~ Glossary ~ Clavis ~ Vocabulary ~

~ An Explanation of some Terms and Abbreviations used on this Website ~

Continually under construction as the rest of this site grows ~ December 2021

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A measure of square area = 4046.86m2 ~ Often used to describe space available in an area which could be small <1m2 but has room for additional objects "There are acres of space" ~ Truly large areas are often described as "An area the size of Wales" which is part of the UK with an area ≈ 4M acres
Alternating Current or voltage ? ~ Used as a prefix or suffix to indicate that a Voltage or Current value alternates or changes direction at a fixed rate ~ The basic form of a.c. is a sine wave which alternates equally positive and negative ~ more complex a.c. forms like music which vary in rate and amplitude are referred to as waveforms or signals

A device for making a physical parameter larger ~ In electronics amplifiers increase Voltages or Currents by a fixed amount but only within the limits of the supplies used to produce the larger quantity and within a limited bandwidth ~ Audio power amplifiers convert a voltage to another voltage but with the capability to supply a greater current into a lower resistance load and without loading the source

Audio amplifiers are often segregated into 'stages' to describe their operation or role ~ input stage ~ output stage ~ pre-amp ~ phono stage etc. ~ each stage type has its own requirements and limitations

Amp ~ Ampere
Unit of electrical current ~ See Wikipedia for this weeks description
The term auto-bias applies to any method of automatically adjusting a bias required to keep a system operating within defined limits ~ It may be mechanical or thermal or when biasing thermionic valves ~ electrical ~ A resistor placed in series with a valve cathode (Rk) has a voltage across it which is proportional to the cathode current (anode current if a triode) ~ With the valve grid referenced to 0V ground the voltage across Rk gives the required negative grid bias wrt to the cathode

Valve auto bias is a simple feedback system that tries to maintain the correct bias as the valve ages ~ Rk is often bypassed with a capacitor of suitable value to prevent a.c. feedback lowering the gain while still providing d.c. gain (bias) correction ~ Some 'designers' use semiconductor voltage regulators or more complex arrangements in place of Rk and these also require bypassing at a.c. which can cause conflicts between the 2 feedback systems and unlike a simple resistor many regulators have unsuitable minimum and maximum voltage limits

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Bandwidth BW
The range of frequencies used for transmitting an electronic signal or the range of frequency which a system responds to or can operate effectively within ~ It is also now used for the transmission capacity in bits or lots of bits of a computer network or other digital telecommunication system

Audio bandwidth is normally defined between 20Hz and 20kHz based on the maximum discernible frequency range a normal (young) human can hear but there are superhumans who can hear the audible differences between gold plated and standard mains fuses

The dictionary figurative definition is "the breadth of a person's interests or mental capacity" ~ It is also used figuratively to describe a lack of manpower by 'managers' who cannot mange their manpower and also often have no interest or mental capacity for their job role

Is a physical constant k or kb named after the Austrian physicist Ludwig Eduard Boltzmann that relates the energy of atoms or molecules to absolute temperature ~ In electronics it appears in the formula for thermal noise voltage Vn-rms=√4kTBR where k=1.38062259 x 10-23 J/˚K
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Comité Consultatif International Télégraphique et Téléphonique ~ Or in English the International Telegraph and Telephone Consultative Committee ~ as with many of these international groups the French based acronym was chosen to keep the Entente Cordiale ~ through the 1920s the terms CCI and CCIF were used and in the 1930s CCITT and ITU all of which is best explained here
A coupling capacitor connects stages of an amplifier or pieces of equipment so that direct current and low frequency a.c. is not passed from one to the other but a required a.c. signal is ~ How much low frequency is passed depends on the value of the capacitor and the load resistance of the following stage

Sometimes ~ especially in old radio articles ~ coupling capacitors are referred to as 'blocking capacitors' or their full self explanatory title 'd.c. blocking capacitors' but after the introduction of transistors these titles appear to have been dropped

In good amplifier schematics the interstage coupling capacitors are drawn horizontally to represent the a.c. signal flow from left to right ~ de-coupling capacitors are drawn vertically with one end connected to chassis ground or 0V but both in practice are in the signal path

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Deci-Bel ~ Prior to 1924 telephone line quality was quantified by comparison against a 1 mile length of standard line at a frequency of 800Hz and the reference became known as the '800 cycle mile' ~ The attenuation of current or voltage along the line was found to be an exponential function of the distance d

