy is converted to heat. (I had mistakenly said 50 watts would be converted to heat). Thanks to our ever-helpful eagle-eyed community for correcting me.

According to Wikipedia, the classes of amplifiers are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period. A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse width modulation output is obtained from the stage.

What’s a valuable takeaway from the above droll few sentences is this: a Class B amplifier only draws power from the AC wall socket when a signal is present, where a Class A amplifier is drawing wall power “through all the period of the signal”, including the zero-crossing point where, technically, there is no signal.

Keep that thought in your head as tomorrow we’ll come back to that.

There is a hybrid amplifier we’re all familiar with. This topology shares traits from both Class A and Class B and is appropriately named, Class A/B.

In a Class A/B amp when there is no signal there are still a few watts of power being drawn from the wall. This is because the A part of the Class A/B means the amp is always on—at least a little. This always-on time is called bias, a technique of applying just enough always-on power that we’re not relying upon the application of an audio signal to get things started (eliminating a type of distortion known as crossover notch). The amount of that always-on bias varies from amp design to design. In some amps, like the BHK series, it’s fairly high, which generates a fair amount of constant heat regardless of whether or not a signal is present. In other designs, there’s just enough always-on bias to keep the amp warm to the touch.

The only time the heat sinks of a class A/B amp get good and toasty is when it’s been working out delivering loud music to hungry speakers. That’s the opposite of what happens with a Class A amplifier.

Tomorrow, the strange world of pure Class A.

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Paul McGowan

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