John Curl's Blowtorch preamplifier part II - Page 3851 - diyAudio
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Old 24th April 2013, 12:01 AM   #38501
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Cascodes? What was the advantage of breaking up middle resistor into 4 resistors plu pot. I can see the pot for adjustment of contribution of each half, but why split the others? I like being the only guy that doesnt know.
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Old 24th April 2013, 12:15 AM   #38502
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OK, once again, here is the minimum, and then the more versatile version:
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File Type: jpg 4Qnew1.jpg (210.4 KB, 221 views)
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Old 24th April 2013, 12:17 AM   #38503
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Quote:
Originally Posted by buzzforb View Post
Cascodes?
That's one way, but I think that would take more than 4 more parts. I also think there's a better way. Let's see if John has gone there yet.
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Old 24th April 2013, 12:37 AM   #38504
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Of course there is the 'folded cascode version, which I use with Vendetta and the CTC Blowtorch, but it is actually easier to look at this basic (not primitive) version of the 4Q input stage, and learn what each of the resistor sets do. Here, for example, is a more typical approach that I use with Parasound and Constellation.
First the original, second the typical:
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File Type: jpg 4Q aka JC-2.jpg (232.1 KB, 201 views)
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Old 24th April 2013, 12:52 AM   #38505
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Buzzforb, your questions are revealing.
First, some resistors give feedback, others give bias, some do both. The only obvious resistor function on the input is the variable pot.
It can, acting alone, control the total current of the entire 4 devices, forcing the n channel pair current to match the p channel pair current, due to the fact that there is no ground reference, and the only place for the current in all 4 devices to flow, is with the variable pot, YET the bias pot is not really necessary, just a convenience.
(By the way, I will talk about locally connected cascodes later, SY.)
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Old 24th April 2013, 01:06 AM   #38506
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Quote:
Originally Posted by john curl View Post
I have never heard the device being criticized at this time. I have no opinion of it, at this time.
You're right. I misread what you wrote. My apologies.

se
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Old 24th April 2013, 01:32 AM   #38507
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......................
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Old 24th April 2013, 01:44 AM   #38508
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Quote:
Originally Posted by john curl View Post
Here is the 'minimum' version for discussion at the moment:
Does this version contain feedback or is it just another way of loading the output?
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Old 24th April 2013, 02:14 AM   #38509
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This version uses the 1K 'load' resistors also as feedback resistors.
What is missing is the BIAS resistors. This simple design runs on the Idss of the input devices and there is NO control their current, so extreme selection would be necessary if we used the design this way, BUT it is the most simple way to show it in full 4 Quadrant symmetry.

Last edited by john curl; 24th April 2013 at 02:40 AM.
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Old 24th April 2013, 02:52 AM   #38510
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Here is the CTC Blowtorch way of input current control:
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File Type: jpg 4Q-min.jpg (205.6 KB, 123 views)
File Type: jpg 4Qbasic-with bias r.jpg (218.6 KB, 122 views)
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