John Curl's Blowtorch preamplifier part II - Page 441 - diyAudio
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Old 21st May 2010, 02:50 AM   #4401
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Frequency and time domain are interelated in FFT analysys. An exception is noise. How about a train of halve sinewaves for testing of amplifiers that have only positive or negative polarity ? An FFT analyser could not make sense of that.
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Old 21st May 2010, 03:23 AM   #4402
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Quote:
Originally Posted by Joachim Gerhard View Post
Frequency and time domain are interelated in FFT analysys. An exception is noise. How about a train of halve sinewaves for testing of amplifiers that have only positive or negative polarity ? An FFT analyser could not make sense of that.
Why not? Even noise has a Fourier transform- we talk about 1/f, for example. The signal you drew looks roughly periodic, and if you select any finite time interval for sampling, ANY signal can be treated as periodic, with the period being the sampling interval.
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Old 21st May 2010, 04:09 AM   #4403
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In a mini summation of audio design, we have found that negative feedback should be limited or avoided, if possible, and if used, one should at least try to use the most linear circuits possible, including adding extra parts to IC's in order to make them more open loop linear, if necessary. It would appear that you want the feedback to do as little as possible, so that it does NOT show itself in another form, such as frequency modulation.
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Old 21st May 2010, 04:47 AM   #4404
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It might be pointed out that the 'topology' of the 'Otala' amp was pretty good, we had designed better topologies, in the USA, but where we went astray was in the AMOUNT of feedback used. 20dB once again seemed to be the magic number. More, and you got lower open loop bandwidth, lower slew rate, even if you got better IM and harmonic distortion specs. At first, we thought that TIM was the only culprit, but we found out later, that even amps with 100V/us, could still sound marginal, IF the open loop bandwidth criterion was severely restricted. You see, it is relatively easy just to remove load resistors from the first stage and substitute current sources to get 100-10,000 times more open loop gain. However the open loop bandwidth will diminish correspondingly. This is where we get today's IC op amps. It seems to change the sound, even if other parameters are improved, and slew rate is maintained. This is the puzzle at hand.
As seen in this link the feedback needed to be increased to 30dB before the Otala design started to perform and sounded good:
Story of "Otala" Amp

A small series of the "Otala" amp sold in the mid eighties by Terje SandstrÝm was upgraded with JFET input transistors, improved power supply and if I remember correctly 40dB feedback and was the best sounding version of the "Otala" amplifier.
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Old 21st May 2010, 04:58 AM   #4405
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It was mostly new management in later years that changed the Otala amp. They are not as well regarded today, as they first were.
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Old 21st May 2010, 07:46 AM   #4406
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John - you haven't said much about layout design (in particular grounding) so far in this thread. Do you pay a lot of attention to it? The reason I ask is I've been playing around with an active speaker and got a substantial improvement in clarity just by rejigging the grounding, its gone from being 'upper mid-fi' to high end (within the limitations of the drive units) in my estimation. Have you had any similar experiences with changing a design's grounding/decoupling?
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Old 21st May 2010, 07:49 AM   #4407
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Quote:
Originally Posted by SY View Post
"Traditional audio measurement approaches are based in the frequency
domain, using FFT technology and the steady state test tones it relies
on."

As soon as you see that, you can be well prepared for what's ahead.

I was particularly taken by the comparative spectrographs on the top of page 4, clearly showing the "tweak" setup to have more HF noise and hash. That, of course, is not mentioned....
SY, I can't read the scales on those graphs and they don't appear amenable to enlargement with any clarity BUT the hash and noise is clearly lower in level on the tweak setup at least to my eyes ,though what is ther appears hashier in nature.
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Old 21st May 2010, 04:41 PM   #4408
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Is there anything in the type of testing explained here do you think?

http://www.nordost.com/downloads/New...easurement.pdf

The article is marketing and has inconsistencies but the type of testing reported may have some merit I suspect.
I saw their presentation at RMAF last year. I was underwhelmed to say the least. I don't think it will stand up under scrutiny and there were myriad procedural flaws in their process. If they get rigorous in the test process and publish enough that the tests can be duplicated we may learn something. What I saw was marketing hype with no substance.
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Old 21st May 2010, 04:49 PM   #4409
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Originally Posted by jarrod View Post
do you have the service manual in pdf on the 5173 im trying to do a performance test
Which 5173? Quan-Tech?
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Old 21st May 2010, 05:01 PM   #4410
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Based on my experience, I cannot support the idea that feedback should be avoided as a rule.

If the feedback was closed around circuit with very linear and very fast JFET input stage, high current VAS, and high idle class A output stage, high OLG corner, I have not found any sound degradation even with 60dB feedback (in a preamp). On the contrary, it sounded 'better' than no feedback and low feedback designs with higher distortion. IMO it is very difficult to express any general statement about contribution or sound degradation by feedback, it is strongly circuit dependent. I do not believe in the PIM or FM theory, as it has never been proven for a very linear and very fast circuit. uA741 measurements are not any proof, this was swept into a historical dust bin. We do not work with 0.5V/us circuits, but >100V/us circuits in link stages.
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