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What about so called audio output transistors ?
What about so called audio output transistors ?
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Old 18th October 2019, 06:03 PM   #21
georgesgiralt is offline georgesgiralt  France
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Join Date: Jul 2008
Quote:
Originally Posted by cbdb View Post
Did you readjust the output transistor bias after replacing them? It probably needs doing, and might make a big difference.
Oh, yes ! This amp had a bias problem on one channel. It turns out that the couple of input trans where quite shot. So I had to put a couple of selected equivalents with the highest same gain for both. It was then easy to set the bias.

I've found the Sanken and bought a couple of sets. So I be able to install them when I have to open this amplifier.
Thanks for your help !
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Old 18th October 2019, 06:52 PM   #22
academia50 is offline academia50  Argentina
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Join Date: Jan 2013
Quote:
Originally Posted by georgesgiralt View Post
...... So I had to put a couple of selected equivalents with the highest same gain for both. It was then easy to set the bias.

I've found the Sanken and bought a couple of sets. So I be able to install them when I have to open this amplifier.
Thanks for your help !

Well done, I see that my advice was useful, regards !
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Old 21st October 2019, 01:28 PM   #23
georgesgiralt is offline georgesgiralt  France
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Join Date: Jul 2008
Hello
I checked the datasheet, and found that the PNP and NPN have very dufferent Hfe. One is at about 25 and the other is around 150. How come they can be used as a push pull ?
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Old 22nd October 2019, 03:38 AM   #24
steveu is offline steveu  United States
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Join Date: May 2005
Location: Colorado Springs
Quote:
Originally Posted by georgesgiralt View Post
Hello
I checked the datasheet, and found that the PNP and NPN have very dufferent Hfe. One is at about 25 and the other is around 150. How come they can be used as a push pull ?
Which data sheets? The ones I found both say 55 to 160 at 1 Amp. It's probably about 25 at 10 Amps.
In any case it's important to understand that hfe is normally very poorly defined and only very bad circuits care. Normally the current available to the base is several times the actual base current so it makes little difference whether the base draws 2% or 10% of that current. Circuits are designed for the worst case and if it happens to be better that's fine. A good example is a voltage follower which is the most common audio amp output configuration.
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