Line loss being proportional to ed led to the CCI 1927 introduction of 2 measurement units ~ the 'Neper' using natural logarithms (ln) and the Bel using log base 10 (lg) which made calculations of powers related to line distance much easier ~ A Bel is defined as the logarithmic reference to a 10:1 power ratio where Bels = lg(P1/P2) or Nepers = 0.5 ln(P1/P2)

In 1968 the CCITT recognised one tenth (deci) of a Bel or the dB as the prime way to express signal transmission gains and losses where dB = 10 lg(P1/P2) such that positive dB values show signal path gain and negative dB values indicate a loss or attenuation and the dB values are simply added to give the overall transmission path loss or gain ~ dB is a relative measurement

Deci-Bel relative to a carrier signal ~ Often used in radio frequency measurement because the carrier signal is the only reference available ~ the level of any interference or the amplitude of sidebands is relative to the carrier amplitude

dBm or dB(mW) is a dB measurement relative to a power of 1mW ~ This requires the impedance in which the power is developed to be known and for telephone transmission systems this impedance was standardised at the average of early long cables and was 600Ω ~ Using a standard impedance amplifiers and transformers could be optimised for gain and frequency response and S/N and the measurement of power levels was done by simply measuring voltages where P=V2/R and R was 600Ω ~ 0.775Vrms across 600Ω is a power of 1mW and other voltages relative to this reference are expressed in dBs as 10 lg(V2/0.7752) or 20 lg(V/0.775) ~ For radio frequency transmission the cable impedances were standardised at 75Ω and 50Ω with the same 1mW reference but in all cases dBm is a measure of absolute power ~ unlike engineering management
Deci-Bel relative to something where the 'something' must be defined or made clear ~ The units and method used to measure the 'something' and what it is relative to must be the same ~ dBr prevents the need to creating a myriad of dB suffixes like 'dBov' which have to be defined in context anyhow
dBu or dB(0.775V) is used to refer a voltage measurement to a reference of 0.775Vrms but unlike dBm the voltage is not necessarily across a 600Ω termination impedance and the suffix 'u' is used to denote 'unterminated' ~ dBu is an absolute measurement of voltage

dBV with a capital 'V' is used to refer a voltage measurement to a reference of 1.0Vrms it is simply a voltage without reference to any termination impedance ~ dBV is an absolute measurement of voltage
Direct Current or voltage ~ Used as a prefix or suffix to indicate that a Voltage or Current measurement or value is or should be steady with time as in d.c. to daylight ~ Also used for pulsed Voltages such as rectified a.c. where the direction of current does not reverse ~ Sometimes used to represent 0Hz which raises the question where does d.c. stop and a.c. begin ?
In an audio amplifier a de-coupling capacitor is used to return a.c. signals to 0V ground so they are not passed on to further stages ~ they are often used across bias resistors to prevent the gain of an amplifier stage being reduced by 'local feedback' and from d.c. supply lines to ground

In good amplifier schematics the de-coupling capacitors are drawn vertically to represent the a.c. signal flow down to ground 0V ~ coupling capacitors are drawn horizontally to indicate the signal flow from left to right but both in practice are in the signal path

Nowadays this could be a Transistor using Diamond as the semiconductor ~ From the 1990s diamond material has been used in photo–diodes for detecting and measuring high power ultraviolet light ~ The term Diamond Transistor as used by Burr Brown etc. refers to Voltage controlled Current sources that operate bi–directionally in all 4 quadrants of a V–I graph
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Noise Factor


The noise factor F of a system of electronic amplifiers and attenuators is defined as the S/N at its input divided by the S/N at its output and as such cannot be better than 1 ~ The radio frequency Engineers definition often states each S/N is derived from power measurements at room temperature but this is not practical in many cases especially for audio but then neither is a noise factor for an audio amplifier often required ~ For low noise audio amps like pick–up cartridge inputs it is often nice to know how much additional noise is added by the amplifier or how near perfect it is and this is often expressed as a dB worsening of the output S/N which is referred to as a Noise Figure NF
A device to separate things like coffee grounds from the wanted drink or to remove impurities from water or oil ~ Electronic filters are used to limit the bandwidth of signals or suppress unwanted noise or interference ~ Filters cannot remove noise and distortion within the required frequency band and although they are used extensively in the audio recording process they are frowned upon in some reproduction systems ~ Filters are not equalisation despite what the authors of some (valve) RIAA pre-amplifier 'designs' say
When analysing the harmonics of a repetitive signal the Fundamental is the lowest frequency that forms the signal and can be referred to as the 1st harmonic or F0 (not F1) ~ When performing distortion tests like IMD using more than one frequency both are considered fundamental frequencies (by me)
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Gain Margin


