difference between D and AB amps

[PSturmer 10+ year member]

D amps are more efficient than AB amps. so if they are two amps, one AB and one D, both put out 1000 watts rms. the AB amps draws 100 amps of current and the D draws 80 amps. what happens to the extra 20 amps? is it dissipated in the heat sink or is it returned to the ground?

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[helotaxi 5,000+ posts]

The difference in efficiency is going to be power given off as heat. The Class A/B will have more residual heat to get rid of.

 

[thch 10+ year member]

the issue is in implementation.

Power is Voltage * Current.

in class A/B, if you want to have 50% output voltage, then the speaker see's 50% of the DC "rail" voltage, and the output transistor see's the other 50%. the current that flows through the speaker also flows through the output transistor. so the amp is dissipating an equal amount of power as the speaker. Since audio is rapidly changing, the output is rarely near the highly efficient peaks (eg, 100% on the speaker, 0% on the output device), and thus the efficiency is low.

the class D amp acheives 50% output voltage but switching the output device to an on state for 50% of the time. the switch has two states -- fully on (high current, no voltage) and fully off (high voltage, no current). in both states there is minimal power loss.

For this reason, class D tends to have high efficiency over a wide range of power output levels, while class AB does not.