MICROPHONES AND AMPLIFIERS

Introduction
A microphone is an acoustic-to-electric transducer or sensor that
converts sound into an electrical signal. A microphone may be passive or active.
The electrical power output of a passive microphone is derived solely from the
acoustic power it absorbs, while an active microphone controls an external source
of power.
Further microphones have been classified into two
categories

1) PRESSURE TYPE: In such microphones only one side of the diaphragm is

exposed to the striking sound waves. It consists of a box, enclosing a volume of air
held at a static atmospheric pressure. The diaphragm forms the 'lid' over the box
can move in response to sound waves, and acts as a simple pressure detector that
compares the fixed internal air pressure with the varying external pressure
variations due to sound. Some examples of pressure type of microphones are
Dynamic microphone, Condenser microphone, Carbon Microphone etc.
2) PRESSURE-GRADIANT TYPE: In these types of microphones the diaphragm
mounting is open to passing sound waves on both sides. In such situation the
diaphragm will only move when there is a pressure difference (or pressure
gradient) between the front and rear faces depending upon the path lengths and
phase differences of the passing sound waves giving rise to following conditions:

a) Sound source is directly to the sides of the diaphragm: The sound waves
will be identical front and back; net pressure difference will be nil and so the
diaphragm won't move. The result will be that there will be no electrical output
either, and so the mic is almost deaf to side sounds.
b) Sound source is moved around to the front of the mic: The path length for
sound waves arriving at the back of the mic will be longer than for those arriving
at the front resulting in a phase difference, creating a pressure difference, which
causes the diaphragm to move and provide an electrical output signal. The
maximum output will be generated when the sound source is directly in front of
(or behind) the diaphragm due to longer path difference. The example of pressure
gradient microphone is Ribbon Microphone.
Characteristics of Microphones
1) Sensitivity: It is defined as the output of a microphone in millivolts
(or in dB below 1volt) for the sound pressure of 1 microbar (or o.1 Pa) at 1000
Hz. It is given by:
S = 20 log 1/ EO, where S=Sensitivity, EO = output
2) Signal to noise ratio(S/N):
It is defined as the ratio in dB of the output of the microphone with Sound
pressure level (SPL) of 1 bar to the output in absence of sound. It is given by –

S/N = 20 log Output in presence of sound

Output in absence of sound
3)Frequency response –
It is defined as the output level or sensitivity of the
microphone over its operating range from lowest to highest frequency. Although
the audible frequency range of sound is 20 to 2000 Hz, the frequency range is
judged for flat response from 40 to 15000 Hz. The graph below has frequency in
Hertz (Hz) on the x-axis and relative response in decibels (dB) on the y-axis.

Flat frequency response Shaped frequency response

A microphone whose output is equal at all frequencies has a flat frequency
response. They reproduce a variety of sound sources without changing the original
sound.
Some microphones have peaks or dips in certain frequency areas known as shaped
frequency response. A shaped response is usually designed to enhance or restrain a
specific sound source in a particular application. For example, a microphone may
have a peak in the 2 – 8 kHz range to increase intelligibility for live vocals which is
called a presence peak.
4) Directivity:
Directivity (D) of a microphone is defined as the ratio in dB of the actual
output when placed in a direction of maximum responce to the output which an
Omni-directional microphone in the same direction would have given. It is
given by –
Where E = Actual output when placed in a direction of maximum responce.
D = 20 log = E/Eo Eo = Output which an Omni-directional microphone in the
same direction would have given.
Microphone Polar patterns.
The polar patterns represent the locus of points
that produce the same signal level output in the microphone if a given sound
pressure level is generated from that point. Polar pattern of microphone
indicates its sensitivity to sounds arriving at different angles about its central axis.
It also determines the Directivity of the Microphone
Types of Polar Patterns

Omni directional; Figure of 8:

