Secrets of Home Theater and High Fidelity
Volume 1, Number 1, 1994
7. Connections (Revised June, 2000)
Index for Connections:
Introduction Electrical Properties of Cables
Speaker Placement Professional Installation Budget Plan
Now that you have your equipment choices made, you will need to be able to connect them all together. Cables for connecting audio equipment are probably one of the most underestimated factors that can affect sound quality. If all cables made specifically for high fidelity were inexpensive, everyone would use them just in case they resulted in an improvement. The problem is that good cables can cost a great deal of money. Each cable design is unique. Some are flat like the model shown on the left, and most are round, like the models shown on the right. And just because one is thicker, more elaborate in the way the wires are wound, or more expensive, does not mean it will sound better than less elaborate cables. They will sound different from one another, however, because cables, just like amplifiers and speakers, have their own sound personality. This personality is determined by three basic electrical properties which combine to form the total impedance: resistance, capacitance, and inductance. Because all cables have values greater than zero for all three factors, they are filters. Resistance (resistive impedance, measured in Ohms) is usually not a problem, even with inexpensive electrical cable that is sometimes used for speaker connections, because copper is a good conductor (silver is used in some high-end audio cables, and silver is also a very good conductor). Most cables, inexpensive or expensive, have a resistance of about 0.1 Ohm to 0.02 Ohm for an 8 foot section (taking into account two signal paths). The real culprits in cable transmission are capacitance, measured in picofarads or pF (trillionths of a farad) per foot, and inductance, measured in microhenrys (µH), or millionths of a henry per foot. These two factors (capacitance and inductance) are called reactive impedance, and they vary widely even in high quality cable. There is some evidence that for interconnects (between source and preamp, and between preamp and power amp), low capacitance is particularly important, while for speaker cables (between power amp and speakers), low inductance is more important.
Electrical Properties of Cables
Any two surfaces having a difference in electrical potential between them, and are in close proximity with each other such that an electrostatic field exists, would be defined as a capacitor. Capacitors for electronic circuits consist of two metal foils wrapped around one another, but separated (insulated) from one another by a plastic sheet (dielectric). When a voltage is applied to the two foils, an electrostatic charge builds up on the dielectric. (This electrostatic charge is measured in Farads.) In fact, capacitors are one of the two methods for storing electricity in circuits, the other way being with magnetic fields (inductors). This is desirable with capacitors inside the amplifier, but not in the cables. Depending on the insulating material (dielectric), some of the electrical signal passing through the cable is transferred to the insulator, stored as energy (electrons), then released back into the cable where it causes a degradation in the sound quality. The type of insulator has a direct effect on the capacitance.
Various insulators are used in high fidelity cables, and, in decreasing dielectric constants (the lower, the better), they are PVC, followed by polyethylene, polypropylene, and finally, Teflon, which is the best (except for air or vacuum - see below). Usually, Teflon insulated cables are the most expensive, partially because it is a difficult material to work with. Typical values of capacitance with high end audio cables vary from 6 pF to 50 pF per foot. There are some new designs that have air as the dielectric (dielectric constant of 1), and even some with the conductors being surrounded by a vacuum.
Any object in an electronic circuit, having a magnetic field, is an inductor. This includes single wires in the signal path, since a magnetic field is produced when current flows through a wire. Inductors for electronic circuits are made by coiling the wire, which increases the inductance value (in henrys). This is a reason why you should not coil extra length of cables on the floor. Since cables usually have two leads, each conducting in the opposite direction to complete the circuit, high inductance can cause the flow of current in one lead to interfere with the flow in the other lead. Inductance values in high end audio cables vary from about 0.1 microhenrys to 0.6 microhenrys per foot. If the inductance is high, a loss in reproduction of high frequencies can occur.
Combining capacitance and frequency produces capacitive reactance, measured in Ohms. Combining inductance and frequency produces inductive reactance, also measured in Ohms. The lower the frequency, the more capacitive reactance, and this is why capacitors are used as high pass filters in crossover networks. Correspondingly, the higher the frequency, the more inductive reactance, and this is why inductors are used as low pass filters. Together, the resistive impedance and reactive impedance combine to be the total impedance.
