THD+N measurements

Hello All,
I posted this in Rec.Audio.Tech as well, and have received some excellent replies. I would like to see what other replies I may get
to see if I can generate a concensus.
My question deals with the validity of my test setup and if THD+N is actually being measured. The setup deals with a DUT configured as a SE ended amplifier, BTL output. The input of the DUT is stimulated with a 1V@1kHz signal. The power supply rail is actually from a power amp superimposing a 100mVp@20Hz to 20 kHz ripple voltage on a 5Vdc signal. The DUT is being measured by an Audio Precision 2700, and the AP is setup to measure THD+N.
Would the ripple voltage on the rail be considered 'noise' in the THD +N measurement; therefore, making this a valid THD+N measurement?
Thanks in advance.
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In article

If you are trying to measure the effect on the output, of ripple on the power supply rail, it is neither distortion or noise but power supply rejection ratio. You will need a tuneable measuring device tuned to the same frequency you are injecting on the power supply to measure it.
If you are trying to measure THD&N you should use the power supply the amplifier is to be normally used with, otherwise your measurements will have no meaning - or only relative meaning within your lab if you use the same PSU to test all your amplifiers. Yes, some of the noise on the power supply rails will find its way to the output and, depending on your measuring equipment, be lumped in with the noise generated by the amplifier components themselves. Switchable notch filters before your measuring equipment, at power supply frequency and harmonics, will allow you to measure the effect of this contribution.
--
Stuart Winsor

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Kingcosmos schrieb:

The THD+N measurement is often accomplished by filtering the base wave of the input signal through a notch filter and in building the ratio of the remaining level and the origininal input level. The result contains all signal components found in addition to the base wave. The mentioned modulation of the supply rail will be found as well in the THD+N measurement as long as the base frequency is apart from the notch filter frequency. Another way which is often less accurate is digitizing, FFTing and ignoring the base wave frequency. The sum of all remaining spectral parts could be the THD+N...
- Udo
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