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Old 21st March 2007, 09:19 PM   #1
blunt is offline blunt  Denmark
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Default tweaking a class AB amp

Hi all.

I am new to this forum so bare with me. i am kind of a noob when it comes to audio but i have build a class AB amp based on BD 139/140 transistors.

it amplifies arround 60 watt and plays very good. But i talked with my teacher and he said that it could play even better if i had better effects transistors but i dont know how to see if a transistor is better for the purpose(this is problerly still because i am a noob) so i was hoping that someone could reccomend me some. prefereable would be arround 100-150 watt output per channel.

if you need schematics or any other information about the amplifier please say so. I hope this can be a good start for me making alot of DIY audio projects so it would be nice if my first amp would perform good.

BTW i am building a pre-amp for it atm based on the tda2415A if that can help.

Cheers
Anders "blunt"
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Old 21st March 2007, 11:30 PM   #2
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Anders,

Quick calculation shows that with BD139/BD140 about 20W output will be maximum within the ratings - not sure where the 60W can come from. Presume we are talking per channel?

In a case of my own 80W amplifier I use the BD139/140 to drive my power transistors (TIP35/36). But over where you are quite a larger variety of transistors would be available than here (South Africa). I was restricted to 42V + 42V supply because of the transistor maximum safe ratings (that is for 8 ohm loudpseaker impedance).

I am sure others will reply here.
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Old 22nd March 2007, 04:17 PM   #3
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Hi Anders,

Can you post up your circuit ?

Cheers ....... Graham.
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Old 22nd March 2007, 04:24 PM   #4
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bd139/140 are too weak to use them in output stage
in practice they can diffuse something about 5W of power (per one transistor)

as Johan P. said max power of amp with bd139/140 in output stage is 20W

ok, first show scheme of this amp ...
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Old 22nd March 2007, 05:31 PM   #5
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Hej Anders

Welcome to this forum.

The BD139/140 are good drivers, but poor output devices.
You may want to look at BD911/912 as outpot devices
- easy to get in Denmark and cheap
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Old 22nd March 2007, 05:50 PM   #6
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Quote:
Originally posted by ACD
The BD139/140 are good drivers, but poor output devices.
Think again, not if you put 20 of them in parallel.
More amusement : a power amp manufacturer overhere even did it for series production.
(not counting 1 diy twit who got as far as 20 per channel)

And a very cheap output stage up to 75 watt at that, 40 of the good pieces could be as little as $10.
I seem to recollect NP mentioned he thought of going as high as 50 low power devices in parallel.
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Old 22nd March 2007, 05:57 PM   #7
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Jacco

In Denmark it would be cheaper to use two pairs of BD911/912 than fourteen pairs of BD139/140......
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Old 22nd March 2007, 05:58 PM   #8
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I forgot the
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Old 22nd March 2007, 06:01 PM   #9
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True, but 20 BD139 in parallel gives an output stage with an;

Ft of nearly 200MHz
hFe of 200
30 amps continuous, 40 amps peak.
A total Pd of 320 watts.
Lower Cob than 1 single big Sumo wrestler.

BD911/912 already were major suck in the previous century.
Ft of 3 Mhz, yikes
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Old 22nd March 2007, 06:24 PM   #10
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Default I am afraid that those BD139 base to emitter capacitance can create some trouble


Maybe this can be a problem too.... in parallel, i think, you will have much bigger capacitance if compared with a single unit

regards,

Carlos
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