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I am learning about headphones and microphones and I have 2 questions.

I bought a few pairs of headphones from the dollar store and took one pair apart. There is a weak little magnet in the center with very thin insulated wire coiled around it and there is a very delicate plastic sheet covering this little assembly. Now, the headphones worked before I took them apart so it's very exciting to see the guts. I've also studied this design and I think I understand the physics of it.

My questions are:

  1. Should I be able to use these kinds of headphones as microphones? I've thus far been unsuccessful.

  2. The headphone jack is tip-ring-sleeve. If I connect sleeve (which I think is common ground) to either tip or ring (and thus completing the circuit), can I use my multimeter to measure the current/voltage if I yell into the headphone?

My goal is to see (detect) this thing at work. I've learned a bit about op-amps too, and I wonder if I cannot do what I want without making a slightly more advanced circuit.

Kindly,

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  • \$\begingroup\$ It should also work as a mic. A multimeter will not register that small of a signal. Apply a small voltage across tip to ring, listen at one as you speak into the other. \$\endgroup\$ Dec 13, 2016 at 3:47
  • \$\begingroup\$ If I add a battery to the circuit I described, would a multimeter be able to detect changes in the voltage? Is the voltage produced by the headphone/mic below the threshold the multimeter can detect? Or is the change itself too small to be detected (by my pretty inexpensive multimeter, that is)? \$\endgroup\$ Dec 13, 2016 at 4:01
  • \$\begingroup\$ Sorry if my "apply a small voltage" added confusion to the mix. Richard Crowley (bellow) gave a more complete answer. \$\endgroup\$ Dec 13, 2016 at 15:51

2 Answers 2

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In most cases, headphone transducers will also work as dynamic microphone transducers. However, they put out a VERY SMALL amount of signal which you will never see on a regular meter. No it cannot be "amplified" by simply applying a DC voltage (from a battery or whatever). You need a high-gain AC amplifier. Specifically a microphone preamp to bring a "mic-level" signal level up to "line-level" which can be seen on a meter. Typically 40dB (10,000 x) of gain. There are hundreds of examples of mic preamp circuits online.

Most small microphone capsules used with computers, etc. these days are electret condenser microphones which are quite different than dynamic microphones.

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  • \$\begingroup\$ Oh I see. I have an quad op-amp ic (LM324). I will try to feed the very weak output from one earbud/mic into a non-inverting amplifier, and feed that output to the input of the second earbud/mic. I can't try this for several hours so before I can get around to it, does this sound about right? \$\endgroup\$ Dec 13, 2016 at 14:29
  • \$\begingroup\$ LM324 is OK for fooling around and learning about audio. But most people use better-quality op-amps for serious audio applications. Such as TL071 or NE5532, etc. \$\endgroup\$ Dec 13, 2016 at 15:29
  • \$\begingroup\$ I think it's fair to say I'm fooling around. I hooked up a pair of headphones to the lm324, but I got a bunch of static in one bud. That I hear anything is a minor victory. In the simplest construction of a non-inverting amplifier configuration of an op-amp, what kind of resistors should I use? I'm using the formula Gain=1+R1/R2 (where R1 is the feedback resistor). \$\endgroup\$ Dec 15, 2016 at 4:51
  • \$\begingroup\$ I would aim for something at least a voltage gain of 100x. Mic preamps with gain of 1000x are not unusual especially for less-sensitive dynamic microphones (such as you have). \$\endgroup\$ Dec 15, 2016 at 5:03
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Plug the headphones into the 'line in' socket on a PC. Use the sound-in application to select that input, and to use as high a gain as possible for that input.

Use an application like Audacity (free, cross platform), to record, view, and further amplify the sounds.

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  • \$\begingroup\$ I didn't think of that but I do have Audacity installed. I'll give this a shot later tonight. \$\endgroup\$ Dec 13, 2016 at 14:31
  • \$\begingroup\$ The line-input is not sensitive enough for a microphone-level signal. There is typically no gain control on the line input. But there is on the mic input. However the mic preamp circuit inside the computer is probably the worst (most noisy) of anything on the planet. Any gain you add inside the computer (either hardware or software) will be very noisy. That is why people use external (USB) sound interface and mic preamps, etc. \$\endgroup\$ Dec 13, 2016 at 15:26
  • \$\begingroup\$ My mobo-integrated sound has the same amplifier options for all inputs, line, mic, but I know plenty won't, hence 'use as high gain as possible', and follow it by using Audacity to further amplify it. As a zero-investment experiment, it's worth a go. I understood from the question that the OP wanted to play with it, rather than record CD quality with it. \$\endgroup\$
    – Neil_UK
    Dec 13, 2016 at 15:44

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