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I have two audio sources, both using 3.55mm "headphone jack" outputs.

I have one set of powered stereo speakers.

I need to connect both sources to my single pair of speakers. Ideally, I'd like to be able to hear both sources at the same time, but this isn't completely necessary. I ABSOLUTELY do not want to have to press a button to manually switch.

I don't need anything fancy—-no individual volume control or microphone inputs or anything like that. But I do want to avoid any significant loss of audio quality.

How can I do this for under $25? Or if that's completely unreasonable, how can I do it for as little money as possible?


While I suspect this won't matter since the audio is already analog, I play a lot of rhythm video games, so anything that introduces latency is a no-go.

  • Are the speakers powered (active)? – Mick Apr 9 '17 at 22:44
  • @Mick Yes they are. – Wowfunhappy Apr 10 '17 at 22:56
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If you're somewhat decent with electronics, you can build a simple passive mixer with just the appropriate connectors and some 10k resistors from the parts section of RadioShack (or whoever sells them near you):

schematic

The Grounds can all tie together directly.

The resistors prevent the inputs from being shorted to each other, so that none of them go into overload protection, and the technically weaker drive strength is still negligible for the line input that's going to receive this combined signal.

What you end up with is the average of all the active signals (even if they're actively silent), rather than the sum, but the only real difference is the eventual volume of the final mix, which you can easily compensate for by turning it up slightly.

What might be annoying about this is that if you unplug something, whatever's left will jump up a little bit in volume...unless you use normalled input jacks. Normalled means they have switches built into them that are opened by the presence of a plug. For example, a headphone jack might be normalled to some built-in speakers, but when you plug in the headphones, it opens that switch and the speakers stop. In your case, you'd wire it to ground that input when the plug is removed, thus keeping it actively silent when you unplug something. Then the volume stays the same no matter what's plugged in or not.


And, of course, you can expand this idea as far as you want, in terms of channel count, features, etc.:

schematic

This one adds independent volume and mute functions, and it has three inputs instead of two. Only one channel is shown here; you'll need two for stereo: one for left and one for right, and the corresponding controls should probably be ganged as one stereo part each.

You might notice that the mixing resistors have changed from 10k to 2.2k. The value really isn't all that critical - in fact, you can even make them unequal on purpose to pre-bias it one way or another - just stay above 1k for all of them and you're probably fine. However, the ratio between them and the volume controls is important to get a decent audio response out of a linear (cheap) knob or fader. (also called "B-taper", as opposed to "A-taper" for audio but is often approximated worse than what this trick does with parts that are cheaper anyway)

The inverted triangles are all connected together - it gets messy pretty quick to actually draw all of those connections, so just know that they're there. Thus, the Grounds are all tied together as before.

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  • I wish I had the skills to wire stuff like this myself. It would make a lot of tasks less pricey. – Wowfunhappy Aug 2 '18 at 21:03
  • 1
    @Wowfunhappy what's stopping you from acquiring them? I just learned how to weld (I'm a programmer). – php_nub_qq Oct 2 '19 at 18:38
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What you need is something like this, maybe a cheaper one with two inputs, I couldn't find anything, but you might be able to using a little Google Fu.

https://www.bhphotovideo.com/c/product/373759-REG/Behringer_MX400_MX_400_MicroMix_Four.html

You can then grab a couple adaptors, such as the ones linked below to convert your 3.5 to a quarter inch input and output, which then you connect your speakers to.

https://www.bhphotovideo.com/c/product/851064-REG/Comprehensive_SPP_MJS_ADAPTER_ST_1_4_PHONE_PLUG.html

This breaks your budget by a couple of dollars, but you could shop around and get it from Amazon or somewhere cheaper like that.

EDIT: This is a mono mixer, so you will need a mono to stereo 1/4" phono adapter for the output, such as this one here from amazon.

I will look for an option that supports true stereo, but i should imagine this would fit your needs.

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  • I really don't like the supposed stereo-to-mono adapters that can't be tested easily for their actual circuit. I suspect that most of them are simply shorted together between left and right channels, which probably won't blow things up because of modern devices' short-circuit protection, but probably won't give you a predictable mix between the original left and right channels either. – AaronD Jun 28 '17 at 5:01
  • (I thought I commented on this a year ago?) The lack of stereo made this a no go. Unless I was missing something, I would have needed two separate adapters, one to mix the left channels and one to mix the right ones. This would have mostly erased the price advantage, while adding a lot of complexity I wanted to avoid. – Wowfunhappy Aug 2 '18 at 20:59
  • Your question was updated so the stereo and mono issue no longer exists. a cheap mixer would still be a viable option. :) – Edward Nunn Aug 30 '18 at 5:17
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If you are using Windows and are on a desktop, you can use the line-in and microphone jacks and go to the audio settings to hear both devices. You would then have audio control over those two inputs as well.

