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calculating speaker wire current
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|
|
| A co-worker asked me a question tonight and I was unable to answer it.
He has a 1000 watt subwoofer and a 1000 watt amplifier. For purposes of
discussion, assume we are not applying maximum power to a maximum spec.
speaker.
His question: how to calculate which size wire to use. He assumes it's 1000
watts divided by 12 volts equalling 83 amps.
I guess this has been a variation of my curiosity. I've been curious what
calculations are used to spec an amplifier of being 1000 watts.
I understand a home speaker is 8 ohms. Using P= I^2(R) gives a current of
12 amps.
Is this a safe calculation for maximum current?????
| |
| charles 2008-01-25, 3:25 am |
| In article <Xns9A2FEDA8C1B2Fnobodynobodycom@216.196.97.136>,
Steve <nobody@nobody.com> wrote:
> A co-worker asked me a question tonight and I was unable to answer it.
> He has a 1000 watt subwoofer and a 1000 watt amplifier. For purposes of
> discussion, assume we are not applying maximum power to a maximum spec.
> speaker.
The question to ask you colleague is: what measurement system is used to
come up with 1000 watts. Many amplifiers use some 'interesting'
measurement system which seems to be based on "think of a number and
multiply it by another number" to get the wattage.
> His question: how to calculate which size wire to use. He assumes it's
> 1000 watts divided by 12 volts equalling 83 amps.
Why 12v? is it an in-car system? If it really was providing 1000w then
you'd need a 100A supply to the amplifier.
> I guess this has been a variation of my curiosity. I've been curious what
> calculations are used to spec an amplifier of being 1000 watts.
> I understand a home speaker is 8 ohms. Using P= I^2(R) gives a current of
> 12 amps.
Home speakers can be 15 ohms.
--
From KT24 - in "Leafy Surrey"
Using a RISC OS computer running v5.11
| |
| Palindrome 2008-01-25, 9:25 am |
| Steve wrote:
> A co-worker asked me a question tonight and I was unable to answer it.
>
>
> He has a 1000 watt subwoofer and a 1000 watt amplifier. For purposes of
> discussion, assume we are not applying maximum power to a maximum spec.
> speaker.
>
> His question: how to calculate which size wire to use. He assumes it's 1000
> watts divided by 12 volts equalling 83 amps.
>
> I guess this has been a variation of my curiosity. I've been curious what
> calculations are used to spec an amplifier of being 1000 watts.
>
>
> I understand a home speaker is 8 ohms. Using P= I^2(R) gives a current of
> 12 amps.
>
>
>
> Is this a safe calculation for maximum current?????
>
You really ought to be asking on a hifi newsgroup - when it comes to
things like speaker wire, decisions are rarely made on mere engineering
prinicples and logic.
However, let's look at some of those boring principles..
If the power output really is 1000W rms, which it mostly isn't and the
speaker impedance at that frequency really is 8 ohms, which it won't be,
then the current would be about 12A.
However, not only does P=I^2 x R, .....P=V^2 /R. Hence V will not be
12v, but nearer 90.
Which all ties together, because P = I x V.
Where does that 90v come from in a car? Simple, one of the biggest and
most expensive part of a car high power amplifier is nothing to do with
the audio - it is the converter that steps the 12v supply up to a
suitable higher voltage at high power. Useful to know, poking around in
a running "12v" amplifier could easily connect you to a high enough
voltage with enough power to kill.
Now, when it comes to sizing the speaker wires, audio purists wouldn't
dream of using cable designed to handle *mains* at 12A, for that
distance. An electrical engineer might, because it would be perfectly
safe and putting in anything better would not be cost effective.
However, the audio guy would point out that *there* would be power lost
in the wire. That audio signals cover a far higher frequency range and
so it isn't merely a problem of dc resistance. That impurities in the
copper can produce both a non-linear response and non-linear current
distibution..
So, rather than use 12A mains cable, he will use something akin to 200A
welding cable composed of thousands of strands of extremely high purity
copper wire, individually silver coated and Litz woven...
--
Sue
| |
| Stuart 2008-01-25, 9:25 am |
| In article <X6imj.87940$WT3.66500@fe07.news.easynews.com>,
Palindrome <me9@privacy.net> wrote:
> However, the audio guy would point out that *there* would be power lost
> in the wire. That audio signals cover a far higher frequency range and
> so it isn't merely a problem of dc resistance. That impurities in the
> copper can produce both a non-linear response and non-linear current
> distibution..
I remember reading an article in "Wireless World", now known as
"Electronics world", by no less an authority than Douglas Self, about why
he uses "Woolworths" mains cable to wire up his loudspeakers! [1]
He is the designer of a number of high performance amplifiers, author of
any number of papers and articles on audio amplifier design and his
website can be found at:
http://www.dself.dsl.pipex.com/ampins/ampins.htm
Stuart
[1] Electronics World Oct 1997 p831
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Dave Martindale 2008-01-25, 8:25 pm |
| me@palindr0me.plus.com writes:
>Now, when it comes to sizing the speaker wires, audio purists wouldn't
>dream of using cable designed to handle *mains* at 12A, for that
>distance. An electrical engineer might, because it would be perfectly
>safe and putting in anything better would not be cost effective.
>However, the audio guy would point out that *there* would be power lost
>in the wire. That audio signals cover a far higher frequency range and
>so it isn't merely a problem of dc resistance. That impurities in the
>copper can produce both a non-linear response and non-linear current
>distibution..
It's not quite *that* silly to use larger wire. The amplifier has quite
a lot of feedback, and acts as a (controlled) voltage source with very
low impedance. If the damping factor is 100 with an 8 ohm load, the amp
output impedance is 0.08 ohm, and that helps to control the cone in the
speaker (which is not a resistive load, but closer to a permanent magnet
DC motor). Any speaker cable resistance is in series with the amplifier
output impedance, and degrades the damping correspondingly.
How much resistance you can tolerate is a matter of argument.
For that matter, someone wiring 12 V auto electrics will use a larger
diameter wire to carry 12 A than you'd use for 12 A at household line
voltages. A 5 V drop in the wires is not a big deal when the supply
voltage is 120 V or 240 V, but it's a big deal when the supply is only
12 V. So even if you're only talking about DC, and only resistive
loads, you use heavier wire for the same current at lower voltage.
Except for monstrously large amplifiers, speaker circuits are lower
voltage than line voltage.
Dave
(who, I must admit, uses 16 or 18 gauge speaker wires)
| |
| Paul Hovnanian P.E. 2008-01-26, 3:25 am |
| Palindrome wrote:
>
[snip]
> Where does that 90v come from in a car? Simple, one of the biggest and
> most expensive part of a car high power amplifier is nothing to do with
> the audio - it is the converter that steps the 12v supply up to a
> suitable higher voltage at high power. Useful to know, poking around in
> a running "12v" amplifier could easily connect you to a high enough
> voltage with enough power to kill.
