[AMPS] Audio amp power

[Derek K7XD]

No  9db for room gain?

Derek K7XD



> 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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