Instructables

My car stereo has only one 'Aux' input, but I have several gadgets that I like to hear on a trip without swapping cables: MP3 Player, talking GPS, Blackberry, XM player, Kindle, etc.

I couldn't find a commercial solution, so I came up with this little passive audio mixer that takes 1/8" headphone outputs from up to four devices and safely mixes the signals into one line that can feed a car stereo Aux input, powered speaker or any other amplifier that can take 1/8" stereo output.

No batteries are required, it's sonically transparent and most audio gadgets have their own volume control so the mixer can be done very simply and cheaply.

This little mixer also works great for connecting multiple computers to one set of amplified speakers and has many other possible uses. Note that this device is NOT RECOMMENDED FOR HEADPHONES!!!! Whatever you plug into the output needs to have its own amplification, or the volume will probably be too low.

Note: Soldering is required. If you don't know how, please search for Instructables on how to solder, as that's beyond the scope of this project.

 
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Step 1: Materials

Materials:
Qty 1    ---       Altoids Tin. See Note 1.
Qty 5    ---     1/8" (3.5mm) Stereo input Jacks, Radio Shack part 274-249 or equiv.
Qty 8    ---      1k ohm Resistors, Radio Shack part 271-004 (5-pack) or equiv. see Note 2.
1 foot   ---      22-30 gauge solid hookup wire, stripped bare



Note 1: A normal Altoids Tin can take 4 or more inputs, this example assumes 4 inputs. The Altoids Gum tins can fit 3 inputs comfortably. Adjust the number of jacks according to:

       # of Jacks = 1+ # of inputs

and number of resistors by:

       # of Resistors = 2 * # of inputs

Note 2: All resistors (2 per input channel) should have the same value, which can be anything between 1k  and 10k ohms. Higher values result in a higher volume drop. Also, the resistor power rating can be 1/8 or higher. This circuit runs no power through it, so there's no need for bigger than 1/8 w, but use whatever you have handy. Bigger power rating = bigger size.

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operationkhaos made it!16 days ago

This was exactly what I was looking for. Thank you for making the instructions clear and actually including a circuit diagram that wasn't made in MS Paint. It works perfectly for what I need, and every input comes out the other end crystal clear, even when multiple sources are playing at once.

I ended up using a Radioshack project box instead of an Altoids tin, mostly because I just didn't have one sitting around and the box was actually cheaper than a tin of Altoids. This also makes the drilling significantly easier and prevents any unintentional shorts.

Thank you again for taking the time to make this useful, clear and concise guide.

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richfiddler11 (author)  operationkhaos15 days ago
Hey, nice job! Glad you were successful and thanks for posting the pictures!
syberdiver6 months ago

works pretty nice! great idea!!!

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richfiddler11 (author)  syberdiver6 months ago

Nice job! I like the jacks you used, where did you get them?

Rich

hi from Germany. I found the Jacks at www.conrad.de but I will make an other one with 6,2mm jacks :-)

vanderaalle made it!9 months ago

thanks for the great idea! I did a mixer/splitter for an experimental music project

mixersplitter1.jpgmixersplitter2.jpg
richfiddler11 (author)  vanderaalle6 months ago

Hey Vanderaalle, great job!! Looks like you're using 1/4" jacks and I *love* the tin (although not a smoker myself). Hope this works well for you.

-Rich

chipmonger4 years ago
Made one and it works great. Thanks.
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why did you put the resistors , can we do it with out them ?

richfiddler11 (author)  costumatx9 months ago
Please read FAQ on step 15. You can do anything you like, but unless you want to use this as a signal splitter, leaving out the resistors is a bad idea.
richfiddler11 (author)  chipmonger4 years ago
Wow, really nice job! Good idea to use resistor networks!

Would you care to share the artwork?
Jacks are Mouser 161-3508-E. Resistor networks are Mouser 652-4608X-1LF-1K 1k ohm. Other resistor values are available. Board was fabricated by ExpressPCB.com.
Mixer PCB artwork.jpg
Can this image be used to have a board made or do we need the ExpressPCB file?
richfiddler11 (author)  jkotecki1 year ago
Looks like a 2-sided board with the silkscreen mask drawn in so you would either need the file or just reproduce it on your own. It isn't mine and I don't have it. Maybe chipmonger would nice enough to provide if you asked.
This is a 2 sided board and I don't know if the image is scaled correctly once you download it. I do have the ExpressPCB file if you want it. Email me at steve"at"PowerSwitchTail"dot"com.
richfiddler11 (author)  chipmonger4 years ago
Thanks for the artwork and again, really nice job!
EET19821 year ago
I made mine. Not as pretty as yours but sure gets the job done! Stuck with the 1k resistors. Thanks for the great 'structable!
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everend1 year ago
Thanks for posting. Here is my version, it works perfectly.

