Re: What/who defines "unfit for purpose"?


"Anthony R. Gold" <not-for-mail@xxxxxxxxxx> wrote in message
news:bqhga45p7dc5nssvtf15nia0iunbh45e94@xxxxxxxxxx
On Sun, 17 Aug 2008 16:38:08 +0100, johannes
<johs@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:

Yes, there is a hell of a difference between RMS and the ill-defined PMPO
(Peak Music Power output) A 5 Watt RMS can easily be 20 Watt PMPO.

It's all pure technobabble.

RMS watts? WTF does one get by extracting the root of the mean square of
power? Apparently it is the term used by non-engineers as the product of
RMS volts and RMS amps. But that's plain old Watts not special RMS Watts.


The reason they are called that is that there is more than one way of rating
the power of an amplifier system. The RMS watts isn't simply the maximum
RMS volts times the maximun RMS amps. The RMS output power is the maximum
power that the amplifier will continuously deliver while outputting a
(usually) 1 kHz sine wave and is less than the figure calculated above*.
The usually more useful power rating is the 'music power' which recognises
that the peaks of the music can be of a higher power than the RMS power
simply because they aren't continuous.

The often quoted rating, the PMPO (Peak Music Power Output) or just plain
'peak' rating is a useless rating as it is a theoretical power caculated
using the maximum power transfer theorom when the amplifier is terminated in
its output impedance (always a fraction of the speaker impedance). It's
advantage for the amplifier manufacturer is that it gives a rating far
higher than any other rating. Although the amplifier is capable of
delivering these large powers they will rarely do so for longer than a
millisecond or two (assuming there is no current limiting to prevent it).

*It will of course be the actual RMS volts times the actual RMS amps being
delivered.

RMS volts and RMS amps are extremely useful quantities which have physical
significance and for which there are many manufacturer of meters that can
measure them. RMS power is both meaningless and unmeasurable.


RMS power is very measurable indeed, but in practice is usually measured by
measuring the RMS volts across a fixed resistor. The power is the the
votage squared divided by the resitance.

As to PMPO, this is equal to PREAL * k where PREAL is defined as above and
k is explained in more detail here: http://sound.westhost.com/power.htm
:-)


That calculation probably relates to their products where 'k' is amplifier
dependant.

Regardless of that, the PMPO is actually calculated by

(V(s) - (2 * V(d)) / (2 x sqrt(2)))**2
__________________________
Z(o)

where:

V(s) is the output stage supply voltage

V(d) is the output transistor volt drop (typically 0.7V for bipolar - almost
0V for MOSFET)

Z(o) is the output impedance of the amplifier (much less than the speaker
impedance - 0.1 ohm is a typical value).

** means to the power of (that is **2 means squared)


.