Stop trying to be smart, switch!
August 6, 2014 12:27 PM   Subscribe

I have this USB DAC/headphone amp (which is great). I just started using the RCA outputs for a pair of powered monitors (also great). The problem is that the DAC switches between the outputs based on whether the headphones are plugged in. This makes sense, but it's annoying. I want to hack it and switch between them manually. Electronics whizzes, check my reasoning.

Assume that I know how to solder (but not super well) and that I took physics with circuits about ten years ago, plus some tinkering.

I'm guessing that there is a mechanical switch like this inside the headphone jack whose function I want to replace with an external switch. (I don't want to have both active simultaneously; I just want to be able to switch the outputs manually.

This gallery of images shows five sides of the innards with enclosure removed. The last two images show the continuity with headphones plugged and unplugged as tested with a multimeter.

If it were a single connection that switched, I probably would go ahead and try it. With two, I figure it's better to ask.

Ignoring the difficulty of soldering anything new onto these posts and mounting it on the small enclosure (non-trivial, I realize), it seems like I could control it via a DPST switch? Wire 6-7 on one side of the switch and 1-4 on the other? Am I overlooking something stupid?

Next step would be to plug the USB, headphones, and monitors in and try to jump the terminals to reproduce the function of the theoretical switch, but again, thought I'd play it safe.

posted by supercres to Grab Bag (9 answers total) 3 users marked this as a favorite
Sorry, this gallery of images.
posted by supercres at 12:28 PM on August 6, 2014

Also, any tips on the finicky soldering that would be required here? I have a fairly decent soldering iron.
posted by supercres at 12:29 PM on August 6, 2014

The two connections represent the tip and the ring of the TRS headphone adapter. My guess is that 7 is the tip, and is the only one that the electronics use. It's easy to check though:

Put a short headphone extension cord in there, and it will route audio to the headphones. Then short out 3 and 7 with a wire or whatever, and see if it sends it back to the RCA jacks. If it doesn't, try 1 and 5. If neither works on their own, I guess you'll need to do that DPST switch.

Whichever works, solder one wire to each pin and connect them to the switch. Leave the headphone extension cord in place to hold the switches in the headphone jack permanently open, and plug your actual headphones into the extension cord.
posted by aubilenon at 1:24 PM on August 6, 2014

If neither works on their own, I guess you'll need to do that DPST switch.

Er, but check with temporary short before you solder anything of course!
posted by aubilenon at 1:33 PM on August 6, 2014

I actually plugged in a 1/8" to RCA cord to figure out the exact pinout.

1: Right live
2: Right live
3: Left live
4: Ground
5: must be the switch terminal
6: Left live
7: must be the switch terminal

So 2-5 and 3-7 would probably be the circuits to close to route audio to the RCA output. Eh, I'll give it a shot. Hopefully I won't short circuit and fry it.
posted by supercres at 1:44 PM on August 6, 2014

Reporting back: shorting 2-5 makes the right monitor come on. 3-7 for the left. But the headphones are still going. That's fine, just not as simple as I wanted.

I'm sure it's messing something up with current splitting/impedance/whatever. (I'm an E, but not an EE.) Sounds the same through the headphones, though (Sennheiser HD280, so I think I'd hear it).

Anyone have any thoughts about how to accomplish both at once? Is it probably just an SPDT built into the port?

It occurs to me that I could use a DPDT (or two separate switches) and route the headphones through it before it even gets to the port on the DAC's PCB, but that seems like a bit much.
posted by supercres at 2:23 PM on August 6, 2014

I'm sure it's messing something up with current splitting/impedance/whatever.

I would think they'd necessarily be splitting the signal before the headphone amplifier. Can you hear anything messed up about it?
posted by aubilenon at 2:36 PM on August 6, 2014

Oh, good point. No, it sounds fine.

Thanks for your help. Now to work up the courage to do the soldering...
posted by supercres at 2:40 PM on August 6, 2014

This should be super easy soldering. Good luck!

Oh and after you're done and it works, put some hot glue gun glue or electrical tape or something on it, for strain relief.
posted by aubilenon at 3:03 PM on August 6, 2014

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