Splitter as a merger?

October 22, 2010 7:03 PM

Is it possible to use an all female 3.5mm splitter backwards to allow two inputs into one set of speakers?

This might seem a little weird, but if it works, its the most slip stream solution to what I'm trying to do. I just need two 3.5mm inputs into my set of speakers. I don't need to be able to switch between the two, if I'm using one input I won't be using the other.

Right now, I have my 2.1 speakers plugged into my computer. The right and left speakers plug into the sub which then has a hard wire that plugs into my computer. I want to be able to plug my recently bought DJ mixer (with a built in sound card) into those speakers as well. From what I see, Ableton and Traktor will not let me output sound through my computer speakers. Whatever sound either outputs will have to come out of my mixer. There probably is a way to have both programs play though my computers speakers, but that setup is not how it would work out at an actual Live DJ set.

I was thinking about using a 2x male RCA (red and white cables) to a male 3.5mm jack coming from my mixer. I would then run a male to male 3.5mm cable from my computer. Both would plug into a female to 2x female splitter. But in this case, I would user the cable as a merger. Then I would have the hardwire coming from my speaker plug into the opposite end of the adapter. My thought its, its just all electricity and it should all work out. But...am I missing something? could merging cables like this cause a surge in electricity and potentially blow out my speakers? Sure I can just get a audio switcher, but those are so much more expensive and overly complicated to what I'm trying to achieve.

So can such a wiring work? Or am I just a big idiot? :D

Thanks.

posted by NotSoSiniSter to technology (9 answers total)

The problem that you'll have is that you'll get the signal from one source running into the other source at the very end. Your speakers would probably be fine, but you run the chance of damaging your sound cards. Depending on how robust your sound cards are you might get away fine, or you might damage one or both sound cards when too much return voltage comes back.
posted by Benjy at 7:20 PM on October 22, 2010

I don't think it will blow up your speakers, but it won't do what you want. In order to mix audio signals like that, you need a simple additive circuit. You can easily build it yourself if you want (I did so last year, didn't take long, works ok). But if you're looking for a quick solution and don't want to solder wires together, you can probably pick up a box that does it for cheap.
posted by Maximian at 7:23 PM on October 22, 2010

I'm not so sure on the circuit Maximillian links; it does nothing to really isolate the two inputs from each other at DC (which is the real source of any potential problems) except act as a voltage divider.

Personally, I'd use capacitors (something like ~100μF at an educated guess; to make a 100% exact decision, you need to know the nominal input and output impedances of each connection). One in each leg (signal & gnd) of the source connections, like this (if my ASCII art skillz work ;-)

   INPUT 1 ---------| |-------+
           ---| |-+           +------------ OUTPUT
                  |           |       +----
   INPUT 2 ---------| |-------+       |
           ---| |-+                   |
                  |                   |
                  +-------------------+

That'll totally isolate both the signal and grounds of each source, minimising earth loops and any chance of damage to the source outputs due to DC imbalances between them. Ideally the caps would be non-polarised, but at the signal-level voltages involved it probably doesn't matter much.
posted by Pinback at 8:21 PM on October 22, 2010

Maximillian Maximian
posted by Pinback at 8:22 PM on October 22, 2010

Bad idea. Circuitry designed to provide an audio output almost always assumes that it is in sole control of the input it's connected to. If you wire things as you've suggested, you will end up with two outputs fighting with each other for control of the speaker input.

Are you sure there's no way you could connect the computer's line-out socket to an input on your mixer? That way you could make your mixer the sole device driving your speakers, and you could mix in as much computer output sound as you liked.
posted by flabdablet at 8:27 PM on October 22, 2010

I have had a similiar problem with a desktop machine with an onboard unremoveable soundcard to which I added another, better soundcard. The solution I found was to make it always use the better soundcard, and connect the speakers to that; where necessary setting the computer to always ignore the other one. If your DJ mixer has an onboard soundcard, it may well be substantially better than the stock one in your computer, so why not just make your computer always use that as the default soundcard? The method will vary depending on your OS, but it really ought to be possible. Then it will work with Ableton, Traktor, any other audio software and so on.
posted by motty at 10:43 PM on October 22, 2010

I'd recommend against it, as it will have either reliability or performance impacts.

Reactive components (such as capacitors) will have non-linearities that affect the low end.

Resistive summation of the signal will affect maximum volume available, but will affect the tone less.

A big multi-pole switch is the best thing to use, if you can remember to flip it and don't need both sources simultaneously. If you do, then summation of the outputs with resistors is probably the best way to do it, and not affect the performance excessively, while offering some protection to the competing outputs.
posted by FauxScot at 7:02 AM on October 23, 2010

Does your DJ mixer have an aux input that you can plug your computer into?
posted by davey_darling at 7:37 AM on October 23, 2010

There's a reasonable chance that this would work just fine, depending on what the output circuitry of the inactive sound card looks like; there's a reasonable chance that you'll get distorted sound, and there's a smaller chance that it'll damage one or both of your sound outputs. Probably won't damage the speakers though.

I think the best solution is to use an auxiliary input on the DJ mixer if you've got one, as flabdablet et al said.

Since you say the signals are coming from RCA connectors originally, presumably they're "line-out" levels, which are 1Vp-p, usually into a 100Ω impedance, and don't carry any power to speak of. The signal pin will be capacitively coupled already (which is what Pinback's circuit is doing), but the ground/shield may not be— I don't know what's typical for that— which could mean you'd end up with current in the ground/shield wire, which would give you hum and/or equipment damage. The other problem I can think of is that the output stage of the inactive sound card might not look like a simple high impedance; this wouldn't damage anything, but would distort the audio.
posted by hattifattener at 12:16 PM on October 23, 2010