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I've been refurbishing a RCA BN-2A mic preamp "Remote Amplifier" (basically an OP-6 and OP-7 combined on the same chassis) for a friend who's a recording engineer.
I started thinking about the XLR output jack he'd asked me to add in relation to the output transformer (T4). This thing was originally designed to drive phone lines back to a radio station, so it was grounded to a local ground and hooked to a twisted pair. In the stock configuration, the output transformer secondary center tap is not connected to anything.
For modern use as a mic preamp, what would be the optimal way to connect the ground of the balanced XLR output jack and/or the output transformer's secondary center-tap (Terminal 8 on the schematic)? The output balance is better with the OPT's secondary center-tap grounded to the main chassis ground, but should the center tap be connected to the unit's main ground, or should the secondary center tap go directly--and only--to the XLR ground pin, keeping the OPT secondary completely ground-isolated from the rest of the preamp and grounding it only to whatever it's driving?
The only similar circuit I'm familiar with is the Hammond Organ preamp, and its preamp OPT secondary has its center tap grounded to the preamp's main ground.
Follow Ups:
-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.
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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