JBL Class D Training Workbook Service Manual

harman consumer group

SERVICE CENTER TRAINING GUIDE

Servicing Class D Amplifiers

And The Theory of Operation

Introduction to Class D Amplifiers

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I. A Brief Description of Other Audio Amplifier Classes

Class A

• Class A amplifiers are biased. The signal operates in the linear
region between cutoff and saturation.

• The output devices conduct continuously, the bias current
flows in the output devices at all times.

• In Class A operation, both devices are always on. There is
never a time when one output or another is turned off.

• Class A is the most inefficient of all power amplifier designs.

• Class A amplifiers are large, very heavy and run very hot. All
of this is due to the amplifier constantly operating at full power.

• Class A designs are the most linear, with the least amount of
distortion.

I. A Brief Description of Other Audio Amplifier Classes (cont’d.)

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Class B

• Class B operation is the opposite of Class A. Both
output devices are never allowed to be on at the same
time.

• The output devices have no bias (.6 volts is needed to
bias each device).

• Each output device is on for exactly one half of the time.
Class B designs have high efficiency but poor linearity;
this is due to extreme crossover distortion.

I. A Brief Description of Other Audio Amplifier Classes (cont’d.)

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Class A/B

• Class A/B is a combination of Class A and Class B.

• Class A/B operation allows both devices to be on at the
same time that the outputs have a bias voltage, so
current flows to the output devices more than half of the
time but less than the full time of the output wave form.

This is called bias, and bias eliminates extreme crossover
distortion.

I. A Brief Description of Other Audio Amplifier Classes (cont’d.)

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Class C
Class C is used in single frequency RF applications.

(Radio)

Class F
There are no existing products that use this class of

amplifier. (You could be the first!)

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I. A Brief Description of Other Audio Amplifier Classes (cont’d.)

Class G
In Class G operation two power supplies’ voltages are

used. A Class A/B amp is connected to a low voltage rail
and a diode transistor matrix. When the signal is greater
than the lower voltage rail can supply, a transistor switch
connects the output stage to a higher voltage rail. This
involves changing the power supply voltage from a lower
level to a higher level automatically when a larger output
swing is required for large signal peaks.

I. A Brief Description of Other Audio Amplifier Classes (cont’d.)

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Class H
The Class H design is like the Class G, except that the

power supply tracks the audio input signal. This is the
same as in the Bash power amps that we use in our
amplifiers.

II. What Is Class D Amplification?

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It’s a switched mode amplifier using “Pulse Width
Modulation,” or, PWM.

The output signal is proportional to the ratio of positive and
negative of the square wave. Then it is filtered by a low pass
filter to remove the high frequency content of
the square wave. If the duty cycle is 50% no output from the LC
filter will be produced.

If the positive portion of the square is greater than 50% then the
output will be a positive voltage from the LC network.
If you modulate the square wave the output will vary
proportionally to the modulated signal.