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In the Forum: Audio Discussions
In the Thread: Input choke for Low Currents?
Post Subject: Re: Input choke for Low Currents?Posted by hagtech on: 3/7/2006
Hi Cat. I run the 12AX7s just a hair over 1mA each, and the 12AU7s at 6mA. With differential pairs and bleeders, it ends up at 39mA on each +/-300V rail. That works out to 7H minimum inductance at 60Hz, using the standard formula. You can use my online web calculator as a shortcut (www.hagtech.com/theory.html#choke).
Without enough inductance (the first L), then current can become discontinuous and output voltage a bit uncontrolled. As you point out, it takes a solid load to make these things regulate properly. I do the same thing on the heaters. In addition, I add series resistance with the inductors to tune them for damped response with the loading capacitors. This is very important if you don't want an LC tank oscillation - unstable output impedance. Basically, you have to tune everything simultaneously for current, voltage, and damping.
I added yet another trick with shortened tails on the differential pairs. They match the plate voltage such that if there is a change in line voltage (short duration), the dc voltage out the plate stays constant. I guess you can call this passive regulation. Don't think anyone else has come up with this yet. It's amazing to see it in action. I can put a scope probe on the plate (output) of each 12AX7, vary the ac with a variac and the plate voltage doesn't move. This will not happen with normal SE or traditional balanced stages (worst with current source).
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