Stereo Amplifier Power Specifications
 

Back in the late 1970’s and early 1980’s when I was purchasing quality
stereo amplification equipment for various projects, I became quite familiar
with the techniques used by manufacturers to overstate the power output of
their amplifiers – specifically statements presenting total instantaneous
power, peak power, or other such number-inflating techniques. I had thought
that the practice of requiring power output in RMS to be specified at a
specified distortion level had put an end to that type of power inflation
technique and provided serious buyers with an appropriate yardstick for
comparison.

In those days, amplifiers that were capable of 50 Watts RMS per side or
higher were relatively beefy with massive heat-sunk power transistors, heavy
power transformers, filter chokes and substantial electrolytic capacitor
banks to meet the RMS power requirements. I still enjoy a number of
amplifiers from those days and appreciate their “clean” power delivery,
particularly in the bass range.

While I recognize that advances in power supply design, such as switching
mode, and newer amplifier designs, such as classes G, H and D, have afforded
the ability to eliminate significant weight and size from amplifiers, I’m
concerned that somehow the RMS rating criteria is somehow being bypassed in
current stereo and home theatre products.

For example, I recently trialed a name-brand stereo amplifier that was
specified at 100 Watts per side into 8 Ohms. I really didn’t need a new
amplifier, but couldn’t believe the price for what was claimed to be
equivalent to my 15-year old Akai, which isClass G and rated at 130 Watts
per side into 8 Ohms and 100 Watts per side into 4 Ohms. The name-brand unit
was about ¼ the size and weight of my Akai, which cost five times as much 15
years ago.

When I connected the name-brand unit to my Altech "Studio Monitor" 8 Ohm
speakers and played a favourite CD, I immediately recognized that this unit
was dramatically underpowered, particularly in the bass range – probably
doing no better than 10 Watts in terms of what I associated with RMS power
performance.

When I returned the unit (at the speed of sound) to the big box store audio
expert, he emphatically stated that I needed a “high-current” amplifier,
pointing to a much more expensive / expansive name-brand unit that was in
the same, size, weight and price range as my Akai and Pioneer units.

Still adhering to the belief that Ohm’s Law is applicable to the
specification of power, and assuming a fixed speaker impedance, I could not
understand the relevance of “high current” to getting better sound out of an
amplifier with the same 8 Ohm RMS power rating, but no “high-current”
designation. For a given power output, if the current is lower, the voltage
must be higher, per Ohm’s Law.

I note seemingly incredible claims being made for home theatre
systems –seven channels at 100 Watts RMS output – “simultaneously”, as
promised by a “sales engineer” in a professional audio shop that I recently
visited. I can’t believe that amplifiers of this size, weight and price on
offer at this shop could possibly deliver that type of relatively
undistorted RMS power simultaneously from five channels, let alone seven.

How are the manufacturers and sales agencies getting around the old RMS
power specification “equalizer”? I note reasonable specifications for IMD
and THD associated with these current power claims, so what am I missing???

Comments would be very much appreciated.

Bill Evans

Stereo Amplifier Power Specifications
 

"Bill Evans" wrote in message
...
Back in the late 1970's and early 1980's when I was purchasing quality
stereo amplification equipment for various projects, I became quite
familiar with the techniques used by manufacturers to overstate the power
output of their amplifiers - specifically statements presenting total
instantaneous power, peak power, or other such number-inflating
techniques. I had thought that the practice of requiring power output in
RMS to be specified at a specified distortion level had put an end to that
type of power inflation technique and provided serious buyers with an
appropriate yardstick for comparison.

**Just a point of pedantry: "RMS power output" is and always was a
nonsensical term. The correct term is: "Continuous Watts", or just plain ole
Watts. Continuous Watts are derived from the RMS Voltage across a given
load, or the RMS current flowing through that load. There is no such thing
as "RMS Watts".


In those days, amplifiers that were capable of 50 Watts RMS per side or
higher were relatively beefy with massive heat-sunk power transistors,
heavy power transformers, filter chokes and substantial electrolytic
capacitor banks to meet the RMS power requirements. I still enjoy a number
of amplifiers from those days and appreciate their "clean" power delivery,
particularly in the bass range.

While I recognize that advances in power supply design, such as switching
mode, and newer amplifier designs, such as classes G, H and D, have
afforded the ability to eliminate significant weight and size from
amplifiers, I'm concerned that somehow the RMS rating criteria is somehow
being bypassed in current stereo and home theatre products.

