"switch" to combine stereo channels (mp3 player / portable application)

Discussion in 'General Electronics Chat' started by Spanner_, May 17, 2009.

  1. Spanner_

    Thread Starter New Member

    May 17, 2009
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    0
    I have some noise cancelling headphones for my mp3 player but there are times when it is good to have an ear free to hear things(!). However, it would be nice not to lose a channel (by removing a headphone) - i.e. it would be nice to combine the two channels into one channel that can be output to a single headphone (or both headphones - doesn't matter much).

    At first I thought this would be quite easy with a switch and some resistors to keep the impedences matched but I didn't really consider what effect shorting the two channels together would have or even if the left and right channel of a stereo image can be successfully superimposed ontop of one another without some crazy sound distortions occuring.

    Any comments on the feasability of the device are what I'm looking for really. I'm guessing it's quite hard to achieve given that I can't find one commercially available (for portable applications) but thought I'd ask for some informed opinions before;
    a) giving up (and looking into modifying the software instead)
    b) investing time doing it properly
    or, more likely
    c) breaking my mp3 player by 'playing'.

    Any comments greatly appreciated.

    Also, Hi!
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    An op amp summer amplifier can mix the two channels into one.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I think it is a bad idea to short circuit the outputs of two amplifiers together.
    The output of one amp might try to go positive when the output of the other amp is trying to go negative and the response is smoke if they do not limit the current by causing severe distortion.
     
  4. DonQ

    Active Member

    May 6, 2009
    320
    11
    You can do this:

    Code ( (Unknown Language)):
    1. Channel1---- resistor ------+-------+--- Headphone channel 1
    2.                             |       |
    3. Channel2---- resistor ------+       +--- Headphone channel 2
    The resistors are best if about 70% of the impedance of each headphone channel, but are not very critical.

    You can also get this function already built into a jack. Plug the jack into a stereo socket, it does a resistor mix of the two channels and outputs them to the mono socket at the other end of the plug.

    I think stereo headphones are arranged so that if they plug into a mono socket, it plays mono on both channels. If not, you can also get a mono to 2-channel plug/socket. So now what you have is

    Stereo -<<-Stereo to Mono-<<-Mono to 2channel-<<-Stereo headphones.
     
  5. Spanner_

    Thread Starter New Member

    May 17, 2009
    4
    0
    Thanks for the replies :)

    DonQ's solution seems ideal - and was roughly what I thought of originally but can I check that simply matching the impedences with resistors in those locations will also prevent any short circuit issues - such as those mentioned by audioguru?

    I don't know if it is relevant but the headphones are only small, in-ear devices - 16 ohm 9mm drivers - and don't take much juice to be at an audible volume (about a fith of the player's capacity (according to the volume meter).

    The different mini-jack converters are a viable option but I'd rather have a switch in situ so as nothing needs to be carried around seperately to enable 'one ear mode'.

    Also w.r.t. the summing amplifier - I'd assume this will require power to be readilly available? Whilst that could be arranged, it'd be much better if a passive system could be used that draws as little surplus power from the mini-jack audio output as possible.

    Thanks again.
     
  6. DonQ

    Active Member

    May 6, 2009
    320
    11
    I can't guarantee your soldering skills ;), but as long as you have the resistors inline, there is no "short". The resistors act as the load.

    If the resistors are sqrt(2)/2 of the headphone impedance (~70%), then theory says the load the output sees is the same as the original headphones. If you do something like only have it going to one headphone, then it will be slightly different, or you could calculate new values for the resistors.

    Fortunately, the output driver is able to drive a fairly wide range of impedances, so the value of resistors needed is not very critical. Somewhere in the range of 8-16Ω should be fine.
     
  7. Spanner_

    Thread Starter New Member

    May 17, 2009
    4
    0
    Super, thanks alot.

    My soldering skills are pretty good but I've just pulled apart a mini jack from a 'broken' set of headphones (bad connection at jack) and it's the thinnest wire I've ever seen! No wonder it broke. Good thing I don't mind a challenge.

    Anyway, I'll let you know how I get on once I get those resistors and an appropriate switch.

    Cheers
     
  8. DanTheMan

    Member

    Nov 16, 2008
    10
    0
    try something like this:

    [​IMG]

    There are a few around who've had it work.

    Dan
     
  9. DanTheMan

    Member

    Nov 16, 2008
    10
    0
  10. DonQ

    Active Member

    May 6, 2009
    320
    11
    Figure 1 at this link is the 600Ω version of my post. 70% of 600Ω is 420Ω, they use a 470Ω so that they can use a standard value resistor.

    For 16Ω, you would do the same thing, only using two ~11Ω resistors instead (or something close).

    The high value resistor (10kΩ) is not needed. It's just to make it so that when the inputs are unplugged, there isn't an open circuit on the connection to the right. If the connection on the right was plugged into another amplifier, it would go BBUUUUZZZZZ! without this resistor.
     
  11. Spanner_

    Thread Starter New Member

    May 17, 2009
    4
    0
    Thanks for the link Dan, certainly some good info there. I'm sure the opamp circuit would work very well but power is at a premium for this appication so I'm not persuing that route for now.

    DonQ; what is your take on the 20k resistor to the common 'ground' in figure 2?

    http://www.rane.com/note109.html
    (Reposting so you don't have to change page.)
     
  12. DonQ

    Active Member

    May 6, 2009
    320
    11
    Same as the 10k in Figure 1. If the right side is hooked to an amplifier, and both inputs are unplugged from the left side, it keeps the amp from going buzzz because the input to it would be an open circuit.

    I'm sure you've heard this when people are playing around with a PA system.

    With a permanent installation, or hooked to headphones, or if you don't mind the buzz when you unhook the inputs, these resistors are not needed. They don't hurt, but they only help for something I don't think you're going to do.
     
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