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Old 19th November 2005, 06:14 AM   #1
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Default Butler input pair

I know that some audio-grade opamps use Butler front ends (a pair of BJT and FET transistors in parallel as the input transistors, to try and achieve the best of both worlds), but has anyone tried this for a discrete power amp? What where your results?
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Old 19th November 2005, 06:26 AM   #2
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Hi rtarbel,

What problem to solve? This is probably the question to ask. BJT stages have 'bias cancellation' and FET chips have adequate monolithic matching to suit audio. Could simply be introducing the weaknesses of both.

I don't see a need so wouldn't be looking for this.

Cheers,
greg
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Old 19th November 2005, 11:51 AM   #3
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Just what I always wanted.

High input bias (BJT) and output phase reversal on overdrive (FET).

http://patft.uspto.gov/netacgi/nph-P...&RS=PN/5101126
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Old 20th November 2005, 12:26 AM   #4
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by djk
...............output phase reversal on overdrive.....
Vanishingly unlikely in an audio amp....
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Old 20th November 2005, 01:48 AM   #5
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Right.

Do you have any real experience?

I didn't think so.

Having built an amp with this (and other FET opamps) with this problem is why I bring it up.

The data sheet from Analog Devices (OP275) even talks ways to minimize the problem.

Of course, what do they know? They just designed it, built it, and got a patent on it.

Maybe to be of help you should write us an application note, AD may even pay you for it.
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Old 20th November 2005, 02:33 AM   #6
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by djk
.....got a patent on it.....
...patents can be obtained for virtually anything....

...usefull...or otherwise...

...phase reversal due to overdrive in an audio power amp specifically is a non-issue....
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Old 20th November 2005, 02:42 AM   #7
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by djk
Right.

Do you have any real experience?

I didn't think so.



http://www.diyaudio.com/forums/showt...393#post198393
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