John Curl's Blowtorch preamplifier part II - Page 2474 - diyAudio
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Old 19th July 2012, 09:47 PM   #24731
RNMarsh is offline RNMarsh  United States
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Default yes and ps -

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Originally Posted by john curl View Post
Richard, I think that your 'waterfall plot' of a low pass filter was an excellent example of what is wrong with complex filters, in general. It is true that your example is of a relatively steep filter, but it is still credible, as many filters are made this way.
I knew you'd get it. ( ps - it does show the ringing, as well, in the time scale of the decay)
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Old 19th July 2012, 09:48 PM   #24732
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Joshua, I will try to give a quick explanation of the Sine-Square test.
Basically it starts as a square wave with a frequency in the middle of the audio range, lower can be used, higher can be used, but not as effectively. IF you look at the spectrum of a 3KHz square wave, you will find that it falls at 6dB/octave above the designated frequency of the square wave. Therefore at 30K Hz, for example, you should have harmonics that are about 20dB down from 3KHz, and falling still at higher frequencies. This seemingly innocent waveform, by definition, has no defined rise-time and it could be almost zero, and so only an almost infinitely fast op amp could track it. So, we HAVE to generate a DEFINED rise-time. This can be most easily done with a 6dB/octave roll-off filter after the square wave, at between 20KHz-100KHz.
The higher the filter roll-off, the faster the slew rate of the square wave signal for a given peak-to-peak level. When I first came on the scene, 20KHz was postulated as the 'easy' test, and 100KHz the 'hard' test. However, before we did our paper, Tom Holman (THX) came out with a paper using 30KHz as the 'optimum' test, so we changed up to 30KHz at my suggestion, as it was actually more appropriate. Remember, this was BEFORE the brick-wall filters were implemented for CD, a few years later.
However the Holman test LACKED something important. JUST a square wave will put MOST of the odd order IM products back on to the test waveform, itself, and it is almost impossible to evaluate it properly. Holman got away with it on testing HIS preamp, because he did not build a fully balanced design, so even order harmonics were generated most easily, and he got results. IF you did the same thing with one of my designs, very little would be noted.
So, Matti created a combination of a square wave and an added sine wave. The test frequencies deliberately chosen to give IM byproducts that mostly do NOT add or subtract to the test frequencies. He chose 3.18KHz and 15KHz, the 15KHz signal is 6 dB down in level, from the 3.18KHz square wave frequency (peak-to-peak). This was established by cut-and-try, to give a spread out range of IM byproducts that could easily be noted. I will try to find a picture of a typical TIM spectrum for clarity. (more later)
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Old 20th July 2012, 12:22 AM   #24733
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Originally Posted by john curl View Post
Joshua, I will try to give a quick explanation of the Sine-Square test.
(more later)
What about modern high speed op-amps that pass this waveform like it was DC? So we go full circle, the reasons for the audible differences are not measurable.
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Last edited by scott wurcer; 20th July 2012 at 12:25 AM.
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Old 20th July 2012, 12:29 AM   #24734
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Not TIM, anyway. And were would we be, IF we hadn't forced designer to make faster products for audio?
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Old 20th July 2012, 01:00 AM   #24735
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Thanks, John.
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Old 20th July 2012, 01:28 AM   #24736
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Cordell later showed that 20KHz THD exercizes the same mechanism and could be tested with the >-90dB THD analyzers available in the late '70s. It's not obvious how any of this applies to today's greater tolerance for -40dB THD numbers.

Thanks,
Chris
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Old 20th July 2012, 01:30 AM   #24737
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Not really, maybe 30KHz THD.
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Old 20th July 2012, 02:27 AM   #24738
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Quote:
Originally Posted by Chris Hornbeck View Post
Cordell later showed that 20KHz THD exercizes the same mechanism and could be tested with the >-90dB THD analyzers available in the late '70s. It's not obvious how any of this applies to today's greater tolerance for -40dB THD numbers.

Thanks,
Chris
About 20 years ago I found the answere to your question --- I had made a power amp and was in such a hurry to try it that I forgot to set the bias (was set to min). I heard it and got excited! So, this is what some high end reviewers were talking about -- debt of sound stage all over the place. Spacious sound with all the fundementals there. Then I went to check/set the bias.... and measured the thd... it was higher than i expected it and wanted it to be. I set it to my 'normal' and relistened. That great wide, deep and spacious sound stage was gone. It was a let down. High distortion gives this and enhanced these sonic affects that arent in the original recording. Some tube amps and SET amps which are often high (-40-ish) levels seems to me to be not accurate sounding but it is loved by many. Why? Becuase it more accuarately reminds them of music halls they have heard music played in (?). Thx -RNM

Last edited by RNMarsh; 20th July 2012 at 02:31 AM. Reason: why we like some distortion -
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Old 20th July 2012, 02:41 AM   #24739
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Quote:
Originally Posted by RNMarsh View Post
About 20 years ago I found the answere to your question --- I had made a power amp and was in such a hurry to try it that I forgot to set the bias (was set to min). I heard it and got excited! So, this is what some high end reviewers were talking about -- debt of sound stage all over the place. Spacious sound with all the fundementals there. Then I went to check/set the bias.... and measured the thd... it was higher than i expected it and wanted it to be. I set it to my 'normal' and relistened. That great wide, deep and spacious sound stage was gone. It was a let down. High distortion gives this and enhanced these sonic affects that arent in the original recording. Some tube amps and SET amps which are often high (-40-ish) levels seems to me to be not accurate sounding but it is loved by many. Why? Because it more accurately reminds them of music halls they have heard music played in (?). Thx -RNM
I'm not certain that I'm completely with you about this. Bias, especially, reminds me that I didn't specify that the "modern sound" amplifiers tend to have very good monotonicity, so bad numbers at peak output aren't representative of performance at -50 to -20 VU, where linearity matters more.

But it's fersure true that we're addicted to loudness, and distortion sounds louder. Just another example of the difficulties of listening tests (Pogo: "We have met the enemy, and he is us".)

Much thanks,
Chris
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Old 20th July 2012, 03:24 AM   #24740
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Quote:
Originally Posted by RNMarsh View Post
.... I had made a power amp and was in such a hurry to try it that I forgot to set the bias (was set to min). I heard it and got excited! So, this is what some high end reviewers were talking about -- debt of sound stage all over the place. Spacious sound with all the fundementals there. Then I went to check/set the bias.... and measured the thd... it was higher than i expected it and wanted it to be. I set it to my 'normal' and relistened. That great wide, deep and spacious sound stage was gone. It was a let down. High distortion gives this and enhanced these sonic affects that arent in the original recording. Some tube amps and SET amps which are often high (-40-ish) levels seems to me to be not accurate sounding but it is loved by many. Why? Becuase it more accuarately reminds them of music halls they have heard music played in (?). Thx -RNM
That's an interesting observation, Richard. One can probably get similar results by using this tool.

How many here realize that this one gets used (and abused) quite a lot these days during mixing and at the final stage?

Best,
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