a/b audio switch

Discussion in 'The Projects Forum' started by nicholr, Oct 27, 2009.

  1. nicholr

    Thread Starter New Member

    Oct 27, 2009
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    Hi all, I am so green at this I make grass look pale. I am trying to build a simple A-B switch device that will allow me to select between two stereo audio input devices (Sirrius radio and an I-pod) with a singular stereo output. I had a humilating experience in an electronics wholesaler today where the clerk virtually sneered me out of the store for my asking if he could affer some suggestions. Apprently "leave" was the best he could muster. Any help will be cherished.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    A simple DPDT (double throw double pole) toggle switch will work. A small box and three 3.5mm stereo jacks are all you need for the whole thing.
     
  3. nicholr

    Thread Starter New Member

    Oct 27, 2009
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    Thank you for responding to my very elementary question. I appreciate it. Can I prevail further to ask you how to wire the switch or if you have a wiring diagram. I told you I was GREEN!
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Be patient, or someone may chime in. I am on a satellite and have a huge storm rolling in, so will be off for some hours.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Okay, let's see how this goes. The image attached is of a DPDT toggle switch. You would do fine with a so-called minature size.

    Hopefully, you already have some tools, like a soldering iron and wire strippers, diagonal cutters, small pliers, and even a drill to make mounting holes in a plastic project box.

    You will want to get a 3.5mm male-male cable as well, plus one 3.5mm female socket. The Wikipedia article has illo's and the usual connections - http://en.wikipedia.org/wiki/TRS_connector

    Looking at the switch, the two center terminals are common, and make connection to the outer terminals depending on the position of the toggle.

    To do your switching, cut the 3.5mm cable in half, and strip the cut ends to expose the inner conductors. Those need to attach (solder) to the switch. One input signal to one end of the switch, the other to the opposite end. The braid shielding should be twisted into a wire and soldered together.

    If you have a meter, determine the conductors that go to the tip and the ring, so you can connect them correctly to the female socket. Use a small plastic box for the switch and cables.
     
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  6. nicholr

    Thread Starter New Member

    Oct 27, 2009
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    Thank you so much for the informative link (I need to wire my kit plane for an intercom and headsets and that article will be very helpful) and more importantly thank you for indulging my very basic question. I am baffled by 99% of the communications on this site (I'm mechanical savvy not and not electrical/electronic savvy) but I find the communications very intersting and useful. I appreciate your patience. Thank you !! Ron
     
  7. jeromen

    New Member

    Aug 24, 2011
    4
    0
    Hi,
    I just tried to do this exact project, and it didn't work out. The volume ends up being attenuated. (almost 1/2). I tested with a Ohm meter, and there is no resistance to speak of.
    One thing that I've learned is that the DPDT switches are rated for V and A. Not knowing what I was doing, I bought a 6A 125V unit. I've now read that I should have gone with a 1A unit. Would that make the difference? I am having a difficult time explaining my poor result.

    Thank you.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    The choice of switch should not matter. Can you post a picture? Something mysterious is going wrong and a picture is worth ... well, you know.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    896
    No.
    Electrical switches are rated for 125V at 6A and have silver contacts.
    Silver quickly corrodes and forms a layer of oxide that is an insulator to low voltage and low current audio but gets penetrated by the arc of an electrical device.
    Audio switches and audio relays have gold plated contacts and are rated at 20V at 0.4mA. Gold does not corrode.

    An audio switch or audio relay with gold plated contacts costs the same as an electrical switch or relay with silver contacts.
     
    elec_mech likes this.
  10. wayneh

    Expert

    Sep 9, 2010
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    No argument, but do you really think this is the OP's problem? I'll be surprised, but that happens often enough. ;)
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    If he buys an expensive (but cheap) Chinese electrical switch at RadioShack and it has been corroding in the store for 20 years then IT WILL NOT PASS low power audio.

    My Yamaha receiver uses a cheap rotary switch with electrical silver contacts and the switch frequently fails to pass low power audio.

    ALL the audio projects I have built for the last 47 years (with gold contacts on the switches and relays) have never failed.
     
  12. jeromen

    New Member

    Aug 24, 2011
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  13. wayneh

    Expert

    Sep 9, 2010
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    It's not obvious in your picture how the ground is passed thru, or if it is at all.
     
  14. jeromen

    New Member

    Aug 24, 2011
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    Ah, yes, I missed that step. So, I need to twist the braid shielding to make a wire, and basically solder all three together? Thanks for noticing that I missed that.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Yes, you need 3 conductors to convey stereo audio. You wouldn't normally switch the ground line, only the two hotties. If you get any buzz from your rig, you could mount the switch in a metal box and connect it to the shield wires.
     
  16. jeromen

    New Member

    Aug 24, 2011
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    Of course, that was the problem. I completed the project last night, and it sounds great. I am really thankful to this forum and Wayneh in particular. That you took the time to correct what was in retrospective an incredibly stupid mistake (doh... of course you need 3 wires for stereo...) is really appreciated. Many Thanks.
     
  17. wayneh

    Expert

    Sep 9, 2010
    12,085
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    Even a blind hog finds a mushroom now and then. Glad you're off and running.
     
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