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Old 12th January 2013, 02:08 AM   #11
JMFahey is offline JMFahey  Argentina
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Yes, in general cases this is correct, but in this particular amp the Vbe biasing string is incredibly low resistance.

Idle current across the Vas transistor T7 (and its current source T6) is almost 14 mA.
The biasing string current is conventionally chosen as 1/10th of that, meaning around 1.4 mA would be fine.

Why "1/10"?
So we can use a real world biasing transistor, and not worry too much about its actual Beta/Hfe .

Beta 100 would mean we'd have a base current 1/100 of the collector current , or 1/10 of the chosen biasing string current, for an error of 10%.
Which is no problem at all, that's why part of the biasing string is an adjustable trimmer.
And if we have Beta 500 , as in a BC546C , no big deal, instead of 10% error we would have 2% .

BUT in this particular amp, they did not choose a bias string current of around 1.4 mA, they chose ridiculously high 7mA.

Why ridiculous? Aren't they erring on the safe side?

Well, respect to Beta, maybe, ......, but now *half* of the Vas current is wasted running the biasing string (R13/VR2/R14) , instead of fully passing through Thermal sensor T8.
I bet that throws accuracy away.

Not dissing AKSA's calculation, which is fine in a "normal" amp, but in *this* case I would not lower even more any of those resistors.

FWIW, in my quite similar architecture 300W/4 ohms MosFet Bass amp I sense with a BC556 (yes, a PNP) , and the biasing string is around 7K total , 10X higher than the meager 710 ohms shown here.

Works like a charm, under heavy stage conditions.
*Literally* Heavy, most of my customers are Metal Bass players.
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Old 12th January 2013, 06:09 PM   #12
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So how does one further optimise the Vbe stage in the Nmos200 and its many variants, including the Nmos350 Mk II in which the B-C resistor of the Vbe is increased to 680E and the 200E Trimpot is moved to the B-E string with the variable pin shorted to one end pin of the trimpot, which is in series with the 47E resistor and the Vbe transistor has been changed to a BC550C type?
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Old 12th January 2013, 06:28 PM   #13
JMFahey is offline JMFahey  Argentina
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Can't talk about NMos 350 (although I guess it will be quite similar to my own 300W )
In the case of the NMos 200 , if the original values work, continue with them.
My *personal* preference would be to double or quadruple those values, as I mentioned before, but choosing one or the other is your option.
I'm not trying to preach anything here
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