|Posted on February 28, 2010 at 1:02 AM|
More on cable selection and design HERE's ANOTHER!
How big should the conductors be?
The required size (or gauge) of the conductors depends on threefactors: (1) the load impedance; (2) the length of cable required; and(3) the amount of power loss that can be tolerated. Each of theseinvolves relationships between voltage (volts), resistance (ohms),current (amperes) and power (watts). These relationships are definedwith Ohm's Law. The job of a speaker cable is to move a substantialamount of electrical current from the output of a power amplifier to aspeaker system. Current flow is measure in amperes. Unlike instrumentand microphone cables, which typically carry currents of only a fewmilliamperes (thousandths of an ampere), the current required to drivea speaker is much higher; for instance, an 8-ohm speaker driven with a100-watt amplifier will pull about 3-1/2 amperes of current. Bycomparison, a 600-ohm input driven by a line-level output only pullsabout 2 milliamps. The amplifier's output voltage, divided by the loadimpedance (in ohms), determines the amount of current "pulled" by theload. Resistance limits current flow, and decreasing it increasescurrent flow. If the amplifier's output voltage remains constant, itwill deliver twice as much current to an 8-ohm load as it will to a16-ohm load, and four times as much to a 4-ohm load. Halving the loadimpedance doubles the load current. For instance, two 8-ohm speakers inparallel will draw twice the current of one speaker because theparallel connection reduces the load impedance to 4 ohms.
(For simplicity's sake we are using the terms resistance and impedanceinterchangeably; in practice, a speaker whose nominal impedance is 8ohms may have a voice coil DC resistance of about 5 ohms and an ACimpedance curve that ranges from 5 ohms to 100 ohms, depending on thefrequency, type of enclosure, and the acoustical loading of itsenvironment.)
How does current draw affect the conductor requirements of the speaker cable?
A simple fact to remember: Current needs copper, voltage needsinsulation. To make an analogy, if electrons were water, voltage wouldbe the "pressure" in the system, while current would be the amount ofwater flowing. You have water pressure even with the faucet closed andno water flowing; similarly, you have voltage regardless of whether youhave current flowing. Current flow is literally electrons movingbetween two points at differing electrical potentials, so the moreelectrons you need to move, the larger the conductors (our "electronpipe") must be. In the AWG (American Wire Gauge) system, conductor areadoubles with each reduction of three in AWG; a 13 AWG conductor hastwice the copper of a 16 AWG conductor, a 10 AWG twice the copper of a13 AWG, and so on.
But power amp outputs are rated in watts. How are amperes related to watts?
Ohm's Law says that current (amperes) times voltage (volts) equalspower (watts), so if the voltage is unchanged, the power is directlyproportional to the current, which is determined by the impedance ofthe load. (This is why most power amplifiers will deliver approximatelydouble their 8-ohm rated output when the load impedance is reduced to 4ohms.) In short, a 4-ohm load should require conductors with twice thecopper of an 8-ohm load, assuming the length of the run to the speakeris the same, while a 2-ohm load requires four times the copper of an8-ohm load. Explaining this point leads to the following oft-askedquestion:
How long can a speaker cable be before it affects performance?
The ugly truth: Any length of speaker cable degrades performance andefficiency. Like the effects of shunt capacitance in instrument cablesand series inductance in microphone cables, the signal degradationcaused by speaker cabling is always present to some degree, and isworsened by increasing the length of the cable. The most obvious illeffect of speaker cables is the amount of amplifier power wasted.
Why do cables waste power?
