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measuring output Power

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walters

Banned
How do i measure RMS output power?

How do i measure AC voltage output power?

How do i measure the Peak output power?
 

Russlk

New Member
Do you mean 50 or 60 Hz power? Power = volts times amps. don't confuse power and voltage. Voltage is measured with a voltmeter, current is measured with an ammeter. You can also measure power with a power meter, like used on your house.

If you mean RF power, like 100mHz, It is best to use a power meter that is calibrated for a 50 ohm load. For low RF frequencies you can use an RF voltmeter and calculate the power from: P = E^2/R. I expect a bolometer would be used at microwave frequencies, but I don't go there.
 

audioguru

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Hi Walters,
To measure the power of an audio amplifier you must connect a non-inductive resistor with a value of 8 ohms or whatever is its rated load. The resistor must have a high enough power rating so it doesn't change during the measurement.
The amplifier is fed a continuous sine-wave and the level is increased until slight clipping of its output is seen on an oscilloscope connected to its output. An AC voltmeter measures the output voltage and the power is calculated as V squared divided by the load's resistance. This is called the amplifier's continuous RMS power at a fairly low distortion at the frequency of the measurement.

Peak power is simply the continuous RMS power that was measured, times two. :lol:
 

Nigel Goodwin

Super Moderator
Most Helpful Member
walters said:
Thanks alot guys for the information

Audioguru is assuming a meter that will read RMS, most multimeters are scaled on AC to give the equivalent RMS reading for a sinewave - but you should be aware that most multimeters only have a VERY limited frequency response on AC ranges (generally they are designed for 50/60Hz).

But as you're (presumably?) using a scope, in order to see the onset of clipping, it's EASY to use the scope to do the measurement. Simply measure the peak to peak output on the scope, then divide that by 2.828 to get the RMS value (for a sinewave). As before, you then multiply that by itself, and divide by the speaker impedance.

You should also be very aware that while decent quality amplifiers will withstand this type of full power test, cheaper amplifiers may not! - it's quite easy to kill a cheap amp by driving it this hard.
 

audioguru

Well-Known Member
Most Helpful Member
Nigel Goodwin said:
You should also be very aware that while decent quality amplifiers will withstand this type of full power test, cheaper amplifiers may not! - it's quite easy to kill a cheap amp by driving it this hard.
He, he. I worked for a few high quality audio equipment manufacturers. We boasted that our amps would survive any kind of abuse. I ran them at almost full power output (the mains voltage sagged!) all day long and the only problem was that the test room heated up.
I tested and killed many competitors' "high quality" amps that died after only a few minutes of full output. :lol: :lol:
 

eblc1388

Active Member
audioguru said:
I ran them at almost full power output (the mains voltage sagged!) all day long and the only problem was that the test room heated up.

Using speakers or resistors as output load? What's wrong with the dead amplifiers? Power output stage semiconductor failure?
 

Nigel Goodwin

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Most Helpful Member
eblc1388 said:
audioguru said:
I ran them at almost full power output (the mains voltage sagged!) all day long and the only problem was that the test room heated up.

Using speakers or resistors as output load? What's wrong with the dead amplifiers? Power output stage semiconductor failure?

Obviously resistors, you don't do full power tests into speakers (unless you are testing speakers) - or you have a 'deaf wish' :lol:

Normal failure mode would be the output transistors going S/C, this commonly damages the drivers, and possibly other components as well.

Another reason for not using speakers! - output transistor failure in many amplifiers can take the speakers with them!.
 

eblc1388

Active Member
Nigel Goodwin said:
Obviously resistors, you don't do full power tests into speakers (unless you are testing speakers) - or you have a 'deaf wish' :lol:

Normal people yes, but audioguru most often said he hated amplifiers when clipping so I asked just to be sure. :lol:

So it would a good test to perform on a new amplifier then. If it failed, by law one can return it for a full refund in the UK.

