Measuring amp output for watts RMS
 

I am trying to measure the output of my amps and and getting very very low watts output. I read the way to do it was to meter the speaker outputs to get your volts AC and multiplay that by 10 then divide by your impedance. So... I get a maximum of 8 volts AC from the leads x 10 = 80volts / 4ohms = 20watts and that's divided by 2 i take it since it's going to left and right speakers. Shouldn't I be getting a considerable amount more (at 14.4dc volts since the car is running during tests) considering I am using an alpine 4 chan amp when bridged should do 100rms per channel? am I doing something wrong here?

:dunno:
:eyes: :sly: :eyes: :sly: :eyes:
:dunno:

P = V^2 / R

(8*8) / 4 = 16watts

There is something wrong with your measurements or your system tuning.

regardless 16 watts is what a hu put's out.. should be getting much more with this alpine mrpf200 4chan bridged to 2... no? got gains to a hair over 3/4 w/o clipping

what frequency are you playing?

I'm just playing pink noise as I was told it was better than a test tone. Was I misinformed again..?

Maybe. That amp maybe puts out 30x4 on a good day. And at that you are just 3db over deck power. I like Alpine but have never thought much of the MRP line.

Yes, you should be using a 60 Hz sine wave for power testing. Some cheap meters only work at 60 Hz (+/- a bit) on the AC range so high frequency is out of the question. Pink noise is a broad spectrum signal and wont give you any meaningful measurement with a multimeter.

You only want to find the maximum voltage your amp can sustain without clipping a sine wave program. You can just listen for clipping and turn the volume back down a bit when you hear it. Should be accurate enough for what you want to know. Do you have a 60 Hz test program?

what speakers are you testing? if you are bridging a 4 channel amp I assume you are using it to drive full range main speakers? If you use 60 hz that may be lower than your High Pass setting, plus may bottom out your speakers. I would rather use 200 hz or more and keep a listen for bottoming and clipping. Make sure your high pass is set under 100Hz. Most modern DVMs are pretty flat out to a kHz or more in the AC Volts setting.
why are you trying to measure output power? do you suspect something is wrong? Does it not play loud? Most amps work or dont work. If it sounds good at normal levels (just a couple of watts) but not good at higher it is usually the speakers or the power cabling.

Good point Zoomer, I should have said to disconnect the speakers when testing and disable any signal processing. You would need some other way to detect clipping though.

disconnect the speakers? Then you have no load for the amp to power and all you are doing is measuring open circuit voltage.

The basic question remains why do you want to measure amp power output? do you have any reason to doubt its performance?

Impedance is not going to be 4ohms when you play a tone through the speaker.

Use an oscilloscope for true RMS.

True, I goofed there and posted without thinking. Measuring open circuit voltage is not so bad, it's the lack of being able to tell when clipping occurs. If the amp has a regulated supply the voltage rails would not sag much (if at all) under load and so the open circuit voltage could be used for calculations. Even an unregulated amp would give measurements that are fine for what the OP wants to know. I doubt the he needs a high degree of accuracy.

The rated load (4 ohms) is fine for calculations. A scope is the ideal but anyone who asks how to determine power output isn't likely to have access to one.

This should have been the first comment in this thread.
Unless you can measure current you'll never know what your amp is outputting. At least the scope will tell you if you're clipping.
Speaker impedance varies too much with frequency, which you can ignore if you know the current, and voltage. Without that, you're just wasting your time. You're not going to get any useful numbers, just numbers that you think are useful.

Depends on the degree of accuracy required. The generic shape of a (moving coil) speaker impedance curve is well known. If you have the published Rs you can make a reasonable guess at the optimum test tone frequency to avoid impedance issues. There is a region above resonance where the inductance is not yet significant.

If you only want to know if you are getting close to the advertised output of your amp then a scope is not required.

bunch o' nerds in this thread

LOL... guilty:)