IEC 60268-3:2000

INTERNATIONAL IEC
STANDARD
60268-3
Third edition
2000-08
Sound system equipment –
Part 3:
Amplifiers
Equipements pour systèmes électroacoustiques –
Partie 3:
Amplificateurs
Reference number
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INTERNATIONAL IEC
STANDARD
60268-3
Third edition
2000-08
Sound system equipment –
Part 3:
Amplifiers
Equipements pour systèmes électroacoustiques –
Partie 3:
Amplificateurs
 IEC 2000  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http://www.iec.ch
Commission Electrotechnique Internationale
PRICE CODE
XA
International Electrotechnical Commission
For price, see current catalogue

– 2 – 60268-3 © IEC:2000(E)
CONTENTS
Page
FOREWORD . 5
Clause
1 Scope . 7
2 Normative references. 7
3 Conditions. 8
3.1 Rated conditions and standard measuring conditions. 8
3.1.1 Introduction. 8
3.1.2 Rated conditions . 8
3.1.3 Standard measuring conditions . 9
3.2 Other conditions . 9
4 Classes of operation . 9
5 Interchangeable parts . 10
6 Automatic controls . 10
7 Power supply . 10
8 Position of the volume controls . 10
9 Pre-conditioning for measurements. 10
10 Series of measurements . 11
11 Variable consumption apparatus . 11
12 Marking. 11
13 Operating environment. 12
14 Characteristics to be specified, and their methods of measurement . 12
14.1 Power supply characteristics. 12
14.1.1 Characteristics to be specified . 12
14.1.2 Method of measurement. 12
14.2 Tolerance of (long-term) power supply voltage variations. 12
14.2.1 Characteristic to be specified . 12
14.2.2 Methods of measurement. 13
14.3 Tolerance of power supply frequency variations. 14
14.3.1 Characteristics to be specified . 14
14.3.2 Methods of measurement. 14
14.4 Tolerance of power supply harmonics and ripple. 14
14.4.1 Characteristics to be specified . 14
14.4.2 Methods of measurement. 15
14.5 Input characteristics. 15
14.5.1 Rated source impedance, characteristic to be specified . 15
14.5.2 Input impedance . 15
14.5.3 Rated source e.m.f., characteristic to be specified . 17
14.5.4 Minimum source e.m.f. for rated distortion-limited output voltage . 17
14.6 Output characteristics . 17
14.6.1 Rated load impedance, characteristic to be specified . 17
14.6.2 Output source impedance . 18
14.6.3 Output voltage and power (distortion-limited) . 18

60268-3 © IEC:2000(E) – 3 –
Clause Page
14.6.4 Regulation . 20
14.6.5 Overload restoring time. 20
14.7 Limiting characteristics . 20
14.7.1 Overload source e.m.f. 20
14.7.2 Short-term maximum output voltage and power. 21
14.7.3 Long-term maximum output voltage and power . 22
14.7.4 Temperature-limited output power. 23
14.8 Characteristics of protection circuits . 23
14.8.1 Introduction. 23
14.8.2 Protection against potentially damaging combinations of output voltage
and current . 24
14.8.3 Characteristics of d.c. offset protection circuits . 25
14.9 Sustaining-time for rated (distortion-limited) output voltage or power . 26
14.9.1 Introduction. 26
14.9.2 Characteristic to be specified . 27
14.9.3 Method of measurement. 27
14.10 Gain. 28
14.10.1 Voltage gain and e.m.f. gain . 28
14.10.2 Maximum e.m.f. gain. 28
14.10.3 Attenuation characteristic of the volume control . 28
14.10.4 Attenuation characteristic of balance controls for
multi-channel equipment. 29
14.11 Response. 29
14.11.1 Gain-frequency response. 29
14.11.2 Gain-limited effective frequency range . 30
14.11.3 Distortion-limited effective frequency range . 30
14.11.4 Phase-frequency response . 30
14.12 Amplitude non-linearity. 31
14.12.1 Introduction . 31
14.12.2 Rated total harmonic distortion . 31
14.12.3 Total harmonic distortion under standard measuring conditions . 31
14.12.4 Total harmonic distortion as a function of amplitude and frequency. 32
14.12.5 Harmonic distortion of the nth order under standard
measuring conditions . 32
14.12.6 Harmonic distortion of the nth order as a function of amplitude
and frequency. 33
14.12.7 Modulation distortion of the nth order. 34