Copper is a very good conductor of electricity, but it isn't perfect.It has a certain amount of resistance, determined primarily on itscross-sectional area (but also by its purity and temperature). Thiswiring resistance is "seen" by the amplifier output as part of theload; if a cable with a resistance of one ohm is connected to an 8-ohmspeaker, the load seen by the amplifier is 9 ohms. Since increasing theload impedance decreases current flow, decreasing power delivery, wehave lost some of the amplifier's power capability merely by adding theseries resistance of the cable to the load. Furthermore, since thecable is seen as part of the load, part of the power which is deliveredto the load is dissipated in the cable itself as heat. (This is the wayelectrical space-heaters work!) Since Ohm's Law allows us to calculatethe current flow created by a given voltage across a given loadimpedance, it can also give us the voltage drop across the load, orpart of the load, for a given current. This can be convenientlyexpressed as a percentage of the total power.
How can the power loss be minimized?
There are three ways to decrease the power lost in speaker cabling:
First, minimize the resistance of the cabling. Use larger conductors,avoid unnecessary connectors, and make sure that mechanical connectionsare clean and tight and solder joints are smooth and bright.
Second, minimize the length of the cabling. The resistance of the cableis proportional to its length, so less cable means less resistance toexpend those watts. Place the power amplifier as close as practical tothe speaker. (Chances are excellent that the signal loss in theline-level connection to the amplifier input will be negligible.) Don'tuse a 50-foot cable for a 20-foot run.
Third, maximize the load impedance. As the load impedance increases itbecomes a larger percentage of the total load, which proportionatelyreduces the amount lost by wiring resistance. Avoid "daisy-chaining"speakers, because the parallel connection reduces the total loadimpedance, thus increasing the percentage lost. The ideal situation(for reasons beyond mere power loss is to run a separate pair ofconductors to each speaker form the amplifier.
Is the actual performance of the amplifier degraded by long speaker cables?
There is a definite impact on the amplifier damping factor caused bycabling resistance/impedance. Damping, the ability of the amplifier tocontrol the movement of the speaker, is especially noticeable inpercussive low-frequency program material like kick drum, bass guitarand tympani. Clean, "tight" bass is a sign of good damping at work.Boomy, mushy bass is the result of poor damping; the speaker is beingset into motion but the amplifier can't stop it fast enough toaccurately track the waveform. Ultimately, poor damping can result inactual oscillation and speaker destruction.
Damping factor is expressed as the quotient of load impedance dividedby the amplifier's actual source impedance. Ultra-low source impedanceson the order of 40 milliohms (that's less than one-twentieth of an ohm)are common in modern direct-coupled solid-state amplifiers, so dampingfactors with an 8-ohm load are generally specified in the range of100-200. However, those specifications are taken on a test bench, witha non-inductive dummy load attached directly to the output terminals.In the real world, the speaker sees the cabling resistance as part ofthe source impedance, increasing it. This lowers the damping factordrastically, even when considering only the DC resistance of the cable.If the reactive components that constitute the AC impedance of thecable are considered, the loss of damping is even greater.
Although tube amplifiers generally fall far short of sold-state typesin damping performance, their sound can still be improved by the use oflarger speaker cables. Damping even comes into play in the performanceof mixing consoles with remote DC power supplies; reducing the lengthof the cable linking the power supply to the console can noticeablyimprove the low-frequency performance of the electronics.
What other cable problems affect performance?
The twin gremlins covered in "Understanding the Microphone Cable,"namely series inductance and skin effect, are also factors in speakercables. Series inductance and the resulting inductive reactance adds tothe DC resistance, increasing the AC impedance of the cable. Aninductor can be thought of as a resistor whose resistance increases asfrequency increases. Thus, series inductance has a low-pass filtercharacteristic, progressively attenuating high frequencies. Theinductance of a round conductor is largely independent of its diameteror gauge, and is not directly proportional to its length, either.
Skin effect is a phenomenon that causes current flow in a roundconductor to be concentrated more to the surface of the conductor athigher frequencies, almost as if it were a hollow tube. This increasesthe apparent resistance of the conductor at high frequencies, and alsobrings significant phase shift.
