From Issue #2, Page #68
-Contributed by TechLiving
by Daniel Sweeney
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Squelch electrical noise and get better audio and video
The new home entertainment system is all wired up. You're ready to watch The Matrix Reloaded, but something's not quite right.
There's an annoying hum in the background coming from several speakers. Worse yet, the picture on the projection screen is a tad unsteady, and a series of faint bars appear seemingly behind the image.
In most cases, noise and picture artifacts have their origins in the house's electrical system, or more precisely, in the way that the various A/V components interact with the AC electrical system. Let's look at the simplest concepts and the most basic proven procedures for noise suppression.
Noise Problems
The alternating current transmitted to American homes by utilities is nominally 120 volts and 60 cycles per second. The two parallel blade-shaped prongs on an AC power cord represent hot and neutral, while the third pin represents ground, which rests at the same zero voltage potential as neutral. Electrical noise normally enters an A/V system through the ground paths.
Hot and neutral both connect with the power supply of each component and supply the alternating current to it. Ground, on the other hand, connects to the metal chassis of the component and serves as a path to drain away any voltages that occur on the chassis. It's a safety measure, just like the ground return wire extending down from a lightning rod.
Now in the case of the RCA connector cables used in most A/V components, the outer sheath is grounded on the chassis and thus is part of the same circuit as the third pin ground. In other words, each RCA has a direct electrical connection to the third-pin socket on the wall.
Normally, some residual 60-cycle AC winds up on the chassis, generally at levels of at most a couple of volts. Since AC tends to follow the path of least resistance, anything on the chassis is generally going to follow the very-low-resistance, heavy-gauge third-pin wire in the AC cord and from there ultimately to the grounding point for the house wiring. In some cases, however, an RCA shield represents a lower resistance, or perhaps the third-pin wire on another power cable does, in which case you get a "ground loop" where electrical currents circulate through the ground paths and into the signal circuits via the RCA shield, causing hum.
Ground loops are the most common source of electrical noise, but noise can occasionally enter electrical power supplies through the hot lead, or it can result from the magnetic fields thrown off by power cords, which, incidentally, all AC power cords generate. Higher-frequency electrical noise can also be radiated from A/V connectors in the form of radio waves. In this case, the noise ultimately emanates from high-frequency circuits within the components, such as digital decoders.
Finding the Problem
Noise can manifest itself only in the last components in the signal chain, the video display and the loudspeaker, but it usually enters at the beginning of the chain.
In addressing noise problems, the first order of business is to locate the noise, which one does by progressively removing components, starting at the signal source. If, for instance, the noise stops when you've disconnected the DVD player, you've managed to localize it. When disconnecting equipment, be sure to turn everything off first, then turn the system back on to listen for noise.
Once you've identified where the noise is entering the service, you must find its actual source, which is largely a matter of trial and error. First examine the position of power cords in relationship to the RCA connectors. If power cords cross RCAs, they should always do so at right angles and should never run parallel in close proximity. Make sure the two cables maintain the right-angle relationship.
If that doesn't work, clean RCA terminations thoroughly with electrical contact cleaner. Noise voltages develop across an electrical resistance, and reducing resistance in the ground path may solve the problem.
If noise persists, try a different RCA cable and strive to avoid the use of cables of over two meters in length. Better to reposition equipment and use shorter cables if the cable run is excessive. If this still doesn't cure the noise, then you must resort to more radical and expensive palliatives.
If the two components being connected have provisions for balanced three-pin XLR cables, utilize those in preference to RCAs. XLRs are immune from induced hum from power cables and from radiated noise, and their ground circuits cannot be modulated by noise. You can buy the expensive kind from an audio specialist for hundreds of dollars, or purchase XLR microphone cables from a large electronics retailer like Fry's for about $15. A middle ground would be Monster Cable Pro's Z200i-x Reference ($99.95 per meter pair).
If the components won't take XLRs, consider Jensen's DM2-2RR ISOMAX isolation transformer ($199.95). This excellent device, which is ubiquitous in recording studios, breaks the ground path between components involving the RCA shield, and utilizes transformer coupling to recreate the signal at the input of the next component in the signal chain.
