HUM & PIN #1

By Marty McCann
Copyright 1995

Sooner or later every sound person encounters a ground loop hum' in their sound system. Most people think that adding a Peavey PL2 transformer module to the power amplifier rack will eliminate the problem, but this is not always the case.

In professional audio, the sound audio signal path should only be grounded at a single point. This single point grounding is circumvented when we place the mixing console remotely or away from the power amplifiers. Our USA National Electrical Code (NEC) specifies that all electrical equipment that draws potentially harmful amounts of current from the AC Mains must have its metal chassis grounded to the electrical systems earth' ground.

When the mixing console is in a remote position from the power amplifiers, we now have the sound system's audio signal path connected to ground at two points. If there are separate racks of power amplifiers on both sides of the stage, we may have multiple connections to ground creating paths that also connect the audio signal to the electrical earth' ground.

The resultant hum is directly related to the differences in current that flow in the respective electrical circuits, which all share the same earth ground. If the audio signal path is only connected to ground at a single point, the differences in ground currents will not induce any interference from the power line, so the sound system will not hum.

Many uniformed sound persons remove the grounding pin on the electrical cord of one or more pieces of equipment deemed to be the cause of the hum or ground loop. This is in violation of the NEC and should not be done under any circumstances. Using the little gray three pin to two pin electrical ground lifts and not connecting the ground wire to the electrical ground is not' the correct solution either, and is likewise contrary to the NEC.

The problem lies in the fact that the balanced audio connections between the mixer position and the power amplifier positions have a common audio signal ground that finds its way to the electrical ground via of the multiple chassis ground connections.

The solution is to lift the audio signal or pin ##1 of the balanced connector at the source of the balanced audio signal line. In this case, pin ##1 should be lifted at the mixer position.

Whichever is the last audio system component that acts as the source (send) of the balanced line (mixer, EQ, crossover, compressor, etc.), this source component's send, should have pin ##1 of the connecting balanced cable lifted or floated' from its chassis.

The shield of the balanced line will still terminate or go to ground at the receive end or the amplifier rack. This then eliminates the hum that would be induced if both ends of the balanced line went to ground. You have eliminated the cause of the hum by using this correct approach to signal distribution without violating the NEC. Lifting the electrical ground pin is always in violation of the NEC.

I recommend that each sound person have a number of short (4 to 6 inches) balanced microphone cords made up with pin ##1 lifted or floated in the female side of the connector. This will allow them to float' the balanced lines on any system that they encounter without their having to go into the snake box or the equipment itself to lift' pin ##1 from the chassis ground.

Note: Be careful when cutting the ground wire away in a regular microphone cable. You want to trim any excess wire back to the insulation so it won't short to any of the pins in the encounter.

Also, if you just have to cut a microphone cable, mark it as ground lifted immediately or you'll shoot yourself in the foot at some future gig, especially if you try to use that mic cable with a phantom powered microphone.

In some applications, particularly those sometimes encountered with electronically balanced inputs at the end of a balanced signal line; lifting pin ##1 from ground will eliminate the ground loop hum but you may then experience RFI' (radio frequency interference) in the system.

The solution is this case is to add a small capacitor (such as a .001 mFd) from the shield or ground wire to pin ##1. Adding the capacitor allows the balanced line to be lifted (open) from ground at audio frequencies, while acting as a closed circuit for radio frequencies.

In order to explain in any further detail would require a thorough discussion of electrical power distribution. The foundation of the above tip is: Not allowing the audio signal path to go ground in more than one place.

If you have more than one signal path, then each path should only have its signal path's shield connected to ground (pin ## 1) at one end, usually at the termination or receive end.

The real cause of the ground loops has to do with the way that our electrical power is distributed. In 1996, a new article was written into the NEC code that covers balanced power distribution. If you want more information on this, see my article on balanced power distribution.

Good Luck!