How much distortion is too much?

[dead rat]

I have simulated my class d amplifier in PSPICE.

20kHz sin signal (yellow) is converted to PWM with:

Green: 400kHz triangular wave.
Red: 200kHz triangular wave.
Purple: 100kHz triangular wave.

excuse my ignorance, but how do i find out how much distortion the sine wave can take before it sounds really crappy?


I would like to know this before i spend hours bringing my amplifier into the real world.

[BWRX]

The answer to your question will depend on who you ask, but typically a good THD figure to shoot for is 0.1% or lower at reasonable power levels. How much you can tolerate depends on your ears and whether or not you're trained to listen for it. Obviously, the speaker system used will have a very large part (more so than any decent amp) in this as well.

[dead rat]

Is there any equation/tool i can put my faith in, to help me pick the optimal PWM frequency?

Or is this more like trial and error?

[Eva]

Residual components above 20Khz are not important, they are usually ignored for THD measurement. You should focus on components below 20Khz. This requires a test frequency below 5Khz or 10Khz.

[Gyula]

Residual components above 20Khz are not important, they are usually ignored for THD measurement. You should focus on components below 20Khz.

As long as your speakers are linear. But nothing is linear in reality. The sidebands around the carrier harmonics are also infinite, so they can be above the noise floor in the audible range. It depends on the carrier and the audio frequency.