Picks up all around Picks up in front & behind

Omni directional microphones are sensitive to sound from all directions. They are
good for picking up the ambience and reverb of rooms and tend to sound very
natural and open even when placed close to instruments. Omni microphones don’t
exhibit any proximity effects but obviously are not good when separation is
needed.
ii) Figure of 8 (bi-directional)
Figure of 8 microphones pick up from the front and rear and have
null points to either side. They are good for recording two vocalists facing each
other or for recording something and still capturing the ambience of the room.
Figure of 8 microphones don’t exhibit any proximity effects.

iii) Cardioid
Cardioid microphones are directional have a heart shaped polar
pattern. This means they pick up sound mainly from the front and are least
sensitive to sound from the rear (its null point). Hyper-Cardioid:
Cardioid: Picks up from front,
Picks up from front, more focussed than Cardioid
rejects from behind

They can be positioned to pick up the instruments that you want to

record and “ignore” the unwanted one. Cardioid mics do have problems known as
the proximity effect. The microphone boosts the bass frequencies when it is close to
the object it is recording, resulting in a boomy sound.
IV) Hyper-Cardioid: It has a similar pick up pattern as
the Cardioid mics but are more directional and don’t pick up as much from the
side. Hyper-Cardioid microphones are good at separating specific sounds from
multiple sound sources. They are tightly focussed and are good for drum kits as
they can concentrate on specific drums and reject spill from other sources.
5) Output Impedance It is an important parameter of a microphone used to
determine which type of matching transformer would be required to efficiently
transfer power from the microphone to the transmission line and then ultimately
to the amplifier. It is represented in Ω (ohms).
Example: A dynamic microphone having low
output impedance & hence uses a built-in step-up transformer to match
the line.

DIFFERENT TYPES OF MICROPHONES

1) Dynamic microphone.

It works on the principal of Faraday’s Law which states that an

electrical current is induced within the conductor when it is moved through a
magnetic field. The magnetic field within the microphone is created using
permanent magnets .A coil of wire is attached to a thin diaphragm, and placed
within the permanent ‘annular ring magnet’. When sound pressure waves arrive at
the diaphragm it vibrates. The coil attached to it also vibrates within the magnetic
field, generating an output voltage proportional to the incoming sound. This
output voltage is very low - typically -70dBV, which is further amplified.

Characteristics
1) Sensitivity-30 micro volts(90 dB below 1 Volt)for sound
pressure level of 0.1Pa 2) Signal to noise ration: 30dB.
3) Frequency response: 60 Hz
to 8000 Hz. 4) Directivity:
Basically Omni-directional & Cardioid in series with ribbon microphone.
Applications:1) Speech & PA system.
2) Dramas, (Used in Cardioid pattern in series with a ribbon
microphone)
2) Electrostatic or Condenser Microphones: - (Pressure Type)

Principle & Working.

This type of microphone converts pressure fluctuations into electrical

potentials through the use of changing the capacitance of a capacitor. When the
capacitance changes, the charge on the capacitor tends to remain the same, then
the voltage & capacitance are given by
V= Q/C, & C= kA/d ----(1)
Where,V = Voltage across capacitor. Phantom Powering in condenser microphone
Q = Charge in Coulombs
C = Capacitance in farads Output
A = area of the plates,
d = distance between plates.
k =the dielectric constant.
Phantom power is a DC voltage (12-48 volts)
used to power the electronics of a condenser
From expression (1) we have. microphone. This DC voltage is supplied
Q Qd through the microphone cable by a phantom
V=
kA/d kA ---(2) power mixer or by some type of in-line external
source. For example, Pin 2 is 48 VDC and Pin 3
= is 48 VDC, both with respect to Pin 1 which is
Since Q, k & A are constant
ground (shield). Since the voltage is exactly the
then Q/kA = Constant = K (Say), same on Pin 2 & 3, phantom power will have no
Therefore, V= Kd or V = K X d --(
adverse
3) effect on the microphones signal.

