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I have been struggling to find the best way to connect a stereo headphones output (the socket where you would normally plug your headphones into) to a Lenovo G575 laptop microphone input for recording. I have tried various methods and got terrible wavy distortion even though the level was set correctly and there was no clipping. The laptop sound card does not have the option of converting the microphone input to an auxiliary input. I tried the suggestion of connecting each stereo channel to a 330 ohm resister in series with a 1 micro farad capacitor which are wired to the laptop mic input (ring and tip wired together) but am getting the wavy distortion.

Video

I also tried adding a 1k resistance to this after the capacitors and also tested with 22K but get the same wavy distortion even when the level is set correctly. The distortion is hard to describe. Its not clipping. Its more like getting louder and softer as if someone is waving a microphone.

I note the resistance of a typical laptop microphone is 1K. I think the solution must lie in getting the correct resistance bridge set up. Your advice would be appreciated.

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6 Answers 6

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Speaker levels are in the ballpark of VOLTs and Microphone levels are in the order of millivolts .There are only a few exceptions to this generalisation that are unlikely on a laptop.So whatever you do with resistors lots of attenuation is needed ,why not start with an attenuation factor of 100 and use your capacitors to block DC keeping frequency response flat.If all this doesnt work well after optimising the attenuation factor you could use more caps to lowpass filter at say 16KHZ just in case the laptop speaker output is full of HF crud that you cant hear but could be mucking up the microphone input .Another possibility is ground loop ,noise issues.If you place an audio transformer in your circuit and the problem goes away then you know .proving this can be done with any junkbox audio transformer .Then you can do it properly with good transformer OR diff amp stage.

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  • Thanks for the suggestions. This circuit is what I have so far: dropbox.com/s/ez4djzuk234xfqy/… Is this correct? I am wondering if I need the two x 1micro Farad caps or should just stick with the resistance divider. I am looking for a flat frequency response. Kind Regards Chris.
    – Chris N
    Commented Sep 21, 2015 at 14:42
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You're most likely to get better results with a resistive voltage divider. That is, a relatively large resistor in series with the mic. input, and a lower value resistor in parallel with the mic. input. That way, you can get whatever level of voltage reduction you want, rather than by guessing the impedance if the mic. input on the laptop.

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  • Thanks for the suggestion. I am thinking of using this circuit if I have understood the resistance divider concept correctly. Maybe the caps are not necessary. dropbox.com/s/ez4djzuk234xfqy/… Kind Regards Chris.
    – Chris N
    Commented Sep 21, 2015 at 14:45
  • That circuit looks quite reasonable to me. The two caps are there to block any constant DC voltage that might be present on the headphone output. The 330 ohm resistors mix the two channels to give mono (without shorting them together), and the 100K resistor and 1K potentiometer are the voltage divider. A logarithmic pot may give better control over the volume than a linear one..
    – Simon B
    Commented Sep 21, 2015 at 20:52
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Attempting to turn a line level signal into a mic level signal will not provide the best quality. The easiest method would be a dollar usb sound card. Plug and play line level in.

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  • Thank you for this suggestion. I tried it and it works great. I still have to use 1 μF capacitors and a large resistance of 22K to reduce the signal before using the usb sound card but I can now record without distortion. It seems that not all laptop computer microphone inputs are the same and some laptop mic inputs are more forgiving or have a line in option. Other useful link: electronics.stackexchange.com/questions/60949/… Kind Regards Chris.
    – Chris N
    Commented Sep 19, 2015 at 20:54
  • That's odd. You shouldn't need that
    – cde
    Commented Sep 19, 2015 at 21:32
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Thank you everyone for your helpful replies. I have considered all the answers in this thread and here is a summary of what I have learned. There are several problems to consider when connecting a speaker output to a microphone input.

1- Reducing the powerful speaker output to the lower microphone input. Rather than using a capacitor which will reduce the bass, I believe the best way to do this is by using a resistive voltage divider. (Thank you Simon B for this suggestion)

Please see the circuit diagram below. If the mic volume is set to 100% and the signal is still too weak, then try decreasing the value of the 100K resistor or increase the value of the 1K resistor. (eg. instead of 100K/1K try 50K/1K or 50K/2K; experiment until you find an acceptable solution)

2- The other problem is that many modern laptops do not have a separate microphone input. They just have a 4 pin 3.5 mm jack input which combines the stereo output with a mono condenser microphone input. These microphone inputs are designed to work with condenser microphones and do not work well with other inputs. You are likely to get distortion with other microphone inputs.

The best solution is to purchase a usb sound card which has a separate microphone jack input. (Thank you cde for this suggestion)

3- The other question is to decide if you want to connect the speaker output to a mono or stereo microphone input. All the laptops I am familiar with only have mono microphone inputs. However, you might find stereo microphone inputs on some high-end equipment.

If you wish to combine the 2 stereo channel outputs to mono, the correct way is to use a 475 ohm resistor in series with each R/L channel before combining them. Then use a 20 K ohn resistor between the combined signal and earth. The credit for this method can be found here:

ProSoundWeb "Stereo to Mono Summing Box" https://www.prosoundweb.com/why-not-wye/

After combining the signals this way, then use a resistive voltage divider to reduce the signal before connecting to the mono microphone input.

Circuit Diagram: https://www.dropbox.com/scl/fi/mlhiixxzsza7eja2otfd1/Headphones-Out-To-Mic-In.jpg?rlkey=677eg7i855na9zov1l4s2csyo&dl=0

I wish to thank you all for helping me to find a solution.

Kind regards Chris

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Get a 3.5mm to 3.5mm cable like this, cut it in half, solder in the caps and resistors and then use it. If it doesn't work it's only 99p for a cable.

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I was searching the same topic and encountered this YouTube tutorial. He explains how he does the conversion for headphone Output/STEREO MIX to MIC Input. I hope it helps

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    While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review
    – zx485
    Commented Mar 18 at 3:02
  • Thank you very much Emre for posting the link. Using a 1 micro farad capacitor and 330 Ohm resistors in series with the R/L channels and then combining to mono will work. I have successfully used capacitors and resistors like this before. However, the problem is that the 1 mF capacitors will remove some of the bass. It's not so serious for voice, it will act like a "low cut" filter and might make the voice clearer. However, it will affect the quality of music. You would need to experiment to find the best value for the capacitor.
    – Chris N
    Commented Mar 20 at 13:13

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