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Post by superluxman on Dec 22, 2010 9:19:31 GMT
I measured the resistance of my sound card with a multimeter to perform more accurate scheme. Interesting results:
When the black test lead at ground: left channel ~ 2,5 Ohm, right channel ~ 17 Ohms.
When the red test lead at ground: left channel ~ 18 Ohms, right channel ~ 4 Ohms.
The test was performed while the computer works.
How to evaluate such results?
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Post by Deleted on Dec 22, 2010 9:26:36 GMT
I measured the resistance of my sound card with a multimeter to perform more accurate scheme. Interesting results: When the black test lead at ground: left channel ~ 2,5 Ohm, right channel ~ 17 Ohms. When the red test lead at ground: left channel ~ 18 Ohms, right channel ~ 4 Ohms. The test was performed while the computer works. How to evaluate such results? superluxman You can't get meaningful results when measuring like that with a DMM. The only meaningful measurement you could obtain would be that of any residual DC voltage out of the left and right channel soundcard outputs. You would need to set your DMM to a DC voltage range. Alex |
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Post by superluxman on Dec 22, 2010 14:54:09 GMT
My sound card has two outputs, both outputs works as a headphone amplifier. When the computer is off the residual voltage at the outputs is zero . When the computer is turned on voltage at the first output: left channel - 1.5 mV, right channel - 1 mV; voltage at the second output: left channel - 2.8 mV, right channel - 7.6 mV. What does that mean? How to calculate the resistance?
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Post by Deleted on Dec 23, 2010 8:11:40 GMT
There is only one good way to measure the output resistance of an amplifier but you will need: NOT the cheap multimeters with only 200V AC setting as a minimum but the more expensive ones like a Fluke or an oscilloscope. The reason you can't use the cheap DMM is the fact that they cannot measure a few volt AC accurately because of the voltage drop of the sinngle rectifier diode used in the meter to convert AC voltage to DC which the meter can only display. More expensive meters with 2V or 20V AC settings use different methods of rectifying and can accurately display low AC voltages (up to a certain frequency). a bunch of resistors ranging from a few Ohm to 100 Ohm or 100 Ohm (to max 1kOhm) potmeter. a sinewave of say... 100 Hz (easy to do with a PC and tone generator program and most (good) DMM can measure this frequency very accurate) Play the tone on the computer just below the maximum level where it begins to distort. Connect ONLY the AC voltmeter (no headpones or other loads) and note the voltage. Now in parallel to the voltmeter attach the potmeter or resistors and play with those so you get half the output voltage (without touching the volume control of the computer but by lowering the resistance of the load) When half the voltage is reached (-6dB) take the resistors of the amplifier and use the Ohm settings to measure the resistance of the resistors/potmeter that were attached. The value you read IS equal to the output resistance...  You can't measure the resistance directly as a multimeter uses DC current and also get's thrown off by the DC output voltage (like Alex mentioned) I would be using output 1 (lower DC offset) but I figure output 2 is not the same and might have a different (higher ?) output voltage and probably different output resistance too. |
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Post by gommer on Dec 23, 2010 9:01:07 GMT
Hehe, that's a nice method. As simple and logical as can be, as many simple things are once you've heard them. But you have to think of it.
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Post by Deleted on Dec 24, 2010 17:32:17 GMT
If you don't have an AC voltmeter or oscilloscope but DO have a cassette or tape deck at hand or even a digital recorder with a line input and a meter display you can do a similar thing but use the input of the recorder as measuring device.
Set the 'unloaded' level to 0dB (check if the sound does not distort via the line out or HP socket of the recorder) and lower the voltage by means of the resistors/potmeter to -6dB.
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Post by superluxman on Dec 26, 2010 18:52:44 GMT
I found an easier way to measure the output impedance. Potentiometer in such case is not needed. Only a constant resistor. Output impedance is calculated by this formula:
Rout = Rload * ((U / Uload) -1)
where Rload - load impedance (resistor), U - no-load voltage, Uload - voltage with the load.
This method gives the same results as the method of solderdude. But a simpler, because not necessary to select resistance value.
I have no true RMS multimeter, just a cheap ($ 25) 3 3/4 digits DMM.
But even so the results are as follows: both outputs are almost identical and the resistance is 12 ohms.
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