Amplifier
An electronic amplifier is a device for increasing the power of a signal. It does this by taking power from a power supply and shaping the output to match the (relatively low power) input signal. This process invariably introduces some noise and distortion into the signal, and the process cannot be 100% efficient - amplifiers will always produce some waste heat. An idealized amplifier can be said to be "a piece of wire with gain", as the output is an exact replica of the input, but larger. Different designs of amplifier are used for different types of applications and signals. We can broadly divide amplifiers into three categories - small signal amplifiers, low frequency power amplifiers and RF (radio frequency) power amplifiers. Each of these calls for a slightly different design approach, mainly because of the physical limitations of the components used to implement the amplifier, and the efficiencies that can be realized.

Back-Emf
Literally,, back-voltage, is a phenomena found in all moving-coil electromagnetic systems, but for audio is most often used with respect to loudspeaker operation. This term describes the action where, after the signal stops, the speaker cone continues moving (due to inertia), causing the voice coil to move through the magnetic field (now acting as a microphone), creating a new voltage that tries to drive the cable back to the power amplifier's output. If the loudspeaker does too much of this, the cone flops around unpleasantly. It is not pleasant-sounding. To stop back-emf, the loudspeaker must "see" zero ohms looking backward (a dead short), or as close to it as possible from the output of the amplifier.

Banana Plug
An electrical connector designed to join audio wires such as speaker wires to the binding posts on the back of many power amplifiers or to special jacks known as (surprise) banana jacks. A common configuration of banana plugs is to have two of them molded together and spaced 3/4 of an inch apart, which also happens to be the spacing of the binding post receptacles on the back of power amps. This assembly is commonly called a banana plug, but the more technically correct term is "double-banana plug," or it is sometimes called a "GR" plug, after the General Radio Corporation, which introduced it many years ago.

Binding Post
A type of electrical terminal, a binding post is most commonly found as the output connector on a power amplifier, or as the connectors on a speaker cabinet. A binding post is a very versatile connector, accepting banana plugs, alligator clips, bare wire, and others. Generally, binding posts are color coded, with the black connection going to ground, and the red connecting to hot. Binding posts offer fast, easy connections, and provide reasonably good surface area contact for good conductivity.

Crossover
A crossover is a device designed to divide audio information into smaller frequency ranges to comply with the requirements of different transducers in an audio reproduction system. This is accomplished by running the audio through a set of filters. For example, a two-way crossover may be comprised of a low pass and high pass filter where the low pass filter passes a signal with frequencies more suitable for a woofer and the high pass filter passes frequencies the tweeter can deal with. Crossovers can be passive or active designs. Passive crossovers are usually found inside speaker cabinets along with the speaker components. These often connect to the outside world via a single jack, but sometimes each speaker component also has its own jack in case one wants to bypass the built in passive crossover. Active crossovers are placed before the power amp. In that application each frequency range is given its own power amp and its own drivers. This is where the phrase bi-amping and tri-amping come from. There are a number of different types of filter configurations used in crossovers and they each produce subtly different results. One of the big variables is how steep the roll off is at the cutoff frequency. Common configurations are 12 dB per octave, 18 dB per octave, and 24 dB per octave. Each design has its own strengths and weaknesses, but in general steeper roll offs are considered better in modern applications.

Discrete
Today's WFTD is a special request, as many are. I include the actual request today because I thought it was funny. "Would you please define the term "discrete" as it refers to amplifiers (not extramarital activities)?"In electronics and specifically pro audio equipment the term discrete is used to express that individual components are used as opposed to integrated circuit chips. The advent of the integrated circuit has provided increasingly inexpensive and efficient ways to build common circuits. Rather than having to lay out multiple "discrete" components on a board it is possible to build all of that functionality in to one chip, which is less expensive, runs cooler, and uses less power (see any modern electronic device). In audio, however, composite components like this are often considered to be sonically inferior to their discrete counterparts. Whether this is really true is debatable, but it is certain that when using an off-the-shelf component as the basis for a mic preamp or a power amp 90% of the design work is already done and the builder is locked in to whatever that is. In a discrete design, however, the designer can choose each component carefully and create a personalized design of interactions that may produce a superior sound to a generic integrated amp circuit. As such, discrete designs are considered "better" and that word is thrown around in marketing as one way to differentiate a device from others that may use integrated designs.