From what I've seen of some of the people outfitting their cars this
way, it doesn't seem to kill often enough.
:-/
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Great minds discuss ideas,
average minds discuss events,
small minds discuss people.
| |
|
| That's not right & I'm surprised no one has brought up the fact that a
1000 watts audio signal is very different from 1000 watts of electrical
power.
Audio signal to Speakers are usually in milliamps and very low voltages.
Roy Q.T.
Urban Technician
[I don't make em, I just fix em]
| |
|
| Forgot to mention - Lower than 12VDC
Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]
| |
|
| On Sat, 26 Jan 2008 04:01:42 -0500, ROYKEY@webtv.net (Roy) wrote:
>That's not right & I'm surprised no one has brought up the fact that a
>1000 watts audio signal is very different from 1000 watts of electrical
>power.
>
>Audio signal to Speakers are usually in milliamps and very low voltages.
>
>Roy Q.T.
>Urban Technician
>[I don't make em, I just fix em]
Hello Roy,
Audio signals differ from "electrical power" only in the sense that
they involve different frequencies. But all the laws governing
electric circuts apply fully.
Chuck
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| |
| Palindrome 2008-01-26, 9:25 am |
| Roy wrote:
> Forgot to mention - Lower than 12VDC
>
Now here is a little thing to try:
Take a 1000W rms power amplifier designed for an 8 ohm load, connect it
to a dummy load (to spare the neighbour's ears).
Now stick a sine wave signal generator, set at 50Hz (OK, 60Hz for you)
in as the signal source and wind up its output voltage until the amp is
running at full power (typically 1v rms in).
Now look at the voltage across the load with a scope or even a
multimeter.. Does it look very similar to the mains supply? I wonder why...
If you think that they are wrong and that there is less than 12vAC
(speakers really don't like DC) and only a few mA present, feel how hot
the load gets..
If you still don't believe it, grab hold of the two terminals of the
load, one in each hand. One way or another, after that, you won't be
saying its just a few volts and a few mA.
--
Sue
| |
| gfretwell@aol.com 2008-01-26, 1:25 pm |
| On Sat, 26 Jan 2008 12:56:35 GMT, Palindrome <me9@privacy.net> wrote:
>Roy wrote:
>Now here is a little thing to try:
>
>Take a 1000W rms power amplifier designed for an 8 ohm load, connect it
>to a dummy load (to spare the neighbour's ears).
>
>Now stick a sine wave signal generator, set at 50Hz (OK, 60Hz for you)
>in as the signal source and wind up its output voltage until the amp is
>running at full power (typically 1v rms in).
>
>Now look at the voltage across the load with a scope or even a
>multimeter.. Does it look very similar to the mains supply? I wonder why...
>
>If you think that they are wrong and that there is less than 12vAC
>(speakers really don't like DC) and only a few mA present, feel how hot
>the load gets..
>
>If you still don't believe it, grab hold of the two terminals of the
>load, one in each hand. One way or another, after that, you won't be
>saying its just a few volts and a few mA.
You would then discover how much deception there is in the ratings of
amps.
| |
| phil-news-nospam@ipal.net 2008-01-26, 1:25 pm |
| On Sat, 26 Jan 2008 04:01:42 -0500 Roy <ROYKEY@webtv.net> wrote:
| That's not right & I'm surprised no one has brought up the fact that a
| 1000 watts audio signal is very different from 1000 watts of electrical
| power.
|
| Audio signal to Speakers are usually in milliamps and very low voltages.
At 1000 watts, and mere milliamps of current, that would take quite a LOT
of "low voltage" to do the job ... at least when the sound is cranked.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-01-26-1044@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-01-26, 1:25 pm |
| On Sat, 26 Jan 2008 12:56:35 GMT Palindrome <me9@privacy.net> wrote:
| Take a 1000W rms power amplifier designed for an 8 ohm load, connect it
| to a dummy load (to spare the neighbour's ears).
|
| Now stick a sine wave signal generator, set at 50Hz (OK, 60Hz for you)
| in as the signal source and wind up its output voltage until the amp is
| running at full power (typically 1v rms in).
|
| Now look at the voltage across the load with a scope or even a
| multimeter.. Does it look very similar to the mains supply? I wonder why...
|
| If you think that they are wrong and that there is less than 12vAC
| (speakers really don't like DC) and only a few mA present, feel how hot
| the load gets..
|
| If you still don't believe it, grab hold of the two terminals of the
| load, one in each hand. One way or another, after that, you won't be
| saying its just a few volts and a few mA.
Or get a big monster 7200-watt 8 ohm speaker and just connect it to the
240 volt mains. Or 4 1800-watt ones in a half-series half-parallel setup
would also do. Or 9 800-watt ones. Or 16 450-watt ones. Or 25 288-watt
ones. Or 36 200-watt ones. Or ... :-)
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-01-26-1047@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Palindrome 2008-01-26, 1:25 pm |
| gfretwell@aol.com wrote:
> On Sat, 26 Jan 2008 12:56:35 GMT, Palindrome <me9@privacy.net> wrote:
>
>
> You would then discover how much deception there is in the ratings of
> amps.
Which is why I added "rms" to the 1000W.. As you say, manufacturers do
tend to be a little devious when it comes to describing the output of
their products.
There are, however, reputable manufacturers who do deliver what they
appear to be promising..
--
Sue
| |
| Stuart 2008-01-26, 1:25 pm |
| In article <ntmmp31m3b923na5ege7306t50i81qpkn8@4ax.com>,
<gfretwell@aol.com> wrote:
> You would then discover how much deception there is in the ratings of
> amps.
But Sue was very exact in her specification - 1000W /RMS/
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Michael A. Terrell 2008-01-26, 1:25 pm |
| Roy wrote:
>
> That's not right & I'm surprised no one has brought up the fact that a
> 1000 watts audio signal is very different from 1000 watts of electrical
> power.
>
> Audio signal to Speakers are usually in milliamps and very low voltages.
Only in '60 pocket transistor radios. Ohm's law still applies,
whether you can grasp the concept. Also, high power speakers are
usually lower than 8 ohms, due to the heavier copper needed to make the
voice coil for those stupid high power car stereos.
For instance, a 4 ohm speaker at 576 watts has 12 amps flowing
through the voice coil, and 48 VAC across it. It's enough to kill you.
On the other end of speaker system spectrum there are constant
voltage sound systems with 25, 70, or 100 VAC distribution, to reduce
copper losses. Either of the 70 or 100 volt systems can kill you.
> Roy Q.T.
> [I don't make em, I just fix em]
Apparently you can't even fix them right. Don't give advice, when
you obviously don't know shit about the subject. I'm sure you'll claim
the NEC backs you up, as usual? Do they know that you have to use
WebTV, instead of a computer?
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
| |
| VWWall 2008-01-26, 1:25 pm |
| Palindrome wrote:
> Roy wrote:
> Now here is a little thing to try:
>
> Take a 1000W rms power amplifier designed for an 8 ohm load, connect it
> to a dummy load (to spare the neighbour's ears).
