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AUDIO CLASS-D POWER AMPLIFIER

Status: Proposal
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July 3, 2016, Latest update: November 3, 2016
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A simple, single-channel, low-cost, high-efficiency, high-performance audio class-D power amplifier I designed and constructed some years ago for experimentation and personal domestic use.

Most of the components can be SMD, except for connectors, large capacitors and perhaps power inductors. 

A differential to single-ended input buffer allows both balanced and ground referenced audio inputs.

The fully differential configuration of the PWM modulator, the MOSFET power stage and the feedback network, produces low distortion (particularly on even harmonics) and simplifies supply to a single 6 to 24Vdc (3A minimum). Supply can be obtained from a simple mains transformer + rectifier bridge. The rest of supply voltages are generated internally.

Based on a simple self-oscillating architecture, the modulator (an ultrafast comparator) varies not only the duty-cycle of the PWM signal but also its frequency in response to output voltage error, in order to maintain this error below a fixed limit. The varying frequency helps to keep radiated EMI below acceptable limits.

The default gain is 10, but can be modified changing R11 & R21.

Two prototypes have been working for years without problems as a stereo PC audio amplifier.

As a future improvement and in order to get a fully differential signal path implementation, the input buffer could be replaced by a differential input/output device.

Supply: single 6 to 24Vdc
Input impedance: 22kohm
Bandwidth (-3dB): 7Hz - 70kHz
Default gain: 10
Max. power on 8 ohm speaker (4.4Vpp sinewave input): 30W
 
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