Distribution Amplifier
A Distribution Amp, or DA, is a low power amplifier designed to transparently split an input signal to several outputs. For example, the stereo outputs of a mixing console might be run to a DA, where they would be split to simultaneously feed a cassette deck, a DAT recorder, a CD recorder, and so on. Using a distribution amp prevents various problems caused by passively splitting ("Y"-ing) an output to feed multiple sources.

Field Effect Transistor (FET)
A particular type of transistor, an FET behaves in a similar fashion to a triode (tube). There are actually several types of FETs, a common one in the pro audio world being the MOSFET (Metal Oxide Field Effect Transistor). FETs have a high input impedance, and respond in a linear fashion. This makes them ideal for condenser microphone preamps, as well as for certain power amplifier designs.

Heat Sink
An structure used to dissipate heat in devices that are prone to produce heat faster than they can dissipate it themselves. For example: The transistors in a power amplifier have very large amounts of current running through them and can get quite hot. Because they are relatively small the heat cannot dissipate very quickly and can cause failures. As a remedy they are mounted to a heat sink (usually aluminum) that is designed to have much more surface area in contact with the air. Often these heat sinks can be seen on the back of amplifiers and big power supplies as an array of metal "fins." Some designs employ fans that circulate cooler air over the hot heat sink to further enhance its ability to pull heat away from the components.

Load
In electrical terms a load is something that dissipates power and does some work. The work done may take many forms, including generating heat as almost always happens as a side effect of work being done. Without a load no power can be transferred. A speaker is the load for a power amp. In order for current flow to occur a complete circuit must exist. In order for the circuit not to be a short-circuit (a decidedly bad thing) a load must be present to the power the amp. The power amp drives power through the circuit by way of increasing the voltage at its outputs and as a result the load (speaker) draws current and does work. In this case two major forms of work occur: The speaker moves and generates sound, and heat is produced. Any device you plug into an electrical outlet can be considered a load (toaster, light bulb, etc). Plug in too many devices drawing too much current and you will "load down" the power delivery system (another bad thing). In order to protect against this power delivery systems have fuses and circuit breakers to break the circuit when current flow gets too high. Many power amps employ current limiting devices in their output stages to limit current flow without interrupting the audio. It's sort of a self regulating protection system (back in the old days the amp just blew up). An important thing to understand is that a load will DRAW from an available pool of power all of the current it needs to operate at the given voltage. This is somewhat simplified, but in principle remains fundamentally true for all electrical systems. A speaker's impedance rating is an indication of what kind of load it presents to an amplifier. An appliance's current or amperage rating is exactly the load it will place on the electrical system. The reason a speaker cannot be rated in exact terms of current usage is because the voltage and frequencies presented to it constantly change. Impedance is a way of approximating a speaker's resistance to a varying voltage and frequency signal.Also related to us is acoustical loading. The efficiency of a loudspeaker depends to some extent on the acoustic load placed on it by the way it couples to a cabinet and the surrounding structures. A speaker placed in the throat of a horn, for example, will see a higher acoustic impedance than a speaker placed in a free space.

