Why these harmonics always present? I am using Audio Precision 2700 to test some test vectors.
I have connected resistor banks as class - d power amp's load, and tap from this resistor using a XLR cable to the AP.
When my power amp is powered up, but no song is playing(song is paused, not muted) i checked fft spectrum of the input channel , i get the following spectrum

You can see the three peaks in frequency above 4k. (Likely, the 3 peaks in frontier band upto 200 hz). Which causes this harmonics?
The test vector is playing at 48khz.
Note : There's a passive filter after the power amp's output having cut off at 30khz.
Please point me some insight. Does the passive filter aliases the input signal, so that i get undesired signal at folded frequency?[This is my initial doubt, because this is what i believe should be the cause, if it's not then some one please tell me why this couldn't be the problem]
Thanks folks in advance...
If your class-D amplifier's switching frequency is 5 kHz above a multiple of the sample rate of your audio analyzer, then the aliases of the attenuated harmonics of the switching frequency will march upwards in frequency by 5 kHz across (and around) the spectrum displayed by the analyzer, until they disappear below the noise floor of the system and analyzer.