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John Curl's Blowtorch preamplifier part III
John Curl's Blowtorch preamplifier part III
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Old 13th March 2019, 07:07 PM   #15511
syn08 is offline syn08  Canada
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Originally Posted by john curl View Post
Normally, (there are some exceptions with other problems) a circuit thought to be a successful op amp will have an open loop bandwidth of just a few hundred cycles or much less. This is an open invitation to generate PIM distortion, as is known by researchers, and many audio designers, including me, have found that higher open loop bandwidth is better than low open loop bandwidth, subjectively, when comparing IC op amps and discrete op amps as well. So what do you do, if you are designing the BEST discrete op amp that you can? Do you go for really high feedback, with its attendant LOW open loop bandwidth? Or do you try to raise the open loop bandwidth as much as possible, even if you can then measure some residual 2'nd and 3'rd harmonic especially at higher operating levels, because you have thrown away some feedback? Only subjective listening tests can tell you which way to go. The headphone amp that we recently discussed has control of how much distortion he can allow, before it becomes eminently audible, but at the same time lowering potential PIM (or whatever we hear) when open loop bandwidth is increased significantly. This is a control used by Matti Otala back in 1973, me, and just about any serious audio designer of discrete amps.
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Originally Posted by RNMarsh View Post
I have a slightly different set of audio design criteria than Scott and industry for line level audio. It always sounds and measures great so i keep using it.

Low to moderate GNFB. Low to moderate CLG. High OLG BW. High SR. Lowish noise. Complimentary push-pull topology (generally, CFA). Extreamly low distortions.

How to do that easily with a high OLG IC Opamp? Do it with the IC and buffer IC mentioned and what do you get for distortion?
The same old stuff, packaged in the same old "high end audio" context.

The fact that these legends were debunked ad nauseam is, of course, irrelevant. Every attempt to invoke the laws of physics, the EE body of knowledge, any STEM based rationale, will be countered with the "I know what I hear", "trust your ears", "because I am a famous designer", bludgeon arguments. A war that can't be won.
 
Old 13th March 2019, 07:09 PM   #15512
syn08 is offline syn08  Canada
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You keep harking back ‘35 years ago’
Wrong, it's more like 50 today, some things never die.
 
Old 13th March 2019, 07:12 PM   #15513
john curl is offline john curl  United States
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I'll go with what I hear, thank you. I never used a 741 except for control servos back in 1969. They worked pretty good for that. By 1970, I was using Harris HA-911 op amps with almost 10 times higher slew rate. Richard Burwen found the same IC and used it with the original Levinson LNP-2 preamp. Later, we designed even the HA-911 out for faster and more open loop bandwidth discrete designs. More trouble, but worth it sonically.
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Old 13th March 2019, 07:14 PM   #15514
john curl is offline john curl  United States
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Enjoy your audio, guys!
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Old 13th March 2019, 07:15 PM   #15515
syn08 is offline syn08  Canada
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I'll go with what I hear, thank you.
You are welcome .

Last edited by syn08; 13th March 2019 at 07:17 PM.
 
Old 13th March 2019, 07:15 PM   #15516
john curl is offline john curl  United States
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Trust your ears, they are what counts, not specs.
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Old 13th March 2019, 07:16 PM   #15517
scott wurcer is offline scott wurcer  United States
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Originally Posted by john curl View Post
Dadod's amp looks ideal for open loop bandwidth.
I could apply a little positive feedback via input offset and make the OLG 100dB and guess what, nothing would change.
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Old 13th March 2019, 07:18 PM   #15518
john curl is offline john curl  United States
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Nothing that YOU MEASURE would change, perhaps.
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Old 13th March 2019, 07:22 PM   #15519
scottjoplin is offline scottjoplin  Wales
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Originally Posted by john curl View Post
Trust your ears, they are what counts, not specs.
Is it just your preference or do you compare with natural sounds for accuracy like Richard says he does?
 
Old 13th March 2019, 07:50 PM   #15520
indra1 is online now indra1  Indonesia
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Originally Posted by john curl View Post
... it might be that PIM as defined by Otala is again not the only mechanism detected by the human ear when it comes to how much feedback one uses.
A few years back I have seen a closed loop residual slight increase of H2 and decrease of H3 on introduction of local drain to gate feedback on a mosfet VAS, measured at 1W output. The difference in proportion of residual H2/H3 may have some contribution to the audible difference.
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Trust your ears, they are what counts, not specs.
Yes, but easier for others to understand and verify measured quantities.
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Nothing that YOU MEASURE would change, perhaps.
I strongly suspect that you will find a difference once you take a closer look at your measurement, especially on multitone signal.

Last edited by indra1; 13th March 2019 at 07:54 PM.
 

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