Here’s an oldie-but-a-goodie from the inSync Tech Tip of the Day Archives:
Let’s say you need to run a signal from a keyboard to your mixer. The unbalanced line is quite noisy and you don’t have a balancing transformer. What to do?
It’s not hard to make a pseudo-balanced line from the unbalanced output of the keyboard: Make the run with a standard balanced cable (TRS or XLR). For an XLR cable, at the end where the cable plugs into the keyboard connect a precision potentiometer to pin 3 (cold, or “-“) and to ground/shield (pin 1). On a TRS this would be between ring and sleeve. Connect pin 2 hot, or “+”) as normal. On a TRS this would be the tip.
Turn the equipment on and turn up the gain on your mixer until you hear a good amount of hum and line noise coming in from the keyboard line. Turn the potentiometer (which will change the resistance between “-” and ground) and you will hear the hum and noise level change. With some careful work you should be able to find a “sweet spot” where the hum is minimized. If this is a permanent installation you could then measure the resistance of the pot at that setting and substitute a precision resistor in place of the pot.
What you have done is put a resistance in parallel with the output impedance of your source (in this case a keyboard) from “-” to ground that matches the “+” relationship. Normally, on an unbalanced line, “-” would be shorted to ground, and you give up any CMRR your mixer’s balanced input has.
We’ve heard this make a dramatic difference in certain circumstances. Remember you must use precision components here, as minute changes in value will significantly effect the signal-to-noise ratio.