14.12.8 Difference-frequency distortion of the nth order . 35
14.12.9 Dynamic intermodulation distortion (DIM). 37
14.12.10 Total difference frequency distortion . 38
14.12.11 Weighted total harmonic distortion . 40
14.13 Noise . 40
14.13.1 Characteristic to be specified. 40
14.13.2 Method of measurement . 41
14.14 Hum . 41
14.14.1 Introduction . 41
14.14.2 Characteristics to be specified . 41
14.14.3 Method of measurement . 42

– 4 – 60268-3 © IEC:2000(E)
Clause Page
14.15 Balanced inputs and outputs . 42
14.15.1 Balance of the input. 42
14.15.2 Overload (distortion-limited) peak-to-peak common-mode input voltage. 43
14.15.3 Balance of the output. 44
14.16 Cross-talk and separation in multi-channel amplifiers. 45
14.16.1 Characteristics to be specified . 45
14.16.2 Method of measurement . 45
14.17 Gain and phase differences between channels in multi-channel amplifiers . 46
14.17.1 Gain difference . 46
14.17.2 Phase difference. 47
14.18 Dimensions and mass, characteristics to be specified . 47
Annex A (informative) Balanced interfaces . 53
Bibliography . 54
Figure 1 – Arrangements for measuring input impedance . 48
Figure 2 – Oscillogram when measuring overload restoring time . 49
Figure 3 – Protection against potentially damaging combinations of output voltage
and current. 50
Figure 4 – Arrangement for combining two input signals. 51
Figure 5 – Frequency spectrum below 30 kHz of the signal for measuring dynamic
intermodulation distortion . 51
Figure 6 – Arrangement for measuring the balance of a balanced input. 52
Figure 7 – Arrangement for measuring the internal impedance balance of a
balanced output. 52
Figure 8 – Arrangement for measuring the voltage symmetry of a balanced output. 52
Table 1 – Different rated total harmonic distortion and rated distortion-limited
output power specifications for the same amplifier. 27
Table 2 – Distortion components due to dynamic intermodulation distortion falling
in the frequency range up to 20 kHz . 37

60268-3 © IEC:2000(E) – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOUND SYSTEM EQUIPMENT –
Part 3: Amplifiers
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International
Organization for Standardization (ISO) in accordance with conditions determined by agreement between the
two organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60268-3 has been prepared by subcommittee 100C: Audio, video
and multimedia subsystems and equipment, of IEC technical committee 100: Audio, video and
multimedia systems and equipment.
This third edition cancels and replaces the second edition published in 1988, amendment 1
(1990) and amendment 2 (1991), and constitutes a technical revision.
The text of this standard is based on the following documents:
FDIS Report on voting
100C/147/FDIS 100C/165/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 3.
This part of IEC 60268 shall be used in conjunction with IEC 60268-1 (1985) and IEC 60268-2
(1987).
– 6 – 60268-3 © IEC:2000(E)
Annex A is for information only.
The committee has decided that the contents of this publication will remain unchanged until
2005. At this date, the publication will be
reconfirmed;
withdrawn;
replaced by a revised edition, or
amended.
A bilingual version of this standard may be issued at a later date.

60268-3 © IEC:2000(E) – 7 –
SOUND SYSTEM EQUIPMENT –
Part 3: Amplifiers
1 Scope
This part of IEC 60268 applies to analogue amplifiers, and the analogue parts of
analogue/digital amplifiers, which form part of a sound system for professional or household
applications. It specifies the characteristics which should be included in specifications of
amplifiers and the corresponding methods of measurement.
1)
NOTE The methods of measurement for digital amplifiers and similar equipment are given in IEC 61606. [6]
In general, the specified methods of measurement are those which are seen to be most
directly related to the characteristics. This does not exclude the use of other methods which
give equivalent results.