>
> Now stick a sine wave signal generator, set at 50Hz (OK, 60Hz for you)
> in as the signal source and wind up its output voltage until the amp is
> running at full power (typically 1v rms in).
You'd best use a dummy load! 50Hz or 60Hz is very close to the resonance
of many speakers. In a couple of seconds the cone would be in tatters,
if the voice coil didn't burn out first.
Fortunately, 1000W loudspeakers are as fictional as 1000W (RMS)
amplifiers, not that it would be impossible to make one. In practice,
one would use many speakers and probably distribution at a much higher
impedance than 8 ohms.
In any case Roy would need his insulated shank screwdriver!
--
Virg Wall, P.E.
| |
| Palindrome 2008-01-26, 5:25 pm |
| VWWall wrote:
> Palindrome wrote:
>
> You'd best use a dummy load! 50Hz or 60Hz is very close to the resonance
> of many speakers. In a couple of seconds the cone would be in tatters,
> if the voice coil didn't burn out first.
As in, "connect it to a dummy load", perhaps ? ;)
>
> Fortunately, 1000W loudspeakers are as fictional as 1000W (RMS)
> amplifiers, not that it would be impossible to make one.
Not entirely fictional...TL-1023 + TL-1121...
> In practice,
> one would use many speakers and probably distribution at a much higher
> impedance than 8 ohms.
IIUC, at that level of power in a single speaker, hollow-conductor voice
coils are used and chilled water is pumped through them...
>
> In any case Roy would need his insulated shank screwdriver!
>
I thought old and bold electricians just stood on one leg when working
"live"... the wooden one.. ;)
--
Sue
| |
|
| I never said that Ohms Law didn't apply.
The way I see it., the output voltage & currents will remain low &
safe., now the power consumed by the 1000W amplifier will go up as you
crank the volume...and voltage never killed anyone, it's the current
that kills., though I don't see this system killing anyone., so no need
to use an insulated shaft screwdriver for this as suggested.
Note that I've been fixing stereo amplifiers and such since I was
19yo.....& though I've never worked on a 1000W output system, the
principles are the same.
I'm just guessing here, but I still say the amplifiers current output
will be safe with some monster cable...
Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]
| |
| Palindrome 2008-01-26, 5:25 pm |
| Roy wrote:
> I never said that Ohms Law didn't apply.
>
> The way I see it., the output voltage & currents will remain low &
> safe., now the power consumed by the 1000W amplifier will go up as you
> crank the volume...and voltage never killed anyone, it's the current
> that kills., though I don't see this system killing anyone., so no need
> to use an insulated shaft screwdriver for this as suggested.
>
> Note that I've been fixing stereo amplifiers and such since I was
> 19yo.....& though I've never worked on a 1000W output system, the
> principles are the same.
>
> I'm just guessing here, but I still say the amplifiers current output
> will be safe with some monster cable...
>
Roy, what I suggest that you have done is only work on powered up
amplifiers with speakers connected, instead of dummy loads. A couple of
hundred mW is loud enough in a workshop to confirm that the amplifer is
working and you have never "cranked up the volume" whilst actually doing
the repairs or making measurements.
Things become very different when you connect up dummy loads - no one
could work in a workshop with 1000W of audio coming at them from
speakers a foot or so from their ears.. But you can work with a 1kW
dummy load glowing quietly to itself...
Add a suitable input signal and output transformer to give the required
voltage and a 1kW audio amplifier works extremely well as a 1kW mains
supply. Turn the volume right down an, yes, it is pretty tame..
--
Sue
| |
|
|
Re: calculating speaker wire current
Group: alt.engineering.electrical Date: Sat, Jan 26, 2008, 7:28pm
(EST+5) From: me9@privacy.net (Palindrome)
VWWall wrote:
Palindrome wrote:
Roy wrote:
Forgot to mention - Lower than 12VDC
Now here is a little thing to try:
Take a 1000W rms power amplifier designed for an 8 ohm load, connect it
to a dummy load (to spare the neighbour's ears).
Now stick a sine wave signal generator, set at 50Hz (OK, 60Hz for you)
in as the signal source and wind up its output voltage until the amp is
running at full power (typically 1v rms in).
You'd best use a dummy load! 50Hz or 60Hz is very close to the resonance
of many speakers. In a couple of seconds the cone would be in tatters,
if the voice coil didn't burn out first.
As in, "connect it to a dummy load", perhaps ? ;)
Fortunately, 1000W loudspeakers are as fictional as 1000W (RMS)
amplifiers, not that it would be impossible to make one.
Not entirely fictional...TL-1023 + TL-1121...
In practice,
one would use many speakers and probably distribution at a much higher
impedance than 8 ohms.
IIUC, at that level of power in a single speaker, hollow-conductor voice
coils are used and chilled water is pumped through them...
In any case Roy would need his insulated shank screwdriver!
I thought old and bold electricians just stood on one leg when working
"live"... the wooden one.. ;)
--
Sue
Arg ! Hahaha good one Sue., but No, no wooden leg here., I was taught to
put a few fingers in my pocket & lightly press my crotch.....this leaves
only one extremity out there in the hazardous zone};)
Good heads-up on the water chilled sound equipment.
I may sound a little cocky at times, but I am aware that there are
things out there I have absolutely no knowledge about.
Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]
| |
|
| On Thu, 24 Jan 2008 22:21:43 -0600, Steve <nobody@nobody.com> wrote:
>A co-worker asked me a question tonight and I was unable to answer it.
>
>
>He has a 1000 watt subwoofer and a 1000 watt amplifier. For purposes of
>discussion, assume we are not applying maximum power to a maximum spec.
>speaker.
>
>His question: how to calculate which size wire to use. He assumes it's 1000
>watts divided by 12 volts equalling 83 amps.
>
>I guess this has been a variation of my curiosity. I've been curious what
>calculations are used to spec an amplifier of being 1000 watts.
>
>
>I understand a home speaker is 8 ohms. Using P= I^2(R) gives a current of
>12 amps.
>
>
>
>Is this a safe calculation for maximum current?????
Remember in all of this that RMS watts lacks a specific technical
meaning.
http://www.hifi-writer.com/he/misc/rmspower.htm
Chuck
----== Posted via droptable.com - Unlimited-Unrestricted-Secure Usenet News==----
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----= East and West-Coast Server Farms - Total Privacy via Encryption =----
| |
|
| In article <17069-479AFBBA-307@storefull-3256.bay.webtv.net>,
ROYKEY@webtv.net says...
> Forgot to mention - Lower than 12VDC
>
> Roy Q.T. ~ US/NCU ~ E.E. Technician
> [have tools, will travel]
>
Oh, wow! Roy, go back to sleep. You're dangerous when you're
awake.