If you have a laptop, you can purchase two ADC/microphone to usb devices and use the same method.

  1. Right click on your sound meter in Windows and select Recording Devices.
  2. Right click on your microphone/line-input and click properties.
  3. Click on the Listen Tab.
  4. Click on Listen to this device.
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  • Not a bad idea, but I don't think he mentioned using a computer. – Edward Nunn Jun 26 '17 at 21:38
  • Won't this cause latency? And quality loss, as the non-computer input goes Analog -> Digital -> Back to analog? One of my devices is a computer, but this doesn't seem like a great solution. – Wowfunhappy Jun 27 '17 at 3:08
  • Latency, not noticeable. Quality depends on your computer. I use my motherboard's line-in and record my friend's guitar. The microphone in however, is a lot lower quality. You can give it a listen yourself if you have a computer available and determine if there's significant latency. If you just need to play the two signals out, I recommend purchasing a cheap two channel mixer for $40. Output your speakers via XLR/quarter inch, and get 3.5mm adapters to xlr or rca for channel 1 and 2 of the mixer. – Kaito Jun 30 '17 at 15:27
  • @Kaito which is pretty much what I suggested to a certain extent – Edward Nunn Jul 2 '17 at 21:55
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I ended up buying the Rolls MX51S Mini Mix 2 Four-Channel Stereo Line Mixer. It cost more than twice what I wanted to spend, but I couldn't find anything cheaper that supported stereo, so 🤷‍♂️. It's a "powered" mixer, so it has to be plugged into an outlet.

This mixer created a buzzing sound, so I had to also buy a cheap ground loop isolator (I think it was this model). I've read that these can theoretically affect audio quality, which bothers me, but at least I couldn't personally notice a difference.

There's also a slightly cheaper, unpowered mixer: the Rolls MX42 Stereo Mini Mixer . This is actually what I bought first, but I had to return it. It decreases the volume by a lot, which was too much for my setup. However, I didn't have any buzzing issues.

Both of these mixers use RCA, so I also needed a couple 3.55mm ↔ RCA adapters. These are super cheap and I already had several on hand.


*Interestingly, the amount of buzzing seemed to depend on how heavily I was using my computer's GPU. For casual web browsing, there was almost no buzzing. Old or lightweight video games caused slight buzzing, and intensive games had tons of buzzing.

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  • Glad you found something! I'm surprised that the passive version had that low of an output. I'm sure you had the volume controls all the way up, and passive stuff does attenuate the signal, but it shouldn't be "a lot". – AaronD Aug 2 '18 at 23:45
  • Yes, a cheap ground loop isolator can affect the quality, but you probably won't notice either. They're basically a transformer in a pretty case with convenient connectors. Just like a power transformer, but designed for 20Hz-20kHz instead of just 45-65Hz. The signal gets converted from electric to magnetic and back to electric with no electrical connection from one side to the other, thus breaking the loop. The professional stage equivalent is called a D.I. box, for Direct Injection, and is called that because it replaces a microphone for guitars, etc. – AaronD Aug 2 '18 at 23:52
  • The most common form of distortion is called saturation, which is basically the transformer's equivalent to clipping. Because of the physics involved, this is the most likely for deep bass, and it basically becomes a short-circuit at that point, which is really stressful to the device that's driving it. This is true regardless of price or quality. (i.e. you can't break physics) So listen carefully if you decide to turn up the phone or listen to really bassy stuff. If it still sounds clean, you're good. If not, back off the phone a bit and turn up the speaker instead. – AaronD Aug 2 '18 at 23:54
  • @AaronD Thanks. For what it's worth, if I had to put a number on it, I'd say the passive version cut the volume by about half. (Yes, with the volume all the way up on everything.) – Wowfunhappy Aug 3 '18 at 13:37
  • Well, the perception of "half" is typically around -10dB, and without the normalling switches that I mentioned in my answer, that still seems a bit much. It should be -6dB (electrically half) with just two inputs, one silent and one playing. But if they put a resistor behind each jack that's strong enough to look like a short compared to the mixer itself, but weak enough to be driven easily by a phone, VCR, etc., then a 4-input mixer would give you -12dB (electrically 1/4) for each source, which lines up a little better with what you heard. – AaronD Aug 3 '18 at 16:35

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