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Old 1st January 2003, 09:32 PM   #11
PMA is offline PMA  Europe
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In fact the amp shown above has a serious flaw - very limited output voltage swing, especially in a negative half wave. It can be improved by adding three diodes in series with Q4 emitter lead, but anyway it is impossible to get more than some 2Vrms from this circuit.
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Old 1st January 2003, 09:38 PM   #12
halojoy is offline halojoy  Sweden
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Thumbs up Simple BJT power amps - nice thread!

Quote:
Originally posted by PMA
In fact the amp shown above has a serious flaw - very limited output voltage swing, especially in a negative half wave. It can be improved by adding three diodes in series with Q4 emitter lead, but anyway it is impossible to get more than some 2Vrms from this circuit.
nice thread. I am busy downloading the schematics provided.
RudiStor was a headphone intended amp.

This last is also simple.
I would sabotage this by adding a constant current source
instead of the 8 ohm resistor.

There is no need to have the input at half the voltage supply, 12 V
You can have it much lower (4-6V) , to give the output
more "space" to operate. Gives more output voltage swing.

But nice, very nice.

/halojoy - Beauty can be Simplicity
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Old 2nd January 2003, 05:26 AM   #13
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"Death of Zen" is a workable circuit, but constitutes 3
gain stages. Personally I think BJT's are do-able with 2 if
you select high beta devices and don't get too fussy.

The fact is, though, that it's a lot easier with Mosfets
due to their high input impedances.
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Old 2nd January 2003, 09:35 AM   #14
LennyK is offline LennyK  Poland
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Location: Gdansk - best part of Poland :)
funny thing:

I often read threads with no care about authors. I downloaded simple.bmp, looked and thought that I would do this in a different way. Why he did it that? And then from Gabevee: "Mr Pass,... ". Scroll up .... simple.bmp author - The one and only .



regards

Lenny
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Old 2nd January 2003, 12:04 PM   #15
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Default How's this for simple???

Never built it though.
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Old 2nd January 2003, 01:15 PM   #16
PMA is offline PMA  Europe
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Default simple BJT amp already built :)

I was very curious how the circuit that I had shown here yesterday would work . So I have built it and it runs quite happilly, exactly according to my yesterday's description. Negative output halfwave is very limited, because of the Darlington 2Vbe influence to Vce voltage of Q2. But add 4 diodes (I used 1N5408) in series with emitter of Q4 and you have a working amp. It will give you some 1W into 8 Ohms, then there is a very very soft limitation (mostly second harmonic) to some 10 Vp-p voltage swing. The amp is very quick but not very stable, so put a 470pF cap between base and collector of Q3 and everything runs well. You can increase R5 to some 15 Ohms/10W (if R5=8 Ohm it must be 20W) not to produce so much heat .
Pavel

P.S. to see much better amps please visit my homepage
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Old 14th January 2003, 01:55 PM   #17
halojoy is offline halojoy  Sweden
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Click the image to open in full size.

I will soon present an even Better and Simple Bipolar Amplifier
have to learn how to use these damndish
cicuit drawing programs.
Seems they are anything but
simple to use and Good I really hate them.
They can do spice and all exotic things
but to draw a simple circuit - without swearing abd coursing
seems only in my dreams

This was a good contribution Circlotron, old cardealer

But I think we have to beat it!

/halo
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Old 14th January 2003, 06:09 PM   #18
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This circuit is seriously wasteful of the supply voltage, as
even with perfectly ideal transistors, the maximum output
per speaker will be 1 V rms, which is 1/8 watt for 8 ohm
speakers. A more ordinary output stage would give you
1/2 watt.
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Old 14th January 2003, 09:23 PM   #19
PMA is offline PMA  Europe
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This one will be workable and will give reasonable output o several watts....
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Old 14th January 2003, 10:04 PM   #20
halojoy is offline halojoy  Sweden
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yes, and still only 4 transistors.
And almost 4 Watts Out/ 8 ohm
Needs V+ 35 Volts for 4 W.

Will have a Idle power consumption: 35x2A= 70Watt
Effiency: 4/70= 5.7 %

But this one is not bad ...
I would put Collector of Q4 to 0V-line.
Q4 will work more linear 30V all the time.

I lose a little Current from Q4 into load,
but that is anyway not so much, that it is worth having
such a darlington coupling.
I say let Q4 get some REAL working space.
I promise it will be noticed in performance (distortion)

/halo needs at least 4 T
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