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.

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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syberdiver5 months ago

works pretty nice! great idea!!!

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richfiddler11 (author)  syberdiver5 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!8 months ago

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

mixersplitter1.jpgmixersplitter2.jpg
richfiddler11 (author)  vanderaalle5 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)  costumatx8 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

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.kinder14 days 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 Doc1 month 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 Doc1 month 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)  0nullification1 month 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.
xanderdk1 month 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)  xanderdk1 month 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)  xanderdk1 month 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!1 month 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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carbog1 month ago

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

richfiddler11 (author)  carbog1 month 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.
RobbOlson3 months 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)  RobbOlson2 months 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
timoever2 months 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.

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