Altoids Tin 1/8" Stereo Mixer

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.

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.

Comments

[chipmonger]

Made one and it works great. Thanks.

[richfiddler11]

Wow, really nice job! Good idea to use resistor networks!

Would you care to share the artwork?

[chipmonger]

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.

[richfiddler11]

Thanks for the artwork and again, really nice job!

[rand0mmm]

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.

[richfiddler11]

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

[fergusontea]

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.

[richfiddler11]

Nice! I particularly like the 'Hello Kitty' motif ;^) Thanks for sharing the picture.

Rich

[hang4]

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

[lespaul55]

Could you use 1/4 female jacks? If yes then can you mix between the 1/4 inch and 1/8 inch jacks?

[drluv]

Great idea/design. I've been working on feeding 6 MP3 players into one output that would then feed an old PC speake amplifier with it's output going into my 100 amp radio with it's front stereo jacks. The reason for the amplifier is that the volume output of the MP3 players are not loud enough by themselves.

Do you see any problem in this combination such as the impedance of the PC amplifier, etc.? Would the 1 ohm resistors still be a good choice?

Thanks,

drluv

[richfiddler11]

What you want to do is exactly the use case this mixer was designed for. You just need to add 2 additional inputs that are identical to the other 4 in the example. 1 KOhm resistors should be fine (not 1 Ohm, which I'm guessing was a typo?)

You should try feeding the output of the mixer straight into your car stereo input before even bothering with the additional stage in between. If this is an Aux input and is fairly high impedance (normal line-level input) you probably don't even need additional amplification.

Rich

[rkoschnitzke]

Can you design this circuit with diodes instead?

[richfiddler11]

No. Please read the FAQ section, step 15 for the details.

[rkoschnitzke]

What about a Germanium Diode with a fv of apx .3?

I swear my buddy built one with diodes that worked like a charm. Two sources each a computer sound card... into a box with this circuit and one output to powered speakers.

I'll have to look further.

[richfiddler11]

By putting a diode in line with the signal, you are literally clipping it (everything below 0.3V will be clamped at 0.3V, called half-wave rectifying) and it's the same effect as when an amplifier is driven into overdrive and clips the signal. The smooth peak of the sine wave is replaced with a chopped-off square wave on the negative side which introduces harmonics and distortion.

I assume the reason you don't want to use resistors is because of the signal attenuation. Keep in mind that resistors are highly linear and introduce virtually no distortion, meaning the signal is still a clean copy of the original with a little less amplitude. Amplification can bring this back with a very high degree of fidelity to the original signal.

Diodes are non-linear and once the damage (clipping) to the signal has been done, you can't undo it later on. That's why I would never use diodes on an audio signal unless I was intentionally trying to distort it.

But hey, knock yourself out and try it. You won't hurt anything and maybe you won't be able to tell the difference. But I bet if you do compare the resistor and diode approach side-by-side, you will notice which one sounds better.

[rsmith56]

I would love to use this with my 4 wired lav mics (atr-3350 small battery powers it). Two questions - is it possible to add volume to each input? Would you need to power it? and what about a 3 way switch that would allow left/both/right for each? Would that be very hard to do?

[richfiddler11]

Self powered lav mics would work fine. For volume control see circuit in FAQ section, step 15. power is only required if you add an active preamp circuit. Yes, you can add a left/right switch, I'd probably use a SPDT switch.

[heinsickle31]

Ok, I got both speakers to broadcast (apparently the resistors aren't allowed to touch the can, but if you guys have any info on the volume level, that'd be great.

[richfiddler11]

With a passive design you will always lose some volume. What are you trying to drive? A powered speaker?  A passive speaker? Headphones? The lower the impedance of the load you're trying to drive, the bigger the volume loss will be.

[heinsickle31]

Headphones. Actually, I am currently using it as a headphone splitter if it makes a differance, but the loss happens when I use it as a mixer also. I don't mind the loss of volume, I wouldn't put it much higher anyway, but I would like the possibility to though.

