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Old 26th December 2005, 12:15 PM   #101
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Hi

The below curve shows why many constructors turn to BJT's for good linearity in the output stage, and not Mosfet.

The Linear region of operation starts when the output impedance becomes roughly independent of the load current. In this region the output stage doesn't contribute with any significant distortion.
On the curve this corresponds to a flat horizontall line.
As you can see you only have to run the Ic current at a certain level (depending on the transistor type 70 - 500 mA) to get a Ic independent output impedance.
However in the MOSFET the curve only gets completely flat at something like 5-10A, but it gets closer and closer to horizontal, the higher the Id.
This is why MOSFET's are good for Class A amplifiers, providing you keep the idle current above 5-10 Amps. But MOSFET will never get a low THD in Class A/B amplifiers. I have made bunches of MOSFET amplifiers, (like the ZETA), the good thing about MOSFET is that they are very easy to work with, don't require a lot of gate drive, are rugged, easy to come by and dirt cheap. But they will never be the ultimate output device for high quality amplifiers.
(Well maybe except for Class D of course )

On a more academic level, there is nothing that prevents you from building a Pass amplifier using BJT's instead of MOSFET. Of course it's no longer a Pass Amp, but it should work with no significant complications. I recommend a good BJT from SANKEN 2SC2922. It should fill in nicely for the IRF's, and for idle currents below 5 Ampere is bound to give you a better result than MOSFET.
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Old 26th December 2005, 03:31 PM   #102
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Is there any reason to not use an op-amp for the voltage gain stage? I wipped up a design using an op-amp for voltage gain and a bjt on the output using the common LED/BJT current source to bias it.
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Old 26th December 2005, 06:57 PM   #103
rmgvs is offline rmgvs  Netherlands
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Default pass with bjt's

I understand from previous discussions that just replicating the Pass circuit with BJT's (take for example the mini-aleph, a 2-stage design) is not so easy. One small transistor will not drive a bjt power stage to reasonable levels. Would it be possible to have the circuit here on the forum with some values and numbers, better still with some measured or simulated data? I guess a great deal of diy'ers are interested!
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Old 26th December 2005, 08:04 PM   #104
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Could you find the sch's in question for me?
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Old 26th December 2005, 08:51 PM   #105
boxedin is offline boxedin  Canada
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What we need is power stage with BJT. with two transistor i get load current of 500 ma rms with max input (300mv).

How did you get 5 A on load with just BJT transistors?

Anyway , you all find it . it seems like you all have built so many amplifiers.

I am waiting..... meanwhile who has best simulation program?.


Belated Merry christmas and Happy new year to all.

ALexk
Toronto
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Old 26th December 2005, 09:24 PM   #106
lineup is offline lineup  Sweden
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Schematic
see my post above
with this picture attachment:

http://www.diyaudio.com/forums/attac...amp=1135293820

Output here is a combination of 2 transistors, because like rmgvs says, BJT power transistors are not so easy to drive.
This 2T output device gives an easier load for the voltage amp stage.

Also Q2 shouldnt have too low current.
Like 15-30 mA, depending of how much power amp is to produce.

As said before (R2/R1 + 1) is the voltage gain.
And the resistor R8 should be replaced with a current source.
Could be 1-1.5 A
Voltage supply could be 24-30 V.
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Old 26th December 2005, 09:29 PM   #107
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S 30:

snip Lars
"It's not very special or unconventional..."

Hello Lars
indeed, I meet something in the early 70ths.....
may be I am too old!

snip rmqvs
"Why do you put the speaker to the + volage rail?"

I am so free to answer:
Because to save 30 cent for a R and a C! .....
...for that You get a gruesome powersupplyrejection!

Regards
Heinz!
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Old 26th December 2005, 10:09 PM   #108
boxedin is offline boxedin  Canada
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Are you people are designers ? or just amatuers who are building amplifier by copying others circuit without knowing the circuit?.

How can you connect speaker to power supply ? Even if the speaker can handle the voltage, how do you stop DC noise?

Bad design i think.
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Old 26th December 2005, 10:58 PM   #109
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Hey buddy i am a professional i have designed the pure-a wide
band poweramp that sounds like nothing you have have ever listened before, the thing is it is still being refined and it wont be ready in a series of models for a couple more years
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Old 26th December 2005, 11:01 PM   #110
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Heinz: Right on. But as you must have guessed already it's not about the 30c but about making the simplest solution with the fewest possible parts. (As in the header of the tread).

But like i said before, this is not the same as the best solution........

In fact i can only advise everyone against building the oversimplified amplifier circuits, you will never get a very good result from that, so you are really wasting your time.
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