This page is intended to explain amplifier power specifications
in more detail. I have a BS in Electrical Engineering so I do not know
how much of this the average Joe is going to understand. I am also human
so there may be mistakes below. Feel free to email
me and ask me to clear anything up regarding this page.
Amplifier power ratings are important in determining whether
an amp will satisfy your system's needs or not. It is necessary for the
amp manufacturer to give out a power specification which clear and complete.
Otherwise you are just guessing. An example of a good power amp spec
for a 4 channel amp is:
"50watts X 4 RMS all channels driven
continuously into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
Every part of that spec is important and without any part
of it the power rating is virutally meaningless. Many times amp manufacturers
do not give this much information but you have to judge for yourself whether
they are hiding anything. Head unit power ratings are notorious for being
very misleading. Now I'll go into what each part of the spec means
and why each is important.
"50watts
X 4 RMS all channels driven continuously into 4 ohms with less than 0.1%THD
from 20Hz to 20kHz"
The "50watts" part is the one
we notice first and everything else qualifies how that "50watts"
was measured. Having enough power is what most people look for in an amp.
However, other things come into play. If the you are going to run a load
less than 4 ohms, then the current capability of the amp is definitely
important and most specs do not give a current capability. A power rating
into 2 ohms can help though. If the power doubles into 2 ohms then you
know that the amp is built strongly enough that it can deliver enough current
to drive a 2 ohm load. You may think that this is not important if you
are not going to drive 2 ohm loads but it is important. Speakers (woofers,
midranges, tweeters, etc) are not purely resistive. They have capacitive
and inductive properties as well. Depending on the music and your setup,
the impedance may dip well below 4 ohms for a nominally 4 ohm speaker.
Whether you amp can supply current fast enough to reproduce
the music faithfully depends partially on the amp's slew rate (how fast
its output can change), its damping factor (how easily it can control the
speaker) and its current capability. For these reasons 2 ohm power is important
even when driving 4 ohm speakers. Slew rates of 100V/microsec and damping
factors above 100 (referenced with a 4 ohm load) are good but that information
is usually not given out by the amp manufacturer. I hope it is clear now
that the number of watts an amp can produce is only one factor in determining
whether an amp is capable of the performance you desire.
On a final note on this part of the spec, most head units
use IC (integrate circuits or chips) for the built-in amp's output stage.
Those chips rarely can provide adequate current which is why even most
novices know not drive subwoofers from a head unit. Real amps often have
ICs in them as well but the output stages are almost always discrete, meaning
they are built from transistors, resistors, capacitors and not integrated
together inside tiny ICs. Advances in IC technology always making
them better though.
"50watts X
4 RMS all channels
driven continuously into 4 ohms with less
than 0.1%THD from 20Hz to 20kHz"
The "X 4" implies that the amp
has 4 output channels. The "RMS" stands for
"root mean square" and is a method of measuring an AC waveform. More importantly
here it implies that the power rating is not just a peak rating but continuous.
"all channels driven" means that the power
measurement was made with all channels of the amp driven to their maximum
level at the same time. This means that the power supply is strong enough
to allow all 4 output channels to produce 50watts at the same time.
This is a common place where head unit specs "cheat." They
leave off the "all channels driven" and measure only 1 channel at a time
which often gives a higher number. I've seen head units claming "30x4"
which is meaningless but most people take it to mean that the head unit
produces 30watts each into 4 channels. That's 120 watts from a head unit.
No amp is 100% efficient so let us say it draws 150 watts to do this (80%
efficiency which is still high). With a 12V power input, the head unit
amp's power supply would be drawing 12.5 amps. I guarantee you that it
is not easy to design a power supply that fits into a head unit leaving
enough room for everything else (including the amp stages themselves) for
any reasonable price that can deliver that kind of power. That is one reason
why I say not preferable to use the head unit's power.
"50watts X 4 RMS all channel driven
continuously
into 4 ohms with less than 0.1%THD from 20Hz to 20kHz"
"continuously" implies that
the measurement was made using a continuous (probably sine wave) test signal
and not just a quick burst. An amp capable of producing higher power for
short amounts of time will have a higher power rating if they measure power
with short bursts instead of a continuous input.
The argument can be made that continuous power is not as
important because music by nature is dynamic and therefore the peak power
is what we really should concentrate on. My response to this is that there
is no standardized burst input which all amp manufacturers would use to
measure "peak" power. In the end to make their power ratings look higher
they would use extremely short pulses which would not represent the amp's
performance with music. Because no standard currently exists for peak power
we must rely on continuous power ratings for consistancy and to be able
to compare amps with each other.
"50watts X 4 RMS all channel driven
continuously into 4 ohms
with less than 0.1%THD from 20Hz to 20kHz"
"into 4ohms" means that the
power measurement was done using a dummy 4 ohm resistor as the load. This
is not the same as a 4 ohm speaker but provides a standard which everyone
uses to measure power. Sometimes (but not very often) amp manufacturers
will measure power specs into 2 or 3 ohm loads and not say "into
4ohms" only to make the power rating look bigger than it actually
is but this is rare. 4 ohms is what car audio amp manufacturers almost
always give their power ratings for.
"50watts X 4 RMS all channel driven
continuously into 4 ohms with less than 0.1%THD
from 20Hz to 20kHz"
"with less than 0.1% THD" tells
something about the distortion the amp is producing at this power level.
Most amps have an intrinsic distortion that occurs at a near constant level
for most of its power range and then when the amp starts to get overdriven
the distortion rises quickly. THD stands for "total harmonic distortion"
which is one way of measuring distortion that is standardized.
Often a power spec without the THD number was made with the
amp driven until the THD reached 1% or more. This gives a higher power
rating but you probably would not want to use the amp at that level because
it would be distorted. This is a common ploy used when you see a 400watt
amp for $50 at a flea market or discount store. This is often another way
that head unit amp specs are inflated.
"50watts X 4 RMS all channel driven
continuously into 4 ohms with less than 0.1%THD from
20Hz to 20kHz"
"from 20Hz to 20kHz" tells us
the frequency range into which this amp can produce its rated power. Some
amps have power curves that fall off at low and high frequencies. Having
this part of the spec present gives you reassurance that the amp can produce
its power anywhere in the normal audio range. A power spec that says "into
1kHz" or leaves it off could be inflated. Many amps just put the frequency
response as a separate datum on the spec sheet and not with the power rating.
It should be with the power spec as well. A "+/- 1dB" or something similar
should accompany the frequency response so you know how flat the frequency
curve is.
That is it for amplifer power specs and be careful with incomplete
specs. Even the best manufacturers put out incomplete specs and then it
is up to you to figure out whether the amp is well designed or not but
it should not be too difficult. You get what you pay for but look at the
construction and "feel" of the amp as well to help make your decision.
Also, keep in mind that these explanations are valid for home amplification
equipment as well, although the FTC has more stringent requirements for
power claims of home audio equipment.
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