[richfiddler11]

Yeah, if you read the notes and comments, this is not at all recommended as a headphone splitter. The impedance of headphones is in the 8-32 ohm range (quite low) and this mixer is not going to perform well unless your driving something with an input impedance of 1k ohm or above, like a typical powered speaker input.

For a headphone splitter you would be better off not using the resistors. The resistors are only required for mixing multiple sources together.

Rich

[guywith2names]

im not an expert in electronics and i would like to use this with headphones w/o amplifying it.... what if i lower the resistors to lets say 500 ohm or lower? would that work. Im trying to have 2 3.5 input sound run to 1 pair of headphones. i know if i used a headphone splitter to COMBINE 2 sound sources, and i turn BOTH volumns all the way up, it may damage the equipment. what are you thoughts of doing this?

Ps: sorry for any grammer and errors

[richfiddler11]

In order to run headphones, you'd have to go lower than 500 ohms. Most headphones have an impedance of between 16-32 ohms and typically you want the source (i.e. amplifier) to have a lower impedance than the load (i.e. headphones).

If you want to experiment (at your own risk) and as long as you don't take the resistor values below the resistance of a typical set of headphones (again, 16-32 ohms), then you shouldn't cause any damage to either players or headphones  and you can gauge the sound quality/volume trade-offs for yourself.

[TAYLOR1337]

what sort of resistors would i need for a powered headset?
http://www.roccat.org/Products/Gaming-Sound/ROCCAT-Kave-5-1/ is the headset, im guessing 10KOhm as it says on the tech specs but im not sure. im looking to fuse together the 5.1 3.5mm outputs on both my PC and a decoder box from my xbox, and also the headset signal from the xbox controller through the fronts.

Thanks

[richfiddler11]

1 kOhm would still be fine. You typically want the impedance of the source (mixer output) to be much higher than the impedance of the load (headphone). It's good that you're using a powered headphone. Interesting headphones, do you plan to mix 2 sources for each of the 5.1 channels? In other words you'll have 8 (10?) inputs and 4 outputs?

[TAYLOR1337]

another quick question, does it matter what way round the resistor goes? like gold ring towards the input or output etc..?

[TAYLOR1337]

yeah thats what im looking to do :)

i have a tin that hopfully ill be able to put 4 different circuits in, 1 for each channel. yeah ill have 9 inputs in total as ill have an extra input on the front speakers from the output of my xbox controller, on a adaptor i have for it.

Thanks for uploading the instructions btw :)

[JKPieGuy]

Hi there.
I am currently working on alittle electrical project that I've put off for months due to the fact of not exactly knowing what I'm doing and I was wondering if you could help me out. I saw your project you made with the "Altoids tin audio mixer" on Make Magazine awhile ago and I had thought it would make a good headphone splitter if you reversed the way the audio was going into. (Basically one input feeding 4 outputs.) The problem is that I want to add alittle LED indicator light that automatically turns on when you plug some thing in and I don't know how to exactly wire it so that it could do that. Another question I had is do you still need the resistors if I make it into a headphone splitter instead of a mixer and could it be backwards compatible, like say I wanted to use it as a mixer one day instead of a splitter? I was possibly wondering if there was a way to amplify the signal that wasn't to costly (but if you can't help me out with that it's ok.)
If you could answer my questions I'd really appreciate it.
Thanks.

[richfiddler11]

1) Regarding the LED -- Yes, in theory this could be done but I don't see the point.  The project is entirely passive, so there are no batteries to either A) power an LED or B) die and require replacements or require anything to be switched off.

If you added an LED, a battery would have to be added and would only serve to light the LED.

If you're dead set on adding an LED, google on 'LED indicator circuits' or something like that and you'll find loads of information and circuits online.

2) regarding splitting vs mixing -- the ideal construction of a splitter is to use no resistors, while the ideal construction for a mixer requires resistors.