**In many cases, you are correct. In some, you are not. BTW: There are
really only the following Classes of amplification:

Class A
Class A/B
Class B
Class C
Class D

Class C is not often seen in audio. All the other classes (Class G, H, et
al) are not really about the amplifier at all. They relate to various
schemes surrounding the power supply. Ultimately, the actual amplifying
stages will be one of the usual ones (Class A, A/B, B or D).


For example, I recently trialed a name-brand stereo amplifier that was
specified at 100 Watts per side into 8 Ohms. I really didn't need a new
amplifier, but couldn't believe the price for what was claimed to be
equivalent to my 15-year old Akai, which isClass G and rated at 130 Watts
per side into 8 Ohms and 100 Watts per side into 4 Ohms. The name-brand
unit was about ¼ the size and weight of my Akai, which cost five times as
much 15 years ago.

When I connected the name-brand unit to my Altech "Studio Monitor" 8 Ohm
speakers and played a favourite CD, I immediately recognized that this
unit was dramatically underpowered, particularly in the bass range -
probably doing no better than 10 Watts in terms of what I associated with
RMS power performance.

**How much (in 2006 Squid) did your old amp cost, relative to your new amp?


When I returned the unit (at the speed of sound) to the big box store
audio expert, he emphatically stated that I needed a "high-current"
amplifier, pointing to a much more expensive / expansive name-brand unit
that was in the same, size, weight and price range as my Akai and Pioneer
units.

Still adhering to the belief that Ohm's Law is applicable to the
specification of power, and assuming a fixed speaker impedance, I could
not understand the relevance of "high current" to getting better sound out
of an amplifier with the same 8 Ohm RMS power rating, but no
"high-current" designation. For a given power output, if the current is
lower, the voltage must be higher, per Ohm's Law.

**It is a fatal mistake to assume that lousdspeakers are resistive loads.
Your '8 Ohm' speakers may be something entirely different. Here is, for
instance, a speaker which is rated at "4 Ohms":

www.rageaudio.com.au/kappa9.jpg

Note the dip in the bass region.


I note seemingly incredible claims being made for home theatre
systems -seven channels at 100 Watts RMS output - "simultaneously", as
promised by a "sales engineer" in a professional audio shop that I
recently visited. I can't believe that amplifiers of this size, weight and
price on offer at this shop could possibly deliver that type of relatively
undistorted RMS power simultaneously from five channels, let alone seven.

**Most cannot actually deliver full power from all channels simultaneously.
Most can manage full power from two channels simultaneously.


How are the manufacturers and sales agencies getting around the old RMS
power specification "equalizer"? I note reasonable specifications for IMD
and THD associated with these current power claims, so what am I
missing???

**There are a bunch of issues you need to examine:

* The impedance of your speakers.
* The ability of your amp to cope with the afore-mentioned speaker loads.


Fundamentally, however, you should know that many crappy amps were built,
way back when. There were quite a few decent amps built too. Nowadays, there
are still decent amps being built and, usually, at significantly lower costs
(in real Dollar terms) than in the old days. A good old amp will always
outperform a crappy new amp.


--
Trevor Wilson
www.rageaudio.com.au

Stereo Amplifier Power Specifications
 

In article , Bill Evans wrote:
Back in the late 1970’s and early 1980’s when I was purchasing quality
stereo amplification equipment for various projects, I became quite familiar
with the techniques used by manufacturers to overstate the power output of
their amplifiers – specifically statements presenting total instantaneous
power, peak power, or other such number-inflating techniques. I had thought
that the practice of requiring power output in RMS to be specified at a
specified distortion level had put an end to that type of power inflation
technique and provided serious buyers with an appropriate yardstick for
comparison.

RMS values of the current and voltage are used to calculate the mean (or
average) value of the power, because these values of voltage and current are
equivalent to the DC values that would supply energy at the same rate, so "mean
power level" is the quantity that should really be specified.

The power output, like any varying quantity, will have an RMS value, but it
doesn't relate to any significant physical quantity so it isn't particularly
meaningful or useful to know it.

I suspect that the common avoidance of the correct terms "mean power" or
"average power" is simply down to the fact that advertisers don't like the
sound of them, whereas "RMS power" sounds much more technical and obscure, even
though it's wrong.

Even "maximum continuous mean power" wouldn't always tell the whole story, as
it doesn't always represent the output level that puts the greatest strain on
the output devices of the amplifier itself.