Hopefully now the only noises you will hear will be from the special effects. Enjoy! -Daniel Sweeney
Project Snapshot
Estimated Time to Complete: Varies
Estimated Cost: $5 to $1,000 (see below)
The Problem
You've got a great, big expensive display, but the quality is still horrible! Not spending enough? Not quite. Electrical noise can manifest itself as a hum or other extraneous sounds in the audio production and "hum bars" (stripes) in the video. It's common, annoying and can be frustrating to rectify unless you follow the proper procedures.
Pricing Info
In many cases, a bottle of electronic contact is all you need - and it's only five bucks. If you need a Jensen transformer, you're looking at $200. Power-conditioning devices can be more expensive, around $1,000. The power-conditioning price is usually dependent on power rating.
Project Steps
- Learn how electricity gets to your components.
- Locate the noise.
- Find the source.
- Trial and error.
Other Noise Problems
Your AC system may also be subject to dips and surges in electrical voltage, both of which can affect the sound and picture, and, if severe and/or prolonged, can damage equipment. Finally, the AC power may be subject to what are known as line harmonics - disturbances in the alternating current, which principally result from operating motors on the same circuit as audio/video equipment. Conventional CRT TVs and high-powered amplifiers can also contribute to line harmonics. The practical effect of line harmonics is that available power from the wall is considerably reduced.
When attacking noise, it's best to think of reducing it rather than eliminating it entirely. Even battery-powered equipment emits some noise. With AC, you're lucky if you can reduce noise to a barely audible level.
Most projects are step by step, but eliminating noise is a little different because it doesn't involve a set sequence of tasks. The process can be as simple as moving a wire and as difficult as adding multiple noise-reducing components to the signal chain and placing them where they'll do the most good.
Additional Solutions
Also fairly effective in reducing hum are the Equi=tech (www.equitech.com) balanced transformers, which come in several sizes. These transform the actual AC into balanced power, with a voltage swing from 60 volts positive to 60 volts negative. This accessory reduces hum and cancels out line harmonics generated by attached devices. The Model 1.5R ($1,295) with 1,500 watts of output will suffice for most systems.
For the ultimate in power purification, consider the PS Audio (www.psaudio.com) P1000 Power Plant ($2,995) and the ExactPower EP15A and SP15A combination ($1,995 and $900 respectively). While operating on somewhat different principles, both achieve the same result: the production of a balanced power AC sine wave with rock-steady voltage and no line harmonics whatsoever. Bear in mind, however, that neither product by itself will necessarily eliminate 60-cycle hum, because in either case the third pin safety ground passes right through the system to the wall socket.
About The Writer
Name: Daniel Sweeney
City/State: Burbank, Calif.
Occupation: Hack writer and industry analyst covering the telecommunications industry
Something people would be surprised to know about me: I was a literary scholar with a specialization in French Decadent literature.
Parts List
50-foot wire snake - $99.95
Wiring staple gun - $29.95
Magnetic wand - $5.95
12-inch-long drill bit - $9.95
Labels (for wires; see text) - $5.95
Brother P-Touch label maker (optional) - $119.95
100-foot spool of tape
wire - $99.95-$149.95
Plastic tubing with split -$3.99
Drywall knife
Electric drill
Piece of thin chain
Conduits
Duct tape
Wiring Tip
Most electrical codes prohibit sharing electrical outlets and low-voltage wiring (as in this article) in the same box.
Equipment Tip
Keep in mind that sometimes noise problems arise due to incompetent equipment design or manufacturing glitches. This is more likely to be the case, unfortunately, in very expensive high-end audio products than in the mass-market stuff. Esoteric vacuum-tube components are particularly prone to hum problems. If the equipment itself is malfunctioning, no amount of system tweaks will solve your problem.
Safety Tip
Don't lift safety grounds with cheater plugs. This may reduce hum, but it's dangerous. Safety grounds are there for a reason - safety. Also don't file down power cord prongs to allow a reverse-polarity connection. This is an old recording studio trick that sometimes reduces noise but surely increases the danger of equipment failure.
Money Tip
Save Your Money!
Don't expect the expensive AC line filters sold by specialty dealers to solve serious noise problems, particularly AC hum. They don't.
