# How To Measure Your Amplifier's Wattage



## mrflamboynt (Apr 23, 2011)

not sure if anyone has posted anything like this on diyma yet but here goes......

things you will need;

- digital multimeter (RMS)
- (4) 2.5" - 3" coice coils ($ depends who you get em from, $25 ea? )
- piezo element tweeter ($20 radioshack)
- tupperware container with lid (8"d x 8"w x 5"h should fit everything).
- 1 liter distilled water
- polyurethane adhesive
- solder/soldering iron
- electrical tape, or other insulator/connector for wires
- 6 feet 12-14ga speaker wire
- signal generator (something to generate 60-100Hz individual tones) bass test cd, WinISD has signal generator too. thought i saw an iphone app that had a signal generator.

you will need to solder lead wires (bout 6"-8" long) onto the voice coils. you can connect these together later, in series/parallel to achieve the desired impedence for testing, but use tape when configuring, to allow different impedences.

cut out 4 holes in the lid of tupperware container to snugly fit voice coil formers. if you cut carefully, you can re-use the pieces you cut out to place back into the center of the coil former and use urethane to seal everything into place.

wire in the piezo tweeter, series or parallel to the other voice coils. dont worry about impedence of tweeter, as it will not make a difference to the final load when testing.

after you have fit everything into the tupperware lid, fill the container about 3/4 full of DISTILLED water. (NOT tap water) place lid on top, coil windings should be completely submerged in water.

connect "dummy load" to the amplifier you wish to measure, careful you wired the voice coils correctly. use multimeter to make sure you have everything wired to the correct impedence.

with amplifier gain turned all the way down, play test tone, try to make sure signal comming from generator is as pure as possible. then slowly turn up gain on amplifier until you hear a distinct BUZZZZ come from the tweeter, then back the gain down just until the buzzing stops. 

now measure "RMS" AC voltage at amplifier outputs.... then square it, and divide by ohms. example: 
if you measured 71.5 volts AC @ 2ohms = 2556.125 watts (71.5 * 71.5/2 = 2556.125)
this does not calculate power reducing IMPEDENCE RISE caused by a live sub functioning in an enclosure. but wil give you an idea of what your amplifier is doing at certain ohm loads. also can be used as a diagnostic tool, or just use it to test a bunch of amps....

i have used a setup like this to test about 30-40 different amps.... i would just toss it in the freezer when not in use. the coils would be frozen inside the water, and pretty much ensure the coils never got too hot, no matter how powerfull the amp is. obviously the bigger the coils you use the more power you can measure. but i used 4 2.5" coils, and it easily tested 3-4Kw worth of power.

well i think that pretty much explains it, here is a link to my original thread about 12 years ago. richard clark pretty much breaks it down....

determining actual wattage - CARSOUND.COM Forum


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## Nishani (Sep 15, 2009)

Interesting


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## bass lover (May 2, 2011)

i must try with all these extra subs laying around.Sounds fun.Also i dont think my new brutis is really pushing 1200 at 1 ohm!


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## envisionelec (Dec 14, 2005)

mrflamboynt said:


> - (4) 2.5" - 3" coice coils ($ depends who you get em from, $25 ea? )


A raw voice coil and represents a very poor load as there is no component to give you a correct impedance generated by back EMF...if that's what you're going for. As it is almost purely inductive, you are sending a lot of current back into the output devices where it is dissipated as heat. If you are trying to stress the amplifier, this is how you do it. In fact, I use a similar jig to put amplifiers into protection mode.

If you are testing amplifiers, apples to apples, use a resistor.


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## schmiddr2 (Aug 10, 2009)

Like this for $10? 2 OHM 100 WATT POWER RESISTOR DIVERSION DUMMY LOAD 12V


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## envisionelec (Dec 14, 2005)

schmiddr2 said:


> Like this for $10? 2 OHM 100 WATT POWER RESISTOR DIVERSION DUMMY LOAD 12V


Only as a last resort as they're also quite inductive. A better choice is a non-inductive aluminum DALE NH50 series resistor in the classic gold anodized, finned case. Its interleaved winding negates inductance at audio frequencies. They're also deionized/distilled water-submersible whereas ceramic resistor tend to absorb water and crack when heated.

My personal favorite is a planar resistor by EBG, for example. They're also known as motor brake resistors.


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## mrflamboynt (Apr 23, 2011)

i have connected a wide variety of amplifiers, all differing in quality.... none failed, or showed signs of stress.... could it have been because i would only test for a few seconds at a time? i never did any long-term power tests. dunno, but the guy who told me how to do it seemed to know what he was talking about.... not saying anyone who posted does not know what they are talking about (if anyone would be me).... but like it said, used it probably on LOTS of different amps, no back-fire. but i should probably say 

USE AT YOUR OWN RISK!!! USE OF THIS DEVICE IS NOT INTENDED FOR YOUR AMPLIFIER!!! AND WILL LIKELY VOID ANY WARRANTY!

but yeah, i thought it was pretty cool  and if you own a stereo shop that sells low-end brands (you know who you are!) its nice to have an idea where you can find good cheap power, and steer clear of the dogs... for some reason good cheap power sells better than more expensive amps (generally).... i hate to stereo type but there seem to be certain cultures that prefer bling-bling instead of functional, but they still want something decent. so boss, legacy, lanzar, audiobahn, etc.... this was the device that gave us a general idea of what an amp is really doing (very rough estimate), and only cost me about $10 because Dan Wiggins at Adire donated 4 coils to my project (if memory serves). worked out great...


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## bigdwiz (Oct 7, 2010)

I posted a thread a while back in the "how to" section of this website. See my video here:

On YouTube in 1080p

or embedded below








Also, I found some 120w wirewound resistors for a reasonable price (might have been suggested by envisionelec, I can't recall (sorry Aaron!)):

8 Ohm 120W Power Resistor-MPJA, Inc.


Recent updates to my bench and Soundstream Ref 300 test:


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## hankbot (Jan 6, 2013)

envisionelec said:


> A raw voice coil and represents a very poor load as there is no component to give you a correct impedance generated by back EMF...if that's what you're going for. As it is almost purely inductive, you are sending a lot of current back into the output devices where it is dissipated as heat. If you are trying to stress the amplifier, this is how you do it. In fact, I use a similar jig to put amplifiers into protection mode.
> 
> If you are testing amplifiers, apples to apples, use a resistor.


Uh, I know this is an older thread, but I have to disagree with this. Resistance is resistance. He admitted that this won't tell you what your sub is getting due to impedance, but there is absolutely nothing wrong with what he's doing to measure maximum amplifier power. 

He's effectively made his own "liquid cooled" power resistor. The power being generated gets dissipated as heat in the voice coils, thus the distilled water (a crappy conductor) keeps the coils from melting just like the insulating material on a normal power capacitor. 

There is only one suggestion that I have, use as much water as you can to even out the temp, and take a hot resistance reading immediately after you turn the power off, like within a couple seconds. This will give the most accurate value to use in the power calculation. 

Thanks for the idea, I'm going to make one on these rigs I think.


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