Electricity. It's everywhere. It's in your house, in the computer on which you are reading this, in the air, in your body, blah blah blah. You get the point. It also plays an important integral part of sound systems. From the microphone to the loudspeaker, everything is electrical, and you need to know a few things about it.
ANYONE FOR FROGS' LEGS?
An Italian scientist named Alessandro Volta did experiments with electric charge back in the 1800s; he managed to make dead frogs (yes, frogs) "move" by using them as part of an electrical circuit, and is credited with providing us with a lot of information about electric potential. The volt was thus born. Voltage is defined as the electric charge between two points. In household terms, voltage is akin to water pressure available to do work. It can't do any work until a circuit (the plumbing and a water tap) is complete so that current (the amount of water) can flow. In engineering terms, voltage is the potential energy of a bunch of electrons. Current, measured in Amperes, is the flow of electric charge; we can say that current is the rate of flow of electrons, but technically it is the opposite flow that we call current-- as electrons move in one direction, holes are left behind, and appear to flow in the opposite direction. We should also remember from our physics classes that there is a fundamental equation used in electricity: Ohm's Law.
OHM SWEET OHM
Ohm's Law is the mathematical relationship between voltage, current and resistance. It is named after George Ohm, its founder. Ohm's Law states that the current in a conductor is directly proportional to the voltage across it, and inversely proprtional to its resistance. In simpler terms, the law states that more voltage will produce more current if resistance stays the same, but higher resistance will cause the current to decrease if the voltage stays the same. In our household plumbing example, we can say that Ohm's Law dictates that more water pressure will produce more output if the diameter of the pipe stays the same, but a smaller-diameter pipe will cause the output to decrease if the water pressure stays the same.
With these three basic components, we have electricity. Current is "drawn" from a power supply due to the presence of a voltage placed across a load, or resistance. For instance, a typical electrical outlet has 120 volt potential (pressure- it has the capacity to give you 120 volts), but nothing is connected. If you put some load across its terminals, like a lightbulb or steam iron, you then have a complete circuit in which electrical current can flow. As we said before, a higher resistance will yield less current than low resistance. A direct short will cause a very large flow of current, because there is no resistance. If the supply is capable of delivering enough current, it is likely that the wire will eventually heat up and melt. This is why circuit-breakers and fuses are used. They limit current flow by opening (as opposed to shorting) a circuit before current flow becomes too large-- a 15A circuit-breaker will trip when the loads connected to that circuit try to pull more than 15A of current.
Makes sense, right? Good. Ohm's Law is represented as the rate of flow of electrons, and voltage is represented as the electric charge between two points.
Electricity appears in two forms: alternating current (AC) and direct current (DC). Direct current does not change directions-- the electron flow is always from the negative pole to the positive pole-- although as we mentioned before, the electrons themselves don't really "move," it's the holes that are created that "move." Direct current is almost always what is used inside of electronic devices to power the various internal components, but it is a harmful thing in audio signals, which are alternating current. Alternating current does change direction-- standard household electricity is alternating current, because of its flexibility in traveling long distances. It changes direction at a specific frequency-- 60 times per second, or 60 Hz (in the United States, Japan, and a couple of other countries; in Europe the standard is 50 Hz). Audio signals vary their direction-alternation according to the frequency in question.
When measuring voltage in a complex waveform, engineers use a useful method of deriving an average (not the average) voltage of a given signal, termed Root-Mean-Square, or RMS, for short. The signal value (amplitude) is squared, averaged over a period of time, and the square root of the result is calculated. The RMS value of a periodic function, such as a sine wave, is 0.707 multiplied by the peak value of the wave. It is rather difficult to achieve an accurate RMS value from a complex, not-quite-periodic waveform, such as music, but it works rather well in basic AC circuits.
When measuring resistance in AC circuits, we use the term impedance, which refers to the complex resistance ("real") and reactance ("imaginary") in the circuit. If you remember your vector theory, impedance is the combined reactance vector and the resistance vector, but we'll touch on that a little later.
I'VE GOT THE POWER!
Power is defined as the rate at which work is done, expressed as the amount of work per unit time. We know that current is defined as the flow of electric charge per unit distance, and voltage is defined as the potential difference between two points. The product of voltage and current is power. In fundamental physics, the power is measured in watts, abbreviated W. The watt is defined as one joule-per-second, which equals the power dissipated in heat by one ohm of resistance when one ampere of current passes through a circuit.
In a purely resistive circuit, or in a DC circuit, this is accurate, but when we are discussing power in AC load that is not completely resistive (i.e. has reactive components), we measure power in Volt-Amps, or VA; the actual wattage of a reactive circuit will be slightly less than the volt-amp measurement. A DC circuit, or a purely resistive circuit, has zero degrees of phase shift between the applied voltage and the resultant current, but in an AC circuit with inductive or capacitive reactance, there will be some degree of phase shift introduced by these elements, and this phase shift will influence the actual power available in the circuit.
