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Silencing Hum Tell Your Connections To His Off!

Silencing Hum Tell Your Connections To His Off! image
Parent Issue
Day
13
Month
February
Year
1976
OCR Text

as lO V s Ql. ' o Glenn Gould might hum along with Bach,, but your hi-fi system shouldn't get away with it. Still, the chances are a tle better than 50-50 that your system hums more than it should, despite the fact that hum is easv to cure as anv hi-fi Hl. And snendine a couüle of bucks (at most) and maybe an hour of your time will do more to improve the sound of your hi-fi system than anything short of a hot new amp or pre-amp. Why care about hum if you can barely hear it? Simply because even a little hum not only places an added load on your system's ability to reproduce music but also mixes with low frequencies, causing muddy bass. Remove hum and bass notes will gain a new clarity and a well-defined, spacious texture. Ultimately, hum arises from the fact that almost all hi-fi equipment is powered from the AC line. In each component there is a power supply which converts AC to various direct-current (DC) voltages needed to run the device. This power supply can be designed to work very well (and expensively) or to opérate in a so-so fashion that will allow the manufacturer to sell his product at a higher profit. In addition to cutting corners on the power supply, soine manufacturers are just careless and do things such as route the leads from the phonograph past the hum-inducing fields of the power transformen They know what they are doing; hum is well understood and there are lots of reasons for it- but few excuses. Fortunately, there are some things you can do at home on a rainy day that will minimize your system's hum. First, the obvious things. Space out components. Since the phonograph cartridge will piek up hum from the magnetic field around the transformer in your components, put some distance between it and them. A few inches often make a difference, since the strength of a magnetic field (and therefore the amount of hum it can induce) drops as the square of the distance from its source. Obviously, long phono cables are not necessarily bad. Keep power and signal lines separate. It may be neater to bundie these together as they run between components, but it greatly increases induced hum and noise. Shorting phono plugs look like one end of a phono cable without the cable. They are for sale at most local hi-fi stores and are one of the few bargains in the business. Shorting plugs are inserted in unused amp or receiver inputs to cut noise and they work. For proof you can hear, disconnect your present phone cables, switch to this input and listen to it with and without shorting plugs in the jacks. Sometimes, shorting plugs in open phono jacks will even cut hum on other inputs. Shorting plugs are especially useful in systems with several low-level inputs-for example, a system with two phono inputs or an additional microphone or tape-head input. If your system has more than one low-level input, short any that ■ aren't in use. Ground the turntable to your pre-amp. If you have just put your system together and the phono does nothing but buzz, you haven t followed directions. Run a ground wire of heavy lamp lij .115 __.! cord between your turntable s motor board and your pre-amp. If you have already done this and you still have too much hum, look for a better spot on the turntable assemblv to connect to the eround. Do this by connecting one end of the lamp cord to your preamp or receiver and then by touching various places on the motor board. Do this while the system is on, and, if necessary, with the bass and volume up (but be careful-not too high). You'll find the connection point that gives the least hum easy to spot. Ground your whole system. If your home has honest three wire eléctrica! outlets or cold-water (not gas) pipes, by all means run another heavy wire from your pre-amp or receiver to ground. There is one spot within your system that will give the most hum reduction; find it by connecting one end of the ground wire to the water pipe or screw holding the outlet cover on, and then touching the unconnected wire end to various components and to various spots on each one. Usually, the pre-amp will be the best connection point. Even if a system ground doesn't cut hum- and in rare cases it won't-you owe it to your body to reduce any potential shock hazard. Ground your system anyway. If you have done all this, you should have a lot less hum than you started with. There's now one final trick that will cut hum to the minimum your system can produce, and it may be the simplest step of all- flip your wall plugs. Because each component has its own power supply, and because these supplies interact with each other, it is possible to plug units in so that hum in one component caneéis hum from another. Make hum as obvious as possible during this maneuver by turning up the bass and volume controls. If you have a receiver complete with tuner, try shorting its inputs and listening to it at full volume on its phono input. Ditto for a separate power amplifier: short its inputs and listen to it at ïtnum loudness. Now, quickly turn on the equipment, reverse the line cord in its socket and raise the volume aeain. There's a 50-50 chance j that there will be less hum in the new i tion. If that's the case, get a piece of tape " or some paint and mark the plug so you know which way works best. Obviously, if hum increases, put the plug back the way it was. Do the same as you connect other components to the system-phonograph, tuner, tape deck, etc. There's one wall plug orientation for each component that will yield the least system hum, and it just takes a flip of the wrist to find it. If you have done all this, you can now sit back and listen to the music-and that's the way it should be. . - Stan Perlmutter, by permission of The Boston ' Plioenix.