Having trouble feeding audio from headphone out to microphone in.

Discussion in 'General Electronics Chat' started by mxmaniac, Nov 28, 2014.

  1. mxmaniac

    Thread Starter New Member

    Nov 28, 2014
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    I am a newbie, but I've been stuck on this problem for a year, I keep coming back to it, and I've probably wasted over 20 hours on it.

    I need to feed the audio out signal from my PC headphone out, into the microphone input of my Android phone (a Galaxy S1). The purpose is because I have a fancy voicemail greeting I made in an audio editor on my PC, and want to record it as my voicemail greeting, and simply holding the mouthpiece of the phone up the the pc speakers is unacceptable quality wise.

    Now I've identified the proper pinouts for the mic in, and headphone out, however connecting them doesn't work. From what I've been able to find, you can't just do this, because the headphone signal is too strong, or different.

    I've put a multimeter on the android microphone pins, and it shows about 4 mV voltage going through that circuit. I'm guessing this means you can't necessarily simply feed it an audio signal, I'm guessing its expecting a microphone to cause very small ripples on that 4mV signal it provides.

    Thus I'm sort of guessing I may need some sort of transformer, to hook the primary coil up to the PC headphone output, and run the secondary coil in place of the microphone on the android, inducing the audio signal into it. Just a guess though. This is pretty much where I'm stuck, don't know where to go from here.

    Any advice? I can't believe I'm so stuck on such a seemingly simple problem of plugging 2 audio devices together. Thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Line out level from a PC is about 1vpp
    Mic level into any modern device that likes electret mics is about 20mV.

    You need an attenuation of ~1/50. One way is to take a 10K or 50K pot, and hook it up thusly:

    atten.gif
     
    KJ6EAD likes this.
  4. mxmaniac

    Thread Starter New Member

    Nov 28, 2014
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    0
    Awesome. I finally got this to work, thanks to you guys pointing me in the right direction, also I found another link as well that was helpful I'll post below.

    In case anyone else has this issue, heres the results of my initial experimentation. This is going from a PC Headphone out jack, to an Android microphone in.

    Using the T attenuator with a 10K ohm and 100 ohm resistor combo worked, but resulted in a very tinny sound. I started working my way down in resistor values, and the sound seemed to improve. The sweet spot seemed to be around 1.5K ohm for the main resistor, and then I had a variable pot for the second resistor, and having it set right around the area of 17 ohms worked well. After going down to 1K for the main resistor, and somewhere around 10 ohms for the other one, the quality started seemingly getting worse. So for this application a 1.5K and 17 combo seemed like a good combo (possibly could be fine tuned even more though). I also ended up adding a 10uf electrolytic capacitor in the circuit before the main resistor, as I had seen done elsewhere. Unsure if it actually did anything, or even if 10uf is an ideal value, but it seems like it may have improved the sound a bit so I just left it in. After trying both polarities, I settled on the polarity with the + facing the PC side, and the - facing the android side.

    So far I'm pretty happy. I don't yet know if the sound is quite full quality, it didn't sound as good as straight headphones to the PC, but I think there could have been a lot of quality lost in recording using basic "sound recorder" that was on android that I was using to test with. I need to try a better recorder to see if I get better results. Or if anyone has any suggestions on improvements to the circuit, I'd be glad to hear them, as I only half-way know what I'm doing.



    http://www.epanorama.net/circuits/line_to_mic.html
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The PC line out is designed to drive a low-impedance, like standard PC dynamic headphones which typically have an impedance of 35Ohms or so. I have connected it to various external amplifiers, which have a much higher input impedance. I never noticed any distortion attributable to the load impedance that the PC line-out was terminated in.

    The Mic input to most modern things is designed for an amplified electret Mic, and it backfeeds a DC voltage level that is used to power the preamp built-into the Mic. I'm guessing that if you had put a DC-blocking capacitor between the resistive attenuator and where the tap feeds the Tip of the MicIn jack, the distortion would go away...

    I'm thinking that the DC path to ground through your resistive attenuator was shorting the DC level and screwing with the Mic input circuit in your phone and that caused the distortion...
     
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