Channel Strip
Refers to one channel of a mixing board. Due to the layout of most mixers - channels in columns across the face with the functions of each channel arrayed from top to bottom - a channel is sometimes referred to as a strip. Over the years some mixers have been so highly regarded for their sound quality that a market was developed by marrying a channel strip to a power supply and I/O connections for stand alone use. One could plug a mic directly into the channel strip and record directly to a recorder without having to take the big mixer to the location. Even when such a mixer is available the channel strip approach is often used because it is thought that the simpler signal path of the external strip produces a more pure audio signal to record. This is similar to the popular approach of using high quality stand-alone mic preamps as the only device between mics and recorders, with the major difference being that a channel strip has many more capabilities than a simple mic amp. It may have EQ, compression, gates, and more, depending upon the unit. The idea is that it's everything you're likely to need in the signal path to make a great recording. Channel strips are so popular these days that they are generally considered to be a separate product category from preamps and other processing.
Decoupling
The process of isolating one stage of an amplifier from another. Decoupling prevents unwanted oscillations (see WFTD Oscillator) and other noises that may occur due to unwanted feedback through common power supply connections (see WFTD Coupling). It also provides further filtering of the power supply to reduce any lingering AC ripple, producing a cleaner DC supply for the low-level preamp stages. This decoupling is often accomplished by adding a resistor in series with the power supply to a gain stage and a large-value electrolytic capacitor from the supply to ground after the resistor, however, there are a number of other designs employed as well.
In acoustics decoupling refers to mechanically isolating masses from one another, particularly masses that are vibrating, such as speaker cabinets. This prevents the undesired transmission sound through additional materials that can result in a compromise in sound quality to he listener or at the microphone. DIP Switch
Acronym for Dual Inline Package Switch. A compact electric switch with a slider that may be set to "on" or "off." Because of the small form factor, arrays of multiple DIP switches can easily be incorporated into one package. This type of switch is designed to be used on a printed circuit board along with other electronic components and is commonly used to customize the behavior of devices such as mic preamps, equalizers, mixer inputs, and other specific applications. DIP switches are often used instead of jumper blocks in situations where a setting may need to be changed more than once. Direct Out
A patch point found on many mixing boards for taking an individual output out of a channel as opposed to routing its output through some bussing architecture. Direct outputs are not all the same though. Some take their signal right after the mic preamp and before the EQ or other gain stages, some may take the signal after the EQ, but before the channel fader, and some are taken after the fader. Many mixers provide a method of switching where the direct out signal comes from on a channel-by-channel basis.
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.
Discrete
Constituting a separate thing, with unconnected distinct parts. In the world of audio, discrete is generally used to signify separate components. This may on the level of describing the system, where one might refer to the mixer and recorder as discrete from one another, but usually this term is used at the electronic component level. In a mic preamp, for example, discrete components is understood to specifically mean that no IC's are used. Each element of the preamp is a separate transistor (or tube) stage with its associated discrete components (capacitors, resistors, etc.). Conventional wisdom suggests that discrete designs are better, but like anything else in modern electronics this really depends on a number of other factors. All other things being equal discrete designs do give the designer more flexibility, which can obviously yield favorable results.
Effects Loop
A signal path out of one piece of gear, through an effects unit, and back into the first device. It effectively is a loop, with an effects processor in the middle. When you send a signal out of a mixer on an aux send to a reverb, and then bring that signal back to the mixer you have created an effects loop, though we rarely call it that in those circumstances. Most of the time the verbiage is used in the context of guitar or bass amps, or guitar or bass preamps. Many of these have a dedicated insert point designed to be used with some outboard processor. In most cases they work just like the insert of a mixer: plugging something in breaks the internal signal routing to send the signal through the external loop.