Hence any change in the distance (d) due to variation in

sound pressure, varies the voltage(V).The two plates of the capacitor are
diaphragm (movable) & back plate (Fixed).
Characteristics
1) Sensitivity- Very low hence built-in amplifier is used to raise the output
to 03 mV(50 dB below 1V) at sound pressure of 0.1Pa or 1 bar.
2) Signal to noise ratio: High, about 40 dB.
3) Frequency response: Excellent, 40 Hz to 15000 Hz.
4) Directivity: Basically Omni-directional
Applications: For professional Hi-fidelity recording. Good for music purpose.
 3) ELECTRET MICROPHONE: (Pressure Type)

It is also a Capacitor microphone with a built-in charge facility. Insulating

material like TEFLON can trap & retain a large quantity of fixed charge. A thin
layer of negatively charged TEFLON is coated on the microphone back plate.
A positive charge is induced on the diaphragm due
Metal Supporting
to this back plate negative charge, which diaphragm fixtures
ultimately establishes an electric field across the gap
Teflon layer
resulting in a terminal voltage, which varies in
accordance to the sound pressure variation.
Characteristics:
1) Similar to that of a capacitor microphone
Insulator Back plate
2) No separate Bias supply needed.
An Electret Microphone
. Applications: Being very light due to absence of
separate bias- supply it is used as a tie-clip microphone.

4) RIBBON MICROPHONE (Pressure Gradient type) Built-in

matching transformer
Ribbon Foil

Figure of 8: Output
Picks up in front & behind

A Ribbon Microphone Permanent

Magnet

Microphones operated by the gradient of pressure are called pressure-

gradient type .Ex- ribbon microphone. The output voltage is proportional to the
instantaneous difference in pressure on the two sides of the diaphragm. These
microphones are also known as “velocity microphones”. A ribbon microphone
consists of a few microns thick, few cm long & 2 to 4 mm wide strip of aluminium
foil, suspended to vibrate between the poles of a permanent magnet. A built-in
matching transformer is used to step-up the impedance of the microphone.

Characteristics
1) Sensitivity: About 3μV or 110 dB below 1V for a sound pressure level of 0.1 Pa.
2) Signal to noise ratio: High, about 50 dB.
3) Frequency response: Excellent, 20 Hz to 12000 Hz.
4) Directivity: Bi-directional (figure of 8).
Applications: a) Suitable for Dramas, due to its Bi-Directional property.
b) Good for recording two vocalists facing each other.
 5) Wireless Microphones :
A Wireless microphone system consists of a microphone
connected to a miniature radio transmitter, and a receiver designed to receive
only that signal. Some are fixed tuned - that is, they use a quartz crystal for
determination of the operating channel. Most modern products are tuneable --
they add a frequency synthesizer circuit to allow multiple operating channels
from a single crystal. The output is designed for connection directly to the
microphone or line input of a mixer.
Wireless Microphones Transmitters are available in three basic
packages- ANTENNA MICROPHONE

1) Handheld wireless ELEMENT ELEMENT

microphones:
These microphones have conventional
microphone elements mounted to a RF INTERNAL
handle into which a miniature radio INSULATORS PC BOARDS

transmitter and microphone- A handheld transmitter

preamp are built.
2) Plug-on transmitters -
It has a female XL-connector attached to a
compact body that contains the transmitter. Their internal battery provides
phantom power to the microphone and power to the transmitter. A plug-on
transmitter allows the use of virtually any microphone compatible with its
powering circuit.
Antenna
Element

Insulator
Microphone

A plug-on transmitter attached to a mic Body pack

transmitters

3) Body pack transmitters-

It allows the connection of any professional
Electret or dynamic lavalier microphone (It is a miniature microphone
designed to be pinned or clipped to an article of clothing and worn on the
performer). The transmitters are usually a bit larger and contain the same
electronics as the handheld transmitter. Non-lavalier mics and line level
sources may also be used with body pack transmitters with the appropriate
wiring adapters -- this can be a good way to send sound outside to an overflow
system for special events.
Wireless Microphone Receiver:
Receiver is the most important component of a wireless
microphone system, having an operating range of 300 to 600 ft. (100 - 200
meters) under ideal condition. The limiting factors in wireless
microphone performance are-
1) Interference, 2) Reflections, and 3) Range. Wireless
microphones beyond these limiting factors will affect the performance.