C3-PO is an old ball mouse. After removing the guts there was plenty of room for the wiring. For the triple input audio jack I used parts from an old motherboard's sound card. They have five leads on each; 1) ground, 2) closed tip, 3) open tip, 4) closed ring, 5) open ring. When a plug is inserted, the part that touches the tip moves from the open tip lead to the closed tip lead. This must be how a computer knows when a plug is inserted. I used the closed tip/ring leads, leaving the open/tip leads alone. I wonder if the the open leads could be used for anything.
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richfiddler11 (author)  everend1 year ago
Nice! Good recycling!
Thank you. I used pulled parts from trashed sound card and a "Smalls" tin.... I love Old school point-to-point wiring...and Hot glue.
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fergusontea3 years ago
Here's my version of the mixer, using a Lucky Stars Candy tin. I plan to add a "floor" above the wiring so that I can store a short 1/8" patch cord inside as well.
lucky_stars.jpg
richfiddler11 (author)  fergusontea3 years ago
Nice! I particularly like the 'Hello Kitty' motif ;^) Thanks for sharing the picture.

Rich
rand0mmm4 years ago
Made one in less than an two hours, including trip to radio shack for parts, and safeway for MINI altoids tin. Used 1k resistors. Very Small, works fine.
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richfiddler11 (author)  rand0mmm4 years ago
Nice job!

Good idea to run the connectors out of the bottom of the mini altoids tin.

Looks like you used heat-shrink tubing for insulation as well. Heat-shrink is one of the best things ever for building things and professional wiring repairs.

I'm glad it worked for you and thanks for sharing the picture.

- Rich

Great article great project.i read through all the comments but couldn't find an answer to my following question....I'm trying to connect an external bluetooth for my BlackBerry through the aux input on my car radio and recently I'm trying to add an external xm radio through the same input. I've tried using a dpdt switch to disconnect the 2 sources (I have all the grounds tied together....but I still get a bit of feedback when I switch to the BT. XM works great! Any ideas?

richfiddler11 (author)  edward.wolofsky6 days ago
The best troubleshooting advice I can give is:

try changing things around, one thing at a time, while paying attention to conditions when the noise appears and disappears. Swap cables, remove mixer altogether and direct connect, etc. This will help eliminate variables.

What Bluetooth device (receiver?) are you using and how is it powered? Is it really feedback or just noise? (trend is to use these terms interchangeably, but they're very different!)

As soon as a charger/power supply is connected to most devices, audible (and annoying) noise is introduced into the audio. It's because the switching converters used to go from +12V car power to +5V  USB are *very* noisy and hard to filter out. If your Bluetooth device has a battery option, try that and see if the noise goes away.

Proximity to the mobile phone itself can introduce noise, e.g. if my mobile phone is next to my office desk phone, audible noise in present, but it tends to come and go.

Good luck, this type of thing can be tough to get rid of, or you might get lucky and find the cause.

I'm using a Kinivo BTC450 Bluetooth Hands-Free Car Kit for Cars with Aux Input Jack .... no noise when plugged in directly and I noticed minimal noise with the car in the ACC position. Sometimes it's there ... sometimes not when I have the 2 units plugged in ... the power source (the 12 volt plugs) may not be the problem - it could be a shielding issue from the aux input (located in the arm rest) to the radio.
I did come across this unit (saw it on Amazon) :
PAC Ground Loop Isolator for 3.5 Mm Applications - what are your thoughts on that? They want $17.00 on line ... can probably build on for a lot less. Tnx for your feedback (pun not intended!)
hang42 years ago
cool set up. i'm trying to do something similar, but different. I am a hang glider pilot and take in flight videos. I also have a ham radio that I use to talk to my chase driver. I have a mic and speakers wired into my helmet with a three conductor jack that has mic, speaker and ground connections. The mic is keyed with a PTT switch that runs down my sleeve.

