• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

line preamp

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sb****33

Member
Joined 2009
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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R

RichardFlash

Member
Joined 2013
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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