Gain margin often abbreviated as Gm not to be confused with gm which is mutual conductance of a thermionic valve ~ There are various ways gain margin is defined and most assume perfectly monotonic open and closed loop gains but for valve power amps this may not be possible so it is best defined as the amount the normal closed loop gain can be reduced by increasing feedback before oscillation occurs ~ The test may give different results at various power levels and frequencies and where output transformers are used should allow for testing with an open circuit i.e. should be at least 7dB or >2x and not just calculated or modelled



Gain Bandwidth Product ~ Is the product of multiplying the gain of a device (which does not have to be an op-amp) by its bandwidth measured at –3dB or 0.707x points and thus the GBWP is a bandwidth ~ As with other similar parameters the device should be monotonic between –3dB points and these 2 points in the open loop response may be set by a 'dominant pole' so that the open loop is better defined before adding negative feedback ~ As the gain is reduced by NFB the bandwidth 'should' increases proportionally
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InterModulation Distortion ~ Is distortion due to signals "mixing" when passed through a non linear transfer device ~ IMD may be non desirable such as in an audio amplifier or intentional such as a diode mixer ~ The simplest form is the production of sum and difference signals of 2 desired signals and this is the basis of measuring IMD ~ In an audio amplifier the distortion can be from many signals spread over a 10 Octave BW and the IMD products will extend above and below the audio BW and may even be negative frequencies appearing 'aliased' in the low frequency range
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Johnson Noise
Is electrical noise generated by the random movement of charge carriers in a conductor similar to Brownian motion in gases and liquids ~ It was first discovered by John B. Johnson of Bell Labs in 1926 and was analysed by his colleague Harry Nyquist who formulated the rms noise voltage in a specific bandwidth and temperature is Vn = √4kTBnR which is described more practically here
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All at the same height or amplitude ~ A level or 'flat' response means the amplitude of all frequencies in a particular bandwidth are the same at the point they are measured ~ the output response of say a pre-amp or channel of a recording desk may be varied using EQ but should be 'level' when the EQ is cancelled or set 'flat'

The term level is also often used for absolute measurements like "the output level should be 100mV" ~ we will assume an a.c. voltage into a reasonable impedance say 100kΩ but why should we ?

Line Level
In telecommunications or broadcasting may refer to the nominal level of signal at an audio circuit node expressed in one of the dB formats ~ For domestic audio equipment it tends to imply an input or output of an amplifier between 100mV and 1Vrms that has a level or 'flat' frequency response
Long Tail Pair
A term derived from the layout of early valve analogue computers where pairs of valves used as differential 'operational' amplifiers had a common cathode resistor taken to a high voltage negative supply buss running below the platform supporting the valves ~ The cathode resistors were often several resistors in series to get a high value and high voltage rating and these hung below each pair of valves like 'long tails' ~ Beetles finding their way into these computers would often get zapped trying to climb the resistor chains causing errors or 'bugs' in the analogue output ~ Terry Gilliam in his wonderful film 'Brazil' had another take on this story
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Miller Effect
Miller Capacitance

