Does anyone know how to test a stock amp for how many ohms the impedence is? You know these stock amps are always so unorthodox and mysterious when it comes to finding their specs. I know how to test a speaker but have no idea how you would test an amp. None of the labels even hints at what the ohm rating would be. Can you get a meter reading off the amp just like you would on speakers?

 

Nope. It would be useless. Speakers are passive and amp are active. Measuring the impedance output of an amp wouldn't have any value in day to day operations. Different amps are designed differently; but all are meant to drive a range of speaker loads.

 

Nop, cant test the Amp for an ohm rating... most of the time factory amps are designed to work down to a certain ohm rating and thats it... different ohm loads make the amp work at different effeciency... Test the speakers that run on the amp and if you are going to switch the speakers stay with in that ohm rating!!! I say just change the factory amp when you change the factory speakers, youll be much happier

 

I'll give you a crude way that will get you fairly close, and doesn't require a lot of test equipment. It does require an audio oscillator, an oscilloscope, and some power resistors of wattage greater than the amp can put out, so even this might be more complicated than you want.

Pick three frequencies you care about; I'd pick 50 Hz for the bass, 1KHz for the midrange, and 15 KHz for the highs. Hook the genreator to the input of the amp, hook the 'scope to the output, and put a 16 ohm resistor acorss the output. Turn the generator up in level until the sine wave flattens just slightly at the top or the bottom. Measure the peak-to-peak voltage, and divide by 2.8 to get the rms voltage. Square that number and divide it by the resistance to get the power. Do that at each of the three frequencies. Then lower the resistance of the load, until the clipping votage begins to sag. Measure the power of the amp at that resistance. If that's enought power for you, don't go below that resistance. If it's not, you can lower the resistance slightly below that and observe the waveform at the output for any anomolies. If it looks good and the amp stays cool, you're probably OK.

During all this testing, continually monitor the amp for overheating. If it overheats, you need to minimize the amount of time you operate the amp at full output.

Yeah, it's crude. If you want anything more accurate, you'll need a spectrum analyser or a THD (total harmonic distortion) meter.

Jim

 

From your comment about the meter reading I suspect you're testing the speaker with an ohmmeter. That will tell you the DC resistance of the speaker coil. Since the amp, if it's working right, won't ever put DC into the speaker, the coil resistance is not relevant. You need to measure the impedance of the speaker at frequencies of interest to you: low frequencies (but not zero) for woofers and high frequencies for tweeters.

Jim

 

Very nice post CASL55. I tried your technique with an 8 ohm resistive load on an Infinity amp I had handy. It was rated 4 ohms. Using my distortion analyser I computed 3.2 ohms was the most efficient load. Using your suggestion with a 'scope and watching for clipping I got 3.3 ohms, but it only took 30 seconds! I still need my distortion analyzer for analog sound tracks, but for amplifier impedence matching, I can leave that box in the shop. Thanks

 

Thanks. I'll give you the theory behind the method, since you seem, from the language in your post, to be fairly technically sophisticated:

I think you're safe as long as the amp's output is voltage limited, which limitation usually occurs because of the capacity of the power supply. When the current the load is drawing is sufficient to make a siginificant change in the voltage the amp is capable of delivering, it's time to think about not drawing any more current (not making the impedance of the load lower).

You might be able to get away with a lower impedance value than you'd get with this method I came up with, but you'd have to know more about the internal design of the amp to tell.

Jim

 

You could also pull the amp apart and see what final output IC is being used. Then look up the specs. for that IC and you'll have your answer.

Plan B: Contact the manufactuer of the amp, give them its part number and ask them what the imediance should be.