Measuring audio amp output power

Audio load:

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(No mention of tolerance)

Done.

Scopes have a lot of errors. A DVM may work, but pay attention to frequency response. If you stick to a sine wave you won't need a TRMS meter.

Next problem, you need to know the resistor value accurately.

I don't think you want or need want 0.001% accuracy.

Reply to
Ron D.
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** I use the same resistors for all amplifier bench testing.

Submerged in water, they can handle 1000W each.

Using a pair and a simple switching scheme gives 4, 8 and 16 ohms.

** That's a a bit harsh. Many analogue scopes manage 2% accuracy for time and amplitude & modern DSOs have on-screen displays of the same parameters with better than 1% accuracy.
** Most can be trusted to about 1kHz on sine waves.

.... Phil

Reply to
Phil Allison

NO!!!!

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

Reply to
pfjw

The liquid nitrogen was mainly a joke to the extreme of cooling. However if enough power is being used, the resistor its self would be heated to a more normal temperture and would be way above the temperature of the nitrogen. This woudl bring the resistance back up.

Reply to
Ralph Mowery

temperatures.

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.

My Kaye and Laby says: "At low temperatures the effects of impurities etc become increasingly important and these largely determine the value of the residual resistance to which many metals decrease at low temperature."

Mike.

Reply to
MJC

Phil: > Scopes have a lot of errors.

** That's a a bit harsh.

Analog scope days and having error sources and effects drilled into my head. I just really wanted to say, know what your error sources are. At one point in my life numbers like 2E18 and 7E18 were considered "essentially the same".

Reply to
Ron D.

Just one final point. Is a single sine wave sufficient, or will it have to be two, harmonically-unrelated sine waves? ISTR with RF power measurements you need to perform the slightly more complicated 'two-tone' test; just wondering if the same applies at audio frequencies?

Reply to
Cursitor Doom

Former astronomer?

Cheers

Phil "also a former astronomer" Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

** 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".

** Using two tones tests for intermodualtion, always present where there is non-linearity.

.... Phil

Reply to
Phil Allison

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

Reply to
Cursitor Doom

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 
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Reply to
Phil Hobbs

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

Fully agree with you on that!

Reply to
Cursitor Doom

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.

Reply to
Cursitor Doom

** 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

Reply to
Phil Allison

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.

Reply to
whit3rd

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