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Old 25th June 2012, 10:11 PM   #2551
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When you use lateral Fets at the output it is a good idea to go out of the drains. They are fantastic constant current sources. On Gm they are quite poor against bipolars so i think bipolars are a better choice when you want to go out of the low impedance output, emitter in that case. Laterals could be configured driving bipolars from the drain and the bipolars driving the output through the emitters. One problem remains : an emitter or source follower does not like capacitive loading. When you want to drive an ELS with a source or emitter follower you need an output coil and a Zobel.

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Old 25th June 2012, 10:17 PM   #2552
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Here you see what i am talking about.
Drain current is nearly independent from Drain-Source voltage up to 2A idle.
That transfers into a very high output impedance.
It is the 2SK1058.
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Old 26th June 2012, 05:53 AM   #2553
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Quote:
Originally Posted by Joachim Gerhard View Post
When you use lateral Fets at the output it is a good idea to go out of the drains. They are fantastic constant current sources. On Gm they are quite poor against bipolars so i think bipolars are a better choice when you want to go out of the low impedance output, emitter in that case. Laterals could be configured driving bipolars from the drain and the bipolars driving the output through the emitters. One problem remains : an emitter or source follower does not like capacitive loading. When you want to drive an ELS with a source or emitter follower you need an output coil and a Zobel.
I think it is more the positive temperature coefficient that come into account than the poor Gm.

In Sub bas impulses it has time to heat up, and the Gm decreases.?!???
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Old 26th June 2012, 07:45 AM   #2554
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In Sub bas impulses it has time to heat up, and the Gm decreases.?!???
What the hell Feedback is doing ?
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Old 26th June 2012, 08:16 AM   #2555
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Maybe i should rephrase it.

The feedback and feed forward is always tracking the errors. They can only correct them if they exist.

So in time domain the correction through the feedback is always behind. => The bas impulses are in most cases the one with the highest energy. So if the mosfet heats up and starts to produce an lower Gm the feedback tries to fix it. In other words the first edge of those impulses will have a false value.

The perfect amp would be a non feedback amp with an output stage in high class A level. But that is impractical to make. There for we add feedback to compensate. That why we want an as high as possible Openloop gain frequency as possible, so that the delay of the correction through the feedback is as little as possible.
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Old 26th June 2012, 01:23 PM   #2556
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>So in time domain the correction through the feedback is always behind.
No, it is NOT.
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Old 26th June 2012, 01:31 PM   #2557
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Yes it IS - has to be - force of logic in this is undeniable but . . . .

But the delay is so tiny I cannot see how it would sonically impact sub bass.
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Old 26th June 2012, 01:42 PM   #2558
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Yes it IS - has to be - force of logic in this is undeniable but . . . .

But the delay is so tiny I cannot see how it would sonically impact sub bass.
Do you have a another explanation????
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Old 26th June 2012, 01:44 PM   #2559
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As for AF, delays are virtual absent.
Don't confuse phase lag with delay, which is a different kettle of fish.
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Old 26th June 2012, 01:48 PM   #2560
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The open loop phase shift is identical to the delay. That's why when you reach 180 degrees phase shift you starts to add signal rather subtracting the error.
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