In general, the methods are based on the simplest measuring equipment which can provide
useful results. This does not exclude the use of more complex equipment which can give
higher accuracy and/or allow automatic measurement and recording of results.
Rated conditions and standard measuring conditions are specified in order to allow
measurements to be reliably repeated.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 60268. For dated references, subsequent
amendments to, or revisions of, any of these publications do not apply. However, parties to
agreements based on this part of IEC 60268 are encouraged to investigate the possibility of
applying the most recent editions of the normative documents indicated below. For undated
references, the latest edition of the normative document referred to applies. Members of IEC
and ISO maintain registers of currently valid International Standards.
IEC 60065:1998, Audio, video and similar electronic apparatus – Safety requirements
IEC 60268-1:1985, Sound system equipment – Part 1: General
IEC 60268-2:1987, Sound system equipment – Part 2: Explanation of general terms and
calculation methods
Amendment 1 (1991)
IEC 60417-1:1998, Graphical symbols for use on equipment – Part 1: Overview and application
IEC 61000-4-17:1999, Electromagnetic Compatibility (EMC) – Part 4-17: Testing and
measurement techniques – Ripple on d.c. input power port immunity test – Basic EMC
Publication
IEC 61000-4-29, Electromagnetic Compatibility (EMC) – Part 4-29: Testing and measurement
techniques – Voltage dips, short interrruptions and voltage variations on d.c. input power
2)
ports, immunity tests – Basic EMC Publication
––––––––––––
1)
Numbers in square brackets refer to the bibliography.
2)
To be published.
– 8 – 60268-3 © IEC:2000(E)
IEC 61938:1996, Audio, video and audiovisual systems – Interconnections and matching
values – Preferred matching values of analogue signals
3 Conditions
3.1 Rated conditions and standard measuring conditions
3.1.1 Introduction
For convenience in specifying how amplifiers shall be set up for measurement, sets of
conditions are specified in this standard, under the titles of rated conditions and standard
measuring conditions.
A full explanation of the term "rated" is given in IEC 60268-2.
The rated conditions for amplifiers are:
– rated power supply voltage;
– rated source impedance;
– rated source e.m.f.;
– rated load impedance;
– rated total harmonic distortion, or rated (distortion-limited) output voltage or power;
– rated mechanical and climatic conditions.
NOTE 1 Total harmonic distortion and (distortion-limited) output voltage or power are interdependent. Both cannot
be taken as rated conditions simultaneously because normally a given sample amplifier produces less than rated
total harmonic distortion at rated output voltage or power.
NOTE 2 If the power supply frequency is critical, it is also a rated condition.
To obtain the correct conditions for measurements, the values for the above-mentioned rated
conditions shall be taken from the manufacturer's specification. These values themselves are
not subject to measurement but they constitute the basis for measuring the other characteristics.
Methods of measurement for these other characteristics are given in this standard and the
manufacturer is either required or permitted to state 'rated values' for these characteristics in
the specification of the equipment. These include
– rated voltage gain;
– rated distortion limited output voltage or power (when not adopted as a rated condition);
– rated signal-to-noise ratio;
– rated equivalent noise source e.m.f.
3.1.2 Rated conditions
An amplifier, considered as a four-terminal network with regard to a specified pair of input
terminals and a specified pair of output terminals, shall be understood to be working under
rated conditions when the following conditions are fulfilled:
a) the amplifier is connected to its rated power supply;
b) the source e.m.f. is connected in series with the rated source impedance to the input
terminals;
NOTE For an amplifier having several identical channels, the input signal should be applied simultaneously to all
corresponding pairs of input terminals.
c) the output terminals are terminated with the rated load impedance;
d) the terminals which are not used during the measurement are terminated, if necessary, as
specified by the manufacturer;

60268-3 © IEC:2000(E) – 9 –
e) the source e.m.f. is a sinusoidal voltage equal to the rated source e.m.f. at an appropriate
frequency. Unless there is a special reason to the contrary, this frequency shall be the
standard reference frequency of 1 000 Hz according to IEC 60268-1.
