From: tonyb@juliet.ll.mit.edu ( Tony Berke )
Newsgroups: rec.audio
Subject: Re: Do I really need 200W?
Date: 29 Dec 92 17:10:11 GMT
Organization: M.I.T. Lincoln Lab - Group 43


Flame warning:

The posting I'm replying to started out with marginally reasonable
assertions, but it devolved into gibberish to the point where my
usually urge to call people nitwits has been overcome, hence this
reply:


In article <7009@otc.otca.oz> brendan@otc.otca.oz (Brendan Jones) writes:

   in article <1992Dec14.132035.2420@discus.technion.ac.il>, sorin@techunix.technion.ac.il (Goldenberg Sorin) says:
   > What's the difference between the P.M.P.O and R.M.S power ratings ?

   PMPO stands for "Peak Music Power Output" and is essentially a meaningless
   marketing term used to impress those who know little about audio.  It may
   vaguely relate to the headroom of an amplifier, but not in a very scientific
   way and rarely are the distortion figures given at the PMPO level.

Maybe so, maybe not.  Most music has a pretty high crest factor (peak to
average ratio), and most of the peaks are pretty short.  Peak power, depending
how it's measured, can have a lot to do with how loud you can play real music
without clipping.

   In short, I'd ignore any piece of audio equipment that needs to advertise
   itself on the basis of "PMPO".  It's probably junk.

   RMS stands for "Root Mean Sqaure" and is a more realistic measure of the
   output power of an amp, and the one you should look for...

If you're running a PA setup, or maybe buying an amp to run the
subwoofer in a biamped configuration, then I agree.  In both of
these situations, the load on the amp can have a rather high duty
cycle, depending on the program material and the listening habits
of the sytem's operator.

That said, very little music in a home setting consists of loud
steady-state tones, I'm not sure how you can assert that RMS power is
"THE" spec you should look for.  It is "A" spec you can look at.  The
FTC "RMS" power ratings at various impedances can give you a pretty
good idea if the amp was designed with a "brute force" philosophy, or
something a bit less conservative (ie something with a whimpy power supply
and/or marginal heatsinking.  You can gain even more insight if some
sort peak power ratings are presented as well -- a really high ratio
of peak/RMS power ratings can indicate that some sort of "slick"
design was used, or it might just mean that the designer just used a
loosely regulated supply to produce some (potentially useful) peak
power without using huge components.

As the owner of a rather large pile of semi-melted Carver amps, I'm
somewhat wary of designs that are *too* slick, but I also own a
(really large) pile of Crown DC300's, and I frankly think that in a
home setting, they are an inefficient (size, weight, and price) way to
create useful power for a fullrange speaker system.  Unless, of
course, you start getting tricky, like bridging them, or running them
into speakers that you've intentionally designed to have low
impedance.

   It can also help
   to know what class the power amplification stage is (class A, B, AB etc).
   Also, look for what the distortion figures are at the rated RMS power.  IMHO
   anything under 0.1% THD (Total Harmonic Distortion) is fine.

   Class A amplifiers have their output stage always in conduction.  This makes
   them overall very linear, but rather inefficient.

Class A amps are "very linear"?  That kinda borders on a philosophical
statement, and should be stated as such.  Many class AB amps have MUCH
lower steady-state distortion figures than many class A designs.  It's
kinda funy, but many of the huge, expensive, high-end class-A power amps
have puny FTC RMS ratings, but (if they were advertised that way) would
have enormous headroom.  This is usually hinted at by little blurbs like
"200 volt peak-to-peak swings available" and "80A peak current", etc.  Frankly,
I'd rather see some of these amps subjected to some semi-standard forms of
peak power testing, rather than be tempted to (incorrectly) multiply the
200V swing by 80A and start daydreaming about 16,000 watt amplifiers.
Of course, if Krell started publishing dynamic headroom figures, you'd
discount them from further consideration, as they'd suddenly become
"pieces of junk".

   As to how many watts you need - I reckon not many.  I have a 25 W per
   channel true RMS class A power amplifier, and a comfortable listening
   volume is 2 to 3 out of 10.

Here's where you really lose it, big-time.  Without knowing the other person's
listening habits, room size, furnishings, and (really important) loudspeaker
efficiency, you can't possibly have anything meaningful to say about how
much power he/she needs.  My main system is currently in a puny 1500 cubic
foot room, and is biamped, with about 600 Watts per channel total, running into
some speakers with about average efficiency.  That is about enough power for
me, in this room, with these speakers.  My (still unfinished, sigh) listening/
recording space is about 12,000 cubic feet, and I doubt that either the
speakers or the amplification will prove sufficient for all situtions in this
new space.

   Turning it up to 4 to 5/10 is *very* loud, and anything over 5 gets to
   seriously annoy the neighbours :-)  It's only been to 10 once (briefly) at a
   rather raging party I once held - it started to sound a little distorted at
   that level, but it may have been my ears 'cause it was bloody loud!

Oh yea, all my amplifiers go up to 11 !!!  Seriously, what the heck does
the amount of gain in your preamp stage, and/or the output level of your
CD player, have to do with this discussion?

   So from this I think 25 W RMS is plenty, but it also depends on the other
   specs for the amp and the specs for the speakers you're connecting to it.

An interesting debate tactic... put the most important point (the one that
negates all your anecdotal evidence) at the end of your argument!

   My plastic A$5 note worth.

My flaming US$.02 worth,

Tony Berke
Newsgroups: rec.audio