Dehumming & Debuzzing

NOV/DEC 2002 TIP

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Alexander Magazine

Dehum & Debuzz Techniques At Home

Archived Article 1 for Recording Magazine Reader's Tip 5

From The Recording Engineer's Quarterly Archives

BY ROBERT DENNIS

All studios, big or small can have problems with buzzes and hums. These types of problems can quickly make the product put out by the studio unprofessional or even unusable.

These buzzes and hums come primarily from television broadcast signals that get into audio wires and equipment. The interconnecting wires for the equipment have a shield that is supposed to be connected to "ground." The "connection to ground" literally means that the shield is connected to the earth we walk on. This ground is "zero volts." The idea is that a broadcast signal (or other stray signal) can induce buzz in the shield which "shorts out" the buzz. Home studios and professional studios alike get into trouble when grounds or shields are not used or not used properly.

The professional studio can spend a lot of money and effort getting "grounding" correct, but to do a half-way-decent job that works in 99% of the home studios is relatively easy.

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Step 1: Establish Ground

The 3rd pin of a common household outlet is supposed to be "grounded." This ground is good enough for most small studios. Sometimes the outlet is not wired correctly. To establish that you have a ground go to Radio Shack and purchase an outlet tester (about $15) and plug it in. The instructions will tell you what lights are supposed to light if your outlet is correct.

If you have an older 2 prong outlet, buy a grounding adapter (about $1.00) and connect the green wire or green tab under the screw that holds on the plate. Then test the outlet.

"Ground One - Hum None"

Step 2 - Establish ONE Ground Source

In theory, any of the outlets in a building are supposed to have "0" volts on their ground pins. NOT SO. Most outlets have a small voltage (1/2 to 1 volt). So it is important that the SAME ground be used for all of your equipment, including instrument amplifiers that are going to be used. If different outlets are used for some of the equipment the, difference between the voltage on the ground pins of the two outlets can cause hum.

Use one outlet and a bunch of multi-outlet strips (these are about $7 at your building supply company). Make sure everything goes back to the same outlet. Your equipment does not draw a lot of power and 99.9% of the time you won't be tripping circuit breakers because of this. Try to choose an outlet that doesn't have motor-driven appliances likes washers on the same circuit.

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Step 3 - Check For Broken Wires

A piece of equipment can work with a broken ground wire but become an "antenna" for hum & buzz.

The third pin of equipment power cords connects the case of the equipment to ground. One of the pins of the audio cables carries the shield.

You can make a tester to check your cables by taking apart a flashlight (about $1) and taping some scrap wire to the battery & bulb (as shown in the diagram). Connect one wire to one pin of the audio connector at one end of the cable and the other wire to the same pin at the other end. If the light lights, your connection is good. For the 3rd pin on the power plug, connect the second end of your tester to the metal of the case on your equipment.

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