[Roscoe East]

Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

Thanks.

[Scott Dorsey]

Roscoe East wrote:
Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

Yes.

You know, from high school science class, that V=IR. You also should know
that P=IV. This means that P= V^2/R by substitution. Since what you want
to know is V, algebra tells you that V^2=PR, so V=sqrt(PR).

So, if you are driving 100 watts into an 8 ohm load, the rms voltage you
have across the load is sqrt(8 * 100) = about 28V.

In most places, the magic number is 70.7 volts... when the RMS voltage reaches
70.7, the peak potential on a sine wave reaches 100V, and the electrical code
requires the speaker cables to be treated as Class I wiring. Some local
codes may require going to class I at even lower voltages (Hampton, Virginia
does, but the adjacent counties do not, for instance). Low voltage Class II
lines don't require armor or conduit and are a lot cheaper to install.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Richard Crowley]

"Roscoe East" wrote in ...
Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

Yes. It is called Ohm's Law.
http://en.wikipedia.org/wiki/Ohm%27s_law

[Paul Stamler]

"Roscoe East" wrote in message
...
Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

The voltage is the square root of (N times Z). That's only a first
approximation, but it'll do for starters. Double it for safety.

So if an amp is rated at 75W into 8 ohms, well, 75 * 8 = 600, and the square
root of that is about 24.5V rms. Twice that is 49V; I'd use wire rated at no
less than 50V.

Peace,
Paul

[William Sommerwerck]

Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

No, because amplifiers do not output "RMS" watts.

[Richard Corfield[_3_]]

On 2008-05-17, William Sommerwerck wrote:
Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

No, because amplifiers do not output "RMS" watts.

Wouldn't Power = Voltage * Voltage / Impedance help, even if there's
some technical reason it may not be the right answer?

So sqrt(N * Z)

Where N is the power actually being delivered into Z the actual impedance
connected.

Finding the peak is harder as you start making assumptions about the
signal being a pure sine-wave, or connecting an oscilloscope across the
terminals which gives you your direct answer anyway. Whether multiplying
by sqrt(3) gives you the peak available from the amplifier depends
on whether that power rating is determined by the peak voltage out.
Amplifiers I've seen tend to have both nominal and transient ratings
which implies that other things like heat generation and the ability to
provide the current needed for any amount of time come into play.

- Richard

--
_/_/_/ _/_/_/ _/_/_/ Richard dot Corfield at gmail dot com
_/ _/ _/ _/
_/_/ _/ _/ Time is a one way street,
_/ _/ _/_/ _/_/_/ except in the Twilight Zone

[Scott Dorsey]

Richard Corfield wrote:

Finding the peak is harder as you start making assumptions about the
signal being a pure sine-wave, or connecting an oscilloscope across the
terminals which gives you your direct answer anyway. Whether multiplying
by sqrt(3) gives you the peak available from the amplifier depends
on whether that power rating is determined by the peak voltage out.
Amplifiers I've seen tend to have both nominal and transient ratings
which implies that other things like heat generation and the ability to
provide the current needed for any amount of time come into play.

If you want to know the real peak voltage the amp can generate, measure
the supply rail voltages inside the box. Realize the supply will sag
a little bit at full power, and the output will be a junction or
two drops away from the rail at best, so subtract five or six volts to
be safe.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Earl Kiosterud]

"Roscoe East" wrote in message
...
Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

Thanks.

You asked for a formula, and both Scott Dorsey's and Richard Corfield's are correct. The
RMS Voltage is SQRT(N * Z), where N is Watts and Z is impedance. Since amplifiers are rated
with a sine wave (hence "RMS"), just at clipping, the peak voltage would be 1.414 of that.
Delivering typical audio, the peak voltage at clipping would be generally be slightly
higher, due to less power supply loading, though the power would be generally lower.

You don't say why you need this info. If you're going to be putting speaker wiring in the
walls, consider using good old Romex 14-2. The audio purists will howl and flame, but it
works great, and it's cheap. Resistance is low. Since 14 ga wire is good for 15 Amps, that
works out to a max 1800 Watts (15 Amps squared * 8 Ohms) with an 8-Ohm speaker. The Voltage
at such a power would be 120 VAC (SQRT(1800 Watts*8 Ohms), or 15 Amps * 8 Ohms) anyway, so
you're OK there too. Delivering typical audio, such an amplifier would produce RMS Voltage
and Amperage that would be generally lower anyway. There are lots of ways to terminate it
in a regular old outlet box. Just don't get it confused with your house wiring! Mark it
carefully.
--
Earl

[Mark]

On May 16, 9:22*pm, "William Sommerwerck"
wrote:
Or, more specifically, is there a formula to determine (within reason)
what, given a power amplifier output of N watts rms and a speaker
impedance of Z ohms, the voltage across the speaker terminals will be?

