John Curl's Blowtorch preamplifier

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#12,721
I was wondering whether one could somehow measure the output stage bias current of an opamp. If I drive a load, and increase the output load current, I can find the point where the supply current starts to move from class A (being sinusoidal) into class AB (starting to flatten at one side).
Or is there another easier way?

Jan Didden
 
#12,722
janneman said:


Stan Curtis, the guy who put Rotel back on its feet many years ago, claimed the same. His secret, as much as he confided (IIRC; t was an interview in EW I think) was manipulation of the freq response and Zout, or the combination of it. His reviewers hated him, his amps didn't measure particularly linear but they couldn't stop listening to them. An enviable achievement.

Jan Didden
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Well, Lamm doesn't manipulate with linearity. Quite contrary - he says that an amplifying device should have output imedance and freq. response as linear as it could. Also, distortion spectrum shouldnt change through audio freq. response. He is, in fact, great linearity proponent.
 
#12,730
SY said:


No, they don't. They are asserting that something that depends on an uncharacterized parameter (and the designer of that opamp has confirmed this) will work consistently from batch to batch. Without a LOT of supporting data, that's an extremely dangerous assertion. Read Pease for some horror stories about that.
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This is your theory, while this circuit actually works properly in hundreds of units.
Can you explain that?
(There is a valid explanation).
 
#12,732
janneman said:



Joshua,

You sure you want to do this? You know what 'we' will say, whatever your opinion, right? Unless you use a well-controlled, statistically significant DBT....

Jan Didden
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The point is this.
I do all this for my own enjoyment from my sound system.
A DAC with PSUs and I/V amp based on THS4131ID and LM4562MA are due in few days.
Later on I'm going to do some modifications to the DAC's on board "regulators" and hear the difference.
Later on I'm going to build few discrete I/V amps, one based on JC's design (which I posted here some time ago) and between 1 and 3 bipolar ones.
Eventually I'll use the one that will sound best.
Comparing the various amps will be done eventually by controlled DBTs, for I have no desire to fool myself.
Nevertheless, I'm quite sure that some participants in this thread will argue until doomsday that my findings are invalid. Some may claim that I'm half deaf, therefore, even the best controlled DBTs I may conduct are invalid.
 
#12,734
Joshua_G said:



The point is this.
I do all this for my own enjoyment from my sound system.
A DAC with PSUs and I/V amp based on THS4131ID and LM4562MA are due in few days.
Later on I'm going to do some modifications to the DAC's on board "regulators" and hear the difference.
Later on I'm going to build few discrete I/V amps, one based on JC's design (which I posted here some time ago) and between 1 and 3 bipolar ones.
Eventually I'll use the one that will sound best.
Comparing the various amps will be done eventually by controlled DBTs, for I have no desire to fool myself.
Nevertheless, I'm quite sure that some participants in this thread will argue until doomsday that my findings are invalid. Some may claim that I'm half deaf, therefore, even the best controlled DBTs I may conduct are invalid.
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Don't get pathetic. You are perfectly right to do and select what you fancy. Just remember that is will be *your* preference, and as such lots of people will disagree. So don't complain when that happens.

Jan Didden
 
#12,737
janneman said:
Sure, but for audio use, my source would care both about those low value resistors as well as the Ib 😉
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Well, then don't use a 3.9GHz opamp 😉 For almost all +/-15V audio purposes (perhaps except ultra low noise for MC, which would require a few K170 or BF862 FETs), the new OPA211 and/or OPA847 (to name only a few from TI/BB) will perfectly fit and requirements at very reasonable costs.
 
#12,739
janneman said:




Don't get pathetic. You are perfectly right to do and select what you fancy. Just remember that is will be *your* preference, and as such lots of people will disagree. So don't complain when that happens.

Jan Didden
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No doubt the results will be only to my personal preferences. However, I'm quite sure some people here will invalidate even that. We had it before, concerning selecting cables to my system.
 
#12,740
Joshua_G said:



No doubt the results will be only to my personal preferences. However, I'm quite sure some people here will invalidate even that. We had it before, concerning selecting cables to my system.
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No, not at all. Two things were going on. On was that your results could only be seen as personal preferences. The other was my effort to try to explain some of well-known properties of perception.

Jan Didden
 
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