Taken together, these ugly realities introduce various dynamic andtime-related forms of signal distortion which are very difficult toquantify with simple sine-wave measurements. When complex waveformshave their harmonic structures altered, the sense of immediacy andrealism is reduced. The ear/brain combination is incredibly sensitiveto the effects of this type of phase distortion, but generally needsdirect, A/B comparisons in real time to recognize them.
How can these problems be addressed?
The number of strange designs for speaker cable is amazing. Among themare coaxial, with two insulated spiral "shields" serving as conductors;quad, using two conductors for "positive" and two for "negative";zip-cord with ultra-fine "rope lay" conductors and transparent jacket;multi-conductor, allegedly using large conductors for lows, mediumconductors for mids, and tiny conductors for highs; 4 AWG weldingcable; braided flat cable constructed of many individually insulatedconductors; and many others. Most of these address the inductancequestion by using multiple conductors and the skin effect problem bykeeping them relatively small. Many of these "esoteric" cables areextraordinarily expensive; all of them probably offer some improvementin performance over ordinary twisted-pair type cables, especially incritical monitoring applications and high-quality music systems. Inmost cases, the cost of such cable and its termination, combined withthe extremely fragile construction common to them, severely limitstheir practical use, especially in portable situations. In short, theycost too much, they're too hard to work with, and they just aren't madefor rough treatment. But, sonically, they all bear listening to with anopen mind; the differences can be surprisingly apparent.
Is capacitance a problem in speaker cables?
The extremely low impedance nature of speaker circuits makes cablecapacitance a very minor factor in overall performance. In the earlydays of solid state amplifiers, highly capacitive loads (such as largeelectrostatic speaker systems) caused blown output transistors andother problems, but so did heat, short circuits, highly inductive loadsand underdesigned power supplies.
Because of this, the dielectric properties of the insulation used arenowhere near as critical as that used for high-impedance instrumentcables. The most important consideration for insulation for speakercables is probably heat resistance, especially because the physicalsize constraints imposed by popular connectors like the ubiquitous 1/4"phone plug severely limit the diameter of the cable. This requiresinsulation and jacketing to be thin, but tough, while withstanding theheat required to bring a relatively large amount of copper up tosoldering temperature. Polyethylene tends to melt too easily, whilethermoset materials like rubber and neoprene are expensive andunpredictable with regard to wall thickness PVC is cheap and can bemixed in a variety of ways to enhance its shrink-resistance andflexibility, making it a good choice for most applications. Somevarieties of TPR (thermoplastic rubber) are also finding use.
Why don't speaker cables require shielding?
Actually, there are a few circumstances that may require the shieldingof speaker cables. In areas with extreme strong radio frequencyinterference (RFI) problems, the speaker cables can act as antennae forunwanted signal reception which can enter the system through the outputtransistors. When circumstances require that speaker-level andmicrophone-level signals be in close proximity for long distances, suchas cue feeds to recording studios, it is a good idea to use shieldedspeaker cabling (generally foil-shielded, twisted-pair ortwisted-triple cable) as "insurance" against possible crosstalk formthe cue system entering the microphone lines. In large installations,pulling the speaker cabling in metallic conduit provides excellentshielding from both RFI and EMI (electromagnetic interference). But,for the most part, the extremely low impedance and high level ofspeaker signals minimizes the significance of local interference.
Why can't I use a shielded instrument cable for hooking an amplifier to a speaker, assuming it has the right plugs?
You can, in desperation, use an instrument cable for hooking up anamplifier to a speaker. However, the small gauge (generally 20 AWG atmost) center conductor offers substantial resistance to current flow,and in extreme circumstances could heat up until it melts itsinsulation and short-circuits to the shield, or melts and goesopen-circuit, which can destroy some tube amplifiers. Long runs ofcoaxial-type cable will have large amounts of capacitance, possiblyenough to upset the protection circuitry of some amplifiers, causinguntimely shut-downs. And of course there is enormous power loss anddamping degradation because of the high impedance of the cable.
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