Band-pass Filter RF Amplifier First Mixer IF Amplifier 244 MHz IF filter

(243.76 - 244.26 MHz)

(69 – 72 MHz) First local 12 X Multiplier (828 - 864 MHz)

Oscillator

(XLR)
Second Mixer Audio
IF Amplifier 10.7 MHz IF filter DETECTOR Amplifier
(10.6-10.8 MHz)
Matching Transformer
Second local (233.3 MHz)
Oscillator Signal flow in a UHF Wireless microphone receiver
The signals picked by the antenna are sent through a
broadband filter that attenuates signals far off frequency, are amplified, and fed
to the first mixer. A local oscillator, followed by a frequency multiplier stage,
also feeds the mixer. On the principle, of "heterodyne" the two signals "beat"
together to produce new signals at the sum and difference of their original
frequencies. The sum frequency is filtered out, and the difference signal called
an "intermediate frequency," or IF. is amplified and band-pass filtered again
to remove more interfering signals. The operating frequency may be fixed for
crystal controlled and tuneable for synthesised (PLL controlled) receivers.

Microphone Placement of Techniques.

 Always use a microphone with a frequency response that is suited to the
frequency range of the sound.
 Adopt trial and error method. Place the microphone at various distances
and positions until you find a spot where you hear from the studio monitors
the desired tonal balance and the desired amount of room acoustics. As an
audio Engineer you should be satisfied with the sound quality.
 Place the microphone very close to the loudest part of the instrument or
isolate the instrument whenever you encounter poor room acoustics or
pickup of unwanted sounds.
 Always experiment with microphone choice, placement and isolation, on
order to minimize undesirable sounds. Microphone technique is a matter of
personal taste.
1) Vocal Recording
Recording a choral group or vocal ensemble: Vocalists can
be made to circle around an Omni-directional microphone, or two cardioid
microphones, positioned back to back could be used for this same application.

Proximity-Effect-Defined as
Microphone
Vocalists the increase in bass effect with most
unidirectional microphones when
they are placed close to an
instrument or vocalist (within 1 ft.).
Remedies- (1) roll off low
frequencies at the mixer, (2) use a
microphone designed to minimize
proximity effect, (3) Use a
microphone with a bass roll-off
switch, or (4) use an Omni-
directional microphone (which does
not exhibit proximity effect).
For a single vocalist an Omni-directional microphone may be used. If the
singer is in a room with ambience and reverb that add to the desired effect, the
closer the vocalist is to the microphone the more direct sound is picked up relative
to the ambience.

The 3-to-1 Rule

When it is necessary to use multiple microphones or to use them
near reflective surfaces the resulting interference effects may be minimized by
using the 3-to-1 rule. For multiple microphones the rule states that the distance
between microphones should be at least three times the distance from each
microphone to its intended sound source.

Vocalist-1 Vocalist-2

(1-ft.)
(3-ft.)

Microphone-1 Microphone-2
  Area Coverage
Application of choir microphones falls into the category known as
“area” coverage. Rather than one microphone per sound source, the object is to
pick up multiple sound sources (or a “large” sound source) with one (or more)
microphone(s). Obviously, this introduces the possibility of interference effects
unless certain basic principles (Ex- “3-to-1 rule”) are followed..
 Each Microphone placement for a typical choir should be a few feet
in front of, and a few feet above, the heads of the first row.
 Microphones should be centred in front of the choir and aimed at the
last row.
 Spacing between the microphones for each lateral section should be
approximately 6 to 9 feet.

(2- 3 ft.)

Vocalists
(2- 3 ft.)
(2- 3 ft.)

(3 - 6ft.)

