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3.5mm 2 output Audio switch with SHIELDING? Answered

Hi, I'm interested in building a 1 input 2 output (3.5mm) A/B switch. At first I was looking to purchase one, but all the small units like I need have reports of noise being added to the signal because of poor or no shielding. I've found several instructables on building something I can use, but none mention shielding. Any suggestions or ideas on how to achieve this? (See attached pic)

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Jack A Lopez
Jack A Lopez

11 months ago

I suggest a double pole double throw switch (DPDT)
https://en.wikipedia.org/wiki/Switch

inside a metal box, like the kind those mint candies come in.
https://en.wikipedia.org/wiki/Altoids#Tins

The left and right signal wires (of which there are six) connect to the pins on the DPDT switch (of which there are six). The common (aka ground) wires all connect to each other, and to the metal box.

The headphone jacks are the panel mount style, for mounting through a hole in the side of a metal box. Also I think this style of jack will typically connect the common wire to the metal hole it is bolted into, because the outer barrel of the jack is the common.

If you like, I could also draw you a picture of the way I am imagining it.

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Orngrimm
Orngrimm

Reply 11 months ago

I suggest a 3PDT because he also needs to disconnect the unused shield from the unused cable:
- Lets say Speaker-Cable is plugged in but the speaker-cable is NOT in the speakers. Open. = Antenna.
- A high enough field (Like a heavy used power cable with its 50Hz) and induce power in the antenna-shield causing a GND-Hum.
- If the Notebook-Audio-GND is not super heavy and tight grounded (Hint: They normally arent), this hum would be noticeable.

Thats why i suggest a 3Pole and switch both audio-channels + the GND (Shield).

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Jack A Lopez
Jack A Lopez

Reply 11 months ago

I think I see what you mean, about how a long, not perfectly conductive, ground wire could act as an antenna, and why it is preferable to switch all three conductors {left, right, and ground}

By the way, another way to connect all three, or disconnect all three, would be to use a headphone jack, and just plug and unplug things, kind of like an old telephone switchboard.

Although, OP did say he (or she) wanted a switch specifically, rather than a switchboard.

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Downunder35m
Downunder35m

11 months ago

I had a look into my box of junk parts to get a better picture here....
Found these nice headphone connectors with a "switch".
The bubble at the tip opens a tiny switch to disconnect the source.
They are even available with a double throw switch, one either side in reality but not so common anymore.

With those connectors in mind:
If you want the speakers always off with the headphones on you won't need an additional switch.
Just have the ground wire for the speakers go over the headphone switch.
As long as the speakers are off while you use the headphones there should be no added noise or interference.
In terms of going pure and without any risk of added noise or interference I suggest to use a tower switch.
Am sure they are available still from some electronics retailers but I know them from old audio amps (before the digital times).
They are literally stacked rotary switches and come in many configurations.
The most basic you might stil find in some old amp or tuner has 5 sets of switches.

Going steampunk?
Might be a bit outside the box thinking but I think you might like the general idea:
Instead o the switch in your image ue a relay!
I know, adding power means adding problems and you don't want power anyway...
So take the thing apart and add a 3D printed housing for it to fix it into your metal box.
Instead of the coil you use a simple mechanism to MANULLY push the switching plate down onto the coil ;)
This way you can use a cheap relay with three or more contacts to fully seperate all lines from each other.

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Downunder35m
Downunder35m

11 months ago

You can use copper foil in the form of sheets or strips (on a roll) to "seal" a plastic housing.
Requires a bit of soldering as you need to get the ground/shield wire from your devies connected to the copper shield.
Most obvious choice though would be to use a metal enclosure, if in doubt homemade from some old tin cans.
Again proper electrical connections are required to make it work as planned.

Last but not least is the problem of wire lenght.
Even in a metal enclosure yo get noise if your bare wires are too long and too close.
Explains why most of these rF shielded things are designed as small as possible.

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Orngrimm
Orngrimm

11 months ago

At least i within the switch, there will be added some noise as it is very unlikely that your switch is properly shielded. But with proper consideration and other actions, this can be greatly mitigated.

Following is a series of input on how to be sure to shield properly.
- Be sure you not only switch the signals (L & R) but also shielding
- The switch has to properly switch all 3 signals. If you only switch L & R but leave the shield of all 3 connected always, you may end up with a big antenna: A here plugged cable which is not plugged in lets say into the klipsch can act as a receiver of all sorts and generates GND-noise which is added to the system
- Best would be to unplug unused cables, but that defeats a bit the idea of a switchbox ;)
- Be sure your switch is shielded (Metal body) and this metal body is also connected to the shield
- If you make a PCB, be sure you use a 4-layer PCB and have a TOP-Flood and a BOT-flood both on the shield and you go with a midlayer for the signals L & R
- If you wire it "Free-air"" be sure your cables with shield until the very last moment: make the last ammount possible of exposed wires without the shield
- Keep all wires (Headphones, Klipsch, Laptop) well away from each other. Make the wiring "neat" and dont have them close and parallel
- Be sure all your outputs (Headphones and Klipsch) and inputs (Laptop) are the same impedance
- Use a metal case for the project and connect the case also to shield

If you are looking for the switch itself, it will be a 3PDT if you go with a rocker switch (Normally not really shielded) or a 3pole, 2 pos if you go with a rotary (expensive).
If you go with the reasonable priced rocker switch, be very sure you have a good shielding box to put the things into...