The term Miller effect normally refers to the multiplication of the capacitance that exist between the output and input of an inverting amplifier stage which appears as a larger capacitor across the input of that amplifier ~ It can also be used for the change in any impedance connected between 2 nodes exhibiting inverting gain in a circuit ~ The effect was recorded by John Milton Miller in a scientific paper in 1920 when it was shown how the input impedance of single triode valve depended on the gain and the impedance at the output ~ Tetrode valves were already developed to 'eliminate' the capacitance multiplication problem in single stages but Miller made use of it with triode valves to create oscillators and stable amplifiers
Mutual Conductance
The Mutual conductance of a thermonic valve is a measure of change in anode current ∂Ia due to a change in the control grid to cathode voltage ∂Vg1 ~ strictly it is measured with the anode and other non control grid voltages held constant and has the units mA/V
More information and diagrams here
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Noise Figure ~ Is the noise factor F of an electronic system expressed in dB ~ F is used mainly for radio systems or chains of equipment with common impedances ~ NF is best suited to audio amplifiers although not often quoted by manufacturers because it makes comparison of the best and worst designs obvious ~ The input and out impedances of audio amplifiers are often different and stating the decrease in S/N in dB is simpler because the S/N are often already in dB ~ For low noise amplifiers it is the source impedance that dictates the input S/N and the output S/N will always be worse than the thermal noise of the source resistance divided by the source (reference) voltage ~ The series input resistance of a standard QUAD 405 is 22kΩ and the input voltage for 1W output is 0.05V so the maximum possible output S/N even with 0Ω source resistance will be 85dB at 1W measured in the 19980Hz audio Bandwidth
Negative Feed-Back ~ See Negative Feedback in Transistor Amplifiers by S W Amos
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Out-Put Transformer ~ The acronym OPT is easy to quote but not so easy to design or build because an audio output transformer is required to operate over a frequency range of 10 octaves and the winding requirements at each end of this frequency range conflict
The 'Order' of a filter can be and is described in various ways by different people ~ This website mostly refers to the order of a filter based on its construction ~ A single capacitor (or inductor) and resistor filter has a 1st order response where the transfer function in the stopband changes at a rate of 6dB/octave (the voltage or current doubles or halves for each doubling of frequency) which is also 20dB/decade (the voltage or current changes 10x for each 10x change of frequency) ~ The order defines the linear slope of the response in the stopband ~ A 2nd order filter has slope of 12dB/octave and a 3rd order 18dB/octave which is also 60dB/decade (the voltage or current response changes 1,000x for a 10x change in frequency) ~ There is an associated phase shift and delay with any filter which increases with the filter order such that only 1st order filtering (if any) is tolerated by audio purists

Every resonant system has at least one 1st order low pass response ~ If there was no upper frequency limit the Johnson noise voltage from a single resistor would be infinite ~ Microphones have a low frequency response that cannot extend to d.c. or zero frequency but then neither does 'audio' or our ears

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Phono Connector
Abbreviation for Phonograph or record player or gramophone ~ Used to indicate inputs on audio equipment that accept signals from a record deck pick–up ~ With the birth of "Hi-Fi" at the time of the Williamson amplifier the Radio Corporation of America RCA introduced the connector now commonly known as the 'Phono' (and incorrectly as the Cinch) to connect the screened lead from a turntable pick–up to the sensitive 'phono' amplifier and was soon adopted world wide for other external screened connections ~ With the 1960s introduction of DIN plugs for audio connections separate 'RCA' inputs were often still used for the Phono inputs which set the term
Phono Stage
A Phono Stage is an amplifier placed between the output of a vinyl disk pickup cartridge and a 'line level' input of a pre-amp and may be internal or external to the pre-amp ~ As well as amplifying the low level pickup signal often more than 1000x or 60dB ~ the Phono stage provides up to 40db of gain variation across the audio frequency band to correct for the pre–emphasis equalisation used during the record process which has to be corrected for during replay even if a non magnetic transducer like a strain gauge is used
Pre–emphasis is a correction of the frequency response of recorded or broadcast sound to overcome an increase in noise at high frequencies ~ Magnetic tape and vinyl disc reply with magnetic cartridges require that the record level reduces (–6dB/octave) as frequency increases and above about 1kHz the noise of the recording medium would become noticeable ~ Before (Pre) the input signal is passed to tape record head or disc cutter it is filtered such that the signal at high frequencies increases (emphasis) by 6dB/octave starting at some low frequency ~ RIAA and Dolby noise reduction are complicated forms of pre–emphasis and that applied to FM radio broadcast is a basic 6bB/octave above 1kHz such that the transmitter input is about 3dB higher at 3kHz in Europe and Australia and about 2kHz higher in USA and Japan (there are other countries)

Pre-emphasis is also included in the specification for CD recording and playback although it is seldom used ~ It is possible that playing CDs with pre-emphasis via a computer or ripping the files to be stored on a server will give a bright or bass deficient sound ~ copying files from other people is more of a problem because some programs allow outputing of digital audio files with 'personal equalisation'

