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RE: Thanks, Lew

Frank re-stated my own hypothesis for your loss of gain, and I think we're both correct. In simpler terms, the hot pin of the RCA jack is in parallel with pin 2 of the XLR. The ground side of the RCA input is in parallel with pin 3 of the XLR. In a true balanced input, pin 3 carries the opposite phase from pin 2 (by convention, the negative phase, but it's all relative). When you use a shorting plug, you are now also shorting pin 3 of the XLR to ground. Thus the "negative" half of the signal delivered to the XLR is going to ground; you are driving the XLR input in single-ended mode, and you lose gain.

I still wonder why you have significant noise that is cured by a shorting plug.

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