This document provides a guide to understanding amplifier specifications for consumers and sales staff. It explains key specs like wattage, THD, sensitivity and what they mean. It emphasizes that specs can be deceiving if not fully disclosed, such as only measuring one channel instead of all channels driven. The guide cautions that specs outside the normal audible frequency range or without details on testing conditions should be viewed skeptically. It encourages listeners to evaluate amplifiers by how they sound in addition to specifications.

1. Harman International
Sales Primer
Demystifying Amplifier Specs – The Dummy Guide
Ravi Lakkundi
Product Manager, India
Harman International India (Pvt) Ltd
Published on: 20 Sep 2012

2. Harman International
Introduction
Choosing a right receiver/amplifier for one’s home is challenging and an immense task. The obvious
place to start is some review website, magazines and of course knowledgeable trusted friends – After all
you are making a sound investment on/for your ears for years to come! The task of selection finally boils
down to comparing the technical specifications of the receivers one has shortlisted.
Amplifier specifications can actually be quite deceiving, one must know to read and decipher them for
an apt selection of amplifier/receiver system. This paper provides basic level understanding of such
specification sheets, which at times are spurious. Intended for dummies and sales staff, avid readers are
highly encouraged to study the reference materials for technical details.
What to expect in Spec sheet?
Typical Amplifier specifications have
features like THD, Sensitivity,
Wattage/Channel, S/N etc.
An understanding of each of these
terms and how they are measured can
be helpful in selecting a good amplifier
for your home and also will help in
comparing different amplifier based on
price and specs.
A typical detailed AV Receiver specification
“35W x 7 RMS all channels driven continuously into 8 Ohms with less than 0.07% THD from 20Hz to 20 KHz”
All aspects of this specification are very important. Any missing information should be a cause for
concern. Below are examples wherein specifications are unclear and incomplete:
“35W x 7 RMS @ 8 Ohms from 20Hz to 20 KHz”
“35W x 7 RMS @ 8 Ohms, 1 KHz, 1%”
“35W x 7 RMS, 1 Channel driven, 0.07%”

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The Dissection
Watts refers to power. ‘35W x 7’ conveys that the
amplifier is of 35W and ‘x7’ means that the
amplifier has 7 channels.
Higher power is what a consumer expects but
one needs to be careful on how this power was
measured. The remaining part of the spec
explains how this 35W was measured.
Gyaan: Although ‘PMPO’ (Peak Music Power
Output) is a phased out term, but then it’s always
better to be cautious! More about this later in
this section
RMS is Root Mean Square and is a method of
measuring AC waveform. PMPO is generally 5 to
6 times greater than RMS.
‘All channels driven’ signifies that the power
measurement was made by driving all channels,
i.e. 35W measured across all 7 channels, driven
to their maximum level at same time.
Gyaan: There’s a potential for cheating here.
Measuring the power with only one channel
driven, results in obvious higher number for
Wattage!!
A continuous test signal must be used to measure
power of amplifier. An example of such signal
would be a sine wave. An amplifier which is
capable of giving higher power for short amount
of time will have higher power rating if its power
is measured with short bursts instead of
continuous wave.
Gyaan: Watch out for this term in your hunt for
Amplifier, this could be one of the money savers!

4. Harman International
Power generally gets measured across a dummy
resistor, in this case an 8 Ohms resistance load. At
times, the manufacturer measures the power
across a lesser resistive load and spec it as a 4
Ohms load. Measuring the power across a lesser
resistive load will boost the power.
Gyaan: A manufacturer who gives its amplifier
power rating for two different resistive loads, ex:
8 Ohms and 4 ohms is an honest manufacturer,
and if the power rating doubles from 8 Ohms to 4
Ohms, the amplifier is a good amplifier.
THD Stands for ‘Total Harmonic Distortion’. This is
one particular topic which can cover an entire
book on what and how it is measured.
This basically means the distortion that the
amplifier produces at the specified level.
Gyaan: The lesser the better!!!
Every amp has a rated power and a frequency
range within which it delivers its rated power. If
the range is between the audible range 20 Hz to
20 KHz, the amplifier has the ability to faithfully
amplify music.
Gyaan: A power specification which says 1 KHz or
has the frequency range completely absent should
be a contender for skepticism.

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Conclusion
Choosing an amplifier/AV receiver merely on appearance or brand could be a mistake by consumers.
One probably will not be doing justice for his/her own ears for years to come!
The best way to evaluate an amplifier is to listen to it because every amplifier might be tuned differently
for different regions also do have a look at the specifications. This paper tries to give an explanation of
such specification for dummies. This paper is intended to be informative and not completely explanatory.
References
Author is NOT an expert in amplifier technology. This whitepaper is inspired from lot of work which was
done previously by experts and author has tried to summarize such a work (for dummies). For the ones
who wish to read more here are some reference World Wide Web links
[1] http://www.rayfes.com/caraudio/ampspec.php
[2] http://www.buyingloudspeakers.com/Speaker%20Specifications.htm
[3] http://www.audioholics.com/education/amplifier-technology/the-all-channels-driven-amplifier-test-
controversy