--
Keith
| |
| Don Kelly 2008-01-26, 5:25 pm |
| ----------------------------
"Roy" <ROYKEY@webtv.net> wrote in message
news:1710-479B88F3-101@storefull-3252.bay.webtv.net...
>I never said that Ohms Law didn't apply.
>
> The way I see it., the output voltage & currents will remain low &
> safe., now the power consumed by the 1000W amplifier will go up as you
> crank the volume...and voltage never killed anyone, it's the current
> that kills., though I don't see this system killing anyone., so no need
> to use an insulated shaft screwdriver for this as suggested.
>
> Note that I've been fixing stereo amplifiers and such since I was
> 19yo.....& though I've never worked on a 1000W output system, the
> principles are the same.
>
> I'm just guessing here, but I still say the amplifiers current output
> will be safe with some monster cable...
>
> Roy Q.T. ~ US/NCU ~ E.E. Technician
> [have tools, will travel]
>
Actually, in a speaker as in any "motor", Ohms Law doesn't apply because
there is a back emf produced and Ohms Law applies only to linear (resistive
in the true definition) elements. The 8 ohms is a ficticious value based on
V/I (rms) at some frequency, including the effects of the back emf and the
acoustical load at that frequency. Convenient and good enough for most
cases.
Let's see, 8 ohms, 1000 watts, 11A, at 90V (whether working with rms or
peak values) This won't be rms because of the way that speakers are rated
to get big numbers for sales purposes- just like 3HP 120V, 12A lawnmowers.
Hardly mA
As for current killing- true but your statement implies something that is
untrue. In this case the 11A isn't a problem (unless the wire can't handle
it ) but the 90V is a problem because of the current it can drive through
your body- "normally" not lethal but quite annoying.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
|
| In article <4f67501f2cSW_NOSPAM@dsl.pipex.com>,
SW_NOSPAM@dsl.pipex.com says...
> In article <ntmmp31m3b923na5ege7306t50i81qpkn8@4ax.com>,
> <gfretwell@aol.com> wrote:
>
> But Sue was very exact in her specification - 1000W /RMS/
Which begs the question, what exactly is an RMS watt? RMS is used
for voltage and current, which is squared to get power (hence the
'S' in RMS). Why would you square power?
--
Keith
| |
| Palindrome 2008-01-26, 5:25 pm |
| Don Kelly wrote:
> ----------------------------
> "Roy" <ROYKEY@webtv.net> wrote in message
> news:1710-479B88F3-101@storefull-3252.bay.webtv.net...
> Actually, in a speaker as in any "motor", Ohms Law doesn't apply because
> there is a back emf produced and Ohms Law applies only to linear (resistive
> in the true definition) elements. The 8 ohms is a ficticious value based on
> V/I (rms) at some frequency, including the effects of the back emf and the
> acoustical load at that frequency. Convenient and good enough for most
> cases.
IIUC, ratings are normally given for a purely resistive load and are
derived from the rms voltage across the load..Hence rms power. Where
many manufacturers cheat is to hide how long that rms power can be
sustained. For some, it is the 5 second rating..for others, the
continuous rating..
>
> Let's see, 8 ohms, 1000 watts, 11A, at 90V (whether working with rms or
> peak values) This won't be rms because of the way that speakers are rated
> to get big numbers for sales purposes- just like 3HP 120V, 12A lawnmowers.
> Hardly mA
> As for current killing- true but your statement implies something that is
> untrue. In this case the 11A isn't a problem (unless the wire can't handle
> it ) but the 90V is a problem because of the current it can drive through
> your body- "normally" not lethal but quite annoying.
IIUC, it is neither volts nor amps that determine survival, but joules.
A current of many amps through the body is not lethal unless it is
sustained for long enough to transfer the required amount of joules to
kill. A several thousand amp pulse is survivable, if of short enough
duration.
No, I'm not volunteering to try!..
--
Sue
| |
| Palindrome 2008-01-26, 5:25 pm |
| krw wrote:
> In article <4f67501f2cSW_NOSPAM@dsl.pipex.com>,
> SW_NOSPAM@dsl.pipex.com says...
>
> Which begs the question, what exactly is an RMS watt? RMS is used
> for voltage and current, which is squared to get power (hence the
> 'S' in RMS). Why would you square power?
>
The usual measurement is of rms voltage across a purely resistive load
... from which "rms power" is derived. It is just shorthand for a
standard measurement.
--
Sue
| |
|
| In article <38Omj.39166$ee6.7502@fe10.news.easynews.com>, me9
@privacy.net says...
> krw wrote:
> The usual measurement is of rms voltage across a purely resistive load
> .. from which "rms power" is derived. It is just shorthand for a
> standard measurement.
>
RMS volts, squared, times the resistance gives watts, or average
watts, if you must. We are supposed to be professionals here.
--
Keith
| |
| Palindrome 2008-01-26, 8:25 pm |
| krw wrote:
> In article <38Omj.39166$ee6.7502@fe10.news.easynews.com>, me9
> @privacy.net says...
> RMS volts, squared, times the resistance gives watts, or average
> watts, if you must. We are supposed to be professionals here.
>
ITYM rms volts, squared, *divided* by resistance..
When determining the power output of an amplifier into a purely
resistive load, the usual *measurement* to take is the rms voltage and
then derive the "rms power" from that, ie power = v^2/r. Rather than
state "power derived from the rms voltage and load resistance" - the
term "rms power" is commonly used.
AFAIK, this newsgroup is open to anyone interested in electrical
engineering. Even if they don't know Ohm's Law..
--
Sue
| |
|
| In article <65Qmj.130803$XB2.8862@fe02.news.easynews.com>, me9
@privacy.net says...
> krw wrote:
> ITYM rms volts, squared, *divided* by resistance..
Yeah, well we are supposed to be profess... <blush>
> When determining the power output of an amplifier into a purely
> resistive load, the usual *measurement* to take is the rms voltage and
> then derive the "rms power" from that, ie power = v^2/r. Rather than
> state "power derived from the rms voltage and load resistance" - the
> term "rms power" is commonly used.
A *lot* of terms are often used in the audiophool world too but that
doesn't mean they're correct.
> AFAIK, this newsgroup is open to anyone interested in electrical
> engineering. Even if they don't know Ohm's Law..
Did anyone say differently?
--
Keith
| |
| gfretwell@aol.com 2008-01-26, 8:25 pm |
| On Sat, 26 Jan 2008 16:53:25 GMT, Palindrome <me9@privacy.net> wrote:
>
>Which is why I added "rms" to the 1000W.. As you say, manufacturers do
>tend to be a little devious when it comes to describing the output of
>their products.
>
>There are, however, reputable manufacturers who do deliver what they
>appear to be promising..
>
>--
>Sue
In the stereo biz RMS means Relatively Meaningless Specification.
| |
| Fist Fuck 2008-01-26, 8:25 pm |
|
"Palindrome" <me9@privacy.net> wrote in message
news:41Omj.176797$%Q1.172069@fe05.news.easynews.com...