Such a reason could be that the standard reference frequency is outside or near the limit
of the effective frequency range of the amplifier;
f) the volume control, if any, is set to such a position that the rated distortion-limited output
voltage appears at the output terminals;
g) the tone controls, if any, are set to a specified position to give the specified frequency
response, generally the flat frequency response;
h) the balance control(s), if any, is (are) set to the mechanical central position;
i) the rated mechanical and climatic conditions according to IEC 60268-1 are complied with.
Amplifiers for which the rated distortion-limited output power exceeds the rated temperature-
limited output power are likely to be subject to overheating when operated under rated
conditions for an extended period of time. For these amplifiers, rated conditions shall be
maintained for no longer than can be tolerated by the amplifier.
3.1.3 Standard measuring conditions
These are obtained by bringing the amplifier under rated conditions (see 3.1.2) and then
reducing the source e.m.f. to a level of –10 dB referred to the rated source e.m.f.
3.2 Other conditions
If supplementary data of the amplifier are presented, applying to other than the rated or
standard measuring conditions, for example at different frequencies or at different settings of
controls, then the conditions shall be fully defined in the presentation. These conditions shall,
if possible, be chosen according to the recommendations made in the relevant clauses of this
standard.
The procedures for supplementary measurements may be derived from the measurement
procedures given for the standard conditions. If special precautions are necessary to ensure
accuracy, these shall be indicated together with the measurement procedure involved.
4 Classes of operation
Class A: in which the current in each active device supplying the load current is greater than
zero throughout each cycle of the signal for all values of load current up to and including the
value determined by the rated output power or voltage and the rated load impedance.
Class B: in which the current in each active device supplying the load current is equal to
zero for exactly one-half of each cycle of load current.
NOTE In common usage, the term Class B is extended to the case where current flows for slightly more than one
half-cycle.
Class AB: in which the current in at least one of the active devices supplying the load
current is zero for some part of each cycle of load current for some range of values of load
current not exceeding the value defined by the rated output power or voltage and the rated
load impedance.
NOTE At sufficiently low signal levels, a Class AB amplifier usually operates in Class A.
Class D: in which the current in each active device supplying the load is switched from zero
to a maximum value by a carrier signal, modulation of which conveys the useful signal.
NOTE Other classes of operation have been commercialized but no formal definitions of such classes have been
submitted for standardization.

– 10 – 60268-3 © IEC:2000(E)
5 Interchangeable parts
For type measurements, interchangeable parts shall have characteristics as close as is
reasonably practicable to the mean characteristics specified for these parts.
For measurements on a particular sample, the interchangeable parts supplied with that
amplifier shall be used.
6 Automatic controls
The amplifier may contain automatic control circuits such as limiters, compressors, expanders
and electronic fader circuits. These circuits make certain characteristics of the amplifier
dependent either on a signal passing through the amplifier itself or on an external control
signal. When measuring the characteristics of such an amplifier, the automatic control circuits
shall be disabled, except when measuring their characteristics.
7 Power supply
Measurements shall be made with the amplifier connected to rated power supply. Care shall
be taken to maintain the power supply voltage at the rated value during the measurement. If
the manufacturer claims power supply voltage tolerances exceeding ±10 %, then the
characteristics to be specified shall also be stated for the upper and lower limits of these
tolerances.
Additional measurements may be made at the upper and lower limits claimed as tolerable for
the power supply voltage, the power supply frequency and the a.c. power supply harmonics or
the d.c. power supply ripple.
Warning – The power supply voltage tolerances specified by the manufacturer shall not
be exceeded.
8 Position of the volume controls
If a characteristic is measured at only one position of the volume control, the control shall be
at the position corresponding to rated conditions (see 3.1.2), unless a maximum or minimum
position of the control is inherent in the characteristic to be measured.
If the characteristic is to be measured for several settings of the volume control, then
the position for rated conditions shall be included, other preferred settings being maximum,
and –3 dB, –6 dB, –10 dB, –20 dB and –40 dB with respect to the setting for rated conditions.