PMPO
Today's word was a special request from an inSync reader who has been seeing it used on line a lot lately. PMPO: Abbreviation for Peak Music Power Output. A very subjective specification designed to help provide "real world" wattage ratings for power amps and speakers. Lots of number games are played with specifications and very few are held to specific and rigid testing standards. Consequently their value in comparing equipment is often less than ideal. Further, many specifications are obtained in lab conditions that don't translate easily to how the device is likely to perform in the real world, which adds more confusion. For example, power amps are often tested with sine waves, pink noise, and other non-musical signals. These signals can be very demanding (in different ways) for the equipment to reproduce, whereas a "typical" music signal is much easier. The purpose of PMPO is to show a product's performance in real world circumstances that are allegedly more meaningful to the user. An amp that may not compare very well in lab tests to a much more expensive model will often show up as much closer in terms of PMPO. Proponents say this puts the specs on paper much more in line with the actual audible difference in the products. Unfortunately the PMPO measurement is far more subjective than most specs because, in addition to the ambiguities many specs suffer from, there is no standard for what "music" means. Ten different manufacturers can still test their product under ten different sets of circumstances. PMPO is mostly used in the consumer hi-fi industry where deep understanding of specifications is often not a concern.

Power Compression
Speaker voice coils are made of copper or aluminum. As these voice coils increase in temperature during normal operation, their resistance increases. Greater voice coil resistance means less power transfer from the amplifier. As a result, the speaker will not play as loud when it's "warmed up" as it did when it was "cold". Some speakers may exhibit 3 to 6 dB of power compression. A mere 3 dB of power compression is equivalent to cutting the available wattage of your power amps in half. Speaker manufacturers who develop systems for use in demanding applications such as concerts or nightclubs spend a great deal of their research and development energy working on ways to keep speaker voice coils cool while in operation.

Rack Rail
Aside from rack rails, dragsters, and tracks used in railroads the term rail is used in reference to power supplies, though its usage does come from our common sense understanding of the word. A power supply rail is in some devices (especially older ones) literally a metal rail that passes through the device to deliver power to the components. A voltage is applied to the rail from the power supply and various components are connected to it for their power. Usually there is more than one rail, and each one delivers a different voltage and has different components tied to it. If you crack open an old power amp or mixer you stand a pretty good chance of seeing a couple. In most modern and/or low power devices there aren't any actual rails, but the concept lives on. When you hear the phrase 'power supply rail' what is being communicated is some type of bus with a set voltage applied to it for the sake of powering components in the device. The connection may be delivered by wires or traces on a circuitboard.

Voltage Sag
A temporary partial loss of supply voltage in an electrical system. It's akin to the brownout, which is a (often controlled) power reduction where the utility company decreases the voltage on the power lines, so customers receive weaker electric current. Brownouts can be used if total power demand exceeds the maximum available supply, though the typical household often does not notice the difference. Voltage sag is a similar result, but can be more dramatic, of shorter duration, and is not controlled. Voltage sags are usually caused by turning on some device that requires high current to start up. In the second it takes to start up, the supply voltage to other equipment momentarily dips because the source of power either becomes momentarily overloaded or simply can't react quickly enough to the increased power demand caused by the starting device. It is not uncommon for the supply voltage of a device or electrical system to drop when great power (and thus current) demands are place on it. This is what you are observing when you notice the lights briefly dim as you turn on your vacuum cleaner. Turning on a large power amp often causes the same behavior. Minor voltage sags are usually harmless to most equipment, but if severe enough can cause problems, particularly with delicate equipment.

Y Cable
A cable used to split a signal into two parts or combine two signals into one. The term Y Cable is used because the cable is like (and looks like) the letter Y, where there are two parts joined into one, or one split into two, depending upon how you look at it. Y Cables are common throughout audio as a simple and easy way to accomplish these two tasks. There are times, however, where they can be used in inappropriate ways, sometimes with disastrous results. For example, two MIDI signals cannot be combined using a Y cable. Two outputs from power amps cannot (under most normal circumstances) be combined via a Y cable (this will usually result in a substantial repair bill). Even in situations where Y cables can "work" the results are sometimes less than ideal. One must consider the output and input impedances, signal levels, and other factors of the devices in question before indescriminately combining or splitting their signals.