How to determine amplifier output

[mrray13 30]

(the process is easy enough, measure and record dc input to the amp. measure and record ac output to the speakers and multiply the two. ie, 14 dcv input times 130 acv output = 1820 watts)

Curious as to the origin of that math sense it really makes no sense because it totally fails to account for load and thus current which are vital to any electrical power calculation. According to this, 30V on the output at 14V on the input is going to be the same output wattage regardless of load on the output.

i can't reply to that. i don't know the origin, it's what i have read, and what i have been informed to be fairly accurate. but what you said makes perfect sense. without factoring the shown impedence, one is not getting an accurate representation of amplifier power. suppose it would be accurate enough when using a regulated power supply like the jl slash amps. but as you mentioned, without factoring in the current draw/ impedence, one is just getting a general idea (and one not complete) as too power output.

i'll ask this, and i could be way off base, but if one changed impedence, and nothing else, i wonder if output voltage would change. keep gain the same, volume the same, frequency played unchanged. sounds like something i will play with later today.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[helotaxi 46]

i'll ask this, and i could be way off base, but if one changed impedence, and nothing else, i wonder if output voltage would change. keep gain the same, volume the same, frequency played unchanged. sounds like something i will play with later today.

The voltage shouldn't change much. Not enough to account for the change in current. Even with the regulated amps it won't even get you in the ballpark. The only time it would work is in the case where the output current just happened to equal the input voltage i.e 600-700W at 4 ohms, 300-350W @ 2 ohms or 150-175 @ 1 ohm. Get outside these ranges and that equation means nothing. Inside those ranges it's only coincidence.

[Lord Baccus 1]

so what is the formula ???

amprage (Ac or Dc ) x voltage = wattage ?

[helotaxi 46]

so what is the formula ???

amprage (Ac or Dc ) x voltage = wattage ?

P=V*I by definintion.

One of the discussions on here previously was whether, since the coil of the driver shifts the phase of the current in relation to the voltage, you could simply multiply RMS voltage by RMS current and get power or if you have to actually account for the phase shift. My take is that since it is the current that is doing the work and the voltage is simply inducing the current IAW Ohm's law, you can simply multiply them out.

[Lord Baccus 1]

so what is the formula ???

amprage (Ac or Dc ) x voltage = wattage ?

P=V*I by definintion.

One of the discussions on here previously was whether, since the coil of the driver shifts the phase of the current in relation to the voltage, you could simply multiply RMS voltage by RMS current and get power or if you have to actually account for the phase shift. My take is that since it is the current that is doing the work and the voltage is simply inducing the current IAW Ohm's law, you can simply multiply them out.

Ummmmmmm ??? in english please... or something a lil simpler to understand

i dont do math well

[///M5 2,833]

Not too easy to simplify that much more, but:

P = Power

V = Voltage (rms)

I = Current (rms)

In the case here multiply the voltage by the current to get power or as HT said: P = V*I

To measure you will need a voltage measuring device (DMM or O-scope) and a current clamp to measure the current.

[Lord Baccus 1]

ok so say iv never done this and i have a Dmm...

i hook up + / - to ( ?) speaker out on the amp ?

set the Dmm to 20v AC ? DC ?

set peek hold ?

yall are assumming we know what to do with the Dmm...

someone please explain how to go about getting the proper reading.

how is the Dmm used ?

do i disconnect anything ?

all iv ever done with a Dmm is check ohm load & see if my batterys are 12.85v, & checked the alt to see if i was getting 14.4v

Edited March 19, 2007 by Lord Baccus

[JimJ 11]

Measuring voltage in this way is the same thing, except you're measuring AC instead of DC like when you checked battery voltage.

[Lord Baccus 1]

??????????? ummmmmmmmmmmm ??????????????????

how is the process done from start to finish ?

[JimJ 11]

Maybe there's someone locally that can show you.

I'm not sure how to explain this easier...you know how to check battery voltage, this isn't any different except for measuring AC instead of DC.

[Lord Baccus 1]

Maybe there's someone locally that can show you.

I'm not sure how to explain this easier...you know how to check battery voltage, this isn't any different except for measuring AC instead of DC.

so i hook the + / - to the speaker terminals and set the DMM to AC ,

do i set it to Peek Hold ?

then i get the highist # and x 14.6v = accual wattage ?

