Measuring rms output wattage of an amp in vehicle

[capitolj80]

Ok, so I know how to test output using standard bench equipment, but I'm curious how accurately this can be done installed in the vehicle. Once the speakers are hooked up and the gain properly set to match the input signal, I could read the voltages at the output terminal while playing different test tones in the cd player, right? I could play various tones and get the voltage readings at each frequency (using a true rms meter). It seems this would be very accurate because I would be getting voltages based on the subs actual impedance at each frequency and also at different signal frequencies. Only question is what volume level would I play the tones in the player at (3/4, max)? I know that by recording voltages at different frequencies wont give me one universal RMS output, but I WOULD know what it does across the frequency board which could be considered better (besides, I could just use the 60hz reading as my main, right?) To me, it seems like this would be the most reliable way to see what an amp will do IN MY CAR. Am I on the mark with this, or am I overlooking something?

Feedback much appreciated

[00goobs]

I was bored one day (which I am every day) and wondered the same thing. Some of those McIntosh car audio amps had the output meter on there, but I would think it was pretty much a voltage measurement on the speaker outputs expressed in dB which is like a pressure measurement and also what voltage is considered. A wattage measurement would be a product of voltage and amperage, or either and resistance I think.

So if I were to make a wattage meter to have side by side or integrated to an RMD, it would need to take peak measurements periodically of both voltage and amperage then do the math and display the actual wattage. I do think that a voltage measurement and amperage measurement at the power input would do a pretty good approximation of what the amp is using power wise and give 80% of that measurement to actual output and you would get a close measurement of actual output of the amplifier. Maybe?

BTW, what amp are you using? I believe the MS2125, 2250 and MPS2500 have clipping indicators which I found pretty accurate on my scope and you could use that to determine your amps max output, assuming you are using one of those...

[capitolj80]

Well, I was wondering about this as a general future application, but the amp i'll soon be putting in my car is a Vibe Audio BlackBox Bass 5..supposed to do 1500rms @ 2ohm. Fuse rating is 120amps, so I'm curious what the actual output will be. I also wanted to try this with my sister's RSD1200.1 and her hubby's Sony 9001GTR. None of these have clipping indicators, but their gains were set with a DMM (also, there's no audible distortion) and so will mine be. Actually, the Sony has a "dynamic distortion" indicator..i wonder if that's actually a clipping indicator. So, yer saying that if the gain is truly set correctly (just below clipping) that this method will give me a realistic indication of an amps in-car rms capabilities?

[capitolj80]

Also, am I correct in remembering that to get the output wattage, I would square the voltage and divide by the ohm load? i.e if I measure 53.4v at the output terminal, then I would get (53.4x53.4/2ohms) 1,425 watts

[00goobs]

I would say yes a voltage reading could be an accurate measurement of wattage at 50-60hz if the speaker's resistance is consistent at that frequency in that enclosure. A more accurate means would be an amperage and voltage measurement at the output instantaneously, but I don't know how to do that, hehehe... But for kicks, you should be fine with what you have set up. Who actually rates their amplifiers at the actual wattage anyway? ...too many variables...

[capitolj80]

yeah, there are alot of variables to consider...I tried to address as many as i could in this method im curious about the method you describe. IF i can get the amperage and voltage instantaneously, do you know how i would use those to determine wattage? either way, thanks for the input

[ttocs]

I have a dakota digital gauge that uses the inductance created in the power wire to figure how what is being drawn. There are other companies that make similar pieces....

[capitolj80]

So it measures the actual amperage that the amplifier is using? Does anyone know how someone would use that, coupled with the output voltage to calculate the wattage?

[Eric D]

amperage x voltage = wattage

[capitolj80]

Ok, you mean amperage x INPUT voltage, right? for instance 113amps drawn x 13.2 volts= 1,491watts?

[todd217]

Ok, you mean amperage x INPUT voltage, right? for instance 113amps drawn x 13.2 volts= 1,491watts?

That looks right assuming the amp is 100% efficient. But we know they are not.

[capitolj80]

Ahhh..I know what I did wrong there...it's output voltage x output amperage...i.e. 54v x 27.5amps=1,485 watts. I got it

usesful page i found http://www.sengpielaudio.com/calculator-ohm.htm

[ttocs]

just curious, are you wanting to know just for braggin rights or is there a bigger goal in mind? Bragging about how much power your system has is exactly like bragging how much hp your car has. The amount of power has nothing to do with the overall sound/performance, in the end it is up to the user to make the most of it. A 1000 hp car in the hands of the wrong person is just a death machine and doesn't mean it will turn a 10 sec pass. I have heard systems that were in the under 200 watt catagory that blew away any hp system....

[capitolj80]

Well, mostly I just like to be educated..knowledge is power. By knowing more precisely the output capabilities of an amp, I can better choose a subwoofer setup that can be used to the maximum of their (the subs) capabilities as well as leave head room the the amplifier. Also, since all manufacturer specs and vehicle power systems aren't created equal, it seems important to be able to determine what a specific piece of equipment will achieve under user-specific circumstances. You are right, though. The best amp in the world is a waste if you dont use is correctly. That's why I pick the brains of you fine gentlemen to fill in the blanks in my own knowledge