Wish to add phantom power to a mic mixer. I understand non-regulated but filtered DC is ok at 48 volts. Do I use matched pair of 6.8K ohm resistors to each balanced mic line (two wires) at input transformer? I have six transformers (inputs). Any issue regarding cross-talk with a common 48 vdc power bus? Or should I isolate each 6.8K resistor pair with a diode? The mic transformer inputs are center tapped with a .01 ceramic cap to chassis ground.
Add Phantom Power to mixer
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pstamler
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[quote author="amperex"]Wish to add phantom power to a mic mixer. I understand non-regulated but filtered DC is ok at 48 volts. Do I use matched pair of 6.8K ohm resistors to each balanced mic line (two wires) at input transformer? I have six transformers (inputs). Any issue regarding cross-talk with a common 48 vdc power bus? Or should I isolate each 6.8K resistor pair with a diode? The mic transformer inputs are center tapped with a .01 ceramic cap to chassis ground.[/quote]
As long as the resistor pairs are tightly matched there shouldn't be an issue with crosstalk, as any signal appearing on the 48V supply will be common mode and so will be canceled -- ignored, really -- by the transformer. Unregulated supplies, though, will fluctuate as the line voltage changes; you may want to rig up a simple zener-regulated supply.
By the way, you can use 5.62k resistors and get a little extra headroom in some microphones (others won't care).
Peace,
Paul
As long as the resistor pairs are tightly matched there shouldn't be an issue with crosstalk, as any signal appearing on the 48V supply will be common mode and so will be canceled -- ignored, really -- by the transformer. Unregulated supplies, though, will fluctuate as the line voltage changes; you may want to rig up a simple zener-regulated supply.
By the way, you can use 5.62k resistors and get a little extra headroom in some microphones (others won't care).
Peace,
Paul