Choir microphone positions - top view Microphone positions - side

view

 Stereo Microphone Techniques –This is the use of two or more

microphones to create a stereo image will often give depth and spatial
placement to overall recording. There are a number of different methods for
stereo. Three of the most popular are-
1) Spaced pair (A/B) - This technique uses Sound source
two cardioid or Omni-directional
microphones spaced 3-10 ft apart from each
other panned in left/right configuration to
capture the stereo image of the source. The
distance between the two microphones is
dependent on the physical size of the sound (3 – 10 ft.)
source. The drawback to A/B stereo is due to
the relatively large distance between the
microphones and the resulting difference of
sound arrival. Spaced pair (A/B), top view
2) (X-Y configuration), coincident or
near-coincident pair-This technique Sound source
uses two cardioid microphones of the
same type placed either as close as 0
90 – 1350

possible (coincident) or within 12 inches

of each other (near-coincident) and
facing each other at an angle ranging
from 90 - 135 degrees, depending on the
size of the sound source and the
particular sound desired. This technique (X-Y configuration)
is good but may be limited if the sound
source is extremely wide.
3)(M-S) or Mid-Side stereo technique-
This technique involves a cardioid and a
bi-directional microphone elements
usually housed in a single case, mounted Cardioid- (M)
in a coincident arrangement. The
cardioid (mid) faces directly at the source
and picks up primarily on-axis sound
while the bi-directional (side) faces left
and right and picks up off-axis sound. The Bi-Directional-(s) L = M+S
two signals are combined via the M-S
R = M- S
matrix to give a variable controlled
stereo image. This technique is
completely mono-compatible and is (M-S) stereo technique
widely used in broadcast and film
Introduction
applications . to Amplifiers

Amplifiers are devices having the ability to amplify a

relatively small input voltage signal, for example from a microphone,
into a much larger output signal to drive a speaker system or any
other device called load. An amplifier can be thought of as a simple
box containing the amplifying device, and having two input
terminals and two output terminals with the output signal being
greater than that of the input signal, being "Amplified".
An Ideal amplifier
 Characteristics of amplifier
 Gain – It is defined as the ratio between the outputs to the
input signal expressed in decibels. The voltage gain Av the current gain Ai
and power gain Ap are given by :
Av = 20 log V2/V1 --- (1)
Ai = 20 log I2/I1 ---(2)
Ap = Av X Ai =10 log P2/P1 ---- (3)

Where, V2/V1, I2/I1, & P2/P1 are the ratios of output and input voltages,
currents and powers respectively.
The Typical gain of a voltage amplifier is about 60 dB and that of a
Power amplifier is 20 dB respectively.

 Bandwidth – It is defined as the ability of the amplifier to

give a flat responce for frequency range from 16Hz to 20 KHz. For a Hi-Fi
System a flat responce from 40Hz-15 KHz is acceptable.

 Distortion – An amplifier suffers from the following

distortions.
 Frequency distortion-Caused due to unequal amplification of all
frequencies.
 Phase distortion-When the relative phase relationship is not maintained
between the input and the output signals.
 Non-linier or amplitude distortion-Caused due to passage of signal
through non-linear portion of the Characteristics curve of the transistor
resulting in clipping of output signals at the positive and negative peaks.
 Distortion due to self Oscillation-Caused by positive feedback due to
undesired coupling of output of one stage to input of some earlier stage.
These self oscillations overload a stage resulting in severe distortion of
signal.
 Power output- It is the output power which can be taken out
from an amplifier which to be fed to a loudspeaker. The required output power
varies from few watts to several hundred watts. As the output power increases
adequate heat-sink should be used to radiate out the heat generated by
dissipation of power.

 Impedance – For maximum power transfer from the

amplifier to the load, the source impedance (amplifier) must match with the
load impedance. If the impedance of the source is higher than the load then a
step-down transformer with turns ratio of primary to secondary is given by-
np & ns are number of turns in primary and secondary .
np/ns = (Zp/Zs)½
Zp & Zs are impedances of source and load sides’.

Amplifiers can be divided into two distinct types,

 1) Small Signal Amplifiers – These are designed to amplify very small
signal voltage levels of only a few micro-volts (μV) from microphones or audio
signals such as pre-amplifiers, instrumentation amplifiers etc. Small signal
amplifiers are generally referred to as "Voltage" amplifiers as they convert a
small input voltage into a much larger output voltage.