An additional factor, called the "skin effect", which is not easy to quantify, also can result in changes to the signal as it passes through the cable. The term refers to the fact that the signal travels differently at the surface of the conductor than it does near the center. If the conductor is thicker than 19 gauge, the skin effect can degrade the sound quality by smearing the high frequencies. Thin uninsulated conductors twisted together, such as zip cord, would act as if they were one large conductor, and there would be significant skin effect. Thus, thin conductors, each isolated from the others, are desirable. Many audio cables use conductor strands of several diameters, so you can see that the skin effect is controversial.
Such wide variation in electrical characteristics, not necessarily correlating with cost, has probably led to the conclusion and confusion by many audiophiles that high end cables don't improve the sound. Each amplifier may sound best with a specific cable, making the whole problem of cables and high fidelity quite complex. However, good cables do make a difference, and the best way to begin choosing is by testing models that have low resistance, capacitance, and inductance. Your final selection of cables should be used for connecting the sources to the preamplifier, preamplifier to power amplifier (these types of cables are usually called interconnects), and power amplifier to speakers (called speaker cables).
When placing your speakers, the front left/right stereo speakers should be at least 6 feet apart, with the TV in between if you are installing a home theater system. The center channel speaker, if you use one, should be magnetically shielded if it is on top or underneath a direct view television. Otherwise, the magnetic field produced by the speaker will distort the image on the TV (the magnetic field of the speaker driver will bend the electron beam in the picture tube). With a large rear projection TV, the center channel speaker can be placed directly on top, without magnetic interference problems, since the television electronics are in the bottom of the TV cabinet (but test your speaker choice anyway, to make sure). Placement of the rear surround speakers is experimental. They can be on the floor, on the wall, or tucked into corners near the ceiling. They can be to the side of the sitting area (as stated previously, the recommended placement is on the side walls a few feet above ear level) or behind. Try several combinations. Purchase some inexpensive "zip" cable (electrical cable for connecting lamps and other appliances) to use when experimenting with speaker placement. Then, when you have decided on the final placement, purchase the proper length of high quality speaker audio cable. Some of the cable manufacturers listed above carry models that are flat, which are excellent for placing under rugs. The manufacturer can attach the connectors (banana plugs or spade lugs) for you, or you can do this yourself, but with complicated cable designs, it is better to let them do it.
You may also want to consider having a professional installer organize your equipment for you, especially if cables have to go into the walls. Your hi-fi dealer can suggest local companies who perform such installations.
Since home theater systems can be expensive, a budget plan is in order. It is not necessary to purchase everything at once, and you can get nice results right away by just purchasing certain items in stages. Assuming you have the television, and at least one additional video source (VCR, laserdisc, or DVD), and now want to begin building the home theater, you can purchase an integrated surround sound receiver for $300 - $2,600. If this begins to strain your budget, then purchase the rear surround speakers ($100 - $500 per pair) and use them for the front left/right stereo speakers for the time being. Then, you simply use your receiver in the two channel stereo mode until you can purchase the front speakers. Another alternative is to purchase a stereo power amplifier and two full range front speakers. Connect the volume controlled line level output of the TV to the power amplifier, and connect the amplifier to the speakers. This will improve your stereo TV sound until a surround sound preamplifier can be purchased. Additional power amplifiers (at least 100 watts rms per channel) and speakers for the remainder of the surround sound system can be purchased later. The video output of your VCR, laserdisc player, or DVD player can be connected directly to the TV, and the stereo audio outputs are fed to the power amplifier or surround sound receiver. This plan might be spread out over two years. Meanwhile, you are enjoying improved stereo sound for your TV programs and movie rentals. The last choice is to buy everything at once, and even if your budget is limited, satisfactory results can be achieved: $300 for the surround receiver, $500 for 5 speakers, $500 for a subwoofer, and $100 for inexpensive cables. In general, 10% of the budget should be set aside for the cables.
It is obvious that there are many ways to proceed, and that you don't have to rush. Home Theater can be just a good TV with stereo sound built in, or a TV with additional front stereo speakers, or a complete surround sound system including high power amplifiers and full range speakers all around. The first step is to go to several stores, watch and listen to a wide variety of equipment, decide what you will be happy with, and then work out a plan for purchasing it.
© Copyright 1994 - 2000 Secrets of Home Theater & High Fidelity
Return to Home Page.