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'q' is the symbol used for the charge of an electron which is 1.602176487x10-19 Coulombs ~ it occurs in equations such as in=2qIdc  A/√Hz which is the rms 'shot noise' current in a conductor
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Root Mean Squared or Quadratic Mean ~ Used as a prefix or suffix to a regularly varying Voltage Vrms or Current Irms value to indicate that it will have the same effect as an equivalent V or I d.c. when producing power ~ RMS values are derived by squaring all values of the measurement waveform as a continuous function then taking the square root of their average or mean thus allowing for negative and positive values of a waveform producing power in a linear load such as a resistor ~ Wrms RMS power does not exist
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SI Units
Système International ~ The International units of measurement that form the 'metric system' which is ideal for people with 10 fingers and the hard of thinking ~ Although truly international the abbreviation SI is derived from French and most of North America ignores it ~ probably because 1.663x10–4ms–1 is more complicated to use than 'a furlong per fortnight' which is a unit of speed that crosses systems when expressed as '1cm per minute' with an error of only 0.2143% in Europe and damn close in the US
Signal to Noise ~ Most often expressed as a Ratio SNR in dB ~ SNR=20xlog(Vs/Vn)dB or SNR=20xlog(Is/In)dB or SNR=10xlog(Ps/Pn)dB if measurements are Voltage or Current or Power respectively ~ In all cases the signal and the noise should be measured with the same equipment with the same bandwidth ~ A common mistake measuring S/N with a spectrum analyser is to reference the signal peak to the noise floor which gives much higher ratios and especially false figures for some audio equipment because the noise is not integrated in the 10 octave band 20Hz to 20kHz
When referring to electrical filters intended to reduce interfering signals the stopband is the frequency range where the unwanted signals are (or should be) reduced below a particular level ~ For simple 1st order filters the stopband is often referred to as the frequency range outside the –3dB point or corner frequency but specifying a particular level of rejection for unwanted frequencies is better
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Total Harmonic Distortion ~ The sum of harmonic products produced by passing a sinewave through a non–linear system – Expressed as a percentage or in dB below the level of the fundamental i.e. 1% or –40dB THD ~ The measurement bandwidth or number of harmonics should be specified as should inclusion of any weighting filters but sometimes this is not done ~ Swept filter and FFT spectrum analysers can measure each harmonic individually and then mathematically derive the THD

THD plus Noise ~ Measurement of total harmonic distortion as above but including noise present in the non-linear system and the measuring equipment ~ Some THD meters have a sharp notch filter to remove only the fundamental frequency F0 leaving the harmonic and noise energy either side of F0 to contribute to a THD+N figure ~ This type of meter will measure the system noise when the sine input to the device under test is turned off so an approximation of THD alone can be made
Thermal Noise

Thermal noise voltage or current is generated in conductors due to the thermal agitation of electrons within the conducting material ~ it was discovered by John Johnson at Bell laboratories in 1926 and was later formulated by his collegue Harry Nyquist ~ enter Wikipedia stage right
Transient Intermodulation Distortion ~ A form of Intermodulation distortion induced or made worse by the inability of negative feedback around an amplifier to correct distortion of transient or asymmetric signals due to time delays in the feedback path ~ TID may be produced where amplifier stages nested within a feedback loop have a bandwidth less than preceding stages ~ The output stage of a valve amplifier will be limited by the OPT design at both ends of the audio frequency response and preceding stages with greater bandwidth than the OPT should be avoided although ignoring this rule will give the amp a distinct 'sound' ~ See Measuring transient intermodulation in audio amplifiers
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A Variac is an autotransformer that is usually wound on a toroidal former using a single layer of wire with a wiper arm that sweeps around the winding connecting to exposed copper of each turn ~ The common application is with Mains a.c. voltage applied across part of the winding the output voltage from the wiper arm can be varied from zero to more than the input voltage to test Mains powered equipment at specific voltages
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Greek lower case omega used in electronics to represent Angular Frequency which for a frequency ƒ is derived by 2πƒ and is the rate at which the phase of the frequency changes in radians ~ ω and 2πƒ are often transposed to simplify a.c. calculations
Abreviation for ~ With respect to ~ Referenced to
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Italic capital Z is used in electronics to represent Impedance or the opposition to the flow of a.c. ~ The voltage across a resistor is proportional to the current flowing through it whether a.c. or d.c. but if a resistor R is coupled with an inductor L and or capacitor C the relationship between current and voltage is now dependant on frequency and the arrangement of R and L or C in the network as defined by the impedance Z ~ Most inductors or capacitors have some resistance and vice versa and so all components have an impedance even if it is not apparent at low frequencies
A Zobel network is a "filter section" named after Otto Zobel of Bell Labs who developed the principle of "image impedance" for telephone line equalisation ~ The RC network across an audio amplifier output is referred to as a Zobel network but often it is not a calculated image and simply acts to provide a "more resistive" load at high frequencies where it is assumed loudspeakers become inductive

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