in June 2006
Unraveling the Mystery
By Doug Wilkens
Creating a credible pro power amp output ratings
standard is crucial.
Amplifier under test
using low distortion audio signal generator, true RMS meter,
oscilloscope and load bank.
The professional audio
industry does not have its own standard(s) for rating power
Does this statement surprise you?
You would think that the professional audio industry would
be the group that is most interested in having a meaningful
and precise method of comparison between amplifiers in the
What standards or federal regulations
that do exist have been “inherited” from the
consumer audio industry and were developed because of outrageous
claims made by various consumer manufacturers regarding
peak power, peak music power, instantaneous peak music power
and other similar phrases. The FTC rule specifically states
that the rule is for “sound power amplification equipment
manufactured or sold for home entertainment purposes.”
This report is the result of a
number of questions and ensuing tests that were done over
the past couple of years by members of the Live Audio Board
forum on www.prosoundweb. com and also comments and questions
from contractors and consultants associated with the Syn-Aud-Con
“listserv” bulletin board.
Output Rating History
To have a better understanding
of the situation, a review of the history of power amplifier
output ratings is in order.
Naturally, a complete listing of
every attempt back to the earliest days of audio power amplifiers
to make sense of amplifier measurements and ratings would
be too much to include here, but some highlights will help
give an idea of the major accomplishments in this field.
Perhaps the first major attempt
to make sense of amplifier ratings was from the Radio-Electronics-Television
Manufacturers Association (RETMA) in conjunction with the
Electronic Industries Association (EIA) in 1949, when they
published a standard, “Engineering Specifications
for Amplifiers for Sound Equipment” (EIA/RETMA SE-104).
Basically, this was a listing of the various specifications
required for an amplifier, including output power levels
at a distortion of 5%, gain, frequency response, as well
as a number of other electronic and physical measurements
In 1958, the Institute of High
Fidelity (IHF) expanded on the RETMA Standard, with an enhanced
set of conditions and ratings (IHF-A-200). This standard
was updated with a more comprehensive document in 1966 (IHF-A-201).
ANSI (American National Standards
Institute) did some work in the ’70s to develop a
standard for “Measuring Audio Amplifier Power Output
Rating for Institutional Audio Visual Equipment,”
but the standard never went into effect. ANSI apparently
decided to certify the standards developed by the EIA.
The Deutsches Institut für
Normung (DIN) in the past also developed standards for hi-fi
equipment that addressed amplifier power output ratings.
The most recent standard for power amplifier ratings is
61305-3:1995 and is available at www.nor mung.din.de. Thanks
to international agreements, most of the standards that
were developed originally by different standards organizations
in various countries are now part of the International Electrotechnical
Commission (IEC), so you will find this same standard available
as IEC Standard
In the ensuing years, the EIA continued
to work on standards for testing/measurement of audio amplifiers,
including multi-channel amplifiers, and published an interim
Standard in 1981 to try to deal with wildly inflated claims
about amplifier power ratings in the consumer market (the
instantaneous peak music power, et al, mentioned earlier).
The EIA documents are available for purchase at http://global.ihs.com.
Do a search at “Standard Test Methods of Measurement
for Audio Amplifiers” to see the list of documents
available for purchase.
Government Got Involved
Unfortunately, because the consumer-oriented
manufacturers continued down their merry path of inflated
claims, the government stepped in and finally made it the
law for manufacturers to adhere to a clearly defined method
of testing and rating audio power amplifiers. It is not
only the audio industry that has been forced to accept government
intervention into ratings and specifications: Many other
industries have been faced with the same situation because
industry would not self-regulate!
So, in 1974, the Federal Trade
Commission (FTC) came out with its Trade Regulation
Rule regarding “Power Output Claims for Amplifiers
Utilized in Home Entertainment Products.” This FTC
rule has been amended over the years, with the most recent
for download at the National Archives website, www.access.gpo.gov/nara/cfr/
Although various professional amplifier
manufacturers have participated in comments and suggestions
to the EIA and FTC, it seems strange that, thus far, the
professional audio industry (and this certainly is the industry
that should be most interested in a professional standard)
has not worked with the only organization capable of injecting
professionalism into power amplifier ratings for our industry:
the Audio Engineering Society (AES). We all should support
any effort made to create an amplifier power rating standard
that relates to professional products.
