What to do with that third XLR pin when connecting to unbalanced inputs? This inSync reader’s question is a common one we see.
I use two DA-38’s with an original CR-1604 mixer and an SV-3800 DAT. I’ve been connecting the DA-38’s to the 1604 by tying the “-” signal output to ground (for inputs 7-16 which are unbalanced inputs on the Mackie). This was suggested by a tech when I asked about it a couple years ago. I have since heard that it is better to leave the “-” signal floating since the balanced out is actually just two op amps tied to the same output signal, one run through an inverter. Does this sound correct?
There are a number of different types of balanced output designs in use. The inverted output you describe is one common type. They each react somewhat differently to having one side (usually the minus (-) side) tied to ground or unconnected. Therefore when you are connecting balanced outputs to unbalanced inputs it is best to know the configuration of the balanced output device (this may mean consulting a service/owners manual or calling the manufacturer).
A general rule of thumb for tying an electronically balanced output to an unbalanced input is to leave the (-) polarity pin floating. This will work best for both the DA38 and the SV3800. Tying the (-) to ground on some designs can cause the output to distort at lower levels or, in some cases, not work at all.
If the output has a transformer then you MUST tie the (-) to the shield. There is also an output design called “servo-balanced” that can detect when the (-) is tied to ground and will boost the output on the (+) side by 6 dB to compensate for loosing the differential of the (-) side. These designs are not all that common, however, because they are somewhat expensive.
The above rules ONLY apply to balanced outputs. Everything is different for balanced inputs. In general the (-) polarity pin of an electronically balanced input coming from an unbalanced source should be tied to the shield pin at the input.