What I would like to do is be able to record the radio transmissions in both directions by splitting out the mic and speaker wires to a second jack that I can connect to a voice recorder. What would be a real plus is if I could also use the mic to narrate when I am not transmitting. Any thoughts from anyone???? Thanks
hook2k hang49 days ago

hang4, they say better late than never, here's your answer:

You'll need 2 splits and only one mixer to mix radio output (your headphone line) with your voice input (your mic line) onto the output, that connects to camcorder input. However you'd need a mini mic-amplifier, probably battery powered, since your voice from mic is only amplified in radio and too low for camcorder. Mixer assembly you'd plug in BEFORE your PTT switch so that voiceover is mixed to video at all times while only broadcast to radio when switch is pushed. See little scematic I drew for you (sorry for Paint :) )

Glider-Mixer.png

I was looking for something simple, This is all over my head. I need something that will take input from a small MP3 player and a microphone on my collar and output to a speaker. I want to be able to sing along with a karaoke track and amplify it so it can be heard through the speakers. Can anyone help?

richfiddler11 (author)  larry.kinder1 month ago
Are you looking for wired or wireless? Do you have equipment already? (e.g. lapel mic, PA system, etc) What's your budget? What quality do you need?(i.e. home use or pro-quality for paid gigs?). Can you solder? Do you want to build or buy something? I could make suggestions, but they would be just a guess without knowing more.

FYI, the little mixer in this instructable combines stereo low-power signals of similar strength into one stereo signal. It doesn't do any amplification (in fact it reduces the volume a bit), so I don't think it's the right thing for your application.

By the way I want to build it all into altoids tins so I can just clip ti to my belt and move around

Old Doc2 months ago

Could you tell me if I could use mini slider potentiometers instead of the resistors? I'd love to make it look like a real sound board type mixer.

richfiddler11 (author)  Old Doc2 months ago
Yes, of course. Use audio taper slide pots and there's a schematic for volume control with potentiometers in the FAQ portion. Probably tricky to do in an altoids tin though. Both getting small enough slides and cutting slots in the flimsy metal without making a mess will be a challenge. Good luck.

My previous post was confusing so I deleted it. I'm wondering if I could mix 1/4" inputs with 1/8" and if so, in what order. I'm also wondering if it'd be safe to connect all input jacks to a 1/8" output that also flows to a 1/4" output. I would only connect anything to one of the two outputs at a time.

richfiddler11 (author)  0nullification2 months ago
The size of the connector isn't important at all. What *is* important is the electrical power of the signals you're mixing. As long as all the signals are of a similar voltage/power range,  you should be fine and connector size/shape doesn't matter in the least.  

OTOH, if you are trying to mix a weak signal (say, directly from a microphone or electric guitar) with a strong signal (say, from a headphone output of a smartphone, etc.)  the weak signal will be totally lost.

Does that make sense?
Yeah, I think so. I plan on using it for an electric piano at school with a battery powered metronome and at home to mix a TV and PC via headphone jacks. I'll probably use three 1/8" and one 1/4". Now I just need to learn how to solder and buy a soldering iron.
xanderdk2 months ago

Hi i am trying to do the equation to check my values. But i get a really high number.

I calculated the resistance with 4 1000k ohm in series to be 250

And my Zin in is 5.

1000+(1000/1)250*5 = 1251000

What am i doing wrong?

I'am really bad at math but i have to learn it :D

But thanks for a really nice instructable, been looking for weeks for a sound splitter like this.

richfiddler11 (author)  xanderdk2 months ago
Hi Xanderk,

First off, kudos for trying to "do the math" -- that said, you *really* don't have to. Just use resistors in the neighborhood of 1K Ohms (really from a few hundred on up) and you'll be fine.

Also note that If you just want a splitter, you don't need resistors at all, and you can buy commercial devices cheaply. The resistors are necessary when you want to *combine* multiple signals into one.

So now I'm going to correct a few things in case you really do want to learn something (not trying to be mean at all):

1) four 1k resistors in series (connected end to end) have a value of 4k ohms. four 1k resistors in parallel (resistors side-by-side) have a value of 250 ohms. Try googling 'parallel resistor equation' for a good explanation.

2) Not sure where you're getting a Zin of 5 ohms. This would mean you're trying to drive a passive speaker and that is not going to work with a passive mixer (well covered in the instructable and many discussions). Most powered speakers, preamps, etc have Zin values from 1K on up to 10 or 100k ohms.

3) not sure where you're getting the equation you have written out, but it doesn't make any sense to me. When you compute a parallel resistance value, you have to be very careful of where the parentheses go or you'll end up with wacky numbers -- maybe that's what's happened here?

Again, just use resistors of the same value and you'll be fine.

Good luck,

Rich 

Thank you so much for your reply.

I found my mistakes.

Swapped the input/output. The output is the speaker, in the input of coarse the iPod, computer etc. My speakers have a impedance at >5k just forgot the k, 5000 not 5. But no need to calculate this because its the output with no resistor connected. My mistake.

Need to calculate the input devices.

Found a lot of information about resistors in parallel. Trying to learn as i go, thats why i am trying to do the math even if i don't have to. Just for fun :D

Thank you again :)

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