DIY Multi-Input Audio

Edited, see bottom of this post.

What would be the best method for preventing damage to multiple audio input devices that are connected to a home made audio patch panel?

Im wanting to make a device or cable where I can connect multiple 1/8" 3.5mm stereo audio sources, and have them output thru a single lead/port/connector.

My concern is potentially damaging the other sources by having the signal/voltage feed back to the others because all the leads would be interconnected.

The sources would be low power devices like mp3 players and cell phones.

I don't know the terminology, but im basically looking for an electronic "check valve" that would prevent signals from one device back-feeding to the rest, while (preferably) not inhibiting the signal strength going to the output port.

Edit: So what I'm looking to do is make a device or wire for connecting my phones audio out via headset Port, and my xbox ones headset (headphoneout + mic in... On the controller) to my headset with mic, all at one time. The console doesn't support background music, so id like to mix the game audio and phone audio into a single set of headphones, while having the mic feed return to the controller.

The "damage" concern is that I'm worried about damaging my phone by running xbox audio, but not phone audio, and having that signal voltage back-feeding into my phone audio out Port (or vice versa). Idk if that should be a concern or not, but for peace of mind, I would rather not have voltage going into an out Port on either device.

Hence the "check valve" reference. Something that would allow voltage to move forward, toward the headset, but preventing it from running back to the other device. Hope that makes sense.

More or less, I could easily make a Y adapter that has 3 female trrs Jacks, then connect all devices, but im wondering if I should put a resistor or something on two of the leads to stop or lessen voltage from device A going back to device B, but allowing both to flow freely to the Headset.

EDIT 2: I found this diy inout mixer in my searches that seems like it would be what I need. Can you check it out and give me feedback on the design, and if it seems 'proper'? Thanks.

http://www.instructables.com/id/Altoids-Tin-18-Stereo-Mixer/?ALLSTEPS

• This is actually fairly broad. We have to narrow down exactly what threats the circuit is supposed to guard against, and which threats are out of its scope. If one of the input devices is struck by lightning, do you want the others to be protected? Your question does not quite rule this out, in its present state, and it makes a big difference. :)
– Kaz
Jun 24, 2016 at 23:29
• @Kaz: He's just looking for an audio mixer. Jun 27, 2016 at 6:11
• I am looking for just a mixer but the problem is that they don't really make any that carry mic and stereo together, or missing inputs for other devices, or don't take 3.5mm. There not really one that has it all, and I'm not trying to have a mixer and a bunch of adapters and converters, not to mention a ton of money sunk in. Just something very basic with more than one audio input Jun 27, 2016 at 6:36
• @BrianB The "EDIT 2" mixer is essentially the answer given by uint128_t, just with the complete construction explained and including the requirements for stereo implementation. Jun 27, 2016 at 20:39
• Yes, and theyre seponse is what got me looking in that direction, but it gave the specifics as far as resistor rating and such. Also, at the end of the guide, there are notes about adding a mic to the mex as well. So it looks like this woukd fit the bill perfectly. Just wanted to run it by everyone to make sure it looked like the best solution, or see if anyone woukd suggest any mods as far as the resistors or wiring go. But if not, I think we can call this question answered, and I thank yoh all for the advice. Ive learned a lot that I can use in my future tinkering. Jun 27, 2016 at 20:47

3 Answers

The circuit you are looking for is called a "passive summing circuit".

It consists merely of resistors. The circuit does not completely prevent a device from backfeeding the rest, but it ensures that if two devices are driving a signal, they will not be damaged.

There are numerous circuits and references available from a quick web search, but here is an example:

You can add inputs as needed, to a certain extent (a large number of inputs may require an active summing circuit).

In your case, you also want stereo, so you need two circuits, one for each side (L/R). As for values, they're non-crucial, but they should be equal, and in the range of 5kΩ to 25kΩ. Others may have stronger opinions on values.

• This will affect signal strength though. Jun 25, 2016 at 0:32
• True, but for most audio applications, it is sufficient. OP, if you want a completely transparent summer, you need an active summing circuit. Jun 25, 2016 at 0:33
• Or even just an op amp with an RC mixer in front. Just a few extra pieces compared to this. Jun 25, 2016 at 0:36
• The power supply needed for an op amp may be prohibitive when this passive solution is available. Operating at low output impedance and with a ~10k input impedance at line levels, the passive mixer should be just fine. Jun 25, 2016 at 1:53
• Thanks for the comments so far. I edited the question to try to clarify my goals and concerns. The summing circuit looks like it would be pretty close, if not exactly what id be looking for. But more thoughts are appreciated Jun 27, 2016 at 3:49

simulate this circuit – Schematic created using CircuitLab

Figure 1. The simple circuit.

This is the simplest circuit but suffers the reduction in volume as the output signal strength will be given by:

$$V_{OUT} = \frac { V_{PHONE} + V_{XBOX} + V_{OTHER} } {3}$$.

If you want to restore the original volume you'll need to put an amplifier with a gain of 3 feeding each channel of the headset socket. In that case increase the resistors to 10k and use something like the LM386 amplifier chip. There are thousands of example circuits on the web.

• Very nice and easy to understand. So basically, the vokume woukd be rduced by the number of inputs? Is this figured by counting actual devices connected at a given time, or the # of input jacks that are wired to the circuit? Aslo, where would the amp chip be installed.. After all inouts but before the output? And does that require a power source to work (the amp), or no? Jun 27, 2016 at 21:35
• Yes, volume reduced by number of inputs. If you don't need 'other' then omit the resistors and reduction will be less. Amp chips would be required in the bottom right corners of the circuit. Yes, the amplifier would require power. Someone may suggest something that can run on USB power. Thanks for accepting my answer but you should un-accept for a couple of days to see if a better or more complete solution appears. The 'accept' may dissuade others. Jun 27, 2016 at 21:58

Your best option here is probably a simple fixed-gain op amp mixer.

Each input jack will present a constant input impedance, that will be independent of whatever is plugged into other channels. Each input jack will appear to be terminated at (virtual) ground, and will not be able to drive any other input jack.