3.5mm jack signal splitter

Question

I need a solution for listening to music from 2 headphones/earphones on a single mobile phone. Using a speaker could be an option buy my girlfriend wants it this way. Anyway, while looking for a solution, the only thing I could really find is DIY videos of people making a cable like this 3.5mm jack signal splitter. I'm worried this could affect maximum volume, or damage the earphones, maybe even my phone's audio output, as it would put two inputs in parallel, thus making the current larger. For this reason I've been looking for some low-power circuit that i could power with a smaller battery pack, probably with an amplifier with two outputs or something. I've been unable to find any that suit this criteria. I did find this circuit which seems like it could be what I need, but I'm not sure about how portable it is, considering it would probably need a separate power supply or something, since it works at 20-35V.

I would like to know are those 3.5mm signal splitters reliable and safe for both phone and headphones, and if not, I would appreciate if someone could point me to the right direction regarding picking a custom circuit and assembling it. I would very much appreciate if you were to provide me with a suitable circuit diagram.

I have relatively limited knowledge of electronics, one that's mostly theoretical so I would warmly welcome any corrections you make regarding stuff that I said in the question. Same goes for my English, since it's not my primary language.

Answer 1

Depending on how much money and how portable the system can be, the splitter can be a very good solution.

The splitter only parallels the headphones, there is nothing fancy happening inside of it. This can have at least two effects:

• lower the maximum volume
• damage the phone

The maximum volume will be lowered because the amplifier inside the phone needs to drive 2x headphones, so they will need to share the power. Whether this is acceptable or not depends entirely on the amplifier in the phone, and on the "efficiency" of your headphones, i.e. the dB per watt they can output.

Damaging the phone is a very remote and unlikely scenario. The amplifier inside is designed to drive a certain load, and you are demanding more. This can potentially overheat the chip, drain the battery faster, or even damage the chip. Again, this possiblity is extremely remote: the chip is most probably protected from short circuits, so it is impossible to damage it.

One alternative you can explore is to use bluetooth headphones. Some phones allow for multiple headphones to be connected at the same time.

Another option would be to use an active splitter; I don't even know if such a thing exists, but it would require an additional battery/power source.

Since you say that your electronics knowledge is limited, I strongly advise against assembling your own circuit. There is no way you can do better than a mass produced, engineered product.

All in all, your best bet is the splitter.

Answer 2

I think most modern phones are short-circuit protected (don't quote me on that though).

There is one issue that is often overlooked, and that is volume. Different headphones have both different impedance, and different sensitivity.

The different impedance can vary wildly between headphones. For example, in-ear monitors can be around 10 ohm impedance. Big over-ear headphones can go as high as 250 ohm or more. (yes, most consumer headphones will be between 20-30 ohm range I believe, but this is just to illustrate potential issues).

As a result of different impedance, one headphone will see a different load than the other. Add on top of that the fact that different headphones have different sensitivities (how efficiently they turn electrical power into sound) and you get all manner of volume issues with two headphones.

^{simulate this circuit – Schematic created using CircuitLab}

If you want it to be small and light, you can look at 'real' ICs aimed for driving headphones from phones or other mobile devices, but these are usually tiny chip-scale packages. EG, the A22H165M from ST, which offers short-circuit protection and independent control of channels. Two of these could be used to make a tiny splitter, but you need some form of microcontroller.