How to test output watts?


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?

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.


2000-12-31 9:31 am
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.
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.

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.;)
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.
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.

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

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!
Cool! Thanks!


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?


"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:)