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Old 2nd April 2012, 08:26 PM   #2331
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Originally Posted by Salas View Post
I was thinking about substituting the main 3K3 2W feed in BIGBT. Now I understand you talk about those input pair 1K feeding branches I also substituted with BF245B CCSes one steady one variable in my bare bones 40W Latfet one. I saw 20% bandwidth expansion and better tempco for those.
CCS made out BF245 has scary current vs. temp curve. Bias consequently cannot rely on such big temp dependency.

3k3 2W can stay if zeners will be substituted with active ones, only if you want to make "never connected" power supply.
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Old 2nd April 2012, 08:34 PM   #2332
Salas is offline Salas  Greece
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No, it was going NTC while the input pair was going PTC and they cancelled each other during start-up, my bias turn on and offset peak dropped a lot.
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Old 2nd April 2012, 08:39 PM   #2333
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No, it was going NTC while the input pair was going PTC and they cancelled each other during start-up, my bias turn on and offset peak dropped a lot.
In that correlation it is excusable of course but probably not canceled out bias variations in a whole range. Did you thermally coupled input BJT-s and these two fets? Maybe tightly coupled these four would be enough to stabilize complete front-end bias?

Last edited by Lazy Cat; 2nd April 2012 at 08:42 PM.
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Old 2nd April 2012, 08:42 PM   #2334
Salas is offline Salas  Greece
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Close but not tightly coupled. Very steady 175mA bias in the long term operation though. Better be a heat-shrink wrapped up bunch in a final amp of course.
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Old 4th April 2012, 01:29 AM   #2335
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Exclamation TSSA The Simplest Symmetrical Amplifier

Schematics explains itself.
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Old 4th April 2012, 04:04 AM   #2336
Sheldon is offline Sheldon  United States
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...It had its little build story to follow after all. Different but adequate input BJT types we use from our stash would need a little experimentation with the 220R NTC's damper? Thanks for the details, now we got the whole picture. Sheldon is just ready to try that.
Used thermal grease between input parts, then bound with shrink wrap. NTC was bonded to the top of the pair with epoxy. Seems to work well. In my case, it was undercompensated with the 220R series resistor (running 10mA through driver, no driver cascode, so heat output greater than LC's driver). With no series resistor it might be very slightly over compensated.

Now I await the findings on bridge supply zeners and CCS.

Sheldon
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Old 4th April 2012, 04:26 AM   #2337
Sheldon is offline Sheldon  United States
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Rat's nest.
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Old 4th April 2012, 05:54 AM   #2338
sonnya is offline sonnya  Denmark
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Originally Posted by Lazy Cat View Post
Schematics explains itself.
I think Nelson will be proud of you for the simplicity!
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Old 4th April 2012, 07:08 AM   #2339
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I think Nelson will be proud of you for the simplicity!
F5 with input BJT-s.
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Old 4th April 2012, 07:44 AM   #2340
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Hi Sheldon

Nice progress you've made.

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Originally Posted by Sheldon View Post
In my case, it was undercompensated with the 220R series resistor (running 10mA through driver, no driver cascode, so heat output greater than LC's driver). With no series resistor it might be very slightly over compensated.
So NTC is working properly, glad about that. If it is still undercompensated with 220 ohm, than try a little lower value ie. 180 ohm. Without serial compensation resistor it is always overcompensated.

The decision to wait for zeners and CCS is also very wise, I'm working on it.

Last edited by Lazy Cat; 4th April 2012 at 08:03 AM.
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