Volume controls belonging to channels not being measured shall, if possible, be put in the
minimum position, unless otherwise stated.
9 Pre-conditioning for measurements
Before beginning measurements on an amplifier, it shall be operated under approximately
standard measuring conditions for a period of 1 h, or as specified by the manufacturer.
Before operating the amplifier the manufacturer's instructions concerning initial operation
should be studied.
60268-3 © IEC:2000(E) – 11 –
The amplifier is then brought under standard measuring conditions (see 3.1.3). Due to internal
heating, the output voltage may subsequently vary with time. Unless excessive, this effect is
ignored during the pre-conditioning period. When the pre-conditioning period is over, the
amplifier shall be brought under rated conditions or standard measuring conditions, as
required.
10 Series of measurements
If a series of measurements is made, the amplifier should preferably be maintained under
standard measuring conditions in the periods between measurements.
If the amplifier has to be put out of operation for an extended period between measurements,
then pre-conditioning according to clause 9 shall be repeated before each set of
measurements, unless this can be shown to be unnecessary.
11 Variable consumption apparatus
Sound system equipment shall be considered as variable consumption apparatus if it contains
one or more power amplifiers operating in the Class AB or Class B modes, in which the d.c.
power supply for the output stages is either electronically regulated by means of series
control elements or is not regulated.
1)
NOTE 1 Variable consumption apparatus is defined in IEC 60065, fourth edition (1976) as 'apparatus in which
the power consumption can vary more than 15 % due to changes in load impedances of the output circuit or in
signal parameters', but no definition appears in the fifth (1985) or sixth (1998) editions.
NOTE 2 Where the d.c. supply is regulated by shunt control elements, the power consumption is usually, if not
always, substantially constant. The apparatus, however, behaves in some respects as a variable consumption
apparatus, and, in particular, the text of 14.7.4.1 still applies.
All the measurements contained in this standard may be performed on variable consumption
apparatus, in most cases with no special problems. However, certain problems may occur in
the measurement of hum and rated distortion-limited output power, and some additional
measurements are valuable in assessing the performance of such apparatus (see the note of
14.6.3.1 and item c) of 14.14.3).
12 Marking
Principles for marking the terminals and controls are given in IEC 60268-1.
Marking may concern
– personal safety and prevention of spread of fire, in the sense of IEC 60065,
– safety in case of faulty connections,
– indications relating to normal operation, according to IEC 60417.
Marking can neither prevent incorrect operation nor provide complete operating instructions. It
therefore has to be considered in conjunction with adequate means for preventing dangerous
or faulty operation, and with the directions for use included in the user's instructions. Care
should be taken that marking is unambiguous and as clearly understandable as possible.
Terminals for the interconnection of equipment, which are inaccessible without the use of a
tool when the equipment has been installed, shall be clearly and unambiguously identified
with respect to the manufacturer's instructions for installation. It may be assumed that these
instructions are to be read by adequately skilled personnel.
––––––––––––
1)
IEC 60065:1976, Safety requirements for mains operated electronic and related apparatus for household and
similar general use
– 12 – 60268-3 © IEC:2000(E)
13 Operating environment
Measurements, especially those including temperature measurements, shall be carried out
with the amplifier mounted in a situation similar to that in which it is to be used. Restrictions
on mounting and special ventilation requirements shall be stated by the manufacturer and
form part of the rated conditions (see 3.1.2). See also IEC 60065 or other appropriate IEC
safety standard.
14 Characteristics to be specified, and their methods of measurement
14.1 Power supply characteristics
14.1.1 Characteristics to be specified
The following information shall (except where indicated as optional) be stated by the
manufacturer in the locations indicated, for each pair of terminals to be connected to the
power supply and for each position of the power supply voltage selector, if any:
a) the type of power supply (d.c. or a.c.); on the equipment and in the specification
b) the rated power supply voltage (this is a rated condition, see 3.1.2); on the equipment and
in the specification
c) the power supply frequency or range of frequencies (this may be a rated condition, see
3.1.2); on the equipment and in the specification
d) the power drawn, under rated conditions, expressed in watts; on the equipment and in the
specification
e) for variable consumption apparatus (see clause 11), the power drawn from the power
supply may optionally be expressed as a function of output voltage or power from zero to
the rated value, with specified load impedances, including the rated load impedance. This
characteristic is particularly of value for equipment which may be operated from batteries.
