Music Technology 222 Audio Recording

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Music Technology 222Audio Recording

• Spring 2003
• Alex Keller, Instructor
• Week 6

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Overview

• Amplifiers
• From vacuum tubes to transistors
• Properties and specifications
• Types of amplifiers

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Amplifiers

• Amplifiers are the piece of gear used most often
  in a studio
• Mic pre, EQ, aux sends, dynamic processors, pan
  pots, and faders are all amplifiers
• Any time you hear something over headphones or
  through speakers, an amp is driving the speakers

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The valve and the pipe

• In this picture, a small amount of energy (the
  amount required to turn the handle) regulates a
  large amount of energy (the water going through
  the pipe)

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The amplifier

• In an amplifier, a small amount of energy (the
  audio signal) regulates a large amount of energy
  (the current from the AC plug)

base
collector
emitter
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The vacuum tube

• The first amplifier was a modification of the
  first vacuum tube, the light bulb, invented by
  Thomas Edison in 1883
• The light bulb operates on the principle that in
  a vacuum, electrons can be forced to move from
  one piece of metal (emitter) to another with the
  same voltage potential (collector)
• In 1914 Lee DeForest added a wire mesh grid
  (base) between the emitter and collector to
  control the flow of electrons from the emitter to
  the collector with a small voltage

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The transistor

• The transistor was invented at Bell Labs in 1947
  by Dr. John Bardeen, Dr. Walter Brattain, and Dr.
  William Shockley, who received a Nobel Prize in
  1956 for their work
• Advantages of transistors over vacuum tubes
• Smaller
• More efficient
• Generate less heat (because they are more
  efficient)
• More robust

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The transistor

• A transistor varies between conducting and
  blocking electricity when a positive voltage is
  applied to the base, electrons are conducted from
  the emitter to the

collector when that voltage becomes negative,
no electrons are conducted


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Properties of amplifiers

• Linearity
• Unity gain
• Saturation
• Power
• Slew rate
• Signal-to-noise ratio
• Dynamic range

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Linearity

• Linearity in an amplifier refers to the
  resemblance of an output to an input
• An amp with a perfect linear response responds to
  all frequencies equally
• A perfect linear response is referred to as a
  flat response

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Unity gain

• Unity gain refers to an amplifier with 0 gain,
  outputting a signal identical to input
• Used in
• mixing consoles where signals can be combined
  without changing their original gain
• processing gear when device is bypassed, the
  output signal should be identical to the input

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Saturation

• When an amplifier does not have enough power to
  reproduce the signal the amp receives at the
  base, the amplifier becomes saturated and
  clipping results
• We hear clipping as distortion, mostly with odd
  (unmusical) harmonics

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Tube / transistor saturation

• While transistor saturation produces a sound
  perceived as harsh, tube saturation does not.
• Tube saturation creates a greater proportion of
  even harmonics, which we hear as musical or less
  harsh
• As a result, vacuum tube gear is still commonly
  used in the music and audio production industries
• REMEMBER tubes add coloration to a signal. They
  sound nice but are not appropriate when sonic
  accuracy is the goal!

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Power

• An amps power rating (which is expressed in
  watts) can be ideally obtained by multiplying its
  current and voltage usage
• However, since no circuit is 100 efficent (and
  most are a lot less efficent than that) amps
  wont operate at their ideal potential
• The most accurate way to measure an amplifiers
  power output is using Root-mean-square (RMS)

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Power

• RMS Square Root of the Mean (average) of the sum
  of the squares of each point on the waveform
• RMS is used because half of each point on the
  waveform would be a negative value, the average
  of which would be 0
• Using squared values allows us to work with
  positive numbers
• Dont bother looking at an amps peak or max
  power rating - RMS is the most reliable rating
  for an amp

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Slew rate

• Slew rate is the speed at which an amplifier can
  change the amplitude of its output signal
• Measured in V/ms (volts/microsecond) with a
  higher figure being a faster response
• A faster response indicates a more accurate
  representation of the input signal

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Signal-to-noise ratio

• Signal-to-noise ratio expresses the range between
  the amplifiers line-level output and the
  quietest signal it can accurately reproduce
• Expressed in dB

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Dynamic range

• Dynamic range expresses the range between the
  amplifiers maximum output and the quietest
  signal it can accurately reproduce
• Expressed in dB

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Types of amplifiers

• Op amps
• Preamps
• Equalizers/filters
• Dynamic processors
• Summing amps
• Distribution amps
• Isolation amps
• Crossovers
• Voltage-controlled and digitally-controlled amps
• Power amps

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Op amps

• Op amps are components that are used in almost
  all analog audio circuits
• Op amps have two inputs, one of which reverses
  the polarity of the input voltage
• Some of the op amps output voltage is fed back
  into the negative input, which works to
  phase-cancel out any noise output by the circuit

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Preamps

• A preamp takes a signal and amplifies it so that
  it is usable by other electronics
• Examples
• Mic preamps
• phono preamps
• line-level preamps
• Discrete (stand-alone) mic preamps are commonly
  used now as an alternative to console mic preamps

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Equalizers or filters

• An equalizer or filter is an amplifier that
  adjusts the gain of specific frequencies instead
  of the whole signal
• Types
• Graphic
• Parametric
• Fixed (non-user configurable)

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Crossover

• A crossover uses two or more filters to divide a
  full-bandwidth signal into two or more frequency
  ranges, so that specific frequencies can be
  matched to specific speakers
• Any speaker that has a discrete woofer and
  tweeter uses a crossover
• Large PA systems use crossovers to send
  line-level signals to discrete amps

amp
crossover
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Dynamic processors

• Dynamic processors can be referred to as VGAs or
  variable gain amplifiers
• A VGA is an amplifier that will boost or cut
  signal above or below specific amplitudes just
  as an EQ will boost or cut specific frequencies
• Types
• Compressor
• Limiter
• Noise gate
• Expander

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Summing amps

• Summing amps receive signal from a number of
  sources and combine them at unity gain while
  completely isolating the sources preventing any
  leakage from other signals upstream
• Used in mixers or any device where two or more
  analog signals are combined

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Distribution amplifiers (DA)

• A distribution amplifier takes one signal and
  splits it into multiple signals at, usually at
  unity gain
• Useful in bussing circuits and as a general
  studio utility

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Voltage-controlled and digitally-controlled
amplifiers

• Voltage-controlled amplifiers are used to control
  the gain of a line-level signal with an external
  voltage
• Used in synthesis, console automation and VGAs
• Digitally-controlled amplifiers are used to
  control the gain of a line-level signal with an
  external digital source, like a computer or
  dedicated controller
• Used in console automation and VGAs

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Power amplifiers

• Power amplifiers amplify line-level audio signal
  to a signal that will drive speakers
• What we normally think of when we think of an
  amplifier
• require more current than most other audio
  devices
• perform better when receiving clean AC power
• A general rule get an amp with twice as much
  power as your speakers can handle. Under-powering
  speakers results in amplifier clipping.

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Class A amplifiers

• Class A amplifiers use one transistor to amplify
  a signal
• Are the most accurate type of amp
• Are the most inefficient type of amp

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Class B amplifiers

• Class B amplifiers use two transistors for output
• One handles the compression side of the waveform
  and one handles the rarefaction side
• A small click exists at the transition between
  the two, creating a small amount of harmonic
  distortion 

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Class AB amplifiers

• Class A-B amplifiers use both Class A and Class B
  technology.
• Are Class A at low amplitudes and Class B at
  higher amplitudes
• Advantages
• more efficient than A
• more accurate than B