posted
the easist way is to use a resistor and volt meter. all ac volt meters measure rms voltage. the wattage is W = V*V/R. R is the resistance. You need to detemine when the amp is distorting. you can hook up a $5 radioshack piezo tweeter and test the resistor with a 60hz signal. when the amp starts to clip you will hear it in the tweeter.
If you want to test it on a sub and get the real rms power you need to know the dc resistance of the sub (use an ohm meter), and measure the rms current at a singal frequency. for this you need a good clamp on meter that is accurate (not all are). the wattage will be W = I*I*R. Its really not that hard.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Yes i agree with your method mr.dank and i use it each time i need precision.
I know a second way, with less precision, but really faster: Often, amplifier are between 60 and 80% of efficiency. The input voltage is between 12 and 14 volts with amplifier loaded. I chose 65% of efficiency and 13 Volts. Now you just have to check the total fuse value and calculate with P=VI...To keep the max power output possible, the amplifier desingner keep a little margin between the maximun current take by the circuit and the fuse value normally 1/3 more for the fuse. EX:
I measure 12,8V, take 65% eff and my amp have two 30 amp fuse:
30A * 2 = 60A 60A + 1/3 = 80A (current point to blast fuse)(sinwave input-----squarewave output)
Now we need to calculate the RMS output value ...we will use 60A value 60A * 12,8V = 768VA 768VA * 0,65 = 499 ...say 500Watt RMS output
The 500Watt is the max total RMS output of the amplifier. If your amp is a 2 ch with a minimum load impedence of 2 ohms/ch, You can calculate a power around 250W RMS at 2ohms/ch and 125W RMS /ch at 4 ohms (if the damping factor is excellent, the 2X factor is raisonable)
Try it and compare with measured value! the error margin will low...
Doctorbass
------------------ Show me the bass !
Posts: 643 | From: Quebec city | Registered: Mar 2000
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posted
Sorry to put water on the statement that all voltmeters measure rms voltage but they are by default measuring average voltage .636 of peak unless the voltmeter specificately says that it is a true rms voltmeter.
quote:Originally posted by Stroller: Sorry to put water on the statement that all voltmeters measure rms voltage but they are by default measuring average voltage .636 of peak unless the voltmeter specificately says that it is a true rms voltmeter.
Mike
I thought rms was .707 of peak
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Posts: 375 | From: Buffalo NY | Registered: Dec 2000
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Your are absolutely right, rms is .707 of peak, that is why the voltmeter has to be calibrated in true rms or the power calculations are incorrect using an ordinary of the shelf cheap meter. I think just about all Fluke meters read rms but there is only one meter by radio shack that is listed as true rms.
posted
Hi There isnīt a car audio amp out there with 80% efficiency, and thatīs it! Some class D amps exceed 70% near their max power output, and they typically start at about 20-30% at 10 W. A normal AB amp will very seldom reach the 50% mark. We have measured amps from JBL, RF and others, and weīve done it at 4 ohms. So the above fastcalc can be used, but calculate with 40-50% instead.
------------------ Magnus Fredholm "No guts, No glory"
Posts: 18 | From: Stockholm, Sweden | Registered: Mar 2000
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I dont agree with you magnus fredholm I often calculate efficiency of car amp and each time that was at full power (just before _-_-_- ) and y always got result between 50 and 70%...! AT FULL POWER (bridged at minimum and recommended impedance load) That was with powerfull non inductive resistor and 13 volt supply(amp loaded).
WHY calculate efficiency of an amp if it is not at full power??? what is the utility ??? for dbdrag? I am sure that every competitor here always drive their amp at MAX! in this case? noh??
also..I am not talking about internal power supply effeciency of amp now but between DC in and amp output.
Many compagny give just efficiency of the switching power supply but they dont indicate it...
I know no that phoenix gold ever made an amp with real 80% of efficiency in older model..
I hope that you guess that i am not talking about pyramid or sparkomatic fused at 20amp and giving PURE 60W rms (miracle!!)at 10%THD at 14,4v ...and 300W PMPO writed on top.. enh?...
------------------ Show me the bass !
Posts: 643 | From: Quebec city | Registered: Mar 2000
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posted
Hi Again Iīd like to clarify that the measurements were not made for fun, bus as part of a shootout between class D amps. The measurements were made by AudioGraph, the company that manufactures the PowerCube, used by RF, HK and others. The only reason I stated efficiency at low power as well was just because it might be interesting to someone. AudioGraph measured at full power, 13.8 V, THD less than 1%, and the efficiency for a typical class AB amp was 45%. The highest eficiency measured was for the JBL A1201 GTi@74%, a class D amp. Similar efficiency numbers were reached by the Kicker DX350, Lanzar 1200D, Signat Digi Mk II and DLS D1000, but those are also all class D amps. Thatīs what we got, and I believe the numbers can be trusted. But Iīd love a 80% efficient class AB amp any day!
------------------ Magnus Fredholm "No guts, No glory"
Posts: 18 | From: Stockholm, Sweden | Registered: Mar 2000
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posted
Hello gentlemen, I was wondering if it is true,then, if my Kicker ZR 1000 a/b amp is at 45% before clipping, is it possible for that amplifier to be at 70% efficiency at full clipping?
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posted
In amplifier land, clipping means that the output transistors are all the way on more of the time then when there is a regular signal. Transistors are most efficient when they are all the way on, and least efficient when they are part way on. So yes, when your amp clips it is more efficient.
Dont take this wrong and think that you are getting more power to your subs when the amps is clipping badly. You are, but most of it is at a high enough frequency that it will do you no good for spl. If your system is set up correctly and you play the note your car has the best response at, Then in theory your spl will be greatest right at the threshold of clipping. This is when your output voltage will be greatest and all of the output power is going to a single frequency, which is what you want.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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