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Adding a headphone pass-thru to know when headphones are plugged in?

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  • Adding a headphone pass-thru to know when headphones are plugged in?

    Hey guys,

    I've got a speaker project that I want to add a headphone jack to. It currently uses a 15 watt amp to power the speakers. Want I want to do is have the amplified left/right signal go to the headphone audio jack if headphones are plugged in. Is there a simple circuit I can create that can determine when the headphones are plugged in and re-route the output?

    The only thing I can think of is sending a low current through the left or right pin to the ground pin of the headphone jack and then flip a relay if I find continuity between the two. But will that introduce any kind of hum to the audio signal? That would be bad. Likewise can whatever is on the other end of the line-in jack handle my constant test voltage going through it?

    Or do I need to jury rig some kid of split-collar that the jack passes through which would cause the two halves of the collar to be connected?

    UPDATE: I did find this post that lists a couple audio jacks you can purchase that have built-in switches to determine if certain parts of the audio jack are inserted:

    So how do you read from a switch with only one pin? Presumably the voltage is floating when nothing is plugged in and then grounded when the headphone goes in, so you have a pull-up resistor in there to give you a reference?

    Thanks for any help!
    Last edited by Cannikin; 04-25-2013, 07:02 PM. Reason: Added link to superuser forum

  • #2
    Re: Adding a headphone pass-thru to know when headphones are plugged in?


    The answer all depends on the switch you use. Most PC's these days use an audio jack with what is called an "Independent Jack Detection Switch". This is exactly what you described in your update. There are two types of these switches, one has a normal open connection and one has a normal closed connection.

    Basically, study the schematic of the switch and use a pull-up resistor to some voltage that is used to control the rest of your circuit (relay switches, etc.) The switch signal will either be high or grounded depending on the style of the switch.

    A more common method used in some amplifiers (like NAD for example) is to actually route the power amplifier output THROUGH the headphone jack. When a headphone is inserted, the speaker outputs are physically disconnected. You can view this type solution by looking at the schematics posted here:

    Basically, pick out a jack style that works for your headphone size and mounting desires and then design around it. I'd look to Digikey or Mouser for jacks:
    DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
    The Aria's - RS150-4 (or RS150-8) + XT25SC90
    The Mariposa's - TEBM65C20F-8 + ND16FA
    The Canzonetta's - RS100P-8 and ND16FA
    AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods


    • #3
      Re: Adding a headphone pass-thru to know when headphones are plugged in?

      Thank you very much! It turns out that the amplifier I have:

      Won't power headphones properly. There's a Question/Answer on the page that talks about trying to use it with headphones and the consensus is that it doesn't work (impedance too high and not sharing a common ground between channels).

      But this is great info for my next project, thanks again!