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.

Step 2: Tools

Tools:
- Fine Needle-nose pliers
- Fine diagonal-cut wire cutters
- Punch or Awl
- Ruler or Calipers
- Drill and 1/4" drill bit
- Low power soldering Iron & solder

Step 3: Mark Center-line for 1/8" Jacks

Scribe a line on the tin 1/4" from the bottom edge on these two sides:

- the long side opposite the hinge side (input jacks)
- one of the short sides (output jack)

These will be the center-lines for drilling the holes for the jacks.

Step 4: Mark & Pre-punch Holes for Jacks Before Drilling

Mark and pre-punch the positions for drilling the holes on the center-line with an awl or fine marker. Position is not super critical, but should be even if you want your project to look neat.

- Make 4 marks on the long side center line, spaced out by about 3/4 of an inch.

- Make 1 mark on the middle of the center-line on the short side.

- In order to keep the drill from wandering, it's good practice to punch the holes with an automatic punch or awl.

Hint: It's a good idea to put a small piece of wood or a big rubber eraser behind the thin metal to keep it from bending.

Step 5: Drill 1/4" Holes and Clean Ragged Edges

Drill each hole with a 1/4 drill bit. Again, a small piece of wood behind the metal will keep it from bending and make the backside of the holes neater.

Carefully clean up the ragged edges of the holes with the side cutters or a sharp knife like an exacto hobby knife. This is important so the jacks sit flush.

Hint: As always, safety first, follow this rule: never put anything in front of a sharp blade that you don't want to cut!

Step 6: Dry-fit the Stereo Jacks

Remove the retaining nuts from each jack and fit a jack into each hole. You may need to open the holes with a knife, drill bit or small round file.

Don't attach the retaining nuts just yet, as we'll need to remove the jacks to fit and solder the ground wire.

Step 7: Attach Ground Wire to Ground Tabs

1) Locate the ground tab on your audio jacks. This is the one that connects to the metal barrel on the jack, and is usually closest to the case when the jack is installed.

2) Attach the bare 22 gauge solid wire to the ground tab of the single output jack first and solder.

3) After soldering the first jack, replace it into its hole and determine where to make bend/attach the wire to the ground tab on the next jack.

4) Solder the next jack, and so on, until all the ground tabs are attached and soldered.

Hint: I like to wrap the wire around the tab and crimp with the needle-nose pliers before soldering.

Note: Do not worry about the bare copper ground wire touching the case, they are meant to be connected together

Step 8: Install & Tighten Jack Retainer Nuts

After the ground wire is soldered to all the jacks, replace all the jacks into their holes and add the retainer rings, tightening firmly.

Step 9: Attach First Resistor to Middle Tab (Right Channel)

1) Take a resistor and attach to the middle tabs of the output jack (the one by itself) and the closest input jack (see picture).

- Make sure the resistor body is closest to the first input jack.
- solder in place

NOTE: it is very important that there are no shorts with the ground wire, case or other tabs from this point on.

Step 10: Attach Remaining Resistors to Middle Tab (Right Channel)

1) attach a resistor to the middle tab of the next input jack
   - solder & trim excess wire

2) wrap the other lead of the resistor around the lead of the resistor in the previous stage (see photo),
   - make sure that this end has a direct electrical connection to the middle tab of the output jack.
   - solder and trim excess wire.

3) Repeat until all input jacks have their middle tab tied to the output middle tab through a resistor

4) After attaching all resistors to middle tabs (right channel), gently bend the entire string down slightly to make room for the left channel resistors.

Hint: Do the connecting, soldering and trimming of each resistor one stage at a time, this will be a lot easier than fitting and soldering all inputs at once.

Step 11: Attach First Resistor to Outer Tab (Left Channel)

1) Take a resistor and attach to the outer/left-challe tabs of the output jack (the one by itself) and the closest input jack (see picture).

- Make sure the resistor body is closest to the first input jack.
- solder in place

NOTE: it is very important that there are no shorts with the ground wire, case or other tabs from this point on.

Step 12: Attach Remaining Resistors to Outer Tabs (Left Channel)

1) attach a resistor to the outer tab of the next input jack
   - solder & trim excess wire

2) wrap the other lead of the resistor around the lead of the resistor in the previous stage (see photo),
   - make sure that this end has a direct electrical connection to the outer tab of the output jack.
   - solder and trim excess wire.

3) Repeat until all input jacks have their outer tab tied to the output outer tab through a resistor

4) After attaching all resistors to outer tabs (left channel), make sure there are no shorts/connections between left channel and any other wires.

Hint: Do the connecting, soldering and trimming of each resistor one stage at a time, this will be a lot easier than fitting and soldering all inputs at once.

Step 13: Done! Testing and Troubleshooting.

Congratulations, you should now be done. Close up the Altoids tin and test the mixer:

- attach the output jack to a known good powered speaker
- attach an working mp3 player to each input in turn.

Problems?
If something doesn't seem to be working check these things:
1) Do you get sound with mp3 player, cable(s) and powered speaker without the mixer?

2) Double check wiring. Is everything wired correctly, per instructions?

3) Any cold solder joints? (solder joints that look good but don't really connect) Try resoldering any suspect connections.

4) Any shorts? Shorts are accidental connections caused by wires touching each other. Look carefully and pull apart wires that look like they might be touching.