EIN
Abbreviation for Equivalent Input Noise. EIN is a specification we most commonly encounter when looking at microphones and preamps. Because the output of most microphones is so low the amount of self-noise they produce can be important. Apply a lot of gain and any little bit of noise becomes pronounced. Further, any self-noise of the preamp also becomes pronounced under the high amounts of gain required. There are established theoretical noise floor limits for electronic equipment. All devices operating at a temperature above absolute zero produce their own noise. Even a simple resistor, or any source of resistance in a circuit will produce noise. In fact, a 200 ohm resistor on its own produces 0.26 microvolts of noise. Referenced to standard line level signals this is equal to -129.6 dBu of noise (for more on dBu, see our Summit on dBu versus dBV). When a microphone is connected to a preamp you can think of the microphone as a 'source resistance.' 200 ohms is often considered typical, though mics do vary quite a bit, however the EIN specification is supposed to be measured with a 200 ohm source impedance (for the sake of comparing apples to apples). So you start with .0.26 microvolts of noise, and then add whatever noise the preamp has and you get the real working noise of the system (the system being the mic and the preamp). Preamp manufacturers know they are more or less starting at this theoretical noise limit (-129.6 dBu) so the value they quote is Equivalent Input Noise in their specs. EIN basically takes this 'source noise' into account. Therefore the theoretical lowest EIN spec you could encounter with preamps is -129.6 dBu, which would mean the preamp itself produces no noise at all. If the preamp produces the same amount of noise as the source resistance this value will go up by 3 dB to -126.6 dBu (you may also see dBm). Most mic preamps fall somewhere within this range, however, like most things in specs it is fairly easy for the manufacturer to tinker with the methods to produce better results. Consequently it is not unheard of to see values in the -130 to -135 range. This is usually accomplished by measuring with a lower source impedance, or even a direct short across the input. Resistors of lower impedance will produce less noise, but also offer an unrealistically low source impedance to the preamp, which means the measurements don't have as much 'real world' relevance. Occasionally you will see EIN rated in dBV. Be careful there because the dBV standard gives a result that is 2.2 dB better just because it is referenced to a different voltage to begin with (again, see the dBu versus dBV summit). Clearly this information is pretty technical and not for everyone. For those who don't want to digest it all you can sleep at night knowing that most modern preamps you encounter are of such high quality that they are within a tolerable range in terms of their self-noise. In short, don't lose too much sleep over this unless you are recording very low volume sounds.
Electret
A type of microphone design, similar to condenser. Basically, there is a permanently charged plate in the mic element. As the diaphragm moves in response to sound pressure, it creates a changing capacitance with the plate. The big advantage to using electret (also called back-electret, or occasionally prepolarized condenser) technology is that it does not require an external polarizing voltage (battery or phantom power). In some cases, the microphone includes an impedance changing preamp that requires battery or phantom power, but the electret element itself does not require voltage. Electret mics can lose their charge in high humidity and high temperature environments, so some care should be used in storing and using them. If the electret loses charge, the mic's sensitivity will suffer, resulting in an reduced signal to noise ratio.
Equivalent Input Noise (EIN)
A rating of the overall noise performance of an amplifier (typically a microphone preamplifier). Basically, this is a measure of how much noise a mic preamp will add to a microphone's signal. Measurements are normally made with a 150 Ohm resistor on the preamp to simulate the load a mic would present. The theoretical limit on EIN is -130.0 to -131.8 dBm (the thermal noise generated by the resistor). When comparing this spec, keep in mind that larger negative values are better (i.e. -124 is better than -118). But don't place TOO much weight on this spec, most current EIN specs are infinitesimally small (can you REALLY hear the difference between -120 dBm and -122 dBm??)
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.
Gain Structure
When multiple pieces of electronic audio (or video) equipment are used together, the gain structure of the system becomes an important consideration for overall sound quality. This basically refers to which pieces are amplifying or reducing the signal how much. A properly set up gain structure takes maximum advantage of the dynamic range and signal to noise ratio of each piece in the chain. No one piece is doing a disproportionate amount of the amplification unless it is a piece designed for that function (such as a mic preamp). An example of poor gain structure would be a setup where a mixer's master fader is near the bottom, while all of the individual channel faders are near the top. The resulting level out of the mixer is the same as it would be if all faders were at some mid setting, but the chances of distortion are much higher because of limited available headroom in the circuits preceding the master fader, while the S/N ratio of the final output isn't as great as it could be were the master fader at a more appropriate level. Part of assembling a system with good gain structure is making sure all the pieces can operate at the same reference level. This is where people go wrong combining -10 dBV equipment and +4 dBu equipment. It can work under the right circumstances, but sometimes the resulting gain structure severely compromises the signal to noise ratio of the final result (or in some cases causes it to be distorted). Gain structure must be considered to optimize any system where levels can be adjusted in more than one place.
Inverse Square Law
Useful when setting up a microphone or speaker, the inverse square law states that, in a free field the intensity of sound drops by 6 dB for each doubling of distance from the source. Now, none of us ever work in a truly free field (no reflective surfaces), but for most applications these numbers are accepted as workable. In real world terms, this means that for each time you double the distance between your sound source and a listener or microphone, the power of the audio drops by 75% - a fairly significant amount! How much is this in terms of volume? Well, it depends on the source you consult, we've seen both 6 dB and 10 dB convincingly listed as doubling or halving the volume (let's just say it's subjective and leave it at that...) - regardless, 6 dB is a very noticeable drop in level! Consider this the next time you place a microphone or speaker: Rather than just cranking up or attenuating the mic preamp or amplifier level for gain control, look at the distance to your source...
