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Old 24th October 2003, 07:56 AM   #1
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Default Measuring Current Output of an Amp

Hey guys, probably a dumb question but how does one go about measuring the continuous avg current output of an amplifier? Brands like HK rate high current outputs but brands like Onkyo dispute how they come up with those numbers.
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Old 24th October 2003, 08:53 AM   #2
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1 Get a dummy load of 0.5 (or 1) ohms non-inductive

2 Get signal generator which can produce a single sinus period pulse followed by a long rest (= low duty cycle). You can also use a pulse generator. Time for the pulse 1 ms or less. Rest at least 100 ms.

3 Get an oscillocsope

4 Make sure that the amp is going to withstand this sometimes destroying test!

5 Increase the signal until you won't get more out (when you see a clipping of the sinus pulse). Output voltage/dymmy load = Current.

Such tests can destroy amps!
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Old 24th October 2003, 10:13 AM   #3
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Perhaps, we should get clear about what is to be measured?
Shortcircuit current capability? Hardly interesting, or ...?
Current capability into resistive load? What resistance?
Current capability into reactive load? What impedance?
...

The actual measurement is easy in either case. A very low
value resistor in series with the load, and follow Per-Anders
procedure. I would recommend a much lower value than
0.5 Ohms, though.
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Old 24th October 2003, 10:23 AM   #4
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You guys are recommending a hot game ....
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Old 24th October 2003, 10:31 AM   #5
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The most interesting measurement is the Amplifiers capabillity
to deliver current into a capacative and inductive load (like a speaker). A current measurement into a non-inductive resistor alone, will not tell how the Amplifier handles difficult loads (but it sure gives a better rating)
But be carefull, and don't push it tyo hard!!!!!!
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Old 24th October 2003, 02:57 PM   #6
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Yeah hey guys, thanks. 1ohm seems kinda harsh. Its interesting though....I think that's how HK comes up with those big numbers. I'm gonna do some tests on my amp and see what I get into more of a real world situtation based on those calculations. Appreciate the responses.
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Old 24th October 2003, 03:27 PM   #7
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an interesting approach to exercising output of single ended amps is to connect a resistive load between the outputs; diving the 2 inputs with relatively phase shifted signals allows simulating any impedance load

in "Output Resistance and Intermodulation Distortion of Feedback Amplifiers" JAES V 30, #4 4/82 pp178-190 Cherry and Cambrell suggest driving the 2 channels with 5:7 frequency ratio sines to sweep out the I-V characteristics of the outputs and test for "Interface Intermodulation Distortion", a sharp rise indicating ouput stage strain

( any sound card should let you test to < 0.1% IMD )
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Old 24th October 2003, 03:36 PM   #8
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As Christer says, you can take any kind of load, just you know what you are interested in. I gather that it's easiest to have a resistive load.

I'll guess that current capability is like MHz or GHz in computers, says some of the performance.
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Old 24th October 2003, 03:52 PM   #9
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And of course, which I should perhaps have pointed out, with
a purely resistive load it is even simpler to measure. Just
measure the voltage over the load and apply Ohms law.
It is not very interesting to talk about the current for resistive
loads. Usually people are ineterested in power, and for
resistive load the current and voltage is implict from there.

I tend to agree with ACD that the only case (except for
calculations when you are designing an amp) where it is
interesting to talk about current capability is for reactive
loads, where you my need a lot of current also at low
voltage. Highly reactive loads can be fatal for the amp for
this reason, if it does not contain proper protection. Since
the current is high, but the output voltage low, you don't
burn that much power in the load. So where does the rest
of the power go? Right, the output transistor have to take
a high voltage and a high current simultaneously. Many
amps have more two or three times as many output
transistor as might seem necessary if only considering
resistive loads., in order to handle this situation.
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Old 24th October 2003, 05:07 PM   #10
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Default Re: Measuring Current Output of an Amp

Brands like HK rate high current outputs but brands like Onkyo dispute how they come up with those numbers. [/QUOTE]

The confusion is caused by marketing. They created the
term "high current" amplifier to imply they have a better
design. Analogy -> My car has a high horsepower engine.
Both these statements tell me nothing without
a comparison to make the statement valid.

My ABC amplifier design is 'high current' as opposed to
my other XYZ design but ABC is not high current compared
to my Krell amplifier based on tests. /hahahah
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