Rod.

Stereo Amplifier Power Specifications
 

"Roderick Stewart" wrote in message
om...
In article , Bill Evans wrote:
Back in the late 1970's and early 1980's when I was purchasing quality
stereo amplification equipment for various projects, I became quite
familiar
with the techniques used by manufacturers to overstate the power output
of
their amplifiers - specifically statements presenting total instantaneous
power, peak power, or other such number-inflating techniques. I had
thought
that the practice of requiring power output in RMS to be specified at a
specified distortion level had put an end to that type of power inflation
technique and provided serious buyers with an appropriate yardstick for
comparison.

RMS values of the current and voltage are used to calculate the mean (or
average) value of the power, because these values of voltage and current
are
equivalent to the DC values that would supply energy at the same rate, so
"mean
power level" is the quantity that should really be specified.

The power output, like any varying quantity, will have an RMS value, but
it
doesn't relate to any significant physical quantity so it isn't
particularly
meaningful or useful to know it.

I suspect that the common avoidance of the correct terms "mean power" or
"average power" is simply down to the fact that advertisers don't like the
sound of them, whereas "RMS power" sounds much more technical and obscure,
even
though it's wrong.

Even "maximum continuous mean power" wouldn't always tell the whole story,
as
it doesn't always represent the output level that puts the greatest strain
on
the output devices of the amplifier itself.

Rod.


I've also seen the reason for the apparently incredible output levels of the
TescoSonic style home cinema systems, being a lot simpler than all of the
clever explanations above. It's becoming increasingly common for all of the
channels of these systems, to be quoted together, with some clever wording
that makes it sound like they are being quoted seperately. Thus, a six
channel system that appears to do an incredible 120 of whatever flavour of
watts you choose, per channel, is actually 120 watts spread across the six
channels. Inside, it just has a few low quality 20 watt car audio ICs, and a
power supply just about man enough to run them.

You'd weep if you saw inside some of this apparently high specced crap that
I have to fix ...

The biggest offenders for misquoting, have got to be the computer speaker /
amplifier systems, that often claim such nonsense as 120 watts RMS super sub
woofer, and contain a speaker that you would be ashamed to design into a
transistor radio.

Arfa

Stereo Amplifier Power Specifications
 

In article , Trevor
Wilson wrote:

"Bill Evans" wrote in message
...

[snip]

I had thought that the practice of requiring power output in RMS to
be specified at a specified distortion level had put an end to that
type of power inflation technique and provided serious buyers with an
appropriate yardstick for comparison.

**Just a point of pedantry: "RMS power output" is and always was a
nonsensical term. The correct term is: "Continuous Watts", or just plain
ole Watts.

I tend to prefer a term like "mean sustained sinewave" power. Alas, I think
we are now probably stuck with 'RMS' as a result of all the marketing-speak
that has infected power specs in adverts over the years... :-/

[snip]


When I returned the unit (at the speed of sound) to the big box store
audio expert, he emphatically stated that I needed a "high-current"
amplifier, pointing to a much more expensive / expansive name-brand
unit that was in the same, size, weight and price range as my Akai
and Pioneer units.

As Trevor explained, the problem here is that the loduspeaker may be
nothining like a plain 8 Ohm resistor as a load. The thing to keep an eye
out for in reviews is any measurement of the 'peak current' the amp can
deliver. Hence the comment by the 'expert' in the shop.



I note seemingly incredible claims being made for home theatre
systems -seven channels at 100 Watts RMS output - "simultaneously", as
promised by a "sales engineer" in a professional audio shop that I
recently visited. I can't believe that amplifiers of this size, weight
and price on offer at this shop could possibly deliver that type of
relatively undistorted RMS power simultaneously from five channels,
let alone seven.

**Most cannot actually deliver full power from all channels
simultaneously. Most can manage full power from two channels
simultaneously.

Note, though, that some decent multichannel amps can exploit the use of
modern switch-mode power supplies, and may even use 'digital' amplifier
methods. These can allow the amp to be much smaller and lighter (and
dissipate less waste heat) than ye olde fashioned types of psu and amp. As
ever, though, the devil will be in the details when it comes to deciding if
any particular example is good for use. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Stereo Amplifier Power Specifications
 

Roderick said

RMS values of the current and voltage are used to calculate the mean (or
average) value of the power, because these values of voltage and current are
equivalent to the DC values that would supply energy at the same rate, so
"mean
power level" is the quantity that should really be specified.

