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: 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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syberdiver4 months ago

works pretty nice! great idea!!!

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richfiddler11 (author)  syberdiver4 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!6 months ago

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

mixersplitter1.jpgmixersplitter2.jpg
richfiddler11 (author)  vanderaalle4 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)  costumatx7 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)  jkotecki11 months 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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fergusontea2 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)  fergusontea2 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

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.

xanderdk11 days 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)  xanderdk10 days 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 :)

richfiddler11 (author)  xanderdk9 days ago
OK, re-read my own instructable (been awhile) to see you're trying to calculate Zload (the impedance seen by, say, an iPod connected as an input). Again, you don't really need to calculate this, but ...

The formula I gave is Zload = R plus (R/N)//Zin 

(grrr... instructables Rich Editor removes the plus sign, so i have to write 'plus' in it's place)

NOTE: the notation '//' is shorthand for parallel resistance calculation. If you have 2 resistors in parallel, R1 and R2, the equivalent resistance is:

R1//R2 = R1*R2/(R1 plus R2)

So in your example

You're trying to calculate 1000+(1000/1)250*5 = 1251000 but as you noticed, this is way off.

For R=1k=1000, N=4, and Zin=5 the answer for Zload should be:

1000 plus 250*5/(250 plus 5) = 1000 plus 1250/255 = 1004.9 ohms

Hope this helps
paulthegeek made it!17 days ago

Hey, just wanted to let you know I built one of these based entirely on your guide. I plan to eventually fill the box with epoxy or something but it works perfectly as-is! Thanks for a great Instructable!

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carbog22 days ago

Could this be done in a plastic case? Or is the tin meant to couple the grounds of al the jacks?

richfiddler11 (author)  carbog22 days ago
Metal gives you shielding against stray noise, but if you're mixing headphone outputs then shielding isn't that critical and you should be fine with plastic. It's only when you're mixing line-level signals (e.g. Un-amplified microphone outputs, or what cones out of RCA jacks on stereos, etc) that you really need good shielding.
RobbOlson1 month ago

First of all Rich, Great job on the write-up and your willingness to respond to comments. I came across this when trying to figure out how to add an additional aux input jack in my truck and I was so impressed that I signed up to the site to give you kudos and ask my own question.

I am currently building a console and am incorporating the docking station (pictured below) to charge my phone as well as connecting it to the aux jack but I want the wires to be run inside the console to keep it tidy looking. Ideally, I want an input jack inside the console but I would still like to keep the original jack below the radio available though too in case I have someone riding with that may not have an iphone and can then plug into that jack. I have done some searches but am only finding splitters for audio outputs (for 2 headphones) rather than 2 inputs and am wondering if what you did here would work for my application.

Also, I am new to all this electronic stuff and am just learning about capacitors, diodes, resistors, and such. I was hoping you might explain how this device works (eliminating interference between inputs or how it determines which device to use for the output signal)

Thanks again for the great write-up!!

docking station.JPG
richfiddler11 (author)  RobbOlson1 month ago
Hi Robb,

I guess you could build the mixer (with several input ports) into your dock/console and allow guests to connect to the mixer/console. The output of the mixer would take up the input port on your radio. Unless you want to mod your car stereo I don't know what else you could do other than buy a car stereo with a rear 1/8" stereo input port (they do make them that way).

Regarding how/why this works, basically a small amount of resistance keeps the input sources from fighting each other. Why? A piece of wire has nearly zero resistance and if you were to place it across the terminals of a battery, a lot of current would try to move through the wire and "bad things" would happen. If you add enough resistance to the wire, it will limit the current and keep the bad thing from happening. Hard to explain it more simply than that -- does that help/make sense?

Good luck,

Rich
timoever1 month ago

All mine seems to do is give me loads and loads of static noise when I attach it... :( Can't seem to find the problem.

richfiddler11 (author)  timoever1 month ago
You probably have a bad solder joint or possibly a short. Make sure and check that your input and output cables work when connected directly between the source and the amplified speakers or whatever you're driving.

If you have a volt-ohm-meter ($5 at harbor freight) you could check the resistance between the input and output, that would give you some idea of where the problem lies. Sorry I can't be more help.
DJAWB3 months ago
I have a question, is there some way i could make say two sound systems in two separate cars play in unison? The idea of an aux splitter is possible but i have no experience in sound or electrical.. Any suggestions? Thanks in advance
richfiddler11 (author)  DJAWB3 months ago

Hey DJAWB, yes you're correct, your question really has nothing to do with the mixer, but I'm truly curious (i.e. don't read this question with a harsh tone)-- why on earth would you need the two cars to get the audio at exactly the same time?

Without knowing anything about what you're trying to do, what fidelity level is needed (voice or music), whether the cars are close vs. across country, etc. it's hard to say.

If both drivers have smart phones and good data plans, some type of network streaming would probably make the most sense. I'd do some research on setting up an IP-radio server. Maybe you could just use skype on your mobile phones?

If the cars are very close together, you might be able to use some kind of wireless technology like an FM transmitter (might need to be hacked for more power), CB radio, bluetooth, wireless guitar transmitter/receiver, or maybe just power a wifi router in your car so the other car could connect without using 3G data..

DJAWB3 months ago
I know this has nothing to do with this instructable but i can't find anything on this topic so i thought i'd ask in the comment box where people with experience with these things could help
JKPieGuy4 months ago

A few years ago I made a headphone splitter resembling this project. It worked great for when you wanted to share music with your friends without bothering anyone else. Though I was thinking of one day having the feature to switch from "Splitter-Mixer" with the flip of a switch (or a small circuit that would sense it and switch automatically). Was also considering adding a small amplifier circuit to it with a max volume shunt, to prevent people from adjusting it to high and bursting their ear drums. I'll let you know if I ever come though with either one of the two ideas.

piergap4 months ago

Question: do not the volumes go too low? i expected operational amplifiers and a 9V battery needed (.... then turning into an active mixer).

i tried to connect two media players like that (just through resistors), but the output was really low....

richfiddler11 (author)  piergap4 months ago

Hi piergap, please read through the instructable text and discussion, this is addressed many times. If you have questions after reading, let me know and I'll be glad to explain anything that's not clear. The short answer is when driving line-level devices like self-powered computer speakers, the loss is not a big deal. OTOH, when driving devices like normal headphones that require power, the circuit in this mixer will not work well -- there is no free lunch and you have to understand the limitations. In the right scenario, this is an extremely handy little device.

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