The ratio of the total power in watts to the total apparent power in voltamperes is called the power factor. It is best described as a multiplier between zero and one, that one must use to calculate the real power from the apparent power. For example, if you measure the RMS voltage and RMS current of a circuit, and multiply them together, you will obtain the apparent power (note that many manufacturers will call this "RMS Power"; which is actually an inaccurate description). However, you must then multiply the apparent power by the power factor to calculate the true power. If the load is purely resistive, then the phase shift will be zero, and the power factor will be one, and thus the apparent power will equal the true power. Power factor is merely a ratio-- the ratio of watts to voltamperes, of resistance to reactance, and can also be calculated by the phase shift, in degrees, by simply taking the cosine of that angle. If the phase shift is 0, the power factor is 1, and if the phase shift is 90°, the power factor is 0.
GET THE BALANCE RIGHT
The transmission of electrical signals usually falls into two categories: balanced and unbalanced. An unbalanced connection is a two-wire system. One wire carries the audio signal, and the other conductor, called the "shield," is connected to ground, or the electrical reference point. A balanced connection is a three-wire system. Two separate wires carry the audio signal: one inverted in polarity with respect to the other. The third wire is the shield. Balanced connections are more immune to noise, and is by far the preferred method in professional audio. When the signal reaches its destination, the conductor reversed in polarity is righted again and added to the first audio signal conductor. Any noise that has been induced in the cable run, which has been induced equally over both conductors, is cancelled out. This method of canceling out induced noise is known as Common-Mode Rejection. Balanced connections are used for low-impedance microphones and professional line-level interconnects.
Unbalanced connections are used for high-impedance microphones and instrument pickups, and for semi-professional line-level interconnections. The unbalanced system is more susceptible to noise pickup, and when it can be avoided in professional audio, it is. In unbalanced connections, only one center conductor is used (positive). Any induced noise will appear at the input of the equipment-- there is no signal by which it can be canceled out.
LOVE IS A SHIELD
Balanced connections usually use a type of connector first designed and developed by the ITT Corporation, who called it the "XLR" connector, used in conjunction with a twisted-pair cable enclosed in a shield. No, I'm sorry, "XLR" doesn't actually stand for anything- it was just a part number assigned by the corporation. Switchcraft followed suit and made the "Q-G" connector, which they touted as "XLR-type." Neutrik followed soon after with its "NC" series of connectors. The most common XLR connector is the three-pin type, although there are configurations with up to seven pins for data, control, communication, et al., applications. The three pins correspond to the shield of the cable (pin 1), the positive signal, and the negative signal. In wiring an XLR connector, the shield of the cable goes to pin one, which, due to the construction of the female connector, makes contact first, to dissipate any static within the cable. The current AES-EBU-IEC convention stipulates that the positive signal terminates at pin 2, and the negative signal at pin 3. Various types of XLR connectors are available: panel-mounts, cable-end connectors, watertight connectors, etc., etc. Almost all XLR connectors are lockable; some use a "retention spring" to hold the mating connector in place instead of a latch. The strain relief offered by the XLR connector construction makes it ideal for portable applications-- handheld microphones, for instance.
The 1/4" TRS, or three-conductor phone plug, is also used for balanced wiring where space or money is at a premium. Unfortunately, the phone plug is not as rugged as the XLR connector, and only some versions made by Neutrik are lockable. The tip of the connector is usually designated the positive contact, the ring of the connector the negative, and the rest of the connector is connected to the shield.
But what is a shield? A shield is a wrap of a conductive metal covering the inner conductors of a given cable. It can be made of what is essentially aluminum foil or out of thin strands of copper, braided together around the inner conductors. The shield protects the inner conductors from unwanted electrostatic fields that might induce a signal, such as a hum or buzz, across the conductors of the cable, and thus into the audio stream. Similarly, in data applications, this electrostatic interference may cause unreliable data transmission as it introduces noise into the data stream. Since so much of our world is dependent on the accurate transmission of audio signals, it's never a good idea to skimp on cheaply made cable with cheap shield arrangements.
Unbalanced connections usually use a 1/4" TS, or two-conductor phone plug, used in conjunction with a cable that has one center conductor surrounded by a shield (compare with the pair of conductors in a balanced line). Since an audio signal always needs two conductors, one for the signal, and one as a ground reference, the shield is used as the second conductor in an unbalanced arrangement. Again, no noise-cancellation takes place at the input. A bad shield in an unbalanced cable can be even more detrimental to the audio signal as it is an intrinsic part of the signal transmission.
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http://www.harada-sound.com/sound/handbook/basicelec.html - © 1999 Kai Harada. 07.11.1999.