Electronics > Projects, Designs, and Technical Stuff
Analog Disovery as Audio Analyzer
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bson:
I'd suggest an LTspice experiment with a simple op-amp driving an 8ohm resistive load; measure the THD+N.  Then replace the resistor with an inductor and parasitic resistor of say 0.1ohm that gives you |Z|=8ohm at 10kHz.  Check the THD+N.  Then repeat at say 2ohm and 200ohm.  This will teach you resistive loads are useless for design purposes and their only use with audio amplifiers is to generate fantastic-looking numbers for marketing collateral.  (Other than smoke testing of course, without risking destruction of real drivers.)
fpliuzzi:
jaxbird, your change to the IMD test instrument allows great flexibility in setting up the test parameters for the various present, and possibly some future IMD standards (better than just having a couple of presets for the most commonly used ones). It also allows for experimentation, which is always interesting. Thank you for implementing this feature.
fivefish:
jaxbird: Are you using Analog Discover or Analog Discovery 2? 

Side-Topic: What's the advantage of AD2 vs. original AD?  Will Waveforms 2015 run on the original AD?

I'm deciding what kind of AD to get to run jaxbird's software :)
jaxbird:

--- Quote from: bson on December 14, 2015, 07:03:33 pm ---I'd suggest an LTspice experiment with a simple op-amp driving an 8ohm resistive load; measure the THD+N.  Then replace the resistor with an inductor and parasitic resistor of say 0.1ohm that gives you |Z|=8ohm at 10kHz.  Check the THD+N.  Then repeat at say 2ohm and 200ohm.  This will teach you resistive loads are useless for design purposes and their only use with audio amplifiers is to generate fantastic-looking numbers for marketing collateral.  (Other than smoke testing of course, without risking destruction of real drivers.)

--- End quote ---


In essence what you are saying is that there is no point in standardized testing of amplifiers as using resistive loads are not equal to speaker loads.

I cannot say I agree, surely many valuable performance details are revealed using resistive loads. Based on these results it's even possible to make good predictions about the performance when combined with a specific speaker simply by looking at the speaker's impedance behavior, frequency response/dispersion pattern, sensitivity etc. And by using standardized tests with standardized loads the results are comparable.

jaxbird:

--- Quote from: fpliuzzi on December 14, 2015, 07:08:12 pm ---jaxbird, your change to the IMD test instrument allows great flexibility in setting up the test parameters for the various present, and possibly some future IMD standards (better than just having a couple of presets for the most commonly used ones). It also allows for experimentation, which is always interesting. Thank you for implementing this feature.

--- End quote ---


Thanks, it was a great suggestion  :-+. I will probably still add some general purpose spectrum analyzer screen with optional tone generator for more experimentation, but I like to have purpose specific tools for most standard tests.

Maybe at some point even add a scope/tone gen with the option to view signal with the fundamental notched out similar to what you'd get from many of the traditional distortion analyzers.

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