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I am using this class D amplifier SSM2356 in my design.

I am giving same input to the audio input left and right. I play a audio and probe the supply, input audio and the audio output signal (both left and right) at the speaker. Here is the waveform:

Amplifier waveform

Strangely the output is a PWM signal with some ringing, but I believe it should have been amplified replica of the input signal.

Please suggest if my methodology of the audio measurement is correct? I would also like to get suggestions on reducing the ripple on power supply.

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Uh.... have you read the datasheet you link? The output plots for that chip look pretty much like what you're getting. –  Connor Wolf May 25 at 15:28

2 Answers 2

No, this is a "filterless" amplifier that relies on the electromechanical filtering of the speaker itself to produce the correct acoustic waveform from the digital output signal you see on the scope.

Unlike most class-D amplifiers that use PWM to drive the output stage, this one uses delta-sigma modulation. This eliminates the strong tone that occurs at the PWM frequency with conventional amplifiers that needs to be filtered out by additional passive components.

Your power supply needs to be able to provide the transient currents required by the audio signal; you basically need more bulk capacitance close to the amplifier chip(s).

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DISCLAIMER: this is not an answer to your question (as i think Dave was absolutely right), but an idea for a little experiment that might improve sound quality whilst being nicer to your power supply. It's completely based on a hunch, but since it's a matter of crossing a few wires and it can do no harm to your circuit, i thought i'd share.

When feeding amplifiers with two (almost) exactly the same inputs, reversing one of the inputs' polarity (and then reversing it again at the output) could improve stability.

TLDR:

  • cross wires on one of the inputs
  • cross wires on one of the outputs
  • use 'scope. Better sound or stability? Yay!
  • no improvement? Grumble and weep for the loss of 45 precious seconds of your life.
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