Step 14: Final Comments

It may look strange to build an electronics project without a circuit board in this open wiring style. Old timers called it "rats nest" wiring, but despite the negative term it's perfectly OK.

If you construct it right, the wiring has enough strength to stand on its own and the box provides adequate shielding.I've used my mixers in the car and elsewhere with no problems for several years.

However, if you're worried about the open wiring, then you can always cover the bare wires & resistors with electricians tape, just make sure not to break something in the process.

If you really wanted to make the mixer super rugged, you could fill the tin with epoxy, parafin or circuit-grade Silicone RTV.

Note: Don't use the regular Silicone RTV made for caulking and bathroom sealer. This stuff gives off acetic acid which will eat through your wires and ruin your project after a while. Circuit-grade Silicone RTV does not give off acid and is safe for wiring. I've read that GE Silicone II (available at home improvement stores) does not leach acid and also silicone RTV sold at automotive part stores labeled either 'Oxygen-sensor Safe' or 'Type B' should be OK.

I hope you are successful and get lots of good use out of this project, please let me know.

Rich

Step 15: FAQ & Design Tradeoffs

FAQ's

1) Do I really need resistors?

Absolutely, Yes.

It's a very bad idea to connect active outputs together without some resistance in between.

Depending on the design of the devices you're connecting together, the best thing that will happen is mild to severe distortion in the sound. The worst thing that can happen is damage to one or more of the devices. Why risk your expensive equipment to save a few cents on resistors?

2) Can I use diodes instead of resistors?

No.

Here's why: Diodes allow current flow in only one direction, but sound is an AC signal that must move in both directions. Also, a diode will not "turn on", i.e. conduct current in the forward direction, until there is a voltage of .7 volts across it. This means that you will lose .7 volts from your signal and for low-voltage signals, you won't get much out at all.

So sound will be either non-existent or very distorted. In fact, diodes are often used to create distortion effects in older analog guitar pedals.

3) Do you have a schematic for the circuit?

Yes, see the first picture below.

4) Can I add a volume control?

Sure, and there are a couple of options here.

- To add a single master volume control, do something like the second picture below, using 10k ohm stereo audio-taper potentiometers. note: audio-taper and log-taper are the same thing.

- To add individual channel volumes, do something like the 3rd picture below using the same type of potentiometer, stereo 10k audio taper pots.

I don't recommend trying to do both a master volume and individual channel volumes because the signal loss will be too great and would require op-amps or other active amplification.

5) Can I Use it as a Splitter?

Yes, with the mixer as built, you could plug one device into the output jack and then connect multiple amplified speakers, etc via the 4 'input' jacks. You will probably lose some volume and it won't work for headphones very well, just line inputs.

However, if you plan to use it always as a splitter and never as a mixer, you might be better off building it without the resistors. While the resistors are necessary for a mixer, they are not at all required for a splitter.

6) Can I Connect a Headphone to the Output Jack?

Not recommended. This circuit performs the best when driving a high-impedance load. As the load impedance connected to the output gets lower (low impedance = heavy load) the volume drop will get greater and headphones have very low impedance (16 or 32 ohms common).

In other words your headphones will not be very loud, and might be very quiet -- that's just the way it works.

If you want to mix into headphones, build/Buy a CMOY headphone amp. The CMOY headphone amps also come in Altoids tin. Do a search on it, there are tons of plans and people selling them out there. You could even put the mixer circuit in front of the CMOY amp circuit if you built one yourself.

Design Trade-offs

This design does sacrifice signal volume for simplicity, battery-free operation and minimal distortion. This isn't a problem in my experience, since my various car stereos, amplified speakers, etc. have more than enough gain to compensate.

There are some trade-offs you can make to reduce this volume loss. Here are the relationships so you can experiment with factors like number of inputs, resistor value and acceptable distortion:

Volume vs. Resistor Value

The higher the resistor value, the more volume you will lose. The lower the resistor value, the less volume will be lost.

Loading vs. Resistor Value

The lower the resistor value and the more inputs are present, the higher the loading on each device's output.

Distortion vs. Loading

The higher the loading seen by a device output, the higher the distortion. Mp3 players, etc are designed to drive 16 ohm (or so) headphones. However, the distortion will be much higher when driving low-impedance (high loading) headphones than when those same outputs are driving a high-impedance line input (high impedance = low loading) of 1 or 2k ohms.

Load Impedance vs. Number of Inputs

Each additional input device will increase the loading seen by the other input devices slightly, but R (the resistor value) will still be the dominant term.

The load seen by each input device's output is roughly:

Zload = R + (R/N)//Zin

Where:
N=number of inputs-1
Zin = input impedance of aux input or amplifier
// = parallel resistance formula: R1//R2 = R1*R2/(R1 + R2)

A Zload of 1k ohm or higher should give fairly low distortion. Anything lower than that will start to get into increasing distortion levels and anything below the impedance of headphones will be dangerous.

Step 16: Variations...

A lot of folks have asked for variations, I'll start adding them here.

1) The first schematic is an idea to mix a voice chat (phone call, skype, teamspeak, etc) from a smartphone or tablet with game audio from a PS4, WiiU or XBone (with adapter). Note that the mic will not feed the voice chat on the console, but you probably could connect it through a capacitor (so you don't have two devices trying to power the mic).

Introduction: Altoids Tin 1/8" Stereo Mixer