On 12/10/10 19:26, Wayne Stallwood wrote:
On 11/10/10 06:11, nev young wrote:
So assuming that you want to connect a line-out on one machine (probably not a computer) to the mic-in on the laptop all you need is a simple attenuator.
IIRC, the line out gives 1 volt pp and expects a 100Ω load and the mic in expects 100 mvolt pp from a 10KΩ source.
So a few resistors from the junk box[1] should do the trick.
ISTR that the mic input on most laptops expects a Electret microphone with a built in preamp FET and therefore has a DC Bias (phantom power) you'd want to decouple your Line input from.
This is correct. But. The power is (usually) supplied via the third line. That is to say the mic signal connects between the tip and the body of the 3.5mm jack. The power connects to the centre ring and body of the jack. So as long as you leave the centre ring alone there's not going to be a problem.
see http://www.hobby-hour.com/electronics/computer_microphone.php for pretty pictures (not mine).
So a more suitable circuit would look something like
Line in ---C1-------R1---+---Mic Output | R2 | Ground-------------------+---Ground
I would have gone with:
line source Mic input on laptop
-----C1------+----90KΩ-----+------ | | 100Ω 10kΩ | | ground ---------------------------
OK so it's not perfect impedance matching but probably as good as you can get taking into account the tolerance of the resistors.
If you're paranoid about DC levels then a cap C1 can be included but personally I've never needed one. YMMV.