Amp classes

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In the past few days we’ve covered a lot of ground learning about the various amplifier classes like A, AB, B which are the main amplification classes, referring mostly to how the output stages of the amplifiers use power: efficiently or inefficiently. We also covered Complimentary, Push Pull and Single Ended output stages.

There are others, such as Class H and T: H is a Class AB output stage with a variable power supply that goes up or down in stages depending on the size of the input signal. Class T is a marketing term used by TriPath to differentiate their Class D amps which have some unique feedback characteristics. I am sure there are more, but probably none very interesting.

Perhaps the most interesting of them all is Class D, sometimes known as a Digital Amplifier. As I have mentioned before, Class D is not digital it is, in fact, analog but in a very different form.

Traditional power amplifiers take a small input signal, make it bigger and then provide enough horsepower to drive your loudspeaker. To do this they are designed in two successive stages: the input voltage stage and the output current stage. The first stage of a power amplifier takes a small input signal and amplifies it typically 30 times: put one volt of music in and you get 30 volts of music out. Simple.

The second stage of a power amplifier takes the large voltage output of the first stage and adds current capabilities so that large voltage has enough power to drive a loudspeaker. Put 30 volts into this stage and get 30 volts out – the difference is this second stage can also deliver power – measured in watts, to the loudspeaker.

In a Class D amplifier we also have two stages, but the first stage is very different than a traditional power amplifier as is the second stage. In a very weird scenario the input stage compares the music to a reference signal, unrelated to the input, then feeds that information to the output stage which then connects all the watts in the amp’s power supply up to the speaker – for varying amounts of time. This process is called PWM or Pulse Width Modulation.

Tomorrow we will make this strange and wonderful process make sense for you.