John, mind telling us how rms power is calculated for a single tone? The
average power is calculated as Vrms squared divided by R. This average power
is also how much heat is delivered to the load. So what is rms power, and
how is it calculated, and what does it physically mean?
thanks,
Dave
K0QE
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From: John Nelson [SMTP:John_Nelson@compuserve.com]
Sent: Friday, January 14, 2000 4:08 PM
To: Ian White, G3SEK; QRO list
Subject: [AMPS] Audio amp power
Message text written by "Ian White, G3SEK"
>But since you've de-cloaked, John, please could you tell us what
the hi-fi
industry purports to mean by "watts RMS" and "music power"?<
Proper audio engineers (and measuring instruments such as the Audio
Precision family) use the classical heating-effect derivation of RMS
power
in terms of a set value of resistive load, usually 4 ohms. In
engineering
circles the power is always stated in association with a set level
of total
harmonic distortion (THD) and the frequency at which the measurement
was
made. The latter is almost always a single-tone input of 1kHz and
the THD
typically around 0.1%. If you're being rigorous, you also state
whether the
measurement was made with one or both channels of the amplifier
under test
being driven and what the mains voltage, ambient temperature and
heatsink
temperature were. IIRC, there is an IEC procedure setting out the
agreed
method of testing audio amplifiers (IEC1010 rings a faint bell).
Incidentally, you'll probably gather from this why I've always taken
the
view that discussing the power output of an RF linear amplifier
without
referencing it to a set level of distortion (two-tone IMD rather
than
single-tone THD, naturally) is meaningless.
"Music power" is pretty well anything the marketing department wants
it to
be. The usual starting point is twice the RMS power at 10% THD or
when the
amplifier is driven into saturation, whichever is greater. The
latter test
is of course performed into the lowest load impedance possible
without the
unit blowing up. The result is multiplied by two to account for the
two
channels and increased by a further 20% or so because "...you get
more
power output on music than when testing, don't you?". This number is
then
increased by another 50% on the basis that at very low duty cycles
-- shall
we say one microsecond per second? -- the amplifier can generate
more
output than in static sine-wave testing, and finally multiplied by a
variable factor ("normalisation of testing conditions") to give a
result
which is 10W more than the perceived competition. The more
unscrupulous
manufacturers will start with DC input, of course, not RMS output.
It goes without saying that music power (sometimes referred to as
PMPO,
"peak music power output) is never referenced to any other
parameter,
especially distortion or load impedance. Do not worry about this, or
about
such things as whether a heatsink one inch square can really
dissipate
500W. Music power is very a special type of power, and engineering
folks
can't be expected to understand its finer points.
I can recall a recent case where an amplifier developing about 5W
RMS per
channel into 4ohms at 1kHz for 0.1% THD was rated at "100W music
power."
73 John
GW4FRX
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