# Laptop Microphone input / attenuator



## Guest (Aug 17, 2006)

Hi, 
Hope this isn't too dumb a question but could we not use the microphone inputs on our laptops by placing a 40dB pad (and possibly a capacitor to block DC if bias in present on plug) in between the RS meter and the mic input?

Terry


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## brucek (Apr 11, 2006)

> could we not use the microphone inputs on our laptops by placing a 40dB pad (and possibly a capacitor to block DC if bias in present on plug)


The RS meter has no bias present - its outputs standard line level.

I doubt if the mic input would provide a very good signal. The noise floor would be high and I suspect the input impedance is low which would cause some distortion.

Best to use the line input. External USB soundcards are quite inexpensive..

brucek


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## JohnM (Apr 11, 2006)

terrym said:


> Hope this isn't too dumb a question but could we not use the microphone inputs on our laptops by placing a 40dB pad (and possibly a capacitor to block DC if bias in present on plug) in between the RS meter and the mic input?


I did some loopback measurements on a laptop mic input a while back with a suitable attenuator in the loop, aside from the high noise floor Bruce mentioned (less than 40dB S/N) the frequency range was also restricted at both the top and bottom ends. Only tried that one, so others may be better - can't hurt to try a soundcard measurement with the pad in the loop and see how it comes out. If you do try connecting an RS meter you will need the blocking cap else the mic input's 5V bias output pegs the RS meter to the end of its scale.


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## brucek (Apr 11, 2006)

> you will need the blocking cap else the mic input's 5V bias output pegs the RS meter to the end of its scale.


John, do you feel that the blocking capacitors high pass effect would become overwhelming at low frequencies since this probably is a low impedance input? The reactance for example of a 15uf at 20Hz would be ~530ohms. If the input were 600ohm (a guess), this divider would half the input? He should at least use a large enough capacitor to overcome this?.....


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## JohnM (Apr 11, 2006)

The input impedance should be 4k min, 10k recommended. Also the 5V output is on only one of the stereo connections, so with appropriate connections the blocking cap could be omitted. See this diagram, from a useful article at http://www.epanorama.net/links/pc_sound.html#interfacing


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## brucek (Apr 11, 2006)

> aside from the high noise floor Bruce mentioned (less than 40dB S/N) the frequency range was also restricted at both the top and bottom ends


Yea, no kidding... from the article _"AC-coupled tip to implement analog (external to ADC) 3 dB rolloffs at 60 Hz"_



> so with appropriate connections the blocking cap could be omitted


Yea, although the ring would provide voltage to the mic as the plug was inserted, likely causing a quick pegging of the meter - possibly damaging it - even though the supply would be limited? I guess if he avoided the ring connection to the supply, it would be wise to plug the cable in first and then the RS meter last..... on second thought I suppose his attenuator would ameliorate the problem somewhat..

brucek


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