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Below shows a dynamic microphone equivalent circuit:

enter image description here

As you see above, the microphone output is first converted to balanced circuit via the transformer. The signal then travels along the cable until the preamplifier. It seems the signal is splitted to bipolar by the transformer. Here is another example.

But how about in headset microphones such as this one? I guess they are also dynamic mic type. But obviously there is no transformer or another circuit after the headset mic. Does that mean the headset mics are single ended unbalanced transmission? By transmission I mean the cable from the headset mic to the TRRS jack which goes into the smartphone audio port.

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  • \$\begingroup\$ You can remove LF noise from unbalanced mics by using a sharp multi-pole HPF. You need to filter out not just 60 HZ noise but 120 HZ and maybe 180 HZ harmonics from multi-phase power lines. \$\endgroup\$ – Sparky256 Oct 7 '18 at 4:29
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Headset microphones are not dynamic microphones.

They are electret microphones.

The signal from an electret microphone is something like 100 times the signal from a dynamic microphone. The signal from a dynamic microphone is a few millivolts. The signal from an electret microphone is a few hundred millivolts.

That makes the influence from external noise less of a problem for the electret microphone.

The cable length is also far shorter. Your headset cable is maybe 1 meter long. The stage microphone may have a cable run of tens of meters. Less cable, less opportunity for noise to get in. Again, less of a problem for the electret microphone.

The main problem for headset microphones is that they can pick up interference from the radio signals from the phone - and there are effective ways of handling that with out using a balanced connection. (Bypass capacitors, shielding, and ferrite chokes.)

So, really, the headset can do just fine without out a balanced line.

Then there's the down side of balanced lines:

The transformer. That big, heavy, clunky sucker is OK if it is in something that you hold in your hand. It is not so whoopy if it is hanging from your ears.

You can do balanced connections without one, but then you need an electronic balanced line driver - and it needs a power source.

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  • \$\begingroup\$ Thanks for great answer. Is the FET part of the mic inside the headset mic part or inside the smartphone? \$\endgroup\$ – cm64 Oct 7 '18 at 14:48
  • \$\begingroup\$ Electret microphones nearly always have a small FET amplifier as part of the capsule. \$\endgroup\$ – JRE Oct 7 '18 at 15:17
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The advantage of perfectly balanced differential sources is the induced voltage from a common mode stray current nearby is cancelled. The long cables used on mic’s utilize this for low hum with shielded twisted balanced pair cables.

Nothing is perfect so the CMRR can be measured in dB attenuation from a specified interference level to compare the best with the rest. But these tests are usually only done by EMC experts.

Yes FET buffered mic’s are unbalanced and prone to some mobile charger noise from the SMPS PWM carrier modulation hum from the AC LINE input in some cases. This is due to the unbalanced coupling capacitance in the high frequency transformer. Ferrite CM CHOKE donuts help but may not eliminate the hum in all cases with all charger types. You may find these on all VGA cables, some mic’s, some DC adapters and some USB cables.

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