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Old 24th November 2003, 03:48 PM   #1
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Location: missouri
Default measuring output power

I was wondering if someone could explain to me how to determine the actual output power in watts of my aleph. Thanks, Mike
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Old 24th November 2003, 05:56 PM   #2
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There's lots of ways. What test equipment do you have?
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Old 24th November 2003, 05:57 PM   #3
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Thats easy. Measure the true RMS voltage across what ever load you want to test it on and then calculate the square of that voltage, then divide by the load amount.
Example: 40 volts RMS AC measured across 8 ohms is 1600 volts. Divide that by 8 to get 200 Watts RMS.

If anyone does it another way I'd like to know about it. I've always used the above to calculate it as I have several very accurate RMS AC voltmeters.

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Old 24th November 2003, 06:44 PM   #4
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Nelson, I have only a multimeter and a transistor tester, building this amplifier was my first electronic DIY project. By the way thank you so much for sharing your ideas! There aren't alot of people out in the world so generous with knowledge versus making money off that knowledge.

Mark, that sounds simple enough. So I can simply use a resistor for the load? What rating does the 8 ohm resistor have to have so I do not burn it up? Thanks Mike
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Old 24th November 2003, 06:56 PM   #5
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Originally posted by MikeZ
So I can simply use a resistor for the load? What rating does the 8 ohm resistor have to have so I do not burn it up? Thanks Mike [/B]


How about a big fat resistor which can take the abuse of your amp! So for a Aleph 2, at least one which can take 100W for some while! I use two 4 ohm wire wound resistor (8 ohm in serie, 4 ohm a piece, 2 ohm parallel!) which have bought at a dump store for US$1 a piece. I have put 400W (to test my power supply) on to them. After 5 minutes they started to glow, but they did not break down!

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Old 24th November 2003, 07:05 PM   #6
PRR is offline PRR  United States
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> So I can simply use a resistor for the load?

Everybody does. Loudspeakers are not really resistors, but the resistor-load power is a good indication of speaker-load power and a lot simpler.

What rating does the 8 ohm resistor have to have so I do not burn it up?

Bigger than the expected power of the amplifier. Small amps work OK with the 8Ω 20Watt resistor that Radio Shack used to sell. Big power amps need BIG resistors: use arrays of 20W to 100W resistors. Hmmmm... the Aleph 0 is rated 75 watts, but may have a "soft" distortion rise, and may deliver more power before it clips. And some were rated more. Better find over 100 watts worth of resistors that add up to 8Ω.

For testing above 1KHz, ideally you must use "non-inductive" resistors, though I've never seen much difference with the usual wire-wounds at mid audio frequency.

You bring up power until the amplifier distorts "too much". Since you have only a multimeter, you won't know when it is distorting. A scope will show gross clipping or severe warping. A THD meter is necessary for low-THD readings. But you can get the "gross undistorted (several-percent) power" reading by wiring a speaker in parallel with a 0.1Ω resistor in series with your load resistor. You can hear the test-tone in the speaker. When the sound quality changes, gets rougher, back-off until it cleans-up, then measure the output voltage.
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Old 24th November 2003, 07:08 PM   #7
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Can I use a lightbulb for the resistor? If so I could measure the resistance of a lightbulb and hook the lightbulb up to the ouput of my amp, then measure the voltage across the lightbulb?? Mike
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Old 24th November 2003, 07:51 PM   #8
Steven is offline Steven  Netherlands
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Lightbulb resistor will change too much because of temperature changes.

If you don't have big power resistors, just use some 10 or 20 smaller metal film or carbon resistors in parallel. In the past I just use e.g. 10 half Watt resistors in parallel, with the wires twisted together, soldered some leads to it and dropped the whole thing in a mug with water. It is amazing how many Watts you can dissipate with these small water cooled resistors, even 100W for some minutes. At a certain moment the water starts to boil, then get yourself a free refill with cold water. And it has low inductance too.

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Old 27th November 2003, 04:53 AM   #9
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One good source is at Steve there has some 50W, 8 ohm resistors. Put two in series and parallel that with another similar pair and you have something good for 200W. That's $8 worth.

Put it on some cheap heatsinks and you've got a new piece of test equipment for your laboratory.
Vince Harris
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Old 28th November 2003, 03:54 PM   #10
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Something not clear to me.

With a tester and a couple of
resistors I'll be able to measure
the output power.
But how can I notice the clipping
without an oscilloscope?

Under clipping the voltage reading
increases if I increase the volume.
Only when I have a square wave
it stops to increase but, at that point
I probably blow something.

I think one can use the audio
board of the PC and some free
software to see the wave shape.
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