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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.
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Topic - RCA mic preamp (BN-2A/OP-6) - Thermionic27609 08:59:26 07/15/15 (3)
- You would not connect the center tap at all - Ralph 09:58:19 07/15/15 (2)
- RE: You would not connect the center tap at all - Thermionic27609 11:53:22 07/16/15 (1)
- That might be correct - Ralph 12:50:07 07/16/15 (0)
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