[///M5 2,833]

No. You need to measure RMS. Perhaps a quick look through the gain setting tutorial will help you. Either way, my guess is that you don't have a current clamp so you can't do it anyway.

[helotaxi 46]

do i set it to Peek Hold ?

then i get the highist # and x 14.6v = accual wattage ?

Hoping you're kidding about the last part but in case you weren't, forget you ever saw that "equation."

[Lord Baccus 1]

can someone please help:

i need step by step instruction as to how to do it:

1. get out Dmm

2> hook Dmm to ?

3. set Dmm to ?

4. special clamp ?

5.ect ect ect...

i have never done this and it is kind of frustrating not knowing how too, if i need a clamp ill get 1 asap

[///M5 2,833]

If you didn't follow so far you shouldn't try. For your application just use a mic and see what is louder.

[mrray13 30]

can someone please help:

i need step by step instruction as to how to do it:

1. get out Dmm

2> hook Dmm to ?

3. set Dmm to ?

4. special clamp ?

5.ect ect ect...

i have never done this and it is kind of frustrating not knowing how too, if i need a clamp ill get 1 asap

1) yep

2) hook to speaker output

3)set to AC voltage, <200

4) clamp multimeter, get one

5) clamp multi meter over speaker wires

6) set clamp meter to amperage (can't remember exact symbol)

7) turn on system and read peak numbers.

8) multiply peak numbers together.

hope that helps.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[helotaxi 46]

7) turn on system and read peak numbers.

8) multiply peak numbers together.

That will give you peak output. To get RMS you will need either to use true RMS meters for both voltage and current or multiply the peak value by sqrt(2)/2 ot give a pretty close approximation of RMS.

[mrray13 30]

oops, lol. my bad. nice catch.

since i don't have true rms meters, so after i get peak numbers, the equation will look like this, with x=peak number

x*0.707

you know, for us idiots, lol.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[///M5 2,833]

Assuming you are playing sine waves.

Either way I will reiterate the questionableness of the accuracy here. In the right hands it could be for the specific application, but generically speaking you are almost as good off going by published specs or your ear.

[mrray13 30]

one could do it with sine or music. and yeah, it's accuracy won't be precise, but it's a daily application. and the stuff your used to messing with is beyond any normal person. maybe the whole idea is pointless, but for someone who just has to know, roughly, what's going on, it's accurate enough. and then you aren't dealing with some manufacturers "wishful thinking" ratings.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[///M5 2,833]

Buy decent gear and they won't be wishful thinking specs...

[helotaxi 46]

one could do it with sine or music.

To get anything resmebling RMS without using RMS meters, you must use a sine wave. I still question the usefulness, other than repeatability, of fixed load testing. At that point it gets to be like dyno drag racing..."under ideal conditions the amp will do X, under real conditions...well who the hell knows?!?" Even the CEA ratings are pretty bogus considering they are spec'd at 1kHz. What if the amp can't play that high? What freq do they use then? Are the ratings valid at all for a bandwidth limited amp? If you want applicable numbers, you're going to have to do the testing yourself. If you're going to do the testing yourself you may as well use the actual load you plan to drive and the actual power supply you plan to use. IMO that's going to give you the most relevant results.

[mrray13 30]

again, nice catch helo, lol. damn, i suck at this.

and i second helo. fixed load testing is great for paper, but real world, where it's relevent, it's just not accurate. using the actual load with the actual power source will give the real world numbers the OP wants. a car will hardly ever duplicate a test bench.

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[tejcurrent 13]

OK, I'm going to get a true rms meter (didn't get 1 yet because of expense) and try out whats been suggested so far. again, this isn't the most accurate testing, just in car, and to let those (like someone said) who must know have a crack at it. Part of it is also the comment M5 made about getting good equipment you can trust the manufacturers ratings, but even those cea compliant aren't perfect. Example: my mojo 4k is rated 4kwrms @14.4v @ 1khz, but once you figure in I've got it playing between 20-80hz, with a current drop from 14.8 to around 13v, while this tells me I need more power all around, I can get a little more accurate #s with testing it myself in my exact setup.

I was told that every 1v drop you lose around 10% of power, so if I went from 14.8 to 12.8- .2x8000 means I'm losing around 1600watts.

I just want to get a little closer to actual rms wattage than just guessing, or trusting the manufacturers specs.

[///M5 2,833]

CEA is far from perfect. Do realize that you are if lucky going to get a 10% number which of course might be or not closer than the spec anyways.