Two computers output to same speakers causes feedback-ish noise

Discussion in 'The Projects Forum' started by browner87, Aug 13, 2009.

  1. browner87

    Thread Starter Member

    Nov 15, 2008
    10
    0
    I have a 5.1 surround sound speaker set, 1 computer, and one laptop. Both the computer and laptop use the speakers frequently, but not usually at the same time. So, to save a lot of time and effort and because all wires at my desk are run under the desks and zip tied, I wanted to make both able to plug into the speakers at the same time.

    So, what I decided was to plug a Y-adapter into the 3 audio out jacks on my computer, then run a cable from one of each Y-adapter's outputs to the amp, and a cable from the other output of each adapter to my laptop.

    So the wiring sort of looks like this from one end to the other:
    ................Computer
    Laptop=====^^^======Amp==Speakers

    Simply, the laptop is connected to the amp through 2 male-male patch cables coupled by the 2 inputs of a Y-adapter. For the computer the Y-adapter is just a 6 inch extension.

    So, the laptop plugs into the Y-adapters at the computer which are also connected to the amp so that even if the computer is off, I can use my laptop for audio. Likewise, if the laptop is off and the computer is on, it is connected directly to the amp through the Y-adapters.

    The only problem is that when BOTH the computer AND laptop are connected with all the cables, I get sort of a static/feedback kind of noise on the speakers. I've double checked every cable for defects. The problem ONLY occurs when the computers are coupled by the set of audio cables, regardless of whether they are powered on (well, the computer anyways. I haven't tried turning off the laptop while plugged in).

    Is there any simple way to fix this? Is there some problem with common grounds here? Because all 3 devices (PC, laptop, amp) are plugged into the same power bar. I don't really want to start making a real circuit here, but if some resistors or diodes perhaps (maybe caps?) could stop this buzzing, it would be really nice.

    Does anyone know a cheap way to hopefully stop the buzzing when the computers are connected? Thanks :)
     
  2. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    A Y-adapter is meant for multiple output - single input only, not for multiple input which is the case you are trying to do.

    Multiple inputs cause impedance mismatch. The simplest way you can solve this problem is to add a 100n capacitor in series with each input to your amplifier. Leave the ground line as it is as this is the only thing which removes all hummings and feedbacks. You will need a total of 6 capacitors for the laptop and another 6 for the pc. First try this on one speaker before you cut and add caps on all inputs.

    Another option would be a passive mixer using resistors. In that you replace caps with resistors. You will get an idea of mixers here

    http://forum.allaboutcircuits.com/showthread.php?t=26615&page=2
     
  3. browner87

    Thread Starter Member

    Nov 15, 2008
    10
    0
    Well, your first suggestion seems to be working. I went to our not-quite-so-local electronics dealer and picked up 12 100nF caps, and after inserting one at the input to the laptop only, the noise cut at least in half. I'm busy canivering a second one up and if it cuts the noise completely or close enough on the one line, I'll pull out the strip board and wire up something solid. Thanks for the idea!

    BTW, you mentioned using resistors to make a mixer, but the link you gave has schematics with a few more things I think including an external power supply. Is there a way to do it without aux power? I can get transistors and other basic things, but I REALLY don't want to need another power source for it.

    EDIT:

    OK, I wired up the second one and it almost seemed to counteract the first one. I put it like:

    .............................Computer
    Laptop=Caps===Caps=^^^======Amp==Speakers


    As well as

    ......................Computer
    ..........................||
    ........................Caps
    Laptop=Caps====^^^======Amp==Speakers

    I was just pulling it all out and muttering to myself, when I thought I might as well exhaust all the possible placements of the caps. So, I wired it like so:


    ......................Computer
    Laptop=Caps====^^^=====Caps=Amp==Speakers


    It seems to have worked! The noise now is barely audible with a speaker held against my ear, so hopefully when I wire the other 4 speakers like this the total noise will not be noticeable. I'll let you know when I'm done if it is or not. If it is noticeable, maybe we could talk more about this resistor thing (mixer), if not then I'm good to go!

    Thanks again for the help.
     
    Last edited: Aug 15, 2009
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You should never short outputs together. One or both could be damaged.
    Connect a 10k resistor in series with each output then short together the two free ends of the resistors to make a passive mixer then each one drives at least 10k ohms.
    Connect the mixed signal to the input of the amplifier.
     
  5. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    The link I gave provides a mixer with no power supply below the one which needs a supply. It is made only with resistors as Audioguru said. Replacing caps with resistors in your current circuit will make it that way.

    [​IMG]

    Also a capacitor based mixer can be made better to filter out that noise completely, if you know the correct noise frequency band. Then the correct cap value can be used to cut the noise also.
     
  6. browner87

    Thread Starter Member

    Nov 15, 2008
    10
    0
    Well, it seemed to work last night, but trying the exact same thing again this morning, I still hear noise. Unless you are sure the resistor mixer will give a noise free (and hopefully fairly unaltered) output, I think I will just use an old switch I have originally used for switching parallel cables, so it has like 25 inputs and 25 outputs and I'll just flick the switch to go between computer and laptop.

    Also, can you explain why there are variable resistors in the circuit? Are they needed for something, or would the circuit work just fine if the lines basically went like:

    Left In 1---/\/\/\/--\
    ............................>------Left Output
    Left In 2---/\/\/\/--/

    Right In 1---/\/\/\/--\
    .............................>------Right Output
    Right In 2---/\/\/\/--/

    so basically just leave out the variable resistors?

    I'd like to avoid the switch, but I thought I read somewhere about a resistor mixer causing some distortion or noise or quality loss. Is this true? If so I'll go to the switch to keep a perfect sound quality, if not I'd like to just use the resistors.

    Thank you for your time
     
  7. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    A switch is an open device which is not shielded. It is succeptible to pick up more noise than a passive mixer. You can try the resistor trick for 1 channel and if your are not satisfied with what you hear, go for the switch if that sounds better. I cannot gaurantee that the resistor network will work as one device has output impedance and gains different from another. you just will have to test and see. It has worked for me.
     
  8. browner87

    Thread Starter Member

    Nov 15, 2008
    10
    0
    If I go with the switch idea, I will use the switch to connect the shields also (although I know it itself isn't shielded). I'll try the resistor thing right now (I just finished re-drilling my strip board to fit some audio jacks from an old sound card). I'll let you know when I'm done what worked for me.


    EDIT:

    OK, the resistors didn't do anything more than the caps. I'll get out my switch and see how that goes. Otherwise I might have to just find some other way.

    Thanks for all the help people!
     
    Last edited: Aug 15, 2009
  9. browner87

    Thread Starter Member

    Nov 15, 2008
    10
    0
    Yep. I ran all 7 (times 3) conductors (6 channels of audio, 1 for grounding) into the switch and now it switches between what is connected and it all works great. I noticed I had the exact problem I started with if I didn't run the grounds through the switch and just connected them all. Maybe I could have avoided the switch if I'd either put a resistor/cap in the shield as well, or maybe I could have just had a switch for the grounds. Meh. It's done so it's fine.

    Thanks again for the suggestions!
     
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