> Don Kelly wrote:
>
> IIUC, ratings are normally given for a purely resistive load and are
> derived from the rms voltage across the load..Hence rms power. Where many
> manufacturers cheat is to hide how long that rms power can be sustained.
> For some, it is the 5 second rating..for others, the continuous rating..
>
>
> IIUC, it is neither volts nor amps that determine survival, but joules.
>
> A current of many amps through the body is not lethal unless it is
> sustained for long enough to transfer the required amount of joules to
> kill. A several thousand amp pulse is survivable, if of short enough
> duration.
>
> No, I'm not volunteering to try!..
>
> --
> Sue
>
She's right
| |
| Fist Fuck 2008-01-26, 8:25 pm |
|
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:479B6E09.EE09BF6F@earthlink.net...
> Roy wrote:
>
>
> Only in '60 pocket transistor radios. Ohm's law still applies,
> whether you can grasp the concept. Also, high power speakers are
> usually lower than 8 ohms, due to the heavier copper needed to make the
> voice coil for those stupid high power car stereos.
>
> For instance, a 4 ohm speaker at 576 watts has 12 amps flowing
> through the voice coil, and 48 VAC across it. It's enough to kill you.
>
> On the other end of speaker system spectrum there are constant
> voltage sound systems with 25, 70, or 100 VAC distribution, to reduce
> copper losses. Either of the 70 or 100 volt systems can kill you.
>
>
>
>
> Apparently you can't even fix them right. Don't give advice, when
> you obviously don't know shit about the subject. I'm sure you'll claim
> the NEC backs you up, as usual? Do they know that you have to use
> WebTV, instead of a computer?
>
>
> --
> Service to my country? Been there, Done that, and I've got my DD214 to
> prove it.
> Member of DAV #85.
>
> Michael A. Terrell
> Central Florida
| |
|
| All I know for sure is that Don & Keith should go jump in a lake - use a
regular screwdriver or a pick to break through the ice - otherwise
what's the point to all of this anyway } no one has mentioned what
type of wire to use the OP should consider...
Ohms Law Rules - Don't confine the response to speakers - I am thinking
what happens inside the amplifier - yet that doesn't seem to IMPEDE you
two from trying to discredit everything said.
And Frankly My Dears
"I Don't Give a Damn"
wake me up before you go.......
Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]
| |
| Don Kelly 2008-01-27, 3:25 am |
| ----------------------------
"Palindrome" <me9@privacy.net> wrote in message
news:41Omj.176797$%Q1.172069@fe05.news.easynews.com...
> Don Kelly wrote:
>
> IIUC, ratings are normally given for a purely resistive load and are
> derived from the rms voltage across the load..Hence rms power. Where many
> manufacturers cheat is to hide how long that rms power can be sustained.
> For some, it is the 5 second rating..for others, the continuous rating..
>
>
> IIUC, it is neither volts nor amps that determine survival, but joules.
>
> A current of many amps through the body is not lethal unless it is
> sustained for long enough to transfer the required amount of joules to
> kill. A several thousand amp pulse is survivable, if of short enough
> duration.
>
> No, I'm not volunteering to try!..
>
> --
> Sue
When considering impulses, definitely Joules.
When considering continuous currents, look at mA -It doesn't take much in
terms of energy to cause fibrillation and the survival rate for a 1A current
through the body is higher than that of a 200ma current for the same time
because the heart is usually stopped and muscular contractions often throw
the victim clear and the heart restarts naturally. Note that considerable
thermal damage can occur at the contact points- this is definitely related
to local Joule heating but internal damage to organs might be negligable. In
the case of fibrillation at lower currents and energies, outside
intervention is needed. Also while there is an i*t relationship in the
onset of fibrillation, it is not an i^2*t relationship but I*root(t) =165
is commonly used (I in mA and t in seconds) This data is in several places
including EPRI's EHV book. Note that data for noticable, painful and lethal
situations is given in terms of probabilities depending on body size and
current and much safety design is based on this.
Agreed, shock effects (including death) are due to Joules in that a certain
level of energy must exist at a neuron to cause it to trigger
uncontrollably. However this comes down to the current at that point and
this can't be measured so, for practical purposed, external current rather
than external energy is the best measure we have.
This is effectively decoupled from the external energy. eg, with total body
resistance of 5Kohms, vs that at 50Kohms one cannot say that the current
required for electrocution is higher in the latter case-the voltage and
external energy is higher.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
>
| |
| Don Kelly 2008-01-27, 3:25 am |
| ----------------------------
"Roy" <ROYKEY@webtv.net> wrote in message
news:29563-479BEC38-642@storefull-3254.bay.webtv.net...
> All I know for sure is that Don & Keith should go jump in a lake - use a
> regular screwdriver or a pick to break through the ice - otherwise
> what's the point to all of this anyway } no one has mentioned what
> type of wire to use the OP should consider...
>
> Ohms Law Rules - Don't confine the response to speakers - I am thinking
> what happens inside the amplifier - yet that doesn't seem to IMPEDE you
> two from trying to discredit everything said.
>
> And Frankly My Dears
> "I Don't Give a Damn"
>
> wake me up before you go.......
>
> Roy Q.T. ~ US/NCU ~ E.E. Technician
> [have tools, will travel]
>
------------
Not trying to discredit everything that you say- you get credit for the
sensible things that you say but not for the nonsense. By the way, an
amplifier as a source behind impedance (along with feedback loops etc)
doesn't obey Ohm's Law.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
| Don Kelly 2008-01-27, 3:25 am |
| ----------------------------
"Palindrome" <me9@privacy.net> wrote in message
news:38Omj.39166$ee6.7502@fe10.news.easynews.com...
> krw wrote:
> The usual measurement is of rms voltage across a purely resistive load ..
> from which "rms power" is derived. It is just shorthand for a standard
> measurement.
>
> --
> Sue
>
--------------
Rms voltages and currents give average power. That's why we use rms values
The only place that I have seen "rms power" used meaningfully is for motors
with cyclic loads so the rms power is the average of the root of the sums
of the P*t products in each load interval. Useful in choosing motor sizes
for hoisting or other cyclic duties.--
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
|
| On Jan 25, 7:52 pm, da...@cs.ubc.ca (Dave Martindale) wrote:
> It's not quite *that* silly to use larger wire. The amplifier has quite
> a lot of feedback, and acts as a (controlled) voltage source with very
> low impedance. If the damping factor is 100 with an 8 ohm load, the amp
> output impedance is 0.08 ohm, and that helps to control the cone in the
> speaker (which is not a resistive load, but closer to a permanent magnet
> DC motor). Any speaker cable resistance is in series with the amplifier
> output impedance, and degrades the damping correspondingly.
Absolutely. They are attempting to calculate speaker wire as if it
were a power cord. But as you point out there is much more to it than
that. While there is also a sort of religion about speaker wires
amongst the audiophools, generally speaking the rule is that you
should use as heavy a conductor as you can practically use/afford/
install etc.
