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Old 3rd June 2012, 08:39 PM   #991
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Instead of increasing gain, the same thing could probly be done with degeneration.

The input buffer is important if you want to make your amp very fast. Else it will oscillate depending on the source. I've done this with my own amp.
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Old 4th June 2012, 07:21 AM   #992
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. . .However, my tests were made with closed loop gains ranging from 10 to ~20. It might be stable at higher gains, I didn't test it.
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Instead of increasing gain, the same thing could probly be done with degeneration.
With a modern source and with gain at potential, if 30+30vdc rails the feedback resistor should be 20k in real life. Therefore, what, proportionately should the feedback resistor be if 37+37vdc rails?
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Old 4th June 2012, 12:50 PM   #993
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Unless that was a trick question, 30/20k~37/25k.

I kind of prefer to have highish gain, in the practical sense. Having an available boost can helpful.
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Old 4th June 2012, 04:50 PM   #994
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Instead of increasing gain, the same thing could probly be done with degeneration.
The problem is not the input impedance but the bias current and degeneration will not help in this case.

The bias current is dependent on the tail current, which is itself under the control of the servo.
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Old 4th June 2012, 05:20 PM   #995
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I totally understand the input impedance and LTP base current issue. I was meaning for making the amp stable with smaller compensation values, without increasing gain so much. Degeneration is an alternative to this.

I found that using too much phase lead (say 47pF) compensation made the VAS compensation values skyrocket. It was better to use no phase lead or use the NFB network in my first schematic. I used a very small value in my later schematics (1.2pF) but this is not very practical.
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Old 5th June 2012, 05:03 AM   #996
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I don't think the very first stage is the optimum place for modulated bias.
In some ways it is, but it some ways it isn't... It surely makes input Z act
a bit funky...

Modulating the second stage instead, might offer some advantage.
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Old 5th June 2012, 05:14 AM   #997
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Yes, I tried that, But the problem is Bias starts at an indefinite level and drops instead of starting at 0 during startup. Furthermore, I suspect there is some kind of dynamic oscillation coming into play, causing the LTP to turn off and squelch noise when there is no input, based on what Dan has told me, unless he is exaggerating. Taking the LTP out of the servo loop would be great, but as I see there are good reasons for being the way it is in this circuit. Not that I totally understand the servo yet.
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Old 5th June 2012, 10:09 PM   #998
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Default Kean taught me to do stuff with the simulator! Thanks man!!!

Well, I don't know what phase linear is for but this happened somehow when I was trying to follow all of the directions all at the same time. The THD figure has dropped down to 0.002620% even though the bias is also dropped to 100ma and this should be running desirably cool and efficient, so I guess we won't be needing those big fans after all.
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Old 6th June 2012, 01:38 AM   #999
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....Not that I totally understand the servo yet.
All this while, we've been trying to get Elvee to post a good explication for us dumbos! Let's hope he soon answers our prayers ...
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Old 6th June 2012, 02:48 AM   #1000
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All this while, we've been trying to get Elvee to post a good explication for us dumbos! Let's hope he soon answers our prayers ...
Let me reference Dan's above schematic.

I know something that may greatly clarify things for those who are confused. C2 seems to control the open-loop gain of the servo. This is because C1 acts as a high-pass across the BW of the loop. C2 acts as a low-pass, returning feedback phase to normal. This phase conversion causes R17 to act like phase lead compensation for the servo loop. I may be able to find a way to implement phase lag or other types of compensation to make the servo more stable and faster after I understand it well enough.
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