Yes. If you square the root of the mean square you obviously get the mean
square. Since the square is proportional to power, you get mean power.

There is real and meaningful confusion in this RMS power nonsense. Pure
inductors and capacitors dissipate no power, although they may have voltage
across them, and current through them, simultaneously. This is possible because
positive and negative power cancels. RMS power would not cancel, because it
would always be positive. Hence it is not a trivial confusion. It is indicative
of a serious lack of understanding of basic essentials.

My first question in the hifi shop is always "what kind of power is that?" If
man says "RMS" I walk out. Hence I now have to make all my own hifi.

cheers, Ian

Stereo Amplifier Power Specifications
 

Appreciate your comments and those of others on this thread, but most
comments seem to focus on the definition of RMS, not the basic issue of
misrepresenting power output capabilities of modern amplifiers. What I
recall from the late seventies is that the International Institute of High
Fidelity (IIHF) had established detailed technical specifications that
defined the measurement techniques that would necessarily be used to
establish the specified amplifier power output on an RMS, continuous power
basis. This specification process was, as I recall, very rigorously
described.

What happened to IIHF? Is there no accepted industry standard, either in
Europe or North America or elsewhere on rating audio amplifier output power
levels?

"Ian Iveson" wrote in message
. uk...
Roderick said

RMS values of the current and voltage are used to calculate the mean (or
average) value of the power, because these values of voltage and current
are
equivalent to the DC values that would supply energy at the same rate, so
"mean
power level" is the quantity that should really be specified.

Yes. If you square the root of the mean square you obviously get the mean
square. Since the square is proportional to power, you get mean power.

There is real and meaningful confusion in this RMS power nonsense. Pure
inductors and capacitors dissipate no power, although they may have
voltage across them, and current through them, simultaneously. This is
possible because positive and negative power cancels. RMS power would not
cancel, because it would always be positive. Hence it is not a trivial
confusion. It is indicative of a serious lack of understanding of basic
essentials.

My first question in the hifi shop is always "what kind of power is that?"
If man says "RMS" I walk out. Hence I now have to make all my own hifi.

cheers, Ian

Stereo Amplifier Power Specifications
 

In article , Bill Evans wrote:
Appreciate your comments and those of others on this thread, but most
comments seem to focus on the definition of RMS, not the basic issue of
misrepresenting power output capabilities of modern amplifiers.

Quoting an amplifier's output power in "RMS Watts", or as "RMS power", or
simply appending the magic letters "RMS" to some quoted power output value
*is* misrepresenting it, because these expressions, although having tha
apparent authority of technogobbledygook, are actually meaningless.

Rod.

Stereo Amplifier Power Specifications
 

In article , Bill Evans
wrote:

What I recall from the late seventies is that the International
Institute of High Fidelity (IIHF) had established detailed technical
specifications that defined the measurement techniques that would
necessarily be used to establish the specified amplifier power output on
an RMS, continuous power basis. This specification process was, as I
recall, very rigorously described.

Can't comment on the 'IIHF' apart from noting the the real problem is
likely to be that such bodies may have no way of 'enforcing' their
decisions or standards on manufacturers or shops.

What happened to IIHF? Is there no accepted industry standard, either in
Europe or North America or elsewhere on rating audio amplifier output
power levels?

Pass. The IHFA (not IIHF) did set a series of regulations on how to test
powers back in the 1970's. (IHFA-707 IIRC) These were very demanding, and
some of us did use them for a while. However they were so demanding that
people seem to have decided to stop using them.

This is one of the areas where the audio magazines *could* help if they so
chose. They could adopt a suitable set of test measurement standards and
apply them. Then both publish the results, and 'name and shame' makers whos
published specs were twaddle.

Alas, the track record is that mags tend to prefer to publish 'subjective
auditions'... Pretty useless for prospective customers, and apply no
pressure on makers whatsoever... :-/

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Stereo Amplifier Power Specifications
 

In article , Jim Lesurf wrote:
This is one of the areas where the audio magazines *could* help if they so
chose. They could adopt a suitable set of test measurement standards and
apply them. Then both publish the results, and 'name and shame' makers whos
published specs were twaddle.

Alas, the track record is that mags tend to prefer to publish 'subjective
auditions'... Pretty useless for prospective customers, and apply no
pressure on makers whatsoever... :-/

The mags also publish advertisements for the equipment they "review". I wonder
if this could have something to do with the tone of the editorial content?

Rod.