Work? L, R RCA inputs -> 1 XLR out?

RCA (L,R) inputs (pre-amp) = mono XLR output?

Does anyone know if this would work? I want to get a balanced (XLR) pre-amp the cheapest way possible. Is it possible to take a mono XLR input, amplify it via a standard unbalanced RCA stereo pre-amp, and then combine the outputs to a single XLR output? Only custom cables or connectors would be needed which is at my level of expertise :)

I realize I would need two preamps for stereo :) I already have two candidates to experiment upon.
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Most preamps have a differential input, but they make it single ended by connecting the RCA ring to chassis ground. They tend to tie both left and right side signal returns to a common point which is also chassis ground. I turned my Herald RA88a mixer into differential input by putting rubber O-rings around the RCA jacks, cutting the RCA jack buss and rerouting the "ground" wires inside to actually use the op amp differential input. That, and some other things, cut the hum.
On the other hand, my Peavey CS800s amp, which has actual XLR inputs, makes less hum (slightly) if I hook it up with RCA to 1/4 phone plug cables instead of RCA one end (from the floated output of the mixer) and XLR on the other end. A couple of hours making up XLR ends on a coax cable were wasted. Hint. Don't buy SPC21418 XLR plugs. The inserts melt too easily and the shells are too small.
If your preamp has an op amp as an input, or has two transistors tied together at the emitter, one for feedback, you are 1/2 way there. Just some wiring details to make it a differential input. If your preamp is all hooked together with ribbon cables with surface mount part, forget anything I said. The RA88a was hand wiring (kit) to a single sided PWB, and leaded parts on the PWB.
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The cheapest route would be adding two stages to the preamp. One that accepts a balanced signal in and and another that splits it into a low impedance balanced output.

The input receiver is the Texas Instruments INA134
The output IC I use is the Texas Instruments DRV134

Both ICs accurately manage differential signals with great accuracy and low noise/distortion. It is nearly impossible to get the same performance level with discrete components without very careful matching of components. These ICs make the job a piece of cake.

The only advantage to doing this is to accept devices that have differential outputs and to drive devices that have differential inputs.