Audio Precision AP525 and cheap 24-bit/48 kHz ADC test

I have access to an Audio Precision AP525 Audio Analyzer. I don't have much experience with yet, so in order to get more familiar with the instrument I have tested a 24-bit/48 kHz ADC bought on Ebay for €9.
I have tested/looked at:
  • spectrum
  • Frequency response
  • THD+N frequency sweep
  • Jitter

The results are attached to this post. All comments are welcome, both to the the instrument and my setup and measurements. Hopefully, in the future I will use the instrument to contribute to the diyaudio forum.


  • AP525_tests_part1.pdf
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  • AP525_tests_part2.pdf
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You plot THD+N...does the AP do THD only measurement ?.


Good question. I have tried to see what the user manual says and found that it can measure THD ratio and THD level.

This is what the manual says(page 220):

THD ratio
"The THD Ratio result is a continuous sweep measurement that provides a graphical display of the harmonic distortion response of each channel. In this result the ratio of the level of the THD (total harmonic distortion) to the fundamental in the DUT output is plotted against frequency."

THD level
"The THD Level result is a continuous sweep measurement that provides a graphical display of the harmonic distortion response of each channel. In this result the level of the THD (total harmonic distortion) in the DUT output is plotted against frequency."

More about THD
"THD is an abbreviation for Total Harmonic Distortion. As above, this refers to the sum of all the harmonic distortion products in a measurement. THD without the noise was not easily done until the advent of FFT measurement techniques. The APx500 implementation measures only the bins that contain harmonic distortion products, producing a result that is not influenced by DUT noise or spurious interfering signals. In a very low-noise system, the THD result will be the same as the THD+N result. The FFT method that provides THD results also brings the capability to selectively measure discrete harmonic distortion products"

I guess the THD level is the most interesting here?


Paid Member
2004-03-24 5:16 am
SF Bay Area
A THD measurement won't have IM products, by definition unless there is really bad hum or other junk. There is a measurement called spectral contamination which really stresses a device or speaker and all the harmonic and IM products are crafted to miss other tones. You can see a lot with it. I have some spreadsheets or lists somewhere with lists of tones that meet these requirements.


Paid Member
2004-03-24 5:16 am
SF Bay Area
I would use a max resolution FFT and a window that gives good resolution for low level stuff Not flat top for example. For acoustic measuremnts you would like to do some type of coherent averaging. I don't know ARTA well enough to give advice. The ccoherent averaging will seperate random noise from the stuff that is a real effect of the motion. It can take a lot of measurements averaged, possibly 10 to 20 minutes, to get it clean. With experience you can see things like the diaphragm creaking as it flexes. I have used it a little and expect to do a lot on an upcoming project.
Here is the spreadsheet. I made some files with those tones, again somewhere.

I'm not sure how involved programming an AP525 is but this should not be too hard.

I have looked at the spreadsheet and i must say that I don't know how to interpret or read it. Can you give me an introduction or simple explanation of what the test does and how it is or should be done.
Im traveling so this will be brief and may be confusing.
Whatyou do is generate a test signal with a set of tones from the list. Then do an fft of the outout of the dut. Look for im products between the tones. Ideally there should be none.

Sent from my LG-H811 using Tapatalk

Thank you.
I see lists of frequency components, but it says nothing about their amplitude. Does it mean that I'm free to set the amplitude? If yes I assume it's best to have the same amplitude for all frequency components.
I have now done some testing. The AP525 has something called Multitone Analyzer where I can test the DUT with multiple frequency signals. I have chosen the 6 tone signal because it is the simplest one. I have little/no experience with reading this kind of data, so comments are welcome :)

I have also tried to measure noise density spectrum, THD ratio, THD level and impulse response.

The results are attached.


  • Measurements_01.pdf
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Same amplitue for all the components. Check levels with a scope. The crest factor is pretty high.

Sent from my LG-H811 using Tapatalk

AP software has such multi-tone collections build-in. You can adjust the crest factor and it will adjust frequency and phase of each individual tone so that max resolution is obtained and all harmonics and IMD fall in separate bins. It even sets aside some bins that will stay empty and can be used to measure noise in the presence of the signal.

Look up 'multitone testing'; the test signal is actually an ISO standard set and is often named ISO44 for instance for the test tone set for 44.1 sampling etc.

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