Aphex 228 - Manual (Page 14)

Aphex Systems Ltd. Model 228 Page 12
228
instruction Manual
Page 13 Aphex Systems Ltd. Model 228
228
8-channel audio interface
Appendix B: Dealing With Grounds and Hum
Ground Loops
Many people equate this term with hum, and that’s just about the bottom line of it. If you have a ground sensi-
tive system, like unbalanced audio equipment for example, then hum will result from ground currents that flow
from the ac power system. It is sometimes very difficult to isolate and stop ground currents between unbal-
anced equipment, but it is quite easy to clean up balanced gear. That’s why pro gear is always balanced! The
cost of balancing is that of more expensive connectors, cable, and electronics but the cost is worth it when you
depend on your audio quality. Now that we’ve sold you on only using really expensive pro gear, lets show you
how to get away with the really cheap stuff! At least from the standpoint of killing ground hum.
A ground loop is an ac current that has become routed through your audio ground system. The current comes
mainly from ground potential differences that exist between different wall outlets that return to opposite
phases at the power distribution panel. Secondarily, however, many pieces of equipment contain line filters and
transformers that leak a small amount of ac power into the chassis and ground return.
You may once have had the experience of getting zapped by touching two pieces of gear at the same time.
That illustrated the ground loop effect - - straight through you! No matter what you do, you may not be able to
prevent some of your equipment from generating ground currents. The most likely culprits are digital products
because they use switching power supplies that require heavy line filters to prevent conducted EMI from going
out of the box. Filters so employed very often take the ground leakage current right up to the UL safety limits.
Although it won’t kill you, that is a lot of ground loop current for audio cables to handle.
There are basically three ways to attack the problem of a ground loop. First is to eliminate it from its source,
and the second is to re-route it through another path. The third is to balance out your unbalanced audio
interfaces.
Identify the Sources
A good way to identify grounding problems is to use a multimeter to check the ac voltage between the chassis
of your various gear when no audio cables are hooked up and all gear is plugged in and switched on. Just start
touching the two probes to the metal chassis of different pieces of gear. Ideally, you should always see zero
volts. Warning! You may see as much as the whole line voltage between two different chassis! It does happen.
This voltage between chassis will be responsible for your ground loop problems. If you find there is more than
about 1 volt between equipment grounds, you should start looking for a remedy.
Commonize the Power
Try plugging all of your equipment into the same outlet strip. Get one that has enough outlets in one strip or
string more than one together. Of course, you need to make sure you don’t overload the one ac circuit your
strip is plugged into. If the load is too great for one circuit, use a second or third circuit that is tapped off the
same 120 volt phase in your distribution panel. That means all outlets should be on odd or even numbered
circuit breakers. That’s because, as you go down the column, the circuit breakers tap into alternating legs of
your incoming electric power. Be sure you’re always on the same leg. You can tell you’re on the same leg by
measuring the ac voltage between the hot slots of the different outlets you’ve chosen. It should be very low or
zero. That will remedy 50 percent of the cases.
Check the Cord Polarity
For products that have 2-wire power cords, try reversing one of the power cords in the socket. That may reduce
the ground current generated by the internal electronics of the offending gear.
Redirect Ground Loops
Appendices