How to measure real RMS amplifier power without making noise

1. Make a dummy load (set of resistors) joined together the way that you get 8 (or 4) ohms total. Remember how to add resistances correctly! I used 5 resistors of 10 ohms / 10 Watts to create dummy load capable of 50 Watts 8 Ohms. Three paralell, two paralell and the whole set (3&2) in serial. Serial simply adds resistances, paralell adds inverted resistances ie 1/Ra+1/Rb+1/Rc where a,b,c are separate resistors. Remember to take care of power handling and possibly resistors cooling. You will damage 10W resistor quite soon by hooking it up to average stereo amp for more than couple of seconds. You may burn your table as well ;)
2. Hook up a sine wave generator, set it around 50 hz (there are some online java ones). Standard ratings are made usually at 1 khz but this is what we have to compromise. Cheap multimeters work with alternating current that is standard in our power outlets (ie 50 Hz) and they do not at any other (not telling you so!) They will read some results but nowere near reality
3. Drive your amplifier input properly (volume to the max in every place responsible for volume control in your PC). Check with headphones if you do not overdrive (distort) the output
4. Hook up your dummy load
5. Swith on your amp. Drive it to the max (under the level it distorts). Unfortunately there is no way to detect distortion without oscillocope here. So by the rule of thumb use 80% of the volume rocker (unless you are sure 100% does not distort)
6. Measure AC voltage at amp output (the multimeter would calculate RMS voltage of 50 hz alternating current)
7. Square the voltage and divide by resistance, this will bring RMS Power of your amp.
8. This tehnique is not accurate, brings general idea of power you are dealing with. Anyway it is within 15% of laboratory testing. So If the producer claims your amp is 120W RMS @ 8 OHMS (like mine Vieta) and you get 50W then you may be sure they lie.
9. Dummy load keeps resistance at any frequency and power you get from your amp, speaker varies its resistance depending on powet (coil temperature) and frequency (up to 50%!). In general dummy load is much better for measurement.
10 If your not sure if you drive amp input to the full potential - overdrive it for test (not worrying about distortion). If you see that the voltage (and thus power) rises just a bit (eg. 5%), the previous setting was correct. Conclusion? Ovedriving destroys sound quickly and does not really push the amp to a new power limit. The amp has its physical limitsno matter how much mV on the input.
11. If working with subwoofer amps - remember to open the crossover fully (no to block 50 hz partly or fully)
12 Lowering resistance makes the amp pump more power but seldom going down from 8 to 4 ohms doubles power (at least in amateur grade hardware). Be cautious about 2 ohms. Not many amps are capable of working at this resistance at all. Risk of damage etc.

Category:

Travel & Events

Tags:

• Power Measurement
• speakers
• audio
• Car Audio
• RMS power
• amplifier power
• Effective Radiated Power
• Power (physics)