[AMPS] Audio amp power

[Dave D'Epagnier]

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

	----------
	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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