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previous: Jag Okay, here's where I'd really like ... -- 8/13/2004 4:25 AM View Thread

Re: Reduce Hum In Line Operated Amplifier.

8/13/2004 5:54 AM
Dr. Photon
Re: Reduce Hum In Line Operated Amplifier.
not much you can do to de-hum something that's running off the mains.... I'd say convert the thing to a normal, power transformer-operated amp. connect the circuit ground (or grounds if this thing is like Harmony H400 with a ground for the preamp and a ground for everything else) to ground, and then use some transformers to power everything else up. One option would be to use two smallish 1:1 iso-transformers (or one of those spiffy 2X115 volt secondary units), one powering the B+ supply, and annother for the heaters (try to give this one a DC bias, perhaps with the positive bias V being applied to the 12AX7's center tap. A step up would be to grab a normal fil tranny for the 12AX7, and a 35 or 50 volt tranny for the 50C5 and recto (or use a 35C5?). I also recommend improving the power supply. try tossing the 35C5 and going solid state with some more filtering (like a Pi filter) - sag isn't really important in classA. If it were mine, I would grab two 12 volt or 6 volt trannies, wire them back to back with the secondaries connected together (feeding he 12AX7), and the 120 volt winding of the second one would be full wave rected with a diode bridge to make 170 volts (using a kinda large cap), and then a resistor/cap to the normal plate supply of the amp (select the resistor so the final plate voltage is the same as the old amp - aound 110V?). Then I'd just grab a 50 or 35 volt (35C5!) transformer to power the power tube.

Wild Bill Jag, Dr. Photon covered most everyt... -- 8/13/2004 8:26 PM
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