Testing the 1176 for THD

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compresearch

Member
Joined
Nov 9, 2014
Messages
9
How best should i test the 1176 for THD? I am thinking of setting the input at noon, sending a 0dBu signal and adjusting the output to read 0VU and then making measurements from there in 2dB increments up to a limit of around +24dBu. At each increase of gain  will re-adjust the output so it's reading 0VU

Does that sound correct? I can't find any information on this procedure anywhere online.

Many Thanks
 
In the 1176 you trim for symmetry, (if you want so, some people prefer to leave it with some asymmetry) So it's just that, if you have a way to measure 2nd harmonic, then start the test, start to increase the level till you can read that, and then trim till you get the minimum level for it, it shouldn't depend on the level where is the minimum 2nd harmonic or the maximum symmetry. It's also the lowest THD but the different will be smaller since you will be getting higher 3rd harmonic than 2nd at the end so you can get it on the right spot, that's why I suggest going for 2nd harmonic measurement, I've seen they do so with an oscilloscope but I don't know how far you can get that way, if you can set up a THD analyzer maybe with a PC, sending signal, measuring 2nd harmonic level in RTA and that's it.

JS
 
joaquins said:
In the 1176 you trim for symmetry, (if you want so, some people prefer to leave it with some asymmetry) So it's just that, if you have a way to measure 2nd harmonic, then start the test, start to increase the level till you can read that, and then trim till you get the minimum level for it, it shouldn't depend on the level where is the minimum 2nd harmonic or the maximum symmetry. It's also the lowest THD but the different will be smaller since you will be getting higher 3rd harmonic than 2nd at the end so you can get it on the right spot, that's why I suggest going for 2nd harmonic measurement, I've seen they do so with an oscilloscope but I don't know how far you can get that way, if you can set up a THD analyzer maybe with a PC, sending signal, measuring 2nd harmonic level in RTA and that's it.

JS

Many thanks for your reply. Yes i will be using a RTA.
 
compresearch said:
. Yes i will be using a RTA.
What kind of RTA? In order to evaluate THD, you need a high-resolution spectrum analyzer, and then you have to post-process the results.
I would rather use a distortion meter such as the one in RMAA.
When measuring THD on compressors, it is imortant to separate that in two series of measuremnts: measurements with no GR and measurement with GR. In the latter case, the amount of GR and the timing characteristics influence largely the results, in particular at low frequencies, where the artifacts due to the rectification are dominant.
 
abbey road d enfer said:
compresearch said:
. Yes i will be using a RTA.
What kind of RTA? In order to evaluate THD, you need a high-resolution spectrum analyzer, and then you have to post-process the results.
I would rather use a distortion meter such as the one in RMAA.
When measuring THD on compressors, it is imortant to separate that in two series of measuremnts: measurements with no GR and measurement with GR. In the latter case, the amount of GR and the timing characteristics influence largely the results, in particular at low frequencies, where the artifacts due to the rectification are dominant.

I recommended the FFT for doing the calibration, so you trim the preset to get the lowest 2nd harmonic. For that case looks more useful a FFT than a distortion meter since you can focus in the 2nd harmonic wile still looking at the spectrum and noise, and you only need to get the lowest in relationship with it self. For proper THD measurement that's not the proper tool since depending on widowing it has some problems with the actual level so you can't get a  real number for it.

JS
 
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