> How much resistance you can tolerate is a matter of argument.
Yes it is. Some people argue like mad that you can't hear any
difference. But they are Phools. Virtually ANYONE can hear the
difference between super heavy braided speaker wire and say 22 gauge
bell wire found on some cheapo players. Now exactly how much copper
you need until you can no longer hear any difference is a matter of
argument and does depend on how long the wire runs are. Sure, you
COULD "wire" your speakers with 1" copper tubing and they'd sound
great, but it makes it hard to move the speakers around! And on the
other hand wiring expensive components with the ever-popular lamp cord
is sure to make your system sound like you spent a lot less money than
you really did!
Bottom line: Just get as big a wire as you can practically use. Be it
heavy duty power too extension cord, special speaker wire (I've got
some that is rubber covered flat braid...VERY nice and was cheap too).
or anything else you pick up cheap. Don't believe all the crap about
oxygen free, virtually all electrical wire is oxygen free so the wire
remains flexible and won't work-harden.
> Dave
> (who, I must admit, uses 16 or 18 gauge speaker wires)
Usually for shorter runs 16 gauge is getting pretty close to where you
can't hear much of a difference. For longer runs on high-end gear, I
would consider heavier wire.
| |
|
| Benj that is the best reply anyone has given here - Daves wasn't bad
either - you think the op really wants to jot down numbers & work out
formulas? perhaps - but Not a chance - The way the discussion has been
going they would have him wiring up this alleged audio system with high
tension cables since those other phools just want to win some kind of
contest.
You noticed how they didn't even mention rms values (part of the
pertinent formula) until I mentioned the difference between electrical &
audio power - it may not be much, but despite all the arguements I am
not about to energize anything but my speakers with my stereos output
}
Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]
| |
|
| On Sun, 27 Jan 2008 04:17:00 GMT, "Don Kelly" <dhky@shaw.ca> wrote:
> ----------------------------
>"Palindrome" <me9@privacy.net> wrote in message
>news:38Omj.39166$ee6.7502@fe10.news.easynews.com...
>--------------
>Rms voltages and currents give average power. That's why we use rms values
>
>The only place that I have seen "rms power" used meaningfully is for motors
>with cyclic loads so the rms power is the average of the root of the sums
>of the P*t products in each load interval. Useful in choosing motor sizes
>for hoisting or other cyclic duties.--
>
>Don Kelly dhky@shawcross.ca
>remove the X to answer
>
In characterizing power, one can use either instantaneous values which
can provide peak or continuous values, or some time-averaged values.
Neither of these can be characterized as rms power.
RMS power can be computed for any power envelope, but there is no
physical meaning associated with the the computed value.
Don's example suggests the root of a mean value, but nowhere is power
actually squared. Therefore it does not illustrate the use of rms
power.
FWIW, P*t gives units of watt-seconds, or energy. I can imagine
summing energy in a load interval, but taking the root of energy
doesn't seem to have meaning.
Chuck
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| |
| Stuart 2008-01-27, 9:25 am |
| In article <MPG.220595b14e596ff498969d@news.individual.net>,
krw <kkk@kkk.kkk> wrote:
> RMS volts, squared, times the resistance gives watts, or average
> watts, if you must. We are supposed to be professionals here.
Sorry, I assume it was just a slip but power is the square of the voltage
/divided/ by the resistance.
The square of the /current/ multiplied by the resistance also gives power
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Stuart 2008-01-27, 9:25 am |
| In article <MPG.2205764852a913e6989698@news.individual.net>,
krw <kkk@kkk.kkk> wrote:
> Which begs the question, what exactly is an RMS watt? RMS is used
> for voltage and current, which is squared to get power (hence the
> 'S' in RMS). Why would you square power?
It is a way of equivalenceing AC and DC power.
For example. 12V DC into a 12 ohm load will produce a heating power of 12W.
An AC voltage which has a "root mean square" value of 12V will also
produce the same heating power. It is not the same as "average" voltage.
An amplifier which is rated at 1000W should be capable of providing 1000W
of heating into a resistive load equal to its specified load resistance
whether it be 2, 4. 8, 15 ohms or whatever.
The amplifiers I have in my Hi-Fi system are rated to 75W rms into 8 ohm
and they will certainly produce that power because I had them running into
dummy loads whilst testing them following construction.
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| phil-news-nospam@ipal.net 2008-01-27, 9:25 am |
| On Sun, 27 Jan 2008 00:37:34 -0800 (PST) Benj <bjacoby@iwaynet.net> wrote:
| Absolutely. They are attempting to calculate speaker wire as if it
| were a power cord. But as you point out there is much more to it than
| that. While there is also a sort of religion about speaker wires
| amongst the audiophools, generally speaking the rule is that you
| should use as heavy a conductor as you can practically use/afford/
| install etc.
The big issue as I understand it is that an "8 ohm" speak is not really
8 ohms across the whole range of audio frequencies. Things are supposed
to be designed so that a consistent voltage at any frequency gives a
consistent level of sound. Maybe the high end speakers actually achieve
that. The problem is, adding a resistance in the line will reduce those
frequencies that the speaker has a lower impedance for more so than the
the ones it has a higher impedance for. So, the less resistance in the
wire, the better ... but only worth it for the high end speakers that do
achieve a more accurate sound ... so you don't ruin that sound.
Maybe they should have used a higher standard speaker impedance :-)
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-01-27-0836@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2008-01-27, 9:25 am |
| On Sun, 27 Jan 2008 13:24:45 +0000 (GMT) Stuart <SW_NOSPAM@dsl.pipex.com> wrote:
| The amplifiers I have in my Hi-Fi system are rated to 75W rms into 8 ohm
| and they will certainly produce that power because I had them running into
| dummy loads whilst testing them following construction.
That would not be a realistic stress test. "8 ohm" speakers are not
really pure 8 ohms resistive at all frequencies. I've heard of speakers
that vary well above and to some degree below that 8 ohm level, and have
not pure resistance. Can your amplifier, cranked to deliver the full
rated power, handle the variant loads given to it by different speakers?
The dummy load would have been easy for it.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-01-27-0847@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| On 27 Jan 2008 14:43:42 GMT, phil-news-nospam@ipal.net wrote:
>On Sun, 27 Jan 2008 00:37:34 -0800 (PST) Benj <bjacoby@iwaynet.net> wrote:
>
>| Absolutely. They are attempting to calculate speaker wire as if it
>| were a power cord. But as you point out there is much more to it than
>| that. While there is also a sort of religion about speaker wires
>| amongst the audiophools, generally speaking the rule is that you
>| should use as heavy a conductor as you can practically use/afford/
>| install etc.
