On Solid Ground, part 2 (continued) |
|
MIDI cables also have a ground line (pin 2, which is the middle pin), so when you connect them to different boxes, it is possible to create a ground loop. If your equipment is wired according to the MIDI specification, this shouldn't happen, but it is possible and should be kept in mind when debugging your studio. Cable Quality If a cable has a poor shield, it allows EMI to enter the cable conductors. This interference usually manifests itself as a buzz and can be quite loud. There are three common types of shielding: braided, stranded, and foil. The general principle is the same for all three types of shielding: the more physical coverage there is, the better the shield. Wires that need flexibility, such as guitar or microphone cables, usually have a braided shield. Cables that aren't moved much have a foil shield, which provides superior coverage. Stranded shields (containing individual unbraided wire strands) generally provide the least coverage. All braided shields aren't created equal; the coverage depends on how tight the braid is. The cable spec specifies the percentage of shield coverage, which correlates to physical coverage; tighter braid is better. Cheap guitar cords often use loosely braided (and sometimes stranded) shields that provide as little as 70 percent coverage. Don't wire your studio with cables of this type. Some cables with plastic-coated plugs on the end do not run their shields under the plastic. Metal plugs are better because you know the shield is complete from one end to the other. If you must use any kind of adapter, make sure it is all metal. Line Conditioners A good, inexpensive start in the war on RFI is to put ferrite beads around the AC power cables coming out of the duplex wall outlet and around audio cables where they enter the preamp. But AC line conditioners are a more complete solution. (Try both methods together.) Many brands and designs exist, but a good line conditioner should have surge and spike protection, RFI filtering, and EMI filtering. The surge and spike protectors, of course, protect your equipment from power transients and surges. Keep in mind that not all people have RFI problems, as these problems are random and vary depending on your location. But it's a good idea to protect your equipment from as many possible problems as you can. Rack-mounted protection units are especially convenient, as they provide enough AC outlets to power the whole rack. Line regulators are a big step beyond surge and spike protection. These devices smooth out voltage drops and surges and supply a clean, steady 117 VAC. (For a detailed explanation of surge and spike protection, RFI and EMI filtering, uninterruptible power supplies, and line regulators, see "Getting Wired: A Power Primer" in the April 1990 issue of EM.) [Note: also see "Power Hitters" on this Web site.] |
Troubleshooting Since the mixer is the center of your audio system, it should be the central point of our studio grounding system as well; start analyzing the system there. Turn on the mixer and power amp and run the master fader up to see how much noise is in the monitoring system. If you have noise problems, it is possible (though unlikely) you have a loop between the mixer and power amp. Sometimes this doesn't show up until you plug a signal into the mixer. With a signal in the mixer, try unplugging the mixer from the amp and monitoring on headphones. If the hum is still there, the problem isn't between the amp and mixer, so you should go back and make sure your audio cables are of good quality and are placed well away from other types of cables. Once you're satisfied that the mixer and amp are clean, start plugging the cleaned-up racks into the board or patch bay, one piece of equipment at a time, checking to see if the noise level goes up significantly with each piece of equipment. If it does, check the audio cable, replacing it with one you know to be of good quality. If the cable runs through a patch bay, bypass the patch bay and go straight into the mixer. This tells you if there is it ground loop in the patch bay. In a well-designed patch bay, audio grounds should be isolated from the patch-bay chassis, so any ground loops probably are in the audio chain, not the chassis. Still, it's good to test for continuity between an unused patch bay jack and the chassis. If you still have problems, change the polarity on the AC plugs. Go through the whole recording system and check every audio line for hum and noise. If all else fails, and you still have ground loops in the audio lines, you may have to use audio isolation transformers on each of the offending lines. Although you can spend some serious money on high-end audio isolation transformers, you might get by with inexpensive Radio Shack models. However, sometimes these devices can add distortion and pick up RFI, so if they're not really necessary, you're better off without them. The easiest way to find bugs in your recording system is to do a mix. When you are mixing, you probably use every bit of gear you have. In addition, your gear probably will be plugged in through the patch bay in many strange ways, so doing a mix also is a great way to check for ground loops. Murphy Lives Most grounding problems can be dealt with easily using the techniques presented here and a little common sense. The latter, fueled by a healthy dose of caution, is the most important factor. Plan all parts of the system in advance, keep things neat, put safety first, and know when to call for professional help. Do it right, and the only humming you'll hear will come from the singers. |
| (The authors would like to thank Alan Gary Campbell, Chris Meyer, Kevin Kaiser, Larry the O, and Charlie Bolois.) | |
return to top |
<<first <previous | part 1 |