This is a digest of the Audio Engineering Society (AES) conference paper titled, “Distortions in Audio Op-Amps and Their Effect on Listener Perception of Character and Quality.” (http://www.aes.org/e-lib/browse.cfm?elib=16029) As the name indicates, it is a controlled listening test to see if there is audible difference between Op-amps (integrated small amplifiers) when operated below their clipping point.
In recent years, it has become quite fashionable to talk about what Op-amp are used in audio products or swap one for the other in the same vein. While subjective outcomes abound on efficacy of such differences, question becomes if there is any formal, controlled listening tests that can give us reliable indication of audible differences in Op-amps.
The paper in question aims to answer this question. It has good pedigree when it comes to its authors which includes the famous mastering engineer, George Massenburg, and other researchers from various departments of McGill University. If you have not heard of McGill, it is the same college that gave us Dr. Sean Olive of Harman/NRC fame, and is the closest we have to a school contributing to research in sound reproduction.
For the experiment, twenty (20) Op-amps were utilized, seven (7) of which were discrete and fourteen (14) which were Integrated Circuits (ICs). The circuit used for measurements and listening tests is as follows:
Starting from the middle, they use two of the same op-amps, both programmed to have 40 dB of gain. The second one however is in inverting configuration so backs out the actual gain of the first one, resulting in no gain at all.
To compensate for inverting nature of the two-amps in the middle, an inverting "buffer" is used up front. For some reason I don't understand, that component has 9 dB of gain (in which case it is not a "buffer"). They had to then back out that gain with the output "Pad" by the negative -9 dB.
Net, net, the whole thing has "unity gain" meaning it doesn't increase the signal level but launders the audio signal through two of the same op-amp as to obtain their distortion profile.
The resultant circuit resulted in slightly different high-frequency response:
I wish they had compensated for this so that the difference was not there. I assume they wanted to keep all the circuits identical which resulted up to half a dB or so of difference at 20 kHz.
Levels were set such that clipping did not occur:
The full combination of 20 different op-amps would be huge so they made some choices in pairing some op-amps with each other as noted earlier.
For material, being a music school, the had access to raw recordings:
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.
Here is the listening test protocol:
So it is a double blind, AB preference test.
There is a second phase but that did not generate significant results so I won't go into that.
Here is who took the tests:
Let's see the results of the test:
Anywhere you see a "P" value less than 0.05, the results are "statistically valid." See my article on this: https://audiosciencereview.com/forum/index.php?threads/statistics-of-abx-testing.170/
Nine of the outcomes reached this level of statistics. So it seems that differences could be heard in this test under double blind test conditions.
Alas, making sense out of why is complicated. There is little correlation between measurements and listening test results. Where there is some correlation according to the authors (I did not check), higher distortion seems to get the vote.
They also found counter to common understanding, that spectrum analysis of the distortion was not helpful in predicting the outcome:
Likely cause of this is that while later harmonics are more audible (less masked), they are also of much lower amplitude so that takes away audibility.
Sadly they never identify the op-amps so there is no way to do further research on their work, or confirm the results.
Here is their conclusion:
Conclusions
This study is significant in the way it shows audible differences between op-amps at distortion levels that are fairly low. There is a lot of fine print here of course with quadrupling the use of op-amps, and lack of more solid investigation and documentation of the work. It makes for good foundational work for follow on research.
In recent years, it has become quite fashionable to talk about what Op-amp are used in audio products or swap one for the other in the same vein. While subjective outcomes abound on efficacy of such differences, question becomes if there is any formal, controlled listening tests that can give us reliable indication of audible differences in Op-amps.
The paper in question aims to answer this question. It has good pedigree when it comes to its authors which includes the famous mastering engineer, George Massenburg, and other researchers from various departments of McGill University. If you have not heard of McGill, it is the same college that gave us Dr. Sean Olive of Harman/NRC fame, and is the closest we have to a school contributing to research in sound reproduction.
For the experiment, twenty (20) Op-amps were utilized, seven (7) of which were discrete and fourteen (14) which were Integrated Circuits (ICs). The circuit used for measurements and listening tests is as follows:
Starting from the middle, they use two of the same op-amps, both programmed to have 40 dB of gain. The second one however is in inverting configuration so backs out the actual gain of the first one, resulting in no gain at all.
To compensate for inverting nature of the two-amps in the middle, an inverting "buffer" is used up front. For some reason I don't understand, that component has 9 dB of gain (in which case it is not a "buffer"). They had to then back out that gain with the output "Pad" by the negative -9 dB.
Net, net, the whole thing has "unity gain" meaning it doesn't increase the signal level but launders the audio signal through two of the same op-amp as to obtain their distortion profile.
The resultant circuit resulted in slightly different high-frequency response:
I wish they had compensated for this so that the difference was not there. I assume they wanted to keep all the circuits identical which resulted up to half a dB or so of difference at 20 kHz.
Levels were set such that clipping did not occur:
The full combination of 20 different op-amps would be huge so they made some choices in pairing some op-amps with each other as noted earlier.
For material, being a music school, the had access to raw recordings:
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.
Here is the listening test protocol:
So it is a double blind, AB preference test.
There is a second phase but that did not generate significant results so I won't go into that.
Here is who took the tests:
Let's see the results of the test:
Anywhere you see a "P" value less than 0.05, the results are "statistically valid." See my article on this: https://audiosciencereview.com/forum/index.php?threads/statistics-of-abx-testing.170/
Nine of the outcomes reached this level of statistics. So it seems that differences could be heard in this test under double blind test conditions.
Alas, making sense out of why is complicated. There is little correlation between measurements and listening test results. Where there is some correlation according to the authors (I did not check), higher distortion seems to get the vote.
They also found counter to common understanding, that spectrum analysis of the distortion was not helpful in predicting the outcome:
Likely cause of this is that while later harmonics are more audible (less masked), they are also of much lower amplitude so that takes away audibility.
Sadly they never identify the op-amps so there is no way to do further research on their work, or confirm the results.
Here is their conclusion:
Conclusions
This study is significant in the way it shows audible differences between op-amps at distortion levels that are fairly low. There is a lot of fine print here of course with quadrupling the use of op-amps, and lack of more solid investigation and documentation of the work. It makes for good foundational work for follow on research.
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