DIYbanter - View Single Post

M Philbrook

In article , says...

Hi all,

This subject continues to severely bug me. I've just read some audio amp
servicing/repair book where the author states that customers often find
it desirable to know an amp's actual output power, and to return an amp
having measured its output power and noted it down for the customer's
info. The trouble is, customers are familiar with "RMS power" - but
there's no such thing! Irms X Vrms = AVERAGE power and that is all that
matters. BUT - try telling that to a typical customer!

So it must all come down to perception. Customers have an idea in their
minds what say 70WRMS sounds like through listening to dozens of various
VRMS labelled amps over the years. So there ought to be some kind of
notional, informal 'conversion factor' between Waverage and "WRMS" we
could adopt to satisfy them without compromising on the accuracy of the
correct measurement, n'est pas? What would that conversion factor be?
Greater or less than unity for a start?
Your thoughts invited.

Don't get confused, RMS and AVG is not the same

While RMS is working in the 0.707 of the peak, AVG is
in the 0.637 or there abouts.

Since Watts = 1 J(joule)/ Second we have this to look at;

Inject a 1 Hz tone into an amp from a clean sinewave source with
1 Volt Peak coming out going to a 1 ohm load. How do you
measure Watts if Watts involves a J value over a 1 second period?

We know that the SINE wave isn't always at full level of 1 volt during
that time period so that would indicate that we don't have 1 WATT
of energy there, so what do you do ?

In retrospect that, you need to use voltage in your calculations
to form a WATT value at some point. Wouldn't be logical to
use RMS or AVG as the figure, one being a little different than the
other?

Others will argue the fact that RMS power does not exist, I think they
may have fallen off the cliff, turret, edge, mountain what ever it is
they like to hang out on.

When I was schooled in electronics(Long time ago), starting at lower
levels, through highschool, Post grade in a trade shop, college and
adult lectures, there were never any discussion of RMS/AVG power ever
not existing, because of the roots of how it is derived.

I just flip my eye brown when I hear otherwise and move on.

There are some twisted use of Peak, Peak To Peak power for sales
only and they can get away with it. Only because in retrospect, it is
valid, just the end user is getting fooled by the lack of reallity.

To add to this, doing power calculations in AC does bring in the
phase angle but we get back to the power level of WATTS/second which
then puts a fork into it, if you really think about it. So summing all
the points of a sinewave when doing AC power, still comes out back
to what we were talking about above.

P = I cos(V);do that for all 360 points or even closer if you wish and
see where that leads to.

Just my opinion, really!

Jamie