If you plan to split a signal to drive several high-impedance inputs (e.g. powered speaker inputs) then turning the mixer (with resistors) into a splitter will not be a problem. If you're trying to drive several pairs of headphones, then you will not want the resistors.

Cheap headphone splitters are readily available, while cheap, small mixers are not.

3) Regarding amplification -- I've pointed out several times in previous answers that you could easily add this mixer circuit a  front-end to a CMOY headphone amp, which also happens to be built in an Altoids tin. Google on 'CMOY headphone amp' and you'll find tons of info.

[TobaTobias]

Great Project!
I have a question. Could I use a 4 stereo jacks and a mono one, and still use the 1K resistors? Thanks!

[richfiddler11]

Yes, no problem. Just don't tie L & R channels together without the resistors in between.

[heinsickle31]

Sorry for being a little late, but I can't seem to find the problem with mine. I can only get it to play out of the right speaker and it is only about half volume. I did everything that the instructable said and I can't find any shorts. There are only two things I can find that might be an issue. 1) I used different terminals. The link for them are here: http://www.radioshack.com/product/index.jsp?productId=2103452
2) There is one solder point near the output jack where my solder point on the output jack and my solder point where I soldered the 2 resisters together accidently got soldered together (So basically, what should be two solder points are now one). I hope that was a thorough explanation. Also, I am confident that it is the output jack which is the problem, because I tried all of the inputs and they are all working the same. Thanks for any information you may have.

[guywith2names]

if i only have 2 inputs do i still need to amplify the audio signal

[richfiddler11]

It doesn't make any difference if you have 2, 3, 4 ... n inputs ... you can only drive a high-impedance device very effectively with the output of the mixer.

If you are driving an input to a powered speaker, aux input, etc this is no problem (these are examples of high impedance inputs). If you're trying to drive a headphone, speaker, etc (examples of low-impedance inputs) you are not going to be very happy with the sound level.

[richfiddler11]

I meant to say that speakers, etc were *low* impedance devices ... meaning they require some electrical power to drive them. High impedance devices can accept an input with a very tiny amount of electrical power.

[rjones24]

If I wanted to modify this so that one input went to the left channel of the output and a second input went to the right channel of the output, how would I do it?

[richfiddler11]

I'm curious as to why you'd want to do this, but that aside:

Assuming you have 2 stereo inputs, you would first need to mix each stereo channel down to a mono signal. Do this by tying the L and R channels together with 2 resistors in series (1K or so each), then feed the middle tap of the resistors to the L or R output mix.

[rjones24]

I have a couple of police scanners in my car and it can sometimes sound like they're trying to talk over one another. However, if one came out of the left and one out of the right, the overlap might not seem so bad. That aside, you lost me almost right off the bat. I'm very novice but managed to understand your instructable pretty well, I think. But I don't get what you mean by tying the L and R channels together "in series." Sorry, it's been a long time since I did any kind of electronics project.

[richfiddler11]

OK, interesting -- I bet the left/right separation would probably help you keep them apart better in your mind.

Yeah, I might not have explained what I meant in the clearest way. I'll try an ascii art picture and if that doesn't work maybe I can draw a real schematic. Here's what I was talking about. This is assuming the input is coming from the left:


left >----------|
                      |
               1K Resistor      
                      |
                     +---------------------> mono mix output
                      |
               1K resistor
                     |
right>--------+
              

You would tie the mono mix of one input to the L output mix and the other to the R output mix. This is equivalent to panning one input hard left and one hard right.

If you wanted to get *really* fancy, you could implement adjustable panning (i.e. audio positioning) so you could adjust each input's position in the stereo mix. I'd have to think about this but it would probably require a stereo potentiometer on each input channel.

Try the simple case first though -- that will give you the largest separation and if that doesn't make the 2 conversations more intelligible, then there would be no point in adding adjustable panning.