Electronic Circuits and Projects Forum

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.

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.

Are the "speakers" amplified speakers with their own power supply?

Yes, they are amplified speakers, they use a USB port for power.

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?

Use a switch that completely isolates the "speaker".
Break the ground wire too.