I noticed some peculiarity with the OPA2604's distortion graph. This is a chip that is used in some high-end designs achieving lower than -100dB THD+N, so the graph I had wasn't reflective of its true performance. And I was checking this blog's stats on a hunch, and found many traffic sources from Google image search, with the search term OPA2604.
People may get the wrong idea reading that graph, so to reduce the amount of rubbish on the internet, I decided to do a retest with a 12V power adapter this time.
Results are shocking.
The EMU wasn't cooperative that day, and I was in a rush, so I didn't bother getting 0.001% THD. But this is good enough to show the previous measured performance of the OPA2604, which was shocking bad as >0.01%.
The first thing to notice is the numbers for THD dropped by an order of magnitude.
Now it measures okay. Most of the spikes are EMU's "noise floor" (for the lack of a better term that I know).
And yup, this is with 9V battery. So the issue wasn't insufficient voltage. This is an example of why control measurements are important, for if I had measured only 12V scenario, I would had gotten a different and wrong conclusion.
The main difference is the greatly-reduced even harmonics.
My guess for the cause? Is not going to be something groundbreaking - probably a bad contact.
Other graphs are similar, 12V and/or 33Ω load, so I won't go into that. Into 33Ω for both cases THD is 0.005+% but how much of it would be due the the EMU? Even-order harmonics did increase with current draw as with the case of OPA2132. Speaking of which, now I wonder how accurate are the measurements for OPA2132, particularly the 0.015% reading.
So here I declare the previous OPA2604 measurements invalid. Some others might also be questionable.
W A R N I N G !
W A R N I N G !
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