It may be presented as a graph.
NOTE If, in items d) or e) above, the apparent power drawn is significantly greater than the true power, the
apparent power should be stated in addition.
14.1.2 Method of measurement
a) The amplifier is brought under rated conditions.
b) The power drawn from the power supply is measured in watts by means of a wattmeter:
1) with rated source e.m.f., see 14.5.3;
2) with the source e.m.f. according to standard measuring conditions;
3) for variable consumption apparatus, at various values of output voltage or power from
zero to the rated value.
14.2 Tolerance of (long-term) power supply voltage variations
14.2.1 Characteristic to be specified
The tolerance of power supply voltage variation, such that, for any power supply voltage
within the stated limits:
a) the upper limit of working voltage is not exceeded for any condition of normal operation;
this applies particularly to such components as semiconductor devices and electrolytic
capacitors;
b) the tolerances of the heater voltage of electronic tubes used in the amplifier are not
exceeded;
60268-3 © IEC:2000(E) – 13 –
c) the maximum permissible temperature is not exceeded in any component when the
amplifier works under standard measuring conditions – except as regards power supply
voltage – for an extended period of time;
d) neither output nor gain is subject to excessive variations;
e) the signal-to-hum ratio is not reduced by more than 3 dB with respect to the rated value.
Amplifiers designed to take their power supply from the mains are generally not subject to
power supply variations exceeding ±10 %. Such variations do not generally require a special
design of amplifier.
Amplifiers designed to take their power supply from batteries or small converters may be
subject to larger variations of power supply voltage, which may be due to changing load, the
temperature of batteries or the gradual decrease of battery voltage during life or discharge.
The rated value shall be stated by the manufacturer in the specification. If the manufacturer
states that the amplifier tolerates power supply voltage variations not exceeding ±10 %, then
compliance with requirements a), b) and c) is considered to be within the scope of normal
amplifier design. Compliance with requirements d) and e) shall be checked.
If the manufacturer states that the amplifier will tolerate power supply voltage variations
exceeding ± 10 %, the specification shall give details of any special means for compensating
such variations and the proper working of these means shall be checked if possible.
Compliance with requirements a) to e) shall be checked.
14.2.2 Methods of measurement
14.2.2.1 Temperature
a) The amplifier is brought under standard measuring conditions. If a rated temperature-
limited output power is given, the source e.m.f. shall be adjusted so that this value of
output power is obtained.
b) The power supply voltage is adjusted to the upper limit stated by the manufacturer and the
source e.m.f. readjusted, if necessary, to restore the output power to the value specified in
item a).
c) The amplifier shall be capable of working continuously under these conditions for at least
4 h without any component exceeding the maximum permissible temperature.
14.2.2.2 Variations of output and gain
a) The amplifier is brought under rated conditions, the total harmonic distortion being
measured by the method given in 14.12.3.2.
b) The source e.m.f. E and the output voltage U are measured.
s 2
c) The power supply voltage is varied step by step over the range specified by
the manufacturer. For each value of the power supply voltage chosen, the source e.m.f. E
s
is readjusted to obtain the initial total harmonic distortion, and the source e.m.f. E ' and
s
the output voltage U ' are measured.
NOTE Over the range of permitted supply voltage, both E ' and U ' are usually substantially proportional to the
s 2
power supply voltage, where no voltage regulation is provided.
14.2.2.3 Hum
a) The signal-to-hum ratio is measured as indicated in 14.14.
b) The power supply voltage is varied step by step over the range specified by the
manufacturer. For each value of the power supply voltage chosen, the hum voltage at the
output is measured and the signal-to-hum ratio computed. It is expressed as the ratio in
decibels of the rated output voltage to the measured hum voltage.

