From: John Woodgate
Subject: Re: wattage meter exists??
Date: Sun, 8 Sep 2002 11:49:37 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 8 Sep 2002 10:52:18 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Robert Baer
wrote (in <3D7AF3C1.B3A42BAD@earthlink.net>)
about 'wattage meter exists??', on Sun, 8 Sep 2002:
> Assume 6V supply and the speaker drivers go rail to rail (a number of
>modern analog ICs now do this).
> Also assume a full H-bridge driver so the speaker can be driven from
>+6V to -6V (a number of modern analog ICs now do this also).
> That is 12V peak to peak. Now P=(V*V)/R, so assuming 60 watts, R=2.4
>ohms (unlikely as the lowest Z i have seen is 4 ohms).
No way. Power is not peak-to-peak voltage squared divided by resistance.
> Furthermore, power level is nominally specified as RMS, which is 0.707
>of peak if sine wave, which is the other standard for specification.
>This gives P=(0.707*V*0.707*V/R or R=4.8 ohms, which is more like a
Now you have V = 24 V, which seems a mite improbable, no?
12 V peak-to-peak is 4.24 V r.m.s. The 60 W is fictitious; the actual
amplifier power is either 4.24^2/8 = 2.25 W or 4/2/4^2/4 = 5.5 W,
depending on whether the loudspeaker is nominally 8 ohms or 4 ohms. But
that applies only if indeed the amplifier is H-bridge (or BTL - Bridge-
Tied Load, as it's called in audio circles). It probably isn't: in which
case the powers are 1.125 W and 2.25 W.
> You sure as heck will *not* get more power (100W) with less voltage (2
>D cells = 3V supply).
I think we can agree on that.
Actually, all this 'power' business is thoroughly misleading; a
loudspeaker is a voltage-operated device, not a power-operated device,
in the sense that you get the frequency response you want (or are
willing to accept) with constant voltage applied, not with constant
power absorbed. The absorbed power mostly just heats up the voice-coil.
We should rate amplifiers in terms of voltage and minimum load impedance
and loudspeakers in the same terms. It would be far less confusing and
far less open to abuse.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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