In the course of doing research
for this discussion, many amplifier manufacturers were contacted
for comments and specifics about how they test and rate
their professional power amplifiers. Strangely, it appears
that many of them are happy with the status quo, that no
standard exists for professional power amplifier testing/rating.
Why the apparent lack of interest
of the professional audio industry? Why not have a professional
amplifier ratings standard?
It certainly makes sense: If there
is a standard, consultants, contractors and customers can
compare products easily. Everyone would be better served
by having a standard that clearly defines exactly what the
specifications are for power amplifiers, the exact testing
methods, the parameters for the tests, etc. If it were easy,
it probably already would have happened, but some investigation
will reveal all is not so simple.
A large number of power amplifiers
now use switch mode power supplies, pulse-width modulation
(PWM) techniques or other methods of “digital”
power amplification, and many of the manufacturers of these
amplifiers do not want their units tested in the same manner
as power amplifiers that have traditional linear power supplies
and/or analog amplifier circuitry for the output stage.
Most of these manufacturers would
prefer to have a tone-burst signal used as the source material
to drive an amplifier under test, rather than continuous
sine-wave or pink-noise source signals.
Using sine waves or pink noise
as source material, traditional amplifier designs typically
are much more capable of producing continuous power output,
compared with many amplifiers that use switch mode power
supplies and/or PWM (or switch-mode/digital) output circuitry.
In many applications, this does not matter because typical
music program material is not composed of single sine waves,
or even the equivalent of pink noise (with a 10dB crest
An example of how manufacturers
of PWM amplifiers test their products is in the following
description of the test procedure recommended by Lab.gruppen,
a Swedish manufacturer of power amplifiers:
aims to simply point out the main things to be aware of
when putting the amplifiers through their paces. First a
“Quick-guide” list is presented, followed by
more thorough explanations for the individual bullets.
• Max.-output voltage
bench testing quick guide:
1. Use a 1kHz burst signal (33.3ms ON/66.6ms OFF).
A standard continuous test signal will
possibly activate the Automatic
Fuse Saver (AFS), which limits the max.-output voltage.
2. At high-frequencies (more than 10kHz), use a burst
signal to avoid engaging the Very High
Frequency (VHF) protection
circuit. Otherwise, you will not be able to verify the true
output voltage at VHFs.
3. A 22kHz (min 24dB/oct.) low pass filter should
be placed in series with the audio analyzer to
avoid measuring above audible
HF-content in the output signal.
4. Be aware of the gain-switch setting (recommended
5. Set the MLS switches to 0dB; otherwise, max.-output
voltage will not be achieved.
6. Notice the output polarity when measuring.
7. Make sure to use balanced connections and cables
throughout in the test setup.
• Max.-output voltage bench testing extended
When conducting a bench test, there are seven
main areas to be cognizant of:
1. If a sinusoidal signal is used to determine maximum
power output, it is most likely that a protection circuit
called the Automatic Fuse Saver (AFS) will engage. In fact,
instances users have ever reported it engaging are during
tests with sine wave generators. The AFS is designed to
detect and then limit excessive mains current draw to avoid
blowing mains fuses during performance.
The steady high draw that a full-scale
test tone generates is sufficient to engage the protection,
which subsequently limits the mains current draw to a safe
level and hence a reduced output is seen. This has never
been recorded during a music program test. To measure max.-output
power, use an oscilloscope together with a burst signal.
Recommended is a 1kHz signal with 33.3ms on-time followed
by 66.6ms off-time—and so on. This will give you the
true max.-output power.