Line Input
On mixing boards this is an input to a channel that is specifically designed for line level signals. Unlike the XLR microphone input, which is designed for low level mic signals, line inputs are usually 1/4 inch connectors, and are quite often unbalanced, though this will vary depending on the mixer. Line level signals are usually much higher than typical mic level signal and do not need as much amplification to be dealt with by the rest of the mixer. As such, on some mixers, the line inputs actually bypass the microphone preamp stage providing for a pure signal path into the board. Regardless of this, however, line inputs are always capable of handling higher level signals and high impedance signals better than the XLR mic input.
Mic Amp
A type of amplifier specifically designed to amplify signals from microphones. Mic amps come in all shapes and sizes. Usually there are several built in to most mixing boards, but there are hundreds of different outboard units as well. Mic amps are often referred to as preamps, or mic preamps. Either way they are designed to work with the relatively low level and potentially fragile signals produced by microphones (see WFTD Mic Level). Since there are so many different types of mics (condenser, moving coil, ribbon, etc.) signal levels and impedances can vary widely. Besides sounding better in general, a good mic amp is better equipped to maintain the integrity of the signal under these widely diverse conditions. Mic Level
The level (or voltage) of signal generated by a microphone. Typically around 2 millivolts. Compare this with the two normal line levels (1.23 volts and .316 volts), and it becomes apparent just how much amplification is going on in a microphone preamp, and why it is essential that preamps be of as high quality as possible!
Minimum Terminating Impedance
The lowest impedance at which a piece of gear, usually a microphone, can effectively drive a signal into without degradation in performance. If the unit is connected to something with a lower impedance spec, it will usually have a lower output voltage or greater distortion or both. As it relates to microphones and preamps, this does not necessarily imply a bad, or undesirable resulting sound. Moving Coil
A specific type of dynamic (as opposed to condenser) microphone design. Moving coil microphones are among the most commonly used in music and sound production. The ubiquitous SM-58 and SM-57 mics are examples of moving coil design. These mics work on very simple principles. In fact they work just like a speaker in reverse. The diaphragm has a coil of wire attached to its base. This coil is inserted into a magnetic gap. When changes in air pressure cause the diaphragm to vibrate in and out of the magnetic gap it generates an alternating current in the wire that represents the signal.Moving Coil is also one method used in making phonograph cartridges. Moving coil designs were all but replaced by moving magnet designs (same principle, but the magnet moves instead) in the 1970's. Moving coil phonograph cartridges have very low output (requiring a different preamp) and are very expensive compared to their moving magnet counterparts, but there are some sonic advantages to them including lower distortion and better frequency response.
Out of Phase
A phrase used to characterize two or more signals whose phase relationship with each other is such that when one is at its positive peak the other is at (or near) its negative peak. This is also commonly referred to as being 180 degrees out of phase.
Phase is a relative value that is measured in degrees (like angles). 90 degrees out of phase is more out of phase than 80 degrees, but less than 100 degrees. 180 degrees out of phase is completely backwards, which is characterized by one signal's highest peak correlating with another's most negative peak. Most signals are not entirely in phase with each other, and it's just as rare for them to be perfectly (180 degrees) out of phase. But people generally say "out of phase" to mean approximately 180 degrees out of phase. People also frequently say "out of phase" when the more technically correct term to use would be "polarity reversed." Phase implies a time delay, where one signal lags behind another one to some degree. Polarity refers to one signal being "backwards" from another. An example of this would be the "phase" switch on many mic preamps and mixing boards. Generally all this switch does is reverse pins two and three on the XLR connector entering the preamp, thereby reversing the "polarity" of the signal. There is no time delay of the signal. Nevertheless this is often referred to as "out of phase." A similar thing happens when you reverse the polarity of the speaker leads to one speaker in a two-speaker setup. That speaker is now operating with the opposite polarity of the other. No time delay was introduced, yet we often refer to this as "out of phase." This confusion occurs because when viewed on a display like an oscilloscope waveforms that are 180 degrees out of phase with each other will not look any different than two that are polarity reversed. Sonically the difference is generally pretty minute as well. So for all practical purposes the two terms can be used interchangeably.
While it is technically true that any two signals not 100% in phase with each other could be referred to by the somewhat generic phrase, "out of phase," we generally don't use that terminology until the signals approach a 180 degree phase relationship with each other. Pad
- An electronic circuit designed to attenuate the output of a device by a given amount. For example, some microphones have so much output that they can overdrive the input stage of many mic preamps. To prevent this, mic designers will include a switchable "pad" on the output stage of the mic, attenuating, or reducing the mic's output by 10 or 20 dB. While many devices have built-in pads, it is also possible to purchase external pads, which plug in to a device's output and reduce its level.
- A sustainy, "wash" or fill sound, usually used as harmonic background material in a musical arrangement. Arrangers often speak of using a "string pad" during a passage; this would be a section of strings playing long, sustained chords behind the melody. With the advent of samplers and synthesizers, other types of sounds have also become common as pads; just about any sound that can sustain can be used as a pad these days!
