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Trouble with Audio Output Selector Switch

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Stalin828

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Let me preface by saying I am a total noob at this stuff. I work at RadioShack and goof off with it when I am bored at work.

Anyway, I have a problem with my home computer. I have a soundcard that has 1 input and 1 output. However, going into that 1 input I constantly swap between my XBox360 and my microphone. And the 1 output is witness to a constant battle between my headphones and my speaker.

So, one day at work I decided to make a selector switch that I could just plug all of my stuff into and flip a couple switches to choose, instead of all the fuss and muss of pulling my computer out every time I needed to make the switch.

So, what I have is a black box with 2 1/8" TRS jacks on the back. They go to the microphone and headphone jack on my soundcard. On the front of the box is 4 1/8" TRS jacks. All the jacks are the standard open circuit style (when there is nothing plugged in the circuit is open, it is closed by the tip when a device is plugged in). Then I have 2 DPDT switches on the top (one for mic/xbox the other for headphone/speakers obviously).

It works perfectly, except for one minor annoyance. When using my headphones there is very faint noise on the line. The level of the noise doesn't change if I turn the volume up or down on my computer. The noise stops completely if I unplug the speakers from the box though. I have checked and found no shorts. I suspect that it is caused by the way I wired the circuit.

The TRS jack has 3 connections, left channel, right channel, ground. Now, on my switch it is DPDT so there are 2 connections per side of the switch. Thinking it wouldn't be a problem I soldered the ground wire from both of the front audio jacks to the ground wire of the rear output audio jack. I suspect this is the cause of the interference.

Any ideas on the cause of the interference or a way of solving the problem?
 
Why: The buzzing could be from several things, the first suggestion isn't a shirt, but high impedance.

Example: You'll probably relate well to this example, i was in a similar situation and i had this happen to me. Before when you've plug the headphones or speakers in and sometimes you make contact with your hand and the metal jack tip on the edge of the plug and you get that annoying BUZZ? I'm sure it's happened to most if not all of us.

Explanation: Taking the previous example maybe the connections are two close somewhere, or there is too much impedance in the line? I do no believe impedance would increase in volume is why I'm picking that one first.
 
The buzz is from radiated mains hum that is picked up by unshielded input cables or picked up by wiring inside the unshielded box.
 
I find it strange that the problem only exists when both things are plugged in.

When the headphones and speakers are plugged in, I hear the noise on the headphones. The speakers are too low quality to hear any noise. If I am listening to the headphones and then unplug the speakers from the switch, the noise disappears entirely.
 
Maybe the wires to the speakers act like antennas and pickup mains hum.
 
try un-pluging the USB port and see if you still get hum.
I'm with the presumption that its mains hum and the wires are the antenna.
I wonder if adding a capacitor across the outputs of each would help?
 
Unplugging the USB connection for the speakers does make the humming go away.

I haven't tried on my computer speakers, but when I was building it I tried running it through speakers without the ground wire connected and the sound was unintelligible, flat5. Or do you mean disconnect the ground from the speakers and see if the buzz persists in the headphones? I will try this at work tomorrow.

Don't know if it matters, but the humming occurs at the same level whether the speakers are powered on or off.
 
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I reread all the replies and saw audiogurus first response and decided to wiki "mains hum" and that was indeed the problem. My surge protector was directly beind my computer and te audio cable for my speaker was resting directly on top of it. I moved my surge protector about a foot away from my computer and that solved the problem. I still get some interference but it is basically inaudible. I can still hear it but it is low enough that it doesn't drive me nuts. The problem isn't as big now, and I can live with it, but if anyone wants to still try and solve te problem, I intend to keep working on it anyway. Tomorrow I am going to redo the wiring with one of these https://www.radioshack.com/product/index.jsp?productId=2102845. Anyone have any experience with them? It seems like it would be a lot easier than the rats nest of wires I currently have.

Something I read seemed to suggest that since my speakers and headphones are using a different source of power connecting their grounds like that can cause hum. We don't carry a TPTT switch, so any ideas on how to connect the ground in a way that would remove the hum? EDIT: I guess they would have the same ground since the audio cable is plugged into the same computer the USB cable is plugged into, and my headphones obviously don't use external power.

EDIT: This is probably ridiculous overkill, but couldn't I use some sort of relay that will switch which ground is connected based on whether it detects an open or closed circuit on the tip of the headphone cable? I don't know relays very well. Would it mess with audio quality? Hmmm I guess this begs another question. How many volts and amps run across a typical headphone cable (tried googling and didn't find an answer)?

Sorry about the missing "H's" in my post. A failed experiment wit my keyboard has caused te H key to work intermittently.
 
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