From: DPierce@world.std.com (Richard D Pierce)
Subject: Power specs, was: Re: Do I really need 200W?
Organization: The World Public Access UNIX, Brookline, MA
Date: Mon, 14 Dec 1992 16:32:08 GMT

In article <Bz98KE.Irn@dcs.glasgow.ac.uk> dam@dcs.glasgow.ac.uk (David Morning) writes:
>sorin@techunix.technion.ac.il (Goldenberg Sorin) writes:
>
>>What's the difference between the P.M.P.O and R.M.S power ratings ?
>
>One's bogus and makes a 2 watt amplifier appear on paper as if it were a
>400 watt job, the other is slightly less bogus but still open to interpretation
>by unscrupulous sales cretins.
>
>PMPO = Peak Music Power Output
>RMS = Root Mean Square.
>
>The RMS term is the slightly less bogus one but it's meanless without the
>accompanying test conditions that the RMS figure was arrived at.
>

Well, not quite. In the mid 1970's, the Federal Trade Commission issued
regulations regarding the advertising of power ratings for consumer audio
amplifiers. The test method was clearly stated and unambiguous.

In summary, the amplifier must be run at 1/3 of its rated RMS power for
some very long time (at least an hour, as I recall) so that it reaches
some degree of thermal equilibrium. Then the amplifier must be able to
produce its advertised RMS power at or below its advertised distortion
at any frequency within its rated bandwidth continuously. The
advertised specification for the unit must state all of these limits very
clearly, for example: 

   "power output 45+45 watts RMS continuous into 8 ohms
    from 20 Hz to 20 kHz at no more than 0.1% total harmonic 
    distortion." 

This might be a typical specification you would see for a reasonable
styereo integrated amplifier or receiver that's rated at 45 watts per
channel. The spec and the regulation clearly state that the amplifier must
produce 45 watts through both channels simultaneously. It must do it at
any frequency between 20 and 20 kHz at no more than 0.1% distortion. Period.

On the other hand, less than reputable sellers like Lafayette Radio, whose
famed and "award winning" LR-1500 TA receiver was "rated" at 225 watts IHF
peak power struggled desparately to make 30 watts/channel 60 Hz to 15 kHz
at less than 1% distortion. Fortunately, by the time the regs were in
force, Lafayette had effectively disappeared from the face of the earth.
Many other such manufacturers were forced to clean up either their
technical or advertising acts, or they simply followed Lafayette into
their own richly deserved oblivion. 

There were two problems with the spec. The first was that 1/3 power
happens to be right near the point where class AB amplifiers have the
greatest device dissipation. Thus, the break-in period causes most
amplifiers to run much hotter than they normally would in the vast
majority of real listening situations where the actual average power
output is much less usually at less than 1 watt). The net effect is that
it makes class AB amplifiers look, dissipation-wise, more like class A in
the tests, and class C amplifiers get to run a whole bunch cooler through
the tests (Phase Linear 700's did very well, raw-output wise, since they
ran damn near pure class C).

The manufacturers were given an opportunity to respond and comment on this
and other technical aspects, but nobody took the FTC seriously, so the
regs were published and enforced without change. To bad! 

The second problem lies with an absolutely stupid clause in the regs that
says that the amplifier model number cannot be related to the output
power. This means that amplifiers like MacIntosh 2105's (which was a
stereo 105 watt/channel amp) ended up being rated now at 104 or 106 watts.
Marantz 2270's were, instead of 70 watts per channel, called 72 watts per
channel. The Marantz Model 9 (a mono 70 watt tube amp) were safe, though. 
It was a real dumb idea.

Now, having posted this before, it never occured to me that this
regulation MIGHT no longer be in effect. THere was something approaching a
wholesale dismantling of regulations under the Reagan era with the idea of
"unencumbering" business. Unfortunately, this is one of those regulations
that did a pretty good job of protecting consumers from clearly deceptive
claims by many, many manufacturers. Out the window with the bathwater, the
baby doth fly. (followups on this last topic may be followed in
alt.politics.conservative-whining, please). 

-- 
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