audio distortion meter circuit, is it worth building?

[festus]

Hi,

I can't afford a real distortion meter to check out my old valve amp.  Is this circuit worth building?

http://amplifierlab.com/audio-distortion-meter-circuit/2010/05/13

This is pretty simple, which is what I want for DIY project, but do you guys think it be acceptable for the simple task of testing THD for old valve amps?  I was thinking that I could integrate it into my audio load, since part of the project is already based on an audio load.  But if this is a crap design, then it would not be worth the effort.  So that is what I would appreciate feedback on, does this look like a reasonable design?  What do you think is the lowest % of THD you could measure with this, such as 1 decimal point 0.X%, or any chance it could reach down into possibly 0.0X%?

Thoughts welcome, thanx.

[Bored@Work]

Two things:

1) Why? People build and use old valve amps because of their special "tube sound". This special "tube sound" is not the least distortion, often introduced by aging caps. So why do you want to measure something which is be definition "good" when distorted?

2) Whatever that device measures, it is not THD. Especially, not the "H" in THD. It doesn't select the harmonics and doesn't sum them up. It just provides the relation of the base frequency's voltage to the broad voltage of "all the rest" (whatever that ominous AC voltmeter is measuring, above and below the base frequency).

[saturation]

You need a true RMS DMM as the output of the device and it will roughly measure THD using voltage criteria, if it came out near zero it would be meaningful, but measured values will need to be calculated using standard methods.

http://en.wikipedia.org/wiki/Total_harmonic_distortion

A better "voltmeter" would an oscilloscope or something like the Rigol 1052E or equivalent set to the FFT mode.  Its well within the frequency response domain.  You can easily measure each harmonic and perform the IEEE calculation and now have a comparable measurement.

[Bored@Work]

You need a true RMS DMM as the output of the device and it will roughly measure THD using voltage criteria,

It will measure something, but not THD. Notice the harmonics aren't individually filtered for measurement. It'll just measure everything minus the filtered base frequency. So you measure a conglomerate of the quality of the base frequency filter, the accuracy of input frequency, the noise, non harmonic distortions (e.g. mains hum), everything the AV voltmeter can stomache.

I hesitate to call that measurement.

[DavidDLC]

If you are willing to improve the performance of your amplifier by changing the design then it is worth to build something, otherwise I will just think about it.

I will also build it if there is something to learn on the process.

[saturation]

Yes, no question, but if it read near zero its meaningful but I guess my "rough" is too lenient for your stricter taste!  those old style meter designs often showed principle components dominated by harmonics, but you'd only know that with a scope.  I presume the designer substituted an AC voltmeter [ probably averaging too] as low cost solution for hobbyist, better than none.

One could directly connect a Rigol or equivalent 1052E across a resistive load on the amp output and bypass the filter completely and compare A= input  vs B= output waveforms, then get do a FFT on B waveforms to decipher the harmonics, then sum them as described in the Wikipedia link.

One also didn't mention the source of the pure tone needed to perform this test has to be a true low harmonic content quality sine wave!

You need a true RMS DMM as the output of the device and it will roughly measure THD using voltage criteria,

It will measure something, but not THD. Notice the harmonics aren't individually filtered for measurement. It'll just measure everything minus the filtered base frequency. So you measure a conglomerate of the quality of the base frequency filter, the accuracy of input frequency, the noise, non harmonic distortions (e.g. mains hum), everything the AV voltmeter can stomache.

I hesitate to call that measurement.