2. All Lab.gruppen amplifiers are equipped with a
VHF (Very High Frequency) protection circuit. This circuit
detects and mutes the amplifier if there is too much high
frequency energy on the amplifier output. This is a problem
that often is caused by self-oscillation in the audio distribution
system before the amplifier. During normal operation, with
music material, the amplifier will not experience continuous
VHF energy, hence causing the amplifier going into protection.
However, the amplifier will still amplify transient peak-VHF
signals in the music at maximum output power.
The “attack” frequency
at which the protection engages against such continuous
sine waves at maximum voltage is model dependent and falls
between 10kHz and 20kHz. Above the “attack”
frequency, the VHF sensitivity threshold point increases
at 6dB/octave. Chart 1 shows the VHF protection operation
There is a delay—or hold
time—before VHF-protection sets in, which is dependent
on both amplitude and frequency. Higher frequency and/or
higher voltage results in a shorter hold-time to trick the
VHF protection. This makes it possible for the amplifier
to amplify all transients and high frequency bursts that
exist in normal music while still protecting the HF drivers
from destruction if too much non-musical high frequency
energy occurs at the input. The use of an oscilloscope together
with a burst signal will stop the VHF protection engaging,
and thereby enable the measurement of the amplifier at maximum
output voltage across the entire audio frequency spectrum.
3. Most Lab.gruppen amplifiers have switch mode circuits
in the power supply and/or in the amplifier stage. Some
fractions of the switching signals can be found on the output
of the amplifier. To prevent this HF content from interfering
with the meaningful and desired audio measurements, a 22kHz
(min 24dB/oct.) low pass filter should always be connected
in series with the audio analyzer. Signals above 22kHz are
handled according to EMC regulations, and not considered
4. The Lab.gruppen amplifiers are extremely flexible
when integrating into any system. There is a multiple position
gain switch on the rear panel that allows the user to define
the input to output gain of the amplifier, from 41dB to
20dB. During a bench test, it is critical to be aware of
the gain structure of the amplifier. It is suggested that
this is set to unity on all amps (32dB).
5. Many Lab.gruppen models feature the Matching Load
System (MLS). Via a set of switches on the rear panel of
the amplifier, the user is able to control power transfer
into specific speaker loads. Prior to bench testing, these
switches should be set in their maximum settings (0dB);
otherwise, maximum power output may not be obtained.
6. Channel B is always polarity reversed on the input,
but polarity compensated by feeding the minus pin on the
channel B output with the output voltage (output on pin
1 in opposite phase). Channel A is connected in normal polarity
mode. By having channel A and B operating in opposite polarity,
the energy storage in the power supply is more efficient.
This is significant for signals below 100Hz and improves
7. Be sure to use balanced inputs on all measurement
equipment (also oscilloscope probes).
To effectively compare amplifier
power output, you must have standardized source material
and a standardized load, with standardized test parameters,
for a specified time period, in order to have a clear comparison.
The EIA and FTC test procedure does just that, but it is
evident that PWM amplifiers will not perform in the manner
that realistically demonstrates their performance in real-world
applications, if the EIA/FTC testing is used.
Additionally, the bandwidth of
the test system is a major consideration when comparing
traditional analog amplifiers with PWM amplifiers, due to
noise outside the audio spectrum (PWM/digital amplifiers
typically require a test/measurement system that has a very
serious filter to remove signals above/outside the audio
spectrum). For additional reading about this topic, go to
Audio Precision’s website (www.audioprecision.com)
and look for “White Paper: Measuring Switch-mode Power
Amplifiers” by Bruce Hofer.
An additional complication of power
amplifier testing is that the real-world load on an amplifier
is quite different from a non-inductive resistor that often
is used as an amplifier load in a test laboratory. In reality,
the load typically is a loudspeaker system that has impedance
that changes significantly over the audio spectrum. When
you look at a typical loudspeaker impedance curve, you will
see a significant deviation from the “nominal”
impedance rating of the device.
