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Q. Is it safe to insert unbalanced plugs into balanced outputs?

By Paul White
Published June 2000

I know how to wire from unbalanced outputs to balanced inputs using the old '220Ω resistor ploy' but... I am wiring up a 'balanced' patchbay which actually contains a mixture of balanced and unbalanced sockets. It is quite possible that a piece of equipment which has balanced outputs might be connected to a balanced input or an unbalanced input depending on the patching at the time. On the equipment which is unbalanced, where the 'tip' is obviously hot, should I short the 'ring' (cold) to ground or leave it unconnected? I'm sure I've heard that if you get this wrong with certain equipment it explodes and your socks fall off...

Steve Watts

Editor Paul White replies: Your question is a valid one and maybe it's time we looked at this subject again. As a rule, plugging unbalanced sources into balanced inputs causes no problems other than a 3dB level drop in some cases, depending on how the input stage is designed. However, where balanced inputs are available, it's always worth wiring your unbalanced output in a pseudo‑balanced configuration as it can cut down on earth loop problems.

To do this, use a balanced cable with the end feeding the balanced gear wired normally (TSR jack). At the source end, an unbalanced jack is used where the hot conductor is wired normally but the cold goes to the tag normally used to connect the screen. The screen itself is also wired to this tag but via a resistor of between 100Ω and 1000Ω depending on what you have lying around. When feeding unbalanced inputs from balanced outputs, most circuits will let you work unbalanced just by plugging in a mono plug (in effect shorting out the cold), but you may suffer another 3dB level drop if the output stage isn't a fully floating design. As output stages differ slightly, it's worth checking the manual as many suggest a suitable wiring protocol. A few circuits specify you leave the cold leg floating. Worst case, you may have to make up a few special patch leads to accommodate those pieces of gear that don't follow the general rules. Flames and sparks are unlikely though!