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Old 19th December 2009, 06:22   #1
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Default DIY: Testing/Measuring the true power output of your Amplifier.

Hello guys,

Its not been long since I've join the t-bhp team and I cant express even partially how beneficial this site has been to me and how much info I've gathered already. Going through various threads, product discussions and also the pics of what many of you people have done has given me a hell-lot of new ideas for my car. I'll be posting another thread for that in the appropriate section when the time comes, however for this section, I've come up with a new DIY for all you enthusiasts who'd find it interesting. As the topic says, its about testing the power output of your amplifier.

Almost all brands have been categorized as either over-rated or under-rated. However, this is a mild expression cause we do not know for sure. Moreover, all the amps that have been rated to a specific power level, have been done so in an ideal condition - an ideal power supply (14.4v). Ill be testing many of the amps that I possess, once I'm back to my city in around 10 days, in more of a day-to-day environment (around 13.5v).

I wanted to start this thread to see how many of you people would be interested and also if any of you had ever tried doing this before.
I have already purchased most of the equipment required (except a true oscilloscope, a substitute will be used). In addition to testing the output power, I'll also see if I can get you the frequency response graph for those amplifiers.

The amplifiers I'll be testing are:

JBL - px300.4 - This amplifier is the reason for my experiment in the first place. This amp got me really confused about its power rating after I got my hands of two totally different sets of manuals for it.
The first one - rated to be 75 x 4 (4 ohms), 150 x 4 (2 ohms) and 300 x 2 (4 ohms).
The second one - rated 123.5 x 4 (4 ohms), 169 x 4 (2 ohms) and 358 x 2 (4 ohms).

After spending hours and hours over the internet and talking to as many people as I could over here, I finally found out that the cause of this difference was that the old power rating was not done according to CEA standards, and was tested with a power source of 12v while the later was established with a power source of 14.4v (thanks to CEA). This got me wondering, that maybe I should try to test the amp myself. Another factor that determined me was that the px300.4 has a fuse rating of 60 amps. Now, this is a class A/B amp. And to deliver 358 x 2 at 14.4 volts, it would have to be 82.87% efficient. I have never come across a class A/B amp this efficient. This is what made me conclude that I should test the amp out, to get more clear in my mind and for the fun of it.

Other amps to be tested, for the sake of it:

Pioneer PRS-D4100F
Kenwood KAC-8152D


I have some more amps that I'd like to test, if time permits. But these are the top ones on my list.

Guys, this experiment will be done by me for my own interest. I do not expect any of the amps to be even close to their rated power, considering the sole reason that I will not be testing with a power source of 14.4v, rather in-car battery voltage at idle (not to mention the quality of the tools required - I wont be investing alot as I dont plan on going around and offering amp testing services).
Also, in no way am I doing this experiment to compare any of these brands. Ill be stating the final numbers achieved for all amps, but that is not something that gives an overall estimation of that amp's built. There are many factors which are required to justify an amp's true performance, such as its damping factor, THD, channel isolation, delay etc. which will not be tested by me. So, even if this DIY gives you some additional info about an amp, do not make an opinion based on it.
And at last, testing an amplifier is dangerous - please refrain from trying this yourself, especially if you don't know much about circuits else it could result in test equipment failure, amp failure or even worse, an electric shock.

So now that the disclaimer is over with, lets see what you guys have to say.
Let the fun begin!!
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Old 19th December 2009, 10:37   #2
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@CVC2145, this looks to be an intresting experiment.

One question is reg the voltages you mentioned. I recently checked the voltage in my Sumo - at battery only, idling and 2.2k rpm (above cruising speeds), and found the voltages to be 13.2v, 13.5v & 13.5v. I know my alternator and battery are good.

Would like to know why & how the voltage is being considered as 14.4v ?

Last edited by bblost : 19th December 2009 at 21:18.
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Old 19th December 2009, 17:42   #3
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Condor, I am not quite sure about this. But as far as I know, battery voltage at idle should be above 13.5 (depending on the battery charge and alternator regulator voltage) provided you do not have alot of electrical equipment running. You should give the battery atleast 2-3 mins before checking the voltage after starting the car because after you start your car, the alternator would be recharging the energy lost to start the car. I think that the voltage you measured is pretty normal.

As to why the amplifier's are rated at 14.4v, my best answer is that this is the 'ideal' battery voltage in a car at idle. When you have a regulated power supply and run an amplifier which doesn't induce a voltage drop in the battery, you'd probably see the amp being fed in this voltage. Though almost everyone who adds an ICE setup gets a big voltage drop in their circuitry, many people upgrade their components to maintain this 'ideal' voltage. Especially, when you're running with an upgraded alternator or an external power supply.

Maybe someone with more knowledge could help in answering this in more detail, as even I'm not entirely sure about it...
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Old 20th December 2009, 02:35   #4
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Please explain the test procedure you are planning to follow...

I sometime read that this can be measured crudely using a simple multimeter, which can also be used to set amp gains.
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Old 20th December 2009, 17:37   #5
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I suppose you could find the output of your amp using just a multimeter. This would give you a bad approximation though. If you use a multimeter on an amp connected to speakers, you do not know the impedance of the speaker, the point where the signal from the amp starts clipping or even the frequency at which the amp pumps out the most juice. I guess this way the error would be too high for the result to be of any significant value.

The basic setup of what I've thought would go like - connect the load (power resistors) and multimeter on the speaker terminals of the amp, connect a wave generator and find out the frequency at which the amp sends out the most power, connect the sine-wave reader to the speaker terminal and max out the gain until it just reaches clipping and record the voltage there. Then, measure the resistance of the load as resistance changes with temperature and then use some formulas to find the total power output.

If you go ahead with this experiment, do let me know your progress/findings!
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Old 20th December 2009, 20:23   #6
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I just wanted to know how are you planning to go about it. Looks little complicated for a nOOB like me. Once you are done with it, may be you can post a step by step procedure, if it's within my comfort I will try it out...
Till then I will stay with your disclaimer

Quote:
And at last, testing an amplifier is dangerous - please refrain from trying this yourself, especially if you don't know much about circuits else it could result in test equipment failure, amp failure or even worse, an electric shock.
But I am really interested in this as I share similar concerns
Quote:
Now, this is a class A/B amp. And to deliver 358 x 2 at 14.4 volts, it would have to be 82.87% efficient. I have never come across a class A/B amp this efficient. This is what made me conclude that I should test the amp out, to get more clear in my mind and for the fun of it.
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Old 21st December 2009, 05:37   #7
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The disclaimer is no doubt to be taken seriously unless you're sure of what you're doing. If you connect something wrong, there are chances it would result badly. But after you know what you've to do, it gets alot less risky.

An added info about efficiency of amps and their fuse ratings, D-class are generally between 70-90% and A/B between 50-60%. For a faint calculation, take the fuse rating on your amp and multiply it by 10 for a D-class or by 8 for a A/B class. So the JBL working at 82.87% is really astonishing.

Ill be back home soon and I'm hoping this experiment to go well. Ill be posting a dedicated step-by-step instructions for anyone to do this, including a cheap alternative to the oscilloscope. Only thing that might be hard to find is a power resistor. So, if you're planning on doing this, you should start looking.
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