No, because amplifiers do not output "RMS" watts.

correct, there is no such thing (in practice) as RMS Watss.

RMS Volts times RMS Amps = AVERAGE Watts.

RMS is not a correct term to use with Watts.

Average Watts is the correct term.

RMS is correct for Volts and Amps.

Mark

[Scott Dorsey]

Earl Kiosterud wrote:
You don't say why you need this info. If you're going to be putting speaker wiring in the
walls, consider using good old Romex 14-2. The audio purists will howl and flame, but it
works great, and it's cheap. Resistance is low. Since 14 ga wire is good for 15 Amps, that
works out to a max 1800 Watts (15 Amps squared * 8 Ohms) with an 8-Ohm speaker. The Voltage
at such a power would be 120 VAC (SQRT(1800 Watts*8 Ohms), or 15 Amps * 8 Ohms) anyway, so
you're OK there too. Delivering typical audio, such an amplifier would produce RMS Voltage
and Amperage that would be generally lower anyway. There are lots of ways to terminate it
in a regular old outlet box. Just don't get it confused with your house wiring! Mark it
carefully.

In some places, the code specifically prohibits doing this because of the
possibility of it getting confused with house wiring. If you are in a place
like that, use THHN appliance wire instead. It meets the fire code
requirements for use in-wall, although for Class I wiring it will have to be
in conduit or flex. In most cases that's not the case for speaker lines.

I think stranded wire sounds better than solid core wire but I can't give
any actual evidence for this being the case.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[GregS[_3_]]

In article , (Scott Dorsey) wrote:
Earl Kiosterud wrote:
You don't say why you need this info. If you're going to be putting speaker
wiring in the
walls, consider using good old Romex 14-2. The audio purists will howl and
flame, but it
works great, and it's cheap. Resistance is low. Since 14 ga wire is good for
15 Amps, that
works out to a max 1800 Watts (15 Amps squared * 8 Ohms) with an 8-Ohm
speaker. The Voltage
at such a power would be 120 VAC (SQRT(1800 Watts*8 Ohms), or 15 Amps * 8
Ohms) anyway, so
you're OK there too. Delivering typical audio, such an amplifier would
produce RMS Voltage
and Amperage that would be generally lower anyway. There are lots of ways to
terminate it
in a regular old outlet box. Just don't get it confused with your house
wiring! Mark it
carefully.

In some places, the code specifically prohibits doing this because of the
possibility of it getting confused with house wiring. If you are in a place
like that, use THHN appliance wire instead. It meets the fire code
requirements for use in-wall, although for Class I wiring it will have to be
in conduit or flex. In most cases that's not the case for speaker lines.

I think stranded wire sounds better than solid core wire but I can't give
any actual evidence for this being the case.

Long while back some authors in Stereophile mag were going to solid wire.
That sounded best to them.

greg

[videochas www.locoworks.com]

On May 19, 6:48�am, (GregS) wrote:
In article , (Scott Dorsey) wrote:
Earl Kiosterud wrote:
You don't say why you need this info. �If you're going to be putting speaker
wiring in the
walls, consider using good old Romex 14-2. �The audio purists will howl and
flame, but it
works great, and it's cheap. �Resistance is low. �Since 14 ga wire is good for
15 Amps, that
works out to a max 1800 Watts (15 Amps squared * 8 Ohms) with an 8-Ohm
speaker. �The Voltage
at such a power would be 120 VAC (SQRT(1800 Watts*8 Ohms), or 15 Amps * 8
Ohms) anyway, so
you're OK there too. �Delivering typical audio, such an amplifier would
produce RMS Voltage
and Amperage that would be generally lower anyway. �There are lots of ways to
terminate it
in a regular old outlet box. �Just don't get it confused with your house
wiring! �Mark it
carefully.

In some places, the code specifically prohibits doing this because of the
possibility of it getting confused with house wiring. �If you are in a place
like that, use THHN appliance wire instead. �It meets the fire code
requirements for use in-wall, although for Class I wiring it will have to be
in conduit or flex. �In most cases that's not the case for speaker lines.

I think stranded wire sounds better than solid core wire but I can't give
any actual evidence for this being the case.

Long while back some authors in Stereophile mag were going to solid wire.
That sounded best to them.

greg- Hide quoted text -

- Show quoted text -

Had the "Solid Wire Advisory Board" taken out a big ad in the magazine?