Measuring THD+N in audio amplifier

[uoficowboy]

Hi - I'm working on an audio amplifier circuit and would really like to measure the THD+N of it. I know Audio Precision devices are one of the standard ways of making this measurement, but can anybody suggest a method of doing so using more common equipment? Has anybody had success with using a computer's sound card to do it, perhaps?

Thank you!

 

[Audioguru]

You feed the input of the amplifier a sinewave at certain frequency then adjust its level so that the output of the amplifier is almost clipping. Write down the output level.
Then you use a narrow-band notch filter to remove the certain frequency from the output of the amplifier and you measure and write down the level of everything remaining which is the THD+N. Use arithmatic to find the percentage.

 

[uoficowboy]

You feed the input of the amplifier a sinewave at certain frequency then adjust its level so that the output of the amplifier is almost clipping. Write down the output level.
Then you use a narrow-band notch filter to remove the certain frequency from the output of the amplifier and you measure and write down the level of everything remaining which is the THD+N. Use arithmatic to find the percentage.

Hi - thanks for the response! I'm using a Class D so it's tricky to just look for it to be clipping (unless you know something I don't). I had previously (when using a class AB) been using a scopes FFT function to compare the 1KHz output (as I was testing at 1KHz) with other harmonics that popped up as typically they'd be pretty sharp - but with the class D the harmonics are much more broad and I'm not so sure how to calculate the THD.

 

[Audioguru]

I have never played with a class-D amplifier so I don't know what happens when the output level reaches "clipping".