Sound through the earphone is also sent to the amplifier when using a jack splitter after a audio mixer

Question

I implemented the following audio mixer circuit with just 2 input audio sources:

http://www.theorycircuit.com/audio-mixer-circuit/

Both sounds got mixed properly.

Then, at the end of that audio mixer I connected an audio splitter like the one below:

which provides me two female connectors.

Then:

• on one female I connected an amplifier with a speaker
• on the other female I connected a headphones (without microphone)

(for more details, please, check the image below)

My problem is: If I speak through the earphones of the headphones, then I can hear that on the speaker and I don't want that to happens. I just want to listen what's comes up from the mixer but nothing else.

Any idea on how to avoid this to happen? (which is actually weird, I could not expect that to happen).

Thanks in advance!

Answer 1

A dynamic microphone consists of a permanent magnet, and a coil of wire attached to a diaphragm. If you speak into the diaphragm, it generates a small current in the coil.

A headphone speaker consists of a permanent magnet, and a coil of wire attached to a diaphragm. If you pass an alternating current through the coil, it vibrates the diaphragm and makes a sound.

So a dynamic microphone and a small loudspeaker are actually the same thing. This means that you have effectively connected a pair of microphones to the input of your amplifier.

As others have said in their answers, the solution is a small headphone amplifier between the mixer and the headphones.

Answer 2

You don't say how you connected the single output of the mixer with a stereo jack. I suppose (and I have good reasons) it is in parallel (L+R shorted).

Now, when you speak into the headphone, the loudspeaker (of the phone) modulates the line and the amplifier amplifies exactly that. The impedance of the headphone, which should/could be about 70 ohm (I mean, not 10 times less, not 10 times bigger) is too much comparable with the output impedance of the mixer (about 320 ohm?).

I think that the only way to solve your problem is to put an active element in the middle, may be a transistor or another op-amp - but I am not an expert.

Another way is to reduce a lot the sensitivity of the amplifier, and/or use a squelch control if it has it. If the amplifier is able to amplify a microphone, then it is too much sensitive to be used after a "line output", which your mixer is (or should be) supposed to have.

Answer 3

Your headphone is an electromechanical system with some input impedance (can be quite high in high fidelity headphones). Which means that voltage will make it move but movement will induce voltage (with little current sourcing capabilities but still). If you present this movement induced voltage (if you speak into your headphones) to the high impedance input of your monitors amplifier it will amplify it, since it does not requires much current from the source (your headphone) to do so.

What you should do is to buffer the signal going to your headphones. So that the signal presented to the monitors and the headphone sees an high impedance on both side. Which gets buffered on one side to drive a 25-250 Ohms headphone load, and to a 8-16 Ohms speaker load on the other.

I don't know how you could continue using the splitter and eliminate your problem though. It would be simpler to duplicate the output signal, send one to the amplifier (as you have done), pass the second one through a buffer and out another port to the headphone.

Some good audio buffer candidate to drive your headphone could be:

http://www.ti.com/lit/ds/symlink/lme49600.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/AD8397.pdf