On my Soundcraft Ghost mixing console the aux outputs are listed as ‘Ground Compensated’ in the manual. I’ll be connecting these aux outputs to the balanced inputs of my effects gear, which is mostly on XLR and some on TRS. I’ve been using Rane Note 110: Sound System Interconnection as my guide but I can’t determine exactly the options for wiring up these so-called ‘Ground Compensated’ outputs. Do you have any suggestions?
Russell, via email
SOS Technical Editor Hugh Robjohns replies: The ‘ground-compensated’ or ‘GC’ output is a very clever circuit design dating from the 1980s, and Douglas Self, who was a designer at Soundcraft, explains it very well in his book Small Signal Audio Design (reviewed in SOS January 2015: http://sosm.ag/dself-ssad-2nd-ed). The ‘ground compensated’ output is actually a ‘single-sided’ output, but one cleverly designed to work equally well with both balanced and unbalanced destinations. However, its real strength is that it is superb at preventing ground-loop hum problems with unbalanced destinations.
As far as the cable wiring for your balanced destinations is concerned, you simply wire the cables in exactly the same way as for any normal balanced configuration. XLR pin 2 (hot) goes to XLR pin 2 or TRS tip; XLR pin 3 (cold) goes to XLR pin 3 or TRS ring; and XLR pin 1 (screen) goes to XLR pin 1 or TRS sleeve.
If you’re interested in the electronic technicalities, you’ll know that a conventional balanced output provides half the signal on the ‘hot’ connection and the other half, polarity-inverted, on the ‘cold’ connection. As a differential input looks for the voltage difference between the hot and cold lines, it sees and extracts the full-level signal.
This arrangement is often called a ‘symmetrical’ output because the signal is carried symmetrically on both sides, but note that this symmetry is purely a convenience; it has nothing whatsoever to do with the rejection of interference! The interference-rejecting properties of a balanced interface employ the differential nature of a balanced input to cancel out any interference, but that critically relies upon the hot and cold lines having equal (ie. balanced) impedances to ground. Only then will any interfering signals develop exactly equal voltages on both the hot and cold lines, and thus cancel out accurately in the differential receiver.
A GC output doesn’t send a symmetrical signal but instead provides a ‘single-sided’ signal — at full level — only on the ‘hot’ pin of the output XLR (pin 2). There is no output signal on the cold pin, but as a differential input looks for the voltage difference between the hot and cold lines, it will still see and extract the full-level signal. The unusual aspect of the GC design is that not only does the cold line not carry any signal, it’s also not even an output: it’s actually configured as a ‘sensing’ input! The idea of an output socket having an input connection might appear confusing at first, and I’ll return to that in a moment, but first it’s worth considering the simpler but related ‘impedance balanced’ output configuration, which is also commonly employed on a lot of mixer and interface outputs.
The impedance-balanced output also sends a full-level signal on the hot side of the output XLR, and the cold side is connected directly to ground through a resistor, which presents exactly the same impedance as the hot side output’s impedance. This ensures that the system appears as a properly balanced source, and thus retains the normal balanced line interference-rejecting capabilities (see diagram).
The benefits of this design are that it is cheaper to implement (needing only one active line driver instead of two), and the full signal level is received at an unbalanced input instead of being 6dB lower than expected (which would be the case if connecting just one side of a symmetrical output).
However, in a GC output the cold-side impedance-matching resistor is typically split in two to act as a voltage divider. The junction between the resistors is then used to feed part of whatever voltage appears on the cold line back into the output driver where it is added to the wanted output signal (see diagram).
Now, if this GC output is connected to a balanced input, there will be nothing on the cold line and so the system works exactly like a normal impedance-balanced output. But, if connected to an unbalanced input, the cold line will reflect the destination equipment’s ground, which may well be at a slightly different voltage to that of the source equipment’s ground because of a ground-loop. Normally, that difference would result in a ground-loop hum, but the GC output automatically adds any ground voltage difference to the source signal, thus neatly cancelling out any ground-loop hum.
I’m surprised the GC output form is not more widely employed because it is just as simple to implement as the impedance-balanced arrangement, requiring only a slightly different circuit. It’s just as cost-effective and easy to implement, but it provides built-in and extremely effective compensation for ground-loop problems that would otherwise require isolating transformers or other shenanigans to resolve.