Measuring output power

[fp2000]

With the help of a nice oscilloscope one can measure the clean output of an audio amplifier to determine the approximate power rating. If using a sine type wave at 1KHz, how much voltage should be applied to the input in order to simulate signal coming from regular guitar pickups?
Any suggestions? I don't want to put in more than I should

[jaysg]

Start with 100mV peak to peak. At some point, you may want to inject at the input to the phase inverter also.

[fp2000]

what would be the purpose of injecting signal directly to the phase inverter vs just the guitar input?

[jaysg]

what would be the purpose of injecting signal directly to the phase inverter vs just the guitar input?

The output section may be able to produce more clean power than the overall amp, depending on the preamp, that's all.

[ayan]

what would be the purpose of injecting signal directly to the phase inverter vs just the guitar input?

Well, if you did that then you'd be sure that whatever distortion you see at the output on the scope is generated within the power section, for example. If you inject the signal into the front end, there are many places where clipping might occur.

But, you will most likely find out that you need a little input signal to get the power section to start overdriving. Also, every single time I've tried this, when the power section starts clipping you will hear some mechanical noise coming from somehwere within the amp (the OT). I always do these tests with a dummy load, as opposed to a speaker, and watch out since those things will get very hot immediately.

It really is very fun to inject a signal and watch what happens as you go upstream towards the power section.

Cheers,

Gil

[fp2000]

I didn't think about the dummy load. I guess it will be kind of hard trying to get readings with so much noise if you had an actual speaker hooked up.
I guess I will do both, just to verify the total output power of the power section and compare against the overall amp since I am not pluggin my guitar to just the output section.
Thanks so much guys

[BobW]

Gil, any insight on this?
I always assumed a singing transformer that was not potted, and properly loaded was due to mechanical coupling of loose windings, when the tranformer is close to saturation, mostly a safe condition, and an unloaded singing transformer was due to corona induced insulation breakdown, an unsafe condition?

Also, every single time I've tried this, when the power section starts clipping you will hear some mechanical noise coming from somehwere within the amp (the OT). I always do these tests with a dummy load, as opposed to a speaker, and watch out since those things will get very hot immediately.

I agree, my right hand has confirmed how hot.

[fp2000]

I thought about it twice and realized that it would not be fair to measure the output power of an amplifier at the output of the transformer by using a power resistor. I think it would be more accurate to measure the output power by measuring the sound levels out of the speakers with a decibel meter. Has anyone tried this method? different speakers have different efficiency rates so depending on what kind of speakers you get, you will get more or less power. But then one would have to make sure that the signal on the oscilloscope is clean, and perhaps a studio would help since I would not be able to withstand the noise of a 1KHz wave at even 15 watts. Once you have a measurement in dBs then you would translate that into watts by using a little bit of math. Does this sound right?
After all, I think that is why some amps seem louder than others while having the same power rating.

[ampdoc1]

fp2000,

Your idea about measuring the power of your amp with a decibel meter is a misnomer, as that will tell you only what speaker is the most efficient with a given amount of power. Your amplifier's power doesn't change (assuming the loads are equal). A dummy load measurement is the standard way of measuring output.
Not to say though, that your idea isn't more practical if you're trying to achieve maximum volume with a given amp.

ampdoc

[sharkboy]

Another problem with testing from speaker output is that the specsmanship at the various speaker makers (how they do their testing), coupled with variations from speaker to speaker and cabinet efficiencies makes monster variables when trying to achieve a baseline for finding real accuracy.

[glasman]

Well here is my method.

1. connect amp to dummy load
2. Connect dummy load (mine has a 10:1 scope connection) to scope
3. Inject 1kkz sine wave signal at return jack
4. Adjust level from generator until clipping just starts and then back off until the clipping just disappears.
5. Meausre output voltage with a TRUE RMS voltmeter (the scope reading can be used but is not as accurate).

Apply standard (E^2) / R power equation.

Gary

[fp2000]

Fair enough.
Thanks Gary


By the way, there are free programs on the internet that can simulate signal generators with different wave types and frequecies. It would just be a matter of adjusting the output volume of the sound card or any other output device from the computer.
I just installed NCH tone generator.