OTHER TECH

HT Boot Camp: AC Power

Feeding the Beast and Chasing Its Gremlins : A basic guide for harnessing AC power.

There's absolutely nothing worse than putting together an awesome home theater system that's starved for power or buzzing with ground loops. We often take electricity for granted, assuming it will be there when we need it. Unfortunately, that's not always the case. You don't necessarily need an electrician just to connect your audio and video system, but you may need to check out your electrical system before you spend hours, if not days, connecting all your components. The two things you should consider are whether your system is getting enough power and if your components are connected to that power system correctly.

Getting Enough Power
One thing I've learned from measuring amplifiers is that only slight deviations in the amount of electrical power available to the amplifier can drastically change the maximum amount of power available at its audio output. For example, if the AC line voltage sags a few volts from the usual 120 volts, the amplifier could lose as much as 30 to 50 watts at the output. Therefore, it's extremely important that the wall outlet provide all the power that it can. Using good-sized electrical wire (12 awg) or larger (10 awg) over long runs and avoiding skinny extension cables will help ensure that none of the electric company's signal is lost on its way to the outlet. But what if your system's total power consumption exceeds the circuit that the system is connected to?

First of all, a typical 20-amp circuit provides 2,400 watts (20 amps times 120 volts) of power. Some homes have 15-amp circuits or 15-amp power strips, which provide 1,800 watts. One circuit may feed multiple outlets, lights, and other electrical systems. Start by determining the power requirements of the other systems on the circuit. Whenever possible, find a circuit that's either not shared or whose other outlets are not used at the same time as your home theater. Avoid circuits that are shared with lighting systems. Having a single, dedicated circuit for your beast of a home theater minimizes the chance for noise in the system.

Next, make sure that your home theater system doesn't draw more power than the circuit provides, especially when combined with other electrical systems. For the most part, if the system turns on without tripping the circuit breaker, you're probably OK. If you want to be more precise, check each product's manufacturer specifications for power consumption. If you don't have the manual, the back of each component should have a sticker or engraved label that indicates its maximum current draw. This should be listed in watts (W). For example, the B&K Reference 7260 amplifier has a maximum current draw of 1,875 watts. While this is close to the maximum continuous power of a 20-amp circuit, the outlet will handle brief musical peaks. Add a rack of equipment and a projection TV, however, and you'll compromise the performance of the amp. Keep this in mind when shopping for amplifiers, particularly if you're not in a position to upgrade your electrical service.

Ground Loops

Once you're confident that your system has adequate power, you should make sure that the power is clean and that the system is free of ground loops. (While I jokingly refer to ground loops as gremlins, there's usually nothing funny about them.) As I mentioned, making sure the home theater circuit isn't shared with other electrical items, particularly lights and computers, will help keep the AC signal clean. If the signal isn't clean, you may hear hiss, static, or other noise in the background when listening to music, and you may see sparkles or other anomalies in the video image. A line conditioner may help these situations, but it won't get rid of ground loops, which cause significant audible and visible distortion in the system.

A ground loop is created when two interconnected components have an unintended electrical potential through ground wires or the shields of audio or video cables, creating distortion and noise. In audio, a ground loop usually manifests itself as a buzz or hum coming from the speakers, although it can sometimes sound like excessive static. A hum is usually a 60-cycle tone corresponding to the electrical power added to the audio signal. Faulty wiring or power supplies, power cables wrapped too closely to low- and line-level cables, or (most likely) components improperly connected to the electrical service can cause a ground loop.

Likewise, a buzz is usually just a distorted hum that's caused by the feedback from having a lighting dimmer system on the same electrical circuit as your components. If the dimmer is on a different circuit but its power lines run parallel with wiring for the A/V system, the same consequence may occur. A buzz can also come from interconnected components that have poorly shielded transformers. In this case, separating the two components and proper grounding should eliminate the noise.

With video, the ground loop can take on comparable forms. It can appear as a vertically rolling ascending or descending band, or it can look like a grid of diagonal noise. In whatever form it assumes, the result is poor performance.

 ARTICLE CONTENTS

X