BANISH NOISE FROM YOUR STUDIO

It never fails! You set up your studio, and get all set to record. Then you are suddenly aware of "mmmmmmmmmmmm" coming from the speakers. Grounding problems strike again! Then something goes "grickle grickle". There's something foul afoot.

Grounding and noise problems are usually caused by one of the following:

• Something that is not grounded
• Too many grounds: About ground loops
• Bad connections
• Mismatches

Setting up the studio with a systematic grounding system is essential to avoiding these grounding and noise problems. This setup usually works:

1. Get one (or more, if needed) of those multi-outlet surge-suppressor power strips. Plug one of the strips into a wall outlet. Plug all of the other strips (if used) into that one strip. Include any instruments and amps in this (unless they all draw too much power for the house circuit). Then plug all of your devices into the strips, not other wall outlets. This has the following advantages:
   • It gives your entire system a common grounding reference point at the master outlet strip. It keeps your equipment from being plagued with the noises associated with having different grounds (caused by multiple cable runs back to the breaker box).
   • Amazingly, doing this alone gets rid of most grounding problems by itself. If all of the components have 3-wire plugs, this is the equivalent of a star ground system, but without the expense.
   • If more than one circuit is needed to handle the power needed, use ground loop isolation techniques (see below) on cables interconnecting equipment on different circuits. Put the signal handling equipment on one circuit, and the power amps on the others.
2. Never lift the safety equipment ground on the power supply for any device.
   • Never remove the third prong of any power cord.
   • Never insert a ground breaker or 3 to 2 wire adapter in any power cord.
   Any of these could cost you your life. They are also illegal to do in many jurisdictions. If there are grounding problems, they should be taken care of in the signal lines, not the power lines.
3. Turn off any light dimmers, motor speed controllers, fish tank heaters, and other devices that use semiconductors to regulate power to heavy loads. These cause lots of noise.
4. Make sure you use metal screws to mount devices in your rack. This electrically bonds the chassis of each device to the rack. If you have more than one rack, bond them together with heavy wire.
5. Treat rugs and other static generating surfaces with anti-static solutions to take care of any static electricity problems.
6. If hum or other symptoms of ground loops are present, look for places where there is more than one ground path between components. This is called a ground loop. It can pick up stray AC fields as noise. Here are some ways to eliminate these pesky multiple paths:
   • Group all signal carrying lines between two devices into a single cable or bundle. That's what snakes are for.
   • Group your power cords together into a single bundle between the rack and the power strip.
   • Never run power cables close to signal cables.
   • If hum persists, and it disappears when you unplug unrelated cables, you need a ground-loop-breaker signal cable (see below).
   • If power and signal cables must cross, cross them at right angles.
   About ground loops
7. Use a direct box with the ground lifted to connect a guitar amp with a 2-prong plug to other equipment. But it is better to have a guitar amp that came with a two-prong plug and a ground reverse switch rewired with a three prong plug, a three-wire cord, and no ground reverse switch.
8. Keep direct boxes away from "wall wart" AC adapters and power transformers in equipment. The direct box transformer picks up the hum field from the power transformer.
9. Make sure all substantial metal objects (such as a metal table) in contact with the system are electrically bonded to it. Poor contacts can cause noise.
10. Check your signal cables. Some can develop intermittent connections. The page author came across some marginal cables made by GC. The shields were not adequate for hum removal.
11. Keep your contacts clean. The author uses Caig Pro-Gold G5 to remove oxide from marginal contacts. Put some on an audio plug, and insert it into the jack. If the connector can be rotated, twist it. Otherwise, insert and remove the plug repeatedly.
12. Use Caig CaiLube MCL on noisy potentiometers and rotary switches.
13. Watch for impedance mismatches. Adapting the connectors is not enough. You also need to match impedances and levels at both ends of the run. A direct box may be indicated to solve the problem.
14. To convert from low-impedance XLR to a high-impedance input, use a direct box, connected backwards using a female-to-female XLR adaptor.
15. Use balanced lines for signal runs through noisy areas, and for runs over 25 feet in length.
16. Never allow the metal shells of your signal cables to touch other metal objects (other than the jacks they are plugged into). Such touchings can cause noises.

To make a ground loop breaker audio cord:

• Get cords with removable end caps, so you can get inside the plugs.
• Label both ends of each cable you modify as "GROUND LOOP BREAKER" so the cables won't be used elsewhere accidentally.
• On an XLR balanced line cord, disconnect pin 1 at only one end of the cable.
• On a TRS balanced cable, disconnect the shield at only one end of the cable.
• On unbalanced cables, disconnect the shield at one end.
• Where you break the shield, make sure no strands remain, and that the cord clamp does not make contact with the shield.
• If it is possible that all cables except the ground-loop breaker cable could be disconnected, connect the disconnected shield to its terminal through a 10 ohm resistor. It is a good idea to do this anyway.

An alternative ground loop breaker is a transformer of the correct impedance used to isolate the signal. To make this:

• Cut a standard cable in two about two feet from one end.
• Mount the transformer and a terminal strip in a small box, and mount the ends of the cables in strain reliefs.
• Wire the primary of the transformer to one piece of cable, and the secondary to the other in the manner signal is normally carried on the cable.
• Be sure the polarity of each cable is right at the transformer (it will invert the phase of the signal otherwise).
• Connect the transformer case, and/or the box (if metal) to the shield of one of the wires.

This device will pass the signal while isolating the grounds.

Radio Shack 270-054 is a stereo version of this transformer cable with RCA plugs. This is the method the page author uses, and it works quite well.

Other tricks to know:

1. Be careful when intermixing balanced line and unbalanced signals. Use the proper transformer adaptor. Tricky adaptor cables cause problems.
2. Do not connect a speaker or headphone output to a line input directly. It could damage the equipment. Use an attenuator or transformer.
3. A preamp might work better placed at the signal source, rather than at the other end of the cable run.
4. For balanced lines, the better cables introduce a continuous twist in the signal pair as part of the design. This reduces noise pickup.
5. Hum can also be caused by aging filter capacitors in the power supply of a device.
6. Rushing, gurgling, chugging, or fizzling sounds might be leakage from digital signals into audio lines.
7. An irregular buzzing or frying sound might be a video signal, rather than power line hum.
8. Keep video and audio signals separated, except where the same device requires both signals.
9. Keep digital and analog signals apart.
10. Use patchbays for line level signals, but not other kinds of signals.
11. Make adaptor cables from good connectors. Most manufactured adaptors make bad connections.
12. A sudden loss of signal level in a balanced line cable, accompanied by a noise increase, indicates a cable failure.
13. Note that radio transmitters used close to audio equipment can induce noises.

A job the page author once held provided much insight into noise. Electrocardiographs and electromyographs, among other scientific instrumentation, are even more finicky about noise than any audio application is. For the really sensitive measurements, there was a Faraday cage. This is a small grounded copper-lined room that prevents all electric fields from entering it. That is the ultimate in noise removal. It would make a great soloist or drum booth, because it's also pretty dead acoustically in there. But it is quite expensive to make, so it might be a bit too much overkill for studio use.

Links:

• About ground loops
• Mixing your own sound