>
>The big issue as I understand it is that an "8 ohm" speak is not really
>8 ohms across the whole range of audio frequencies. Things are supposed
>to be designed so that a consistent voltage at any frequency gives a
>consistent level of sound. Maybe the high end speakers actually achieve
>that. The problem is, adding a resistance in the line will reduce those
>frequencies that the speaker has a lower impedance for more so than the
>the ones it has a higher impedance for. So, the less resistance in the
>wire, the better ... but only worth it for the high end speakers that do
>achieve a more accurate sound ... so you don't ruin that sound.
>
>Maybe they should have used a higher standard speaker impedance :-)
A speaker is generally a pure resistance at its free-air resonance
frequency. Below that, it is inductive, above it is capacitive. The
impedance at resonance is generally quite high relative to its nominal
impedance and enclosures, etc. are often designed to lower the
resonant frequency so as to extend the speaker's useful lower range.
Above resonance, the impedance tends to be fairly uniform out to some
higher frequency, where it again rises and may even develop another
resonance.
Applying Phil's analysis, adding a series resistance then will have
less effect on the lower and higher frequencies (where the impedance
is above the nominal impedance) and relatively greater effect on the
wide band of frequencies in mid-range at which the speaker is rated.
Still, it is difficult to see how a small fraction of an ohm can
significantly affect the loudness of an 8 ohm speaker, even in
mid-range.
Some interesting plots of speaker impedance can be found here:
http://www.analog.com/UploadedFiles...6207AN843_0.pdf
Chuck
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| |
| Stuart 2008-01-27, 1:25 pm |
| In article <fni5oc42vtn@news3.newsguy.com>,
<phil-news-nospam@ipal.net> wrote:
> That would not be a realistic stress test. "8 ohm" speakers are not
> really pure 8 ohms resistive at all frequencies. I've heard of speakers
> that vary well above and to some degree below that 8 ohm level, and have
> not pure resistance.
Oh certainly. The load presented by a loudspeaker, especially one
involving multiple units and cross-over networks, is exceedingly complex
and very variable over the normal range of audio frequencies.
I remember many years ago making impedance measurements on a driver unit I
was going to design an enclosure for (This was before Theil-Small) and the
impedance peak at the resonant frequency, which was what I was trying to
determine, was huge.
Using a test load is the only way you can make standard, reproducible
measurements which can be repeated and compared.
I have an amplifier that can deliver 75W rms into an 8 ohm test load. The
OP refers to an amplifier which can, presumably, deliver 1000W into a pure
resistive test load, you can therefore compare measurements and
specifications in a meaningful way.
> Can your amplifier, cranked to deliver the full rated power, handle the
> variant loads given to it by different speakers?
Almost certainly not but that isn't part of its specification.
Measurements I have endeavoured to make of peak voltage, suggest peak
powers of only a watt or so at normal listening levels with my
loudspeakers (assuming the load remained constant at 8 ohms). These are
large, fairly efficient (by loudspeaker standards) transmission line
enclosures. This in a normal UK living room, with average furniture, about
17 feet by 12 feet. 75W capability gives plenty of "headroom" for
transient peaks and "funny" loudspeaker effects.
I don't know where to find all the construction details, notes and
performance results right now but "off the top of my head" it is rated for
95W peak into 8 ohm (power supply voltage limitation) and has higher
rating into 4 ohm - all at a specified distortion figure of 0.03%
Douglas Self, who I mentioned earlier in this thred, did produce a design
for a "load invariant" power amplifier but my copy of his book is
currently on loan to someone else ATM so I cannot quote from his
measurements on loudspeakers and the effectiveness of his design.
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Don Kelly 2008-01-27, 5:25 pm |
|
----------------------------
"Don Kelly" <dhky@shaw.ca> wrote in message
news:wnTmj.16042$ow.6715@pd7urf1no...
-- snip----
Sue,
When I put my foot in it, I do a good job of it as in the nonsense below
> ----------------------------
>
>"Also while there is an i*t relationship in the onset of fibrillation, it
>is not an i^2*t relationship but I*root(t) =165 "
What was I thinking of? I looked at an empirical relationship I (ma)
=165/root(t (s)) and rearranged it -but not bothering to square it to
(I^2)*t =constant
Not directly Joules but Joules per ohm>
Apologies
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
| phil-news-nospam@ipal.net 2008-01-27, 5:25 pm |
| On Sun, 27 Jan 2008 16:36:28 +0000 (GMT) Stuart <SW_NOSPAM@dsl.pipex.com> wrote:
|> Can your amplifier, cranked to deliver the full rated power, handle the
|> variant loads given to it by different speakers?
|
| Almost certainly not but that isn't part of its specification.
|
| Measurements I have endeavoured to make of peak voltage, suggest peak
| powers of only a watt or so at normal listening levels with my
| loudspeakers (assuming the load remained constant at 8 ohms). These are
| large, fairly efficient (by loudspeaker standards) transmission line
| enclosures. This in a normal UK living room, with average furniture, about
| 17 feet by 12 feet. 75W capability gives plenty of "headroom" for
| transient peaks and "funny" loudspeaker effects.
I'd be interested in an amplifier than can deliver some rated wattage
into _any_ impedance, high or low, infinite or zero, inductive or
capacitive, continuously, without damage, when sealed in a metal box
and not allowed air flow. Obviously that would be a wattage that is
well below what the marketing department would like to say.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-01-27-1505@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Dave Martindale 2008-01-27, 5:25 pm |
| Stuart <SW_NOSPAM@dsl.pipex.com> writes:
[color=darkred]
>It is a way of equivalenceing AC and DC power.
RMS volts and RMS amps are very useful for that purpose. If you apply
the usual Ohm's law formula to RMS volts and resistance, you get mean
power into a resistive load. Or RMS amps and ohms also gives you mean
power with a resistive load.
But if you multiply volts times amps directly to get instantaneous power
in Watts, you simply average that to get mean power. You *never*
calculate the root mean square of watts - you just measure mean watts.
That's what the previous poster meant. RMS *volts* is useful, so is RMS
*amps*, but RMS *watts* is not.
>An amplifier which is rated at 1000W should be capable of providing 1000W
>of heating into a resistive load equal to its specified load resistance
>whether it be 2, 4. 8, 15 ohms or whatever.
Right, but that is mean watts, probably measured using RMS volts. It
most definitely is not "RMS watts".
I think the terminology came about from audio marketing people wanted to
be able to distinguish power ratings based on RMS volts instead of the
peak voltage of the waveform. The latter seemed to be common for
inflating amplifier ratings. For a sine wave, the waveform peak power
is 2 times the mean power. For a music waveform, it may be many times
larger (because a brief peak can go up to almost the power supply rail
voltage, while several cycles of sine wave will pull down the rail
voltage).
Dave
| |
| Stuart 2008-01-27, 5:25 pm |
| In article <fnirqg1130l@news5.newsguy.com>,
<phil-news-nospam@ipal.net> wrote:
> On Sun, 27 Jan 2008 16:36:28 +0000 (GMT) Stuart <SW_NOSPAM@dsl.pipex.com> wrote:
> |> Can your amplifier, cranked to deliver the full rated power, handle
> |> the variant loads given to it by different speakers?
