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Old 23rd December 2015, 09:30 AM   #1
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Join Date: Jan 2009
Default line preamp

Hi everybody

I found this schematic and constructed the line preamp without the current sources. It works great.
Now I'm planning to add the two B+ regulators, just to see if it's worth. Classic ones.
I would like to figure out, before starting building them, why to limit current with R-3 in the first stage's triode (B+2 regulator). No such thing on the phono part (B+4 regulator).
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Old 23rd December 2015, 10:31 AM   #2
Ketje is offline Ketje  Belgium
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R3 is not for limiting the current, the opposite, drawing extra current.The consumption of the first triode(s) is to small (2x 0.4mA) for the stabiliser to function properly.
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Old 23rd December 2015, 02:40 PM   #3
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could you please be more specific ?
why 2x0.4mA ? How is determined this value ?
what is the stabiliser ? OPAMP U1 or the whole B+ regulator ?
and what is the normal functioning current ?
I don't know where to find all these informations, except here.
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Old 24th December 2015, 01:10 AM   #4
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In the first schematic, V9 only draws 0.4 mA from the supply. I haven't dug too deep into the circuit, but the regulator might need some minimum amount of load current to operate correctly. If the load doesn't draw enough current, then a ballast resistor is added in parallel to the load to bring the total current up to the required amount.

The minimum load current requirement might have something to do with the current the op amp draws. If the load current is too low, the op amp might not have sufficient voltage to operate correctly. The load current just doesn't flow through the pass transistor, it also flows through the opamp and the zener diode D3.

Don't worry too much about the details. Just add the ballast resistor and smile.
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Old 24th December 2015, 07:51 AM   #5
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OK Richard, you are just paraphrasing. I already understood Ketje explanation.
I'll only smile when I'll be perfectly OK.
Is that "minimum current" the Id parameter found in mc34001P datasheet ?
Which is 1.4 mA typ.
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