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In Reply to: RE: RCA mic preamp (BN-2A/OP-6) posted by Thermionic27609 on July 15, 2015 at 08:59:26
-Connecting the center tap will degrade CMRR (Common Mode Rejection Ratio).
The ground pin (pin 1 on the XLR) should be tied to circuit ground but otherwise should not engage the audio signal at all. IOW, the output is occurring between pins 2 and 3 of the XLR and is ignoring ground. As a result, a properly wired transformer used for balanced operation will be a floating winding.
You might want to read AES file 47 about this topic and there is an excellent primer on the Rane website, linked below.
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Thanks, Ralph.
Also, someone else suggested to me that this unit might need a balanced attenuator on it for best operation since it was designed for a 500 Ohm load (what he says was the telephone line Z). He said that running it into a higher Z load like a 10k input on a modern console might result in ringing in the OPT.
You will want to see what the input impedance actually is. It is is 1 or 2K you will be alright (the transformer is designed for 600, not 500 ohms). If the input is 10K you will want to place a resistor across the output of the preamp so the transformer has a proper load. That way you will get the flattest frequency response.
In most cases though you really don't want to run the mic preamp into the console- if at all possible run it directly into the recorder. That works great as long as the preamp has a volume control.
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