Phase Invert
A switch found in the input sections of mixing consoles and mic preamps. The term "phase invert" is actually a misnomer, since what the switch really does is invert the polarity of the signal in that input (correct usage would be "polarity invert"). Its intended use is to correct for balanced lines and mics that are wired backwards. In some cases toggling the phase invert switch may make a sonic difference if signals are out of phase, but doing so will also put that signal out of polarity with the others. (See also WFTD "Phase" and "Polarity")
Phono preamp
Short for phonograph preamplifier, a special type of preamplifier designed to handle the output of phonograph cartridges, which are transducers designed to turn the grooves in a phonograph record into electrical energy that can be amplified for a playback system. The phono preamp is a circuit that boosts the very weak signal coming out of the aforementioned cartridges up to more of a line level so it can be properly handled by the other components in a hi-fi system. Additionally the preamp's job is to apply equalization to the signal to restore it to its original form. In order to make it easier to manufacturer phonograph records, and to make them more universally playable, it was determined years ago that special equalization would be applied during mastering. The RIAA came up with an equalization curve - now known as RIAA Equalization - that, among other things, lowered the level of low frequency information relative to other frequencies (See WFTD RIAA Equalization for more background). In order for records to play back properly the opposite EQ has to be employed in the phono preamp. Not all phonograph cartridges require the same amount of and type of equalization though. High end or audiophile preamps allow the user to set certain parameters to better tailor the response of the preamp to the cartridge being used. Point-to-Point Wiring
A method of connecting electronic components in a microphone, preamp, amplifier or any other piece of equipment in which each component is directly soldered to a tube pin or solder lug or jack. This is essentially the "original" method of making solder connections, which in modern times has largely been replaced by the use of printed circuit boards, on which the wiring has been replaced by conductive traces (usually copper or silver) that run from socket to socket as required for connections.
Original forms of point-to-point wiring used no "boards" whatsoever; key components such as vacuum tubes were often mounted in ceramic sockets for stability. A variation on this employed the use of tag boards - simple templates, often made of thin cardboard with a waterproof coating - on which the location of each component was marked or stamped, to speed assembly. Virtually all soldering performed in point-to-point wiring was (and is) done by hand.
Polarity
In electronics, two points that have opposite electric potentials (one is positive, the other negative). This is not the same as being 180 degrees out of phase (although the results can be similar). Phase implies a relationship with time, polarity does not. What most engineers, consoles and preamps refer to as a "phase" switch is actually a switch reversing signal polarity.Polarity is important when interfacing equipment, particularly speakers - you don't want one cone moving in while the other moves out. Some designers feel that maintaining "absolute polarity" (no polarity reversal in a signal chain) throughout a signal path is important.While tests don't indicate that the ear can hear which polarity is correct, they do show that it may be possible to detect a difference between normal and inverted polarity signals. (Try it for yourself in a critical listening environment: Play a signal though a single speaker, then reverse the speaker wires and play the same signal again - remember to switch the wires back when you are finished!)
Preamp
Short for preamplifier. A type of amplifier specifically designed to amplify very weak signals before they are fed to subsequent gain stages or devices. Preamps are commonly used to bring things like the output of microphones up to a level where more equipment can work with the signal. Similarly, magnetic pickups (as used in guitars and basses), and phonograph cartridges are generally run through a preamp to prepare the signal to be used by other equipment downstream. Preamps are called upon to deliver extremely high amounts of gain while introducing very low amounts of noise and distortion. As such they are a critical component in the audio chain, and in recent years have come under much scrutiny by recording engineers causing many dozens of stand-alone mic preamps to be developed that allege to have superior sonic characteristics.
Re-Amp
The process of running an already recorded signal back through an amplifier (and possibly speakers) of some sort. With the increased popularity and flexibility of DAW systems this has become a popular technique for guitar. An engineer may record the guitar signal dry, or even directly out of the guitar itself along with or instead of any amplifiers, preamps, or effects, and then later process the raw track(s) through a guitar amp or some other preamp or processor. This is accomplished by routing the raw or dry guitar sound (or any other track, for that matter) out of an output of the DAW and into the amp - then out of the amp, usually by way of miking the speaker, back into another channel of the DAW. Sometimes this is also done with plug-ins inside the DAW itself. This final produced sound may then be recorded to another track or simply treated as a live instrument for mixing. This enables artists and engineers the maximum amount of flexibility for the sound as the piece progresses. Often times the guitar sound doesn't get finalized until the mixdown. While this technique is most commonly used for guitar it is also done for bass, keyboards, and sometimes even things like vocals or drums for special effects.
Trim
Found on most mixers, trim controls provide the initial level setting for each channel's input gain. In most cases, trim adjusts gain of the microphone preamp, but it may also apply to line level signals. Optimizing this gain stage will make a tremendous difference in the mixers signal to noise ratio and in gain staging later in the signal chain.
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