nothing to do with mics

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antispatula

antispatula

Active member
I posted a topic like this a while back, but thought I had solved the problem, but have not.........

Ok, so I need to measure the mV coming out of a computers soundcard. I set the multimeter to mV 200 AC. I put the red lead in the red whole, the black lead in the black whole. Without the leads even touching anything, the meter is all over the place! And then when I play the 1 kHZ tone coming from my computer that I need to measure, the measurements are STILL all over the place, and the annoying thing is is they keep increasing streadily, or decreasing steadily. I could measure something, then a few seconds later have a different measurement.

I'm measuring the reading from a 1/8" stereo. When I test ground and the red wire, I get like .078 and when I test the ground and black wire, I get .014....Shouldn't they be the same things, just the right and left channel?!

None of this makes sense. Why are the numbers going up and down all the time. I even switched to DC voltage and would get the same readings. That doesn't make sense.

If you have any ideas, please help me.
 
not sure what you mean by red whole (sic) and black one

if you measure from a stereo line out 1/8" jack, you're talking about 3 circuit components, left out, right out and ground

you need to be able to access each of these 3 separately, you can't just stick a meter probe in the hole

get a 1/8" stereo plug at rs or somewhere and unscrew it from its housing (or if you have a y adapter that's good) and plug it in to the stereo out

measure from left to ground (shield) on the connectors on the plug, and right to ground

with a meter (depending on its quality) you're going to get an rms reading, which is about 0.707 of the peak to peak reading, so multiply by 1.4 if you need the p-p

better would be to use an oscilloscope, but you probably don't have one
 
Woah....I just tried this on a different computer and it seemed to get a more stable reading, and I got exactly what I wanted (I think) 28.5 mV.

Woah, I don't get the rms and pp thing. But this is what I'm doing: All I need to go is get a 1KHz tone to read -18dbm. -18dbm is 28.5. So does that mean all I need is to get 28.5 mV on the meter, or is it more complicated than that?
 
oops, my meter reads in RMS, and the chart I used was PP.....So wait, do I want to calculate -18db at 600ohm?
 
A PC might not make a great source for that test. If the tape deck input impedance is 600 ohms, that could load the PC output and cause the voltage to drop, throwing off your reference level. Confirm that the voltage hasn't changed by testing the leads connected and unconnected to the tape deck.

Edit: the reason your chart may have been off was because it was calculated to 50 ohms, not a 600 ohm standard. Again you need to know the tape deck input impedance to be sure you have the correct reference level.
 
ah, I think I'm getting this now. No, I think the pc one is working fine, and this is why I think that:

When I sent a .097 mV RMS signal into the machine, I was supposed to get a .123 V reading, and I got .1238 reading.....That's a pretty good indication that is right I think.....right? :D
 
how do I know what the OHM's are on the output of my soundcard. Are they usually 600?
 
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