11.4 How big an amplifier do I need?
Description
This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
11.4 How big an amplifier do I need?
Unfortunately, amplifier power ratings and speaker power ratings
are almost always misleading. Sometimes, they are factually
wrong. Speaker ratings are almost useless in evaluating needs.
To start with, sound pressure, measured in dB, often stated as
dB SPL, is a function of the log of the acoustic "sound" power.
Further, human hearing is less sensitive to differences in power
than the log transfer function would imply. This means that the
perceived difference between a 50 watt amplifier and a 100 watt
amplifier, all else equal, is very small! One columnist said
that a 250 watt amplifier puts out twice the perceived
loudness of a 25 watt amplifier, but quantitative statements
about perception should always be treated with caution.
That statement came from Electronics Now Magazine, Jan 1994,
Page 87, Larry Klein's "Audio Update" Column, which is also
good reading on the subject of required amplifier power.
There is a wide variation in the "efficiency" and "sensitivity"
of the various speakers available. I have seen good speakers
with under 80 dB per watt efficiency and have also seen good
speakers with over 96 dB per watt efficiency, measured one meter
from the speaker. This difference of 16 dB represents a factor
of 40 difference in power requirement!
So the first step in determining amplifier requirements is to
estimate relative speaker efficiency. Other factors include how
loud you will want to listen, how large your room is, and how
many speakers you will drive with one amplifier. This
information will give you a rough starting point. For an
example, a typical home speaker will produce 88 dB at 1 watt.
In an average room, a person with average tastes will be happy
with this speaker and a good 20 watt per channel amplifier.
Someone who listens to loud music or wants very clean
reproduction of the dynamics of music will want more power.
Someone with less efficient speakers or a large room will also
want more power.
Past that point, you will have to use your ears. As with all
other decisions, your best bet is to get some candidates, borrow
them from a friendly dealer, take them home, and listen to them
at your normal and loudest listening level. See if they play
cleanly when cranked up as loud as you will ever go, into your
speakers in your room. Of course, it is also important to be
sure that the amp sounds clean at lower listening levels.
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