John Curl's Blowtorch preamplifier part II

[PMA]

Thanks for clarification, John. I highly appreciate your explanation.

Yes, there are big issues with tantalum caps.

[john curl]

DA wise, they are no worse than aluminum, especially with DC voltage across them.

[john curl]

It has been several days since an input has been brought forth on the subject of 'Hirata distortion'. Are there any questions on the subject, or previous subjects so far, serious questions only, please.

[fireworks]

Another way of measuring distortions, which might have been mentioned in this thread, is to record the difference between the output divided by the amplifier gain and the input.
Obviously this difference would incorporate all the distortions, whether harmonic, crossover, Hirata, etc..
Does anyone have experience working this way ?
If the amplitude of this difference is below some threshold, can one say that the amplifier is "perfect" in the sense that it cannot be distinguished from a perfect one in any listening tests ? By perfect amplifier I mean an amplifier for which the difference is zero.

[john curl]

Fireworks, we started to use digital differential subtraction 36 years ago. However, it FAILED to show any problem with discrete circuit designs or IC's, for that matter.
There are two problems: One is having enough bits. Today, that has been solved in principle, going from 10 bits, 36 years ago, to 24 bits, today.
The second is more difficult in making the BYPASS effectively match in level with frequency and phase. This usually limits the test signal to something too benign to be very useful.

[1audio]

An interesting insight I just had is that the standard test for settling time is very close to the input/output cancellation test mentioned above. Settling time is measured to .1% or less. That would loosely translate to .1% distortion and many fast devices can meet the distortion criteria in 20 nS or less. While I think settling time is important it doesn't seem to be a comprehensive indicator of performance. There is a lot on measuring it on the web and its not easy to measure.

[john curl]

I would agree that settling time is not what I really want to DOMINATE a measurement. In audio, it should be a minor factor, but it could measure as a major factor, without care in the test measurement.

[dimitri]

we started to use digital differential subtraction 36 years ago. However, it FAILED to show any problem with discrete circuit designs or IC's, for that matter.

Differential subtraction was used previously in cap measurements: http://waltjung.org/PDFs/A_RealTime_...or_Quality.pdf
If the subtraction technique is not achieving the desired end then these results with discrete cap circuits should be taken with a grain of salt, shouldn't they?

[john curl]

No, because caps were measured by ANALOG differential subtraction, AND the test signal was continuous. Very different.

[john curl]

I think that a personal approach to audio design is appropriate at this point.
It is true that double blind tests and some differential tests make sonic quality to appear rather easy, and all that we should care about is our listening rooms, more or less. This is an alternative belief system that has not convinced me to give up serious audio design, but it has deterred others from going forward.
Now why do I hold to my belief that audio differences, small in actual acoustical error, are still audible? Well, it is because of my experience with comparing different audio products.
People think that level matching was 'invented' by double blind testers, BUT we were doing it more than 40 years ago, seriously. This was especially important to me, because MY audio amp lost, in comparison to a triode amp with the same frequency response, distortion with level, distortion order, and damping factor. In fact, we did a differential subtraction of the two amp outputs, to get the levels matched. YET, I preferred the TUBE amp, and not my own! However, I TRUST my ears, and I didn't give up, I just kept looking further.
When ABX testing came out over 30 years ago, I tried it as well, and I could not hear a difference between one of my own designs and a Dyna tube preamp. What happened, was there no difference? NO, because the differences came back, as soon as I removed the ABX equipment. This was as start as to what the problem was, but not the only thing. However, I am not a psychologist and I can't (or won't try) to explain here what I think is the fundamental problem, but I know personally that double blind tests remove (for me) every vestige of subtlety in the sound. To continue to use double blind tests meant that I had to blind myself to hearing differences, differences that others could hear, and not just me. It just wasn't productive to do so. I stand on the evolution of my designs for the past 30 years as proof that I did the right thing.
Now, when it comes to differential subtraction, it sometimes works well, BUT actually, other tests will bring out more difference, why I don't know. The human ear is an incredible mechanism that seems to hear differences that measuring equipment cannot, so long as people are free to listen openly and compare. Even A compared to B is OK, behind a screen, and that should be enough for anybody. It works for me.