Measuring audio amp output power

[Cursitor Doom]

On Sun, 02 Aug 2015 09:04:54 -0400, Phil Hobbs wrote:

> RMS is an amplitude, e.g. 'volts RMS'.

Yes, I recall 0.707 of the peak signal level. So for example if I set an
amp to some arbitrary output and feed into an 8 ohm load and measure say
10V peak-to-peak on a scope, then I'm getting about 6.25W RMS out.

Phil's right that the term is in

error, but there's so many lies and so much snake oil in the audio
business that this minor absurdity seems a fairly small price to pay to
clear away all the "peak intermittent music power" (PIMP) nonsense.

Fully agree with you on that!

 

[Cursitor Doom]

On Sun, 02 Aug 2015 13:29:38 +0000, Cursitor Doom wrote:

On Sun, 02 Aug 2015 09:04:54 -0400, Phil Hobbs wrote:

RMS is an amplitude, e.g. 'volts RMS'.

Yes, I recall 0.707 of the peak signal level. So for example if I set an
amp to some arbitrary output and feed into an 8 ohm load and measure say
10V peak-to-peak on a scope, then I'm getting about 6.25W RMS out.

Phil's right that the term is in
error, but there's so many lies and so much snake oil in the audio
business that this minor absurdity seems a fairly small price to pay to
clear away all the "peak intermittent music power" (PIMP) nonsense.


Fully agree with you on that!

Should have put the 6.25W RMS in inverted commas, since there's no such
thing as RMS power, of course. It's really all about *average* power.

 

[Phil Hobbs]

On 8/2/2015 7:49 AM, Cursitor Doom wrote:

On Sat, 01 Aug 2015 18:29:58 -0700, Phil Allison wrote:

** A single sine wave is needed for audio power testing.

Amplifier power ratings are based on the *rms* value of a sine wave -
hence the misleading term "rms power".

Well that would make sense with RMS being, IIRC, equivalent to the DC
heating effect in a resistive load.

RMS is an amplitude, e.g. 'volts RMS'. Phil's right that the term is in
error, but there's so many lies and so much snake oil in the audio
business that this minor absurdity seems a fairly small price to pay to
clear away all the "peak intermittent music power" (PIMP) nonsense.

Cheers

Phil Hobbs



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Allison]

Cursitor Doom wrote:

RMS is an amplitude, e.g. 'volts RMS'.

Yes, I recall 0.707 of the peak signal level. So for example if I set an
amp to some arbitrary output and feed into an 8 ohm load and measure say
10V peak-to-peak on a scope, then I'm getting about 6.25W RMS out.

** The "peak" value is referenced to the zero line, so there are negative and positive ones. The "peak to peak" value has no polarity and is easier to read off a scope screen but to get the rms value of a sine wave one divides by 2.83 ( 2 x sq.rt 2)

So in your example, that value is 3.54Vrms and the power is 1.56 watts.


..... Phil

 

[whit3rd]

On Friday, July 31, 2015 at 8:42:40 AM UTC-7, MJC wrote:

In article <ab86ef38-7eac-4270-93f0-017fbea84a34@googlegroups.com>,
pfjw@aol.com says...

Most metallic conductors are Zero-Resistance at cryogenic
temperatures.
So, a wire-wound resistor may drop off to near-zero in liquid
nitrogen.

If only superconductors for liquid N2 temperatures (77K, -196C) were so
common! Even at liquid He (4.22K, -270C) only a minority of metals make
the transition.

Superconducting transition is not the only misbehavior; carbon resistors have a
semimetal/semiconductor transition, and turn high-resistance when cold,
and metal-film resistors move VERY far from their nominal values when
cold. Platinum film RTDs work because they have resistance accurately
proportional to absolute temperature: resistance at 77k is about a quarter of
that at room temperature.