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Old 22nd February 2005, 12:53 PM   #1
udip is offline udip
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Default preamp-cap-buffer

Hi all!


In order to avoid dc at the output of preamps you have about 3 choices:
(I'm talking about preamps that can drive low impedance loads,
like 1k and below, and also has variable gain):

1) Use electrolitic capacitor at the output, and it has to be quite
big if you don't want some bass roll.

2) Use some king of a servo to null the dc at the output.

3) Design a perfect circuit with perfectly matched parts in
order to avoid dc from the beginning.

Well, I don't like options 1 and 2, and option 3 is quite difficult
to obtain (at least for me), specially when the gain is variable.
So I thought why not use a PREAMP-CAPACITOR-BUFFER circuit,
so the capacitor can be a high quality small value one (because
of the hign Z of the buffer), and the buffer is unity gain so
it can be easy to obtain zero offset for dc-coupled output.
I'm not trying to avoid capacitors at all, but just electrolitics.

I'm sure I'm not the first one who thought about that, but I haven't seen circuits using this method, so I wonder if it has shortcomings that I haven't thought of?


Udi.
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Old 23rd February 2005, 03:16 AM   #2
Leolabs is offline Leolabs  Malaysia
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Hi,is me again Your block diagram seems has appeared in some1's article on designing low distortion power amplifier,but i cant recall who's article.

Regarding your "PCB",have you consider the offset of the output buffer itself????
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Old 23rd February 2005, 04:14 AM   #3
Stocker is offline Stocker  United States
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use a buffer with a pair of DC-null pins?
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Old 23rd February 2005, 04:51 AM   #4
Bill Fitzpatrick is offline Bill Fitzpatrick  United States
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?

http://home.c2i.net/semfielec/buffer/buffer.htm
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Old 23rd February 2005, 07:25 AM   #5
Leolabs is offline Leolabs  Malaysia
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As i expected,Bill,FET source follower with current sink loading Quite common but a good design indeed
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Old 23rd February 2005, 04:48 PM   #6
udip is offline udip
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It's quite easy to design a buffer with zero offset, like the
schematic that Bill posted...


Udi.
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Old 23rd February 2005, 04:54 PM   #7
adason is offline adason  United States
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http://www.pha.inecnet.cz/macura/buffer_en.html
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Old 23rd February 2005, 05:42 PM   #8
adason is offline adason  United States
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http://focus.ti.com/lit/ds/symlink/buf634.pdf
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Old 23rd February 2005, 07:35 PM   #9
AudioWizard is offline AudioWizard  France
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Why not 2)?
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