This is a conference paper so it means it has less rigor and usually a lot shorter as compared to papers for the Journal of AES.Is this what the AES has to offer these days?
Amir pointed out:
Notice that they ran the signal through two of the fixtures shown before. So the total picture is the op-amp being in the signal path four times. It is not explained why they had to do this.
Frankly, this test was very amateurish for pro audio engineers. THD+N should never be used as a test when spectrum analysis and multitone (or IM) as mentioned by others can show what is really coming out of a system. The fact that differences could be heard between op amps is nothing new. And folks seemingly preferring distortion, well, just play a sample of 1 KHz from your digital source (that would be a nearly perfect waveform) and play a 1Khz tone from a test LP, and, wait for it, a lot of folks would prefer that LP distorted version. And this applies to a whole lot of tones as in music. Now, however, if one were asked which tone sounded the "cleanest" then the digital would win every time.
Is this what the AES has to offer these days?
This is a very interesting paper, thanks for bringing it up. A couple small flaws I might have pointed out if I were a referee. Several have noted that the frequency responses were not maintained equal- and the test subjects were relatively young. Several have also noted that the distortion was relatively high, which should perhaps also have been dealt with. The conclusion that the detected differences were due to the distortion is not supported by the data- it might be true, but there's no evidence for it. Their decision to not disclose the important detail of which opamps were used is, to me, a major flaw- full disclosure is needed when making claims of this sort.
But the other missing factor takes me back to a simple experiment I mentioned in an article I did in Linear Audio a few years back where I daisy-chained some opamps configured as buffers to see how many it took before I could reliably distinguish whether they were in or out of circuit. If memory serves, it was about five, and I invited the readers to guess what the difference was that I was able to perceive. As far as I'm aware, not one guessed correctly, and at least one more mendacious sort of fellow was highly bothered by the results. I will reveal it here, especially because it relates back to disclosing opamp identities so that a good correlation could be drawn: noise floor.
That's right, though I did not measure distortion in my experiment (it was really meant as a demonstration of a controlled listening test method). It was indeed likely low, but it was not measured nor did I (at least consciously) perceive any distortion.
Like I said, the relatively high distortion in the McGill preprint might be the audible factor, but that's not demonstrated by the data. It could be frequency response and, I'm throwing in noise as another possible variable that the authors should have addressed by disclosing opamp identities.
Again, that could indeed be right, but the authors got to that conclusion without demonstrating this it was right. The frequency response factor could also be part of it. So could noise. That's the part where, to me, the preprint went wrong, not looking at all the factors that could lead to positive identification or at the minimum, providing enough disclosure so the reader can balance the probabilities..
Again, that could indeed be right, but the authors got to that conclusion without demonstrating this it was right. The frequency response factor could also be part of it. So could noise. That's the part where, to me, the preprint went wrong, not looking at all the factors that could lead to positive identification or at the minimum, providing enough disclosure so the reader can balance the probabilities..
Extrapolating particulars from the general (especially a "general" which is pretty old) is not something I'd want to hang my hat on. These guys have some work to do before they can get this past a referee, assuming they ever bother to try.
My idea of testing if audio op-amps sound different or not wouldn't be pushing op-amps to 40dB gain to cause high distortion nor to chain buffers until noise start to appear but to take some standard circuitry (like that one in D10 DAC/preamp where 2 op-amps are doing I-V amplification and the 3rd is doing low pass filtering (I guess) and then you start exchanging op-amps and test if there is a difference in sound. My guess is nobody could hear the difference between op-amps. My guess is also that nobody will make such test any time soon as such test would also imply that there may be no noticeable difference between $100 DAC/preamp, $1500 DAC/preamp and $5000 DAC/preamp, and I'm pretty sure nobody would like to hear that, not in audio manufacturing industry but also not the consumers, who like to think they can "improve" the sound not only by changing the op-amp but also by changing the power cable.
Let me put it this way.
I have a D10 on order (alleged to arrive next week) and I do have a stash of fairly typical and modern audio op-amps on hand. I have test gear with far lower residuals than what I've posted results for, thus far. I have a stash of DIP sockets that sometimes get soldered into existing gear. I also have a suite of files of music that has historically been known for its diagnostic properties...