# How to test output watts?

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#### Edwin Dorre

Hello,

what is the test way to test if my Aleph 4 is pushing enough watts?

I figured the following, however I first wanted to ask you guys before trying...

Take a signal generator on 100hz sinus and connect it to the Aleph. Take a 8 ohm big resistor and connect to speaker and connect a scope to this resistor.

Now pump up the amplitude of the sinus till I see the output sinus clip. Read the +/- voltage of the sinus and do some calculations.

I=voltage-top-top / 8
W=voltage-top-top * I

Is this correct?

Edwin

#### jewilson

Power Measurement

The power calculation is (Voltage Squared) / the load.

Example, if an amp swings 40 volts across and 8 ohm resistor
the power is 40*40=1600/8 =200 watts

If you have an amp that make this much power and you only
have a 100 watt resistor you still use it. The resistor must be
placed in a can of oil to dissipate the heat.

Rise the input voltage till the peaks just start to flatten and stop.
The measurement should made with a scope and meter.

#### grataku

Edwin,
what jewilson said is essentially correct, HOWEVER, since to "swing" means ALOT of things, he should have probably mention "root-mean-square, RMS, output voltage" that is Vpp*(1/(2*sqrt(2)) (Vpp=volt peak-to-peak). So what you get at the end is the very legit watt RMS figure using either the V^2/R or RI^2 formula.
Modern digital scopes (\$\$\$\$) provide direct RMS voltage measurements. So unless you have such capabilities you have to do the conversion. Usually, 'normal' (ie your regular \$100-type) digital multimeters have a very narrow frequency range and are not reliable much above the mains frequency.

#### GRollins

I use a Fluke 8060A (granted, more than \$100). It gives true RMS readings, which simplifies things tremendously. I hook up one of those big gold Dale 8 ohm load resistors and take a direct reading.
To establish clipping, I use a triangle wave since it's a little easier to see when the top and/or bottom flatten. I then switch back over to sine. The voltage reading you get off a triangle wave will not be directly applicable to your usual RMS calculation.

Grey

#### swede

The biggo resistor

So, where do you guys get thoose hefty 8 ohm resistors?

//magnus

#### jackinnj

Physics

Calories per minute equals watts. So if you heat a resistor in a thermos containing one litre of water with your amp and measure the temperature change over a few minutes you can find watts

#### jewilson

Power Resistors

Dale is one good source for high powered resistors.

#### PassFan

Don't know if it's right or wrong, but I always measure my rms output voltage into the actual impedance (i.e. speaker) I will be driving. I always figured the amp would react differently under a real world load versus a resistor. A quick check on the common houshold circuit will tell you if the meter reads true rms. If you see what your supposed to see (120v or thereabouts) your looking at true rms.

#### jewilson

That a NO NO

You can do all types of damage driving a amp to full power across the speaker. At some frequencies, the speaker changes impedance's and so might the amp. When the amp goes non-linear it will pulls the voice coil up on against the magnet blowing your speaker. Even a under powered amp can blow high power speakers this way. Also, the speaker can fail when it bottoms
out from to much power.

The standard is to use a resistor, If you want to measure the speaker impedance over frequency that a different story.

#### wuffwaff

Hi Edwin,

I´ve build myself an adjustable resistor for 8/4/2 ohms. It is made out of 2 heatsinks and 16 8Ohm 25w dale resistors. I can test 1 x 16 ohm, 2 x 8 ohm, 2 x 4 ohm and 1x 2 ohm.
I also use greys method of Fluke (used, about 150 €) and scope and check for clipping on an triangular wave.

For the Aleph 4 I think you could do without a heatsink if you use 4 x 16ohms 25w. This way you can test 8 and 4 ohms power.

william

#### Nisbeth

If you´re having trouble finding large 8 ohm power resistors you can parallel 7 56 ohm or 15 120 ohm power resistors for higher wattages.

/U.

PS: Oh, yes put them in an oil bath for cooling. When the room smells of popcorn, you know it´s time to turn it down

#### jackinnj

Correction to calorimetric method

Too late to edit it I guess -- if you heat a litre of water 1 degree in one minute you have expended 69.7 watts of energy. (The equation is 1 cal/minute = 0.0697 watts). You're probably better off using a good, levelled, random noise source than a sine generator.

#### JBL

Puting the resistor in a bucket of pure water work well also if you don't like oil.

Just make sure it'S pure

#### peranders

Paid Member
When I tested my QRO-amp I used Arcol 300 W non-inductive

http://www.elfa.se/pdf/60/06063002.pdf
http://www.elfa.se/elfa/produkter/se/20/2012534.htm

I put the res on a huge heatsink which came from a locomotive (the swedish RC locomotive type from ABB, ADTranz, Daimler-Benz, Bombardier or what they call themselves these days)! The fins were 6-7" long. This heatsink was placed in a buck of water. 600 W of continious power got the water boiling!

#### Edwin Dorre

Cool! Thanks!

Cool,

I own a TrueRMS voltage meter at least it is saying so on the outside. Do I use the voltage meter to measure the drop over de resistor or the amperage in serie with the resistor? I lost the manual of this critter and I am not that smart with electronics...

Is a 100Hz sinus ok? Or can I better use an other frequency?

Thanks,

Edwin

#### wuffwaff

Hello Edwin,

you should measure the voltage drop over the resistor.
I would go for 1kHz and then you can always test at 10/100 Hz and 10/20/50/100 kHz.

william

#### grataku

I would just leave all this bulls about buckets of oil or water to the local McGyver and limit the discussion to the 'humane' way of measuring powers.

#### PassFan

Jewilson:
"You can do all types of damage driving an amp to full power across the speaker."
How can I hear any sound if I don't hook it up to a speaker? Is your reference to, "full power", peak before clipping, or clipping at full output.
Yes, I understand the speaker must be matched to the amp and pushing an amp into clipping will overheat the voice coil in the speaker and you don't have to be at full power to blow tweeters.
I appreciate your concern and the warning but I think we have to be a little more specific in light of the fact that some, not all, of the new guys don't know about some of this stuff yet.

As for my unorthodox method, it is just to get a general idea of how much I am really pushing. Believe me it isn't easy cause the meter won't stand still. Thanks

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