> | Almost certainly not but that isn't part of its specification.
> | Measurements I have endeavoured to make of peak voltage, suggest peak
> | powers of only a watt or so at normal listening levels with my
> | loudspeakers (assuming the load remained constant at 8 ohms). These
> | are large, fairly efficient (by loudspeaker standards) transmission
> | line enclosures. This in a normal UK living room, with average
> | furniture, about 17 feet by 12 feet. 75W capability gives plenty of
> | "headroom" for transient peaks and "funny" loudspeaker effects.
> I'd be interested in an amplifier than can deliver some rated wattage
> into _any_ impedance, high or low, infinite or zero, inductive or
> capacitive, continuously, without damage, when sealed in a metal box and
> not allowed air flow. Obviously that would be a wattage that is well
> below what the marketing department would like to say.
Sorry but I fail to see the relavance of your comment to the statement of
mine which you have quoted.
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Don Kelly 2008-01-27, 5:25 pm |
| ----------------------------
"Chuck" <nospam@nospam.at.all> wrote in message
news:dh1pp39oeo021a7uqp279vs0kipdn828b9@4ax.com...
> On Sun, 27 Jan 2008 04:17:00 GMT, "Don Kelly" <dhky@shaw.ca> wrote:
>
>
> In characterizing power, one can use either instantaneous values which
> can provide peak or continuous values, or some time-averaged values.
> Neither of these can be characterized as rms power.
>
> RMS power can be computed for any power envelope, but there is no
> physical meaning associated with the the computed value.
>
> Don's example suggests the root of a mean value, but nowhere is power
> actually squared. Therefore it does not illustrate the use of rms
> power.
>
> FWIW, P*t gives units of watt-seconds, or energy. I can imagine
> summing energy in a load interval, but taking the root of energy
> doesn't seem to have meaning.
>
> Chuck
-------------------
Thanks for the succinct correction, I hang my head in chagrin.
Either I should not respond when tired or should do better proofreading.
I should have said root [ (sum of P^2*t terms over the cycle )/(sum of time
intervals in the cycle)]
Example: a crane motor may be run through a cycle
off for 10 seconds, 10HP for 20 seconds, 25HP for 10 seconds, 10HP for 20
seconds and repeating.
So we have Prms =root[(0+2000+6250+2000)/(10+20+10+20)]= 13 HP
The only excuse for "physical meaning" is that all that this does is to
recognise that the existence of thermal lag can be used to advantage as long
as the load variations are fast enough. Some discounting of the "off" time
may be necessary because, at standstill, cooling is poor.
It's a useful design tool in specific applications.
However, while called rms power- it is still really the average power over a
period (60 seconds in the example).
-
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
| Palindrome 2008-01-27, 8:25 pm |
| Don Kelly wrote:
> ----------------------------
> "Don Kelly" <dhky@shaw.ca> wrote in message
> news:wnTmj.16042$ow.6715@pd7urf1no...
> -- snip----
>
>
>
> Sue,
> When I put my foot in it, I do a good job of it as in the nonsense below
>
> What was I thinking of? I looked at an empirical relationship I (ma)
> =165/root(t (s)) and rearranged it -but not bothering to square it to
> (I^2)*t =constant
>
> Not directly Joules but Joules per ohm>
>
> Apologies
>
No apologies needed. To be honest, I didn't understand it at first
reading and didn't try to work out what it meant.
I spend a very, very great deal of effort trying not to get *any*
current flowing through me at all..
If, accidently, some does, again, one day - then I guess I will find out
whether it was over the onset limit for fibrillation..after the
event...or not...
I certainly don't go around touching "live" connections thinking, "It's
ok, if I get a shock from it, it will be I*root(poe^t).. which won't
cause fibrillation...so let's do it"...
--
Sue
| |
|
| From: dhky@shaw.ca (Don=A0Kelly)
----------------------------
wrote
..
-- snip----
Sue,
When I put my foot in it, I do a good job of it as in the nonsense below
----------------------------
"Also while there is an i*t relationship in the onset of fibrillation,
it is not an i^2*t relationship but I*root(t) =3D165 "
What was I thinking of? I looked at an empirical relationship I (ma)
=3D165/root(t (s)) and rearranged it -but not bothering to square it to
(I^2)*t =3Dconstant
Not directly Joules but Joules per ohm>
Apologies
Don Kelly dhky@shawcross.ca
remove the X to answer
-----------------
What's that about your foot in the job and formulas ????? }
Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]
| |
| phil-news-nospam@ipal.net 2008-01-29, 9:25 am |
| On Sun, 27 Jan 2008 21:47:40 +0000 (GMT) Stuart <SW_NOSPAM@dsl.pipex.com> wrote:
| In article <fnirqg1130l@news5.newsguy.com>,
| <phil-news-nospam@ipal.net> wrote:
|> On Sun, 27 Jan 2008 16:36:28 +0000 (GMT) Stuart <SW_NOSPAM@dsl.pipex.com> wrote:
|
|> |> Can your amplifier, cranked to deliver the full rated power, handle
|> |> the variant loads given to it by different speakers?
|
|> | Almost certainly not but that isn't part of its specification.
|
|> | Measurements I have endeavoured to make of peak voltage, suggest peak
|> | powers of only a watt or so at normal listening levels with my
|> | loudspeakers (assuming the load remained constant at 8 ohms). These
|> | are large, fairly efficient (by loudspeaker standards) transmission
|> | line enclosures. This in a normal UK living room, with average
|> | furniture, about 17 feet by 12 feet. 75W capability gives plenty of
|> | "headroom" for transient peaks and "funny" loudspeaker effects.
|
|> I'd be interested in an amplifier than can deliver some rated wattage
|> into _any_ impedance, high or low, infinite or zero, inductive or
|> capacitive, continuously, without damage, when sealed in a metal box and
|> not allowed air flow. Obviously that would be a wattage that is well
|> below what the marketing department would like to say.
|
| Sorry but I fail to see the relavance of your comment to the statement of
| mine which you have quoted.
You assumed the load was a constant 8 ohms. I would not assume that. The
power rating is for 8 ohms. What will the amplifier do at a frequency for
which the impedance happens to be 3 ohms?
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-01-29-0721@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Stuart 2008-01-29, 1:25 pm |
| In article <fnn9bh11m47@news2.newsguy.com>,
<phil-news-nospam@ipal.net> wrote:
> You assumed the load was a constant 8 ohms. I would not assume that.
> The power rating is for 8 ohms. What will the amplifier do at a
> frequency for which the impedance happens to be 3 ohms?
At the voltages I was measuring, the amount of current required would be
/well/ inside the capabilities of the amplifier, hence my statement
"75W capability gives plenty of "headroom" for transient peaks and "funny"
loudspeaker effects"
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
|
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