2) Large Signal Amplifiers (Power amplifiers)-These are designed to

amplify large input voltage signals or switch high current loads, such as audio
power amplifiers or switching amplifiers. Power amplifiers (large signal
amplifiers) are designed to deliver power, which is the product of the voltage
& current applied to the load. The power amplifier works on the basic
principle of converting the DC power drawn from the power supply into an AC
voltage signal delivered to the load.
%-age-Efficiency of the power amplifier is
given by -

η% - is the percentage efficiency

of the amplifier.
Pout - is the amplifiers output
power delivered to the load.
Pdc - is the DC power taken from
the supply.

Power Amplifier Classes- These are classified according

to their circuit configurations and mode of operation being designated
different classes of operation in alphabetical order such as A, B, C, AB, etc.
1) Class-A: In this amplifier
100% of the input signal is used, and Output
the active element remains conducting
(works in its "linear" range) all of the
time. Class-A amplifiers are typically Input
more linear and less complex, but are
very inefficient. This type of amplifier is
most commonly used in small-signal
stages or for low-power applications
like pre-amplifier, microphones, etc.

2)Class-B : In this amplifier 50% of the input signal is used i.e., the
active element (transistor) works in its linear range half of the time and is
more or less turned off for the other half. In most Class B amplifiers there are
two output devices, each of which conducts alternately (push–pull) for exactly
half cycle (or1800) of the input signal.
These amplifiers are subject to crossover
distortion if the transition from one active element (transistor) to the other is
not perfect.

Output

Input
Input Output

Class-B Amplifier Class-B Amplifier (Push-pull)

3) Class-AB: In Class AB operation,

each active device or the transistor
operates the same way as in Class B over
half the waveform, but also conducts a
small amount on the other half, thus
reducing the dead Zone (region where
both devices simultaneously are
nearly off). This way the crossover
Bias
distortion faced in Class-B is greatly Voltage iC
minimised or eliminated. Class AB has
an operating point somewhere between
Class A and Class B and hence is less Class AB Waveform
efficient than Class-B and much more
efficient than class A.

4) Class-C-This amplifiers conduct

less than 50% of the input signal and the
distortion at the output is high, but it has Output
high efficiency (up to 90%). The most Input
common application for Class-C amplifiers
is in RF transmitters, where the distortion
can be vastly minimised by using tuned
loads on the amplifier stage. Class-C Amplifier
 5) Class-D - In the Class D amplifier the input signal
is converted to a sequence of high voltage output pulses whose averaged-over
time power values are directly proportional to the instantaneous amplitude
of the input signal. The frequency of the output pulses is typically ten or more
times the highest frequency in the input signal. The output pulses also contain
inaccurate spectral components which are removed by a low-pass passive
filter to finally obtain the amplified output . Class D
amplifiers can be controlled by either analog or digital circuits. The digital
control introduces additional distortion called quantization error. The main
advantage of a class D amplifier is power efficiency and they do not need large
or heavy power supply transformers or heat-sinks. Therefore they are smaller
and more compact in size than an equivalent Class AB amplifier. They are
widely used exclusively for small DC motors, and also used as audio amplifiers.
Audio Amplifiers-
An audio amplifier is device used to amplify low-power audio
signals (primarily frequencies between 20 to 20,000 hertz) from a
Microphone, CD player, or a pre-amplifier to a level suitable for driving the
next amplifier stage or the Speaker system. The input signal may be only a few
milli or microwatts, its output may be very high up to tens hundreds, or
thousands of watts. There are two types of Audio amplifiers –
Audio-Voltage Amplifier– These are voltage amplifiers (discussed above)
used as pre-amplifier, buffer amplifier, and driver amplifier. Their main
function is amplifying the audio signal voltage sufficient to drive a power
amplifier.

1. Audio-Voltage Amplifier–These are power amplifiers(discussed above)

working in the final stage of amplification used to feed sufficient audio power to
for example the speaker system to convert the audio electrical signal into sound.