On my amplifier spec sheet there is one item that said "Dynamic Headroom into 4 ohms = 2.1dB". What does that means?
 

audioguru

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Most Helpful Member
Hi L. Chung,
I haven't burnt a speaker yet, and I have driven many speakers with clean unclipped power with music, not sine-waves, far beyond their continuous max power ratings. :lol:

Dynamic headroom is "music power". It is the extra power above its continuous full-power rating the amplifier can provide for a moment before the unregulated power supply voltage sags. It is rather meaningless because they don't say how short a time before the voltage has sagged (it starts dropping immediately and exponentially), and you don't know how long a sustained full-power phrase in the music will last. It might be good for gun shot sounds, but 2.1dB is such a small increase that you won't know the difference.

Even speakers have "dynamic compression" at high power when the voice coil heats, its resistance increases, reducing its power a little until it cools. :lol:
 

eblc1388

Active Member
Thanks audio. My listening level is a few watts maximum only so max. power rating of amplifier never concern me.

By the way, how does the manufacturers of the dead amplifier react? They offer refund or replacement?
 

audioguru

Well-Known Member
Most Helpful Member
Hi L. Chung,
In Canada, the warranty says that the manufacturer can repair or replace a defective product. If they replace it, it almost always is a refurb, a repaired one that might have died with the same problem. I usually buy items from a big store that has their own satisfaction guarantee, replacement by a new or newer item that is factory sealed.

My daughter's new laptop computer failed after a couple of months. The big store gave her a newer, better one as a replacement because the "old" one wasn't available anymore.

Many credit card companies refund money for items that fail within a year, of course only if the purchase was with their card. Hey! My new car has a bird dropping on it. I bought it with a credit card so I wonder if....... :lol:
 

Nigel Goodwin

Super Moderator
Most Helpful Member
eblc1388 said:
Nigel Goodwin said:
If it failed, by law one can return it for a full refund in the UK.

Not necessarily true!.

The guarantee covers the unit against 'manufacturing defects', not against abuse - and these types of tests are serious abuse!. Generally the only people who would do these tests are people reviewing the amplifier, in which case they won't have paid for it anyway!.
 

eblc1388

Active Member
Nigel Goodwin said:
and these types of tests are serious abuse!.

If the spec stated 100W rms continuous output at 8 ohms, then loading it to 100W using a 8 ohms resistor could no way be classified as abuse. Am I missing something?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
eblc1388 said:
Nigel Goodwin said:
and these types of tests are serious abuse!.

If the spec stated 100W rms continuous output at 8 ohms, then loading it to 100W using a 8 ohms resistor could no way be classified as abuse. Am I missing something?

If the spec quoted that, fair enough! - but amps that quote that are not very likely to blow up :lol:
 

2ny

New Member
Peak power is simply the continuous RMS power that was measured, times two.


are we talking sine waves? in that case it should be RMS value times the square root of 2.

with sine waves for a constant load, power doubles for every 40% increase in voltage!
 

2ny

New Member
If the spec quoted that, fair enough! - but amps that quote that are not very likely to blow up


the 1975 FTC ruling requires preconditioning for an hour at 1/3 power, after which the amps is tested at full power for 5 minutes at the disclosed power rating by the manufacturer!

this measure afaik is aimed to protect consumers from unrealistic claims by manufacturers as to power output capability!
 

2ny

New Member
On my amplifier spec sheet there is one item that said "Dynamic Headroom into 4 ohms = 2.1dB". What does that means?

this is a measure of how stiff your power supply is!
a fully regulated supply will have a 0db of headroom! becuase the power supply rails are held constant from zero volume to full volume!

designers using unregulated supplies give allowances for low line voltages to insure that their amps can deliver rated power, so that even at full power, there is still some power supply voltage margin available.

power doubling is 3db so that a 2.1db is somewhat less than that!
 

audioguru

Well-Known Member
Most Helpful Member
2ny said:
Peak power is simply the continuous RMS power that was measured, times two.


are we talking sine waves? in that case it should be RMS value times the square root of 2.

with sine waves for a constant load, power doubles for every 40% increase in voltage!
That's correct. The peak voltage and peak current of a sine wave is the RMS value times the square root of 2. Therefore the instantaneous peak power is double the RMS power. :lol:

The quality amps I worked with have an unconditional 5 years guarantee. Nobody could abuse them with their full protection from a shorted load, variable speed fans and overdesign to make them extremely reliable. :lol:
 
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