posted
i was wondering if there was a way to find out what the real wattage of an amp was without bench testing it. like with just using a multi meater. thanks
posted
You can TRY and use a Voltmeter as you're playing a constant test tone(preferably), and see what voltage is produced across the resistance presented at that frequency, then using Ohm's law, you can calculate the power to RMS or average power. Would you like some fries with the meat? "multi meater "
-------------------- E.C. Wuz here Posts: 1057 | From: Vancouver, British Columbia, Canada 'eh | Registered: May 1999
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posted
well what i was thinking of was. there was a big post about this while ago and there was a big equation on how to do it. i know that u can't get way from the ohm's chart just seeing if there was a way without alot of variables.
posted
Use a very large 4 ohm resistor, test tones, and voltmeter. The fixed load keeps you from having to measure current, but let's face it, you have to measure something, otherwise your answer is something you pull out of your....
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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posted
And the problem with measuring a resistor is it will not be the real wattage of your system in use. That's where it counts, unless you can be assured that when you burp your subs, they are at the same resistance as your resistor.
quote:Originally posted by cavRF: i was wondering if there was a way to find out what the real wattage of an amp was without bench testing it. like with just using a multi meater. thanks
thats exactly how you test them, with a dmm
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quote:Originally posted by emu: Use a very large 4 ohm resistor, test tones, and voltmeter. The fixed load keeps you from having to measure current, but let's face it, you have to measure something, otherwise your answer is something you pull out of your....
the problem with measuring current and voltage is that it will not be at the same resistence when using it on a transducer(speaker).
also i do think measuring it on a resistive load is best, mainly because if you use it on a speaker you have way too many changing variables. kinda makes it not accurate. on a resistive load we know exactly what the amp is doing(given the tolerance of the resistence which is recorded) and what it does at that resistence, rather than having it on a changing resistence like on a speaker. you don't know what resistence it is at, so performance will very from install to install.
you should know that an amp will do be doing what it does on a when testing resistively compared to everyday in your vehicle, this is where the whole i need 1 amp alt per 1 amp test draw comes into play. not accurate, never will be accurate. you would be lucky to get 60% of what your amp can do on a resistor than everyday at full volume.
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posted
The reason we use a resistive load is so that we can have something with which to compare two amps with. By controlling certain variables, we get more universal results. If they both claim 1000WRMS @ 4 ohm, but one benches at 1500 while the other is 750, then we can conclude that no matter what sub you're using, the 1500 watter will be our best bet.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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posted
You know, you could just e-mail the company and ask them what they think it is. But ofcourse they would lie to you and plus it would be at 14.4 volt.
-------------------- Team Custom Sound Works Team Texas SPL Posse Team Crossfire Team DD 4 12' DD's 3500 series 4 Vr1000d's crossfire (stolen 5/28/06) 1 VR 302 crossfire (stolen 5/28/06) 1 Ohio generator 4 Optima Red Tops AOL IM = xyphr3 Posts: 6787 | From: Houston | Registered: Oct 2003
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posted
This is getting to be funny. Ok, the reason a resistor is used to test amplifiers and put those specs on paper, is because of the problem that the human eye can't see what the needle/digital display is reading as the speaker is moving @ whatever frequency.(as was mentioned) Now as for Ohm's Law, unfortunatly it can't be dismissed due to lack of intrest, just like the laws of relativity, so, as for specs, leave them on the paper, and listen with your ears,there's more to quantity of power than meets the ear. The last time I had a discussion about power quantity, quality, resistance and distortion, I held the spec sheet to the person's ear and asked what they heard? Keep in mind each amp will "react" differently to each speaker it's connected to, and it's not just because of electrical resistance.
-------------------- E.C. Wuz here Posts: 1057 | From: Vancouver, British Columbia, Canada 'eh | Registered: May 1999
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Is as easy as this. Amps X volts = watts. You need to find out what your drawing for amps x that buy the voltage going into the amp to get the true wattage my clifs draw 400 ea x that by 14.3= 5720 watts... just rember amps x volts = watts.
-------------------- U.S.A.C.I Regional Champ 10/04/03 North East Ct States 2nd plase 09/21/03 10 shows 10 wins in 2003 http://www.sounddomain.com/id/fast_fords Posts: 80 | From: springfield mass | Registered: Mar 2004
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Well, that and power factor! But, the power factor is a function of the sub mostly and not the amp, and shouldn't really change from amp to amp.
Ash
-------------------- ****************************** ****************************** Posts: 1130 | From: Los Angeles, CA | Registered: Aug 2001
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It wouldn't hurt to measure the resistance of the resistor immediately after disconnecting it from the amp as soon as possible in order to get a more accurate result. The heat produced by the test will raise it's resistance.
posted
I read that PDF file about testing using a DMM and a 100Hz test tone. I love the idea of the tweeter being used to find clipping. it actually works extremely well and corresponds to about 2 to 5% distortion.
I have to strongly disagree on the use of the 100Hz sine wave though. Inexpensive DMMs are designed using a known 'compensation factor' to acurately measure the RMS value of a 60Hz waveform. For this test method to work, you will need either a 60Hz wave or a True RMS multimeter.
The water-bucket method was used back in the early 90's for one of Car Audio and Electronics first big amplifier tests. These days, large load resistors aren't overly expensive.
I use eight 225W Ceramic resistors on my test bench, along with two Fluke True RMS meters, a standard meter, a currrent clamp and a scope. I just need to run another electrical circuit from the fuse panel to feed my three 90A Cascade Audio power supplies adequately.
-------------------- Dave MacKinnon Audio Technical Editor - Performance Auto & Sound Magazine - www.pasmag.com Posts: 60 | From: Burlington, Ontario | Registered: Oct 2001
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posted
Voltmeter, an AC Clampmeter and a calculator.
V x A = P V x V / P = I (actual sub impedance at that frequency)
Ben
-------------------- Ben ----- www.EastCoastChoons.co.uk UK SS 1-2 NW Record Holder Team UK - Team ECC - Team RE - Team DD Posts: 2924 | From: Deepest Darkest Norfolk | Registered: Aug 2002
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posted
I is used for current, or amperes. Z is for impedance, complex loads.
That should read, V*V/P=Z
Derived from, R=V/I and therefore I = V/R
P = V*I => P = V*(V/R) => P = V*V/R
which is the same as saying, R = V*V/P.
To get an actual sub impedance, we would need to make use of a scope and some lissajous figures. You need to know the resistive component, the complex component as well as its angle. The angle is where the scope comes in.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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quote:Originally posted by emu: I is used for current, or amperes. Z is for impedance, complex loads.
That should read, V*V/P=Z
Derived from, R=V/I and therefore I = V/R
P = V*I => P = V*(V/R) => P = V*V/R
which is the same as saying, R = V*V/P.
To get an actual sub impedance, we would need to make use of a scope and some lissajous figures. You need to know the resistive component, the complex component as well as its angle. The angle is where the scope comes in.
Yeah isn't power actually something like V * I * cos(alpha)? Do you have any idea how large alpha can be when dealing with a sub amp? I know it would depend on a lot of factors. Will alpha be different at your burp note for a spl box compared to a low tuned daily box? I'm learning about AC now in E. Sci., so I hope I'm not way off with my questions...
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