– 14 – 60268-3 © IEC:2000(E)
14.3 Tolerance of power supply frequency variations
14.3.1 Characteristics to be specified
The tolerance of power supply frequency variation, such that, for any power supply frequency
within the stated limits:
a) the maximum permissible temperature is not exceeded in any component when the
amplifier is under standard measuring conditions – except as regards power supply
frequency – for at least 4 h;
b) the output voltage and the total harmonic distortion of the amplifier are not subject to
significant variations;
c) the signal-to-hum ratio is not reduced by more than 3 dB with respect to the rated value.
The manufacturer shall state the rated value in the specification.
Compliance with requirement a) is considered to be within the scope of normal amplifier
design.
NOTE Increase in temperature of components may occur, particularly in the power supply portion, at the lower
end of the power supply frequency range.
14.3.2 Methods of measurement
14.3.2.1 Variations of output voltage and total harmonic distortion
a) The amplifier is brought under rated conditions, the total harmonic distortion being
measured by the method given in 14.12.4.2.
b) The source e.m.f. E and the output voltage U are measured.
s 2
c) The power supply frequency is varied step by step over the range specified by the
manufacturer. For each value of the power supply frequency chosen, the output voltage
U ' and the total harmonic distortion are measured.
d) Neither the voltage U ' nor the total harmonic distortion shall be subject to significant
variations within the specified power supply frequency range.
14.3.2.2 Hum
The method of measurement is as given in 14.2.2.3, except that the power supply frequency
is varied instead of the power supply voltage.
NOTE The worst signal-to-hum ratio is generally found at the lower limit of power supply frequency.
14.4 Tolerance of power supply harmonics and ripple
14.4.1 Characteristics to be specified
a) The tolerance of power supply harmonics, such that, within the stated tolerance range
1) the variations of the rectified power supply voltage due to changes in the peak-to-
r.m.s. ratio of power supply voltage do not exceed the variation due to the permissible
power supply voltage variations specified according to 2.2.1;
2) the signal-to-hum ratio is not reduced by more than 3 dB with respect to the rated
value.
Requirement 1) implies that the tolerance of a.c. power supply harmonics does not exceed the
smallest tolerance, either positive or negative, specified for the power supply voltage
according to 14.2.1.
60268-3 © IEC:2000(E) – 15 –
b) The tolerance of ripple on the d.c. power supply, such that, within the stated tolerance
range
1) the variations of the power supply voltage do not exceed the variation due to the
permissible power supply voltage variations specified according to 14.2.1;
2) the signal-to-noise ratio is not reduced by more than 3 dB with respect to the rated
value.
The manufacturer may optionally state these tolerance ranges in the specification.
14.4.2 Methods of measurement
a) Power supply harmonics
See the future IEC 61000-4-13. [5]
b) Ripple
See IEC 61000-4-17.
c) Voltage dips, short interruptions and voltage variations
See IEC 61000-4-29.
14.5 Input characteristics
14.5.1 Rated source impedance, characteristic to be specified
The internal impedance, stated by the manufacturer in the specification, of the source
supplying the signal to the amplifier.
Unless otherwise specified, the rated source impedance is assumed to be a constant pure
resistance.
NOTE The manufacturer may also give the range of source impedances which is considered tolerable in practice.
If the rated source impedance is not stated by the manufacturer, the appropriate impedance
specified in IEC 61938 shall be used.
14.5.2 Input impedance
14.5.2.1 Characteristics to be specified
The internal impedance measured between the input terminals:
a) under standard measuring conditions; the rated value shall be stated in the specification
b) at other signal frequencies; this may optionally be stated in the specification, except where
the variation with frequency may be important (such as for inputs for magnetic tape heads
or pick-ups for analogue disc records), in which case sufficient additional data shall be
given.
14.5.2.2 Methods of measurement
In the methods given in 14.5.2.2.1 and 14.5.2.2.2 the modulus of the input impedance is
measured. If more information is required (such as the values of the components of an
equivalent circuit representing the input impedance
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