Because of this fact, there have
been several attempts over the years to define a standardized
test load that emulates the type of load that a loudspeaker
would provide to an amplifier, but it seems without general
acceptance. Most “lab” amplifier testing is
done with non-inductive load resistors. It is difficult
to obtain high-power non-inductive load resistors, so most
high-power amplifiers are tested with wire-wound resistors
of one type or another, including some rather creative loads
using hot water heater elements, electric stove elements,
pizza ovens, etc. Several examples of this type of creative
effort can be found at
Many integrators and consultants
would like to see a variety of different methods of measurement,
testing and ratings used for power amplifiers. For example,
some have stated that they would like to see a chart showing
the current and voltage output of an amplifier, as opposed
to a traditional wattage rating. This (and many other types
of data) can be helpful, but most users have become accustomed
to being able to compare amplifiers by their output wattage
ratings because it is easy.
Crown is one of the few manufacturers
that addresses this issue by including a chart of current/voltage
(V/I) output ranges in its description of operating conditions.
Chart 2 is one sample of many V/I charts that are included
in Crown’s description of operating conditions of
the CTs series of amplifiers.
In addition to the tests performed
at single frequencies (as well as over the entire operating
bandwidth of the amplifier), use of pink-noise source signal
also has been promoted (because pink noise more closely
resembles actual program material with regard to average
and peak signal content).
If our industry decided that rating
a power amplifier by output current and voltage was important,
and that this information should be part of the specifications,
it still would be appropriate to include traditional wattage
ratings as well—and Crown does exactly that.
The measured power output of an
amplifier can be significantly higher if higher distortion
percentages are permitted, so the standard would have to
include what the maximum allowable distortion would be for
the maximum output measurements. Shifting from 0.1% to 1%
makes a considerable difference in output power ratings;
look at various specifications in which the manufacturer
provides both ratings to get an idea of the difference!
To add to the confusion of many
users, the AC mains power requirements of an amplifier often
are listed in the specifications and frequently are also
printed on the rear of the unit. If the amplifier is operating
at full output power, certainly the power requirements for
the amplifier will be much higher (sometimes significantly)
than the output power rating of the amplifier due to the
efficiency of the amplifier.
Fortunately, a few manufacturers
go to the trouble of listing the idle power consumption,
the 1/8th power (or 1/3rd power) consumption and the maximum
power consumption. Unfortunately, non-technical users often
mistake the power requirements to be the output power rating;
just look at some of the classifieds in your local paper.
It is an interesting exercise to
analyze what information is contained on manufacturers’
amplifier specification sheets right now. QSC includes output
ratings according to both EIA and FTC requirements.
Chart 3 indicates the specifications
of the QSC Powerlight Series of amplifiers, showing the
EIA and FTC specifications for 8 ohms, 4 ohms and 2 ohms,
as well as the Distortion, Current Consumption and other
important technical criteria that are useful to consultants
and contractors for making comparisons with similar products.
Yamaha also gives complete specifications,
but adds the power consumption at idle (no signal), as well
as at maximum output, which is quite helpful. Note that,
although QSC uses current consumption, Yamaha uses power
consumption. QSC clearly states it is using EIA and FTC
standards for the power ratings; Yamaha does not state this,
even though it appears the company actually is using the
ratings as specified by the EIA and FTC. Yamaha also includes
a tone-burst rating (1kHz 20ms), which provides an indication
of dynamic headroom. QSC shows the THD distortion at full-rated
power; Yamaha shows THD and IMD at half-rated power. Chart
4 shows excerpts from the specifications for the Yamaha
‘Unraveling the Mystery…’
The title mentioned “unraveling
the mystery…” but by now it may appear the mystery
has deepened. To really remove the mystery, a procedure
with the appropriate measurement techniques and agreed-to
data format must be developed and turned into a standard.
One hopes that, in some small measure,
this discussion will help spur activity on the part of consultants,
integrators, dealers, manufacturers, sound-reinforcement
companies and end users in our industry to work together
to develop a credible standard for professional power amplifier
Doug Wilkens has been involved with the audio industry all
of his life, including many years as an engineer for major
US sound contracting firms, as well as marketing, sales
and management positions for a number of audio manufacturers.