This electret mic is picking up AC mains noise. Why?

[Faranight]

Hello.

I'm rather puzzled what's going on here. I'm doing a thing to record some very faint sounds on an embedded device running Linux. The device has an analog audio input, so all I have to do is attach a mic to the correct pin. At least that was the plan... Since I didn't have a decent mic I thought I'd build my own. I ordered a bunch of electret mics from an online supplier and decided I also want a preamplifier. For this task I went to eBay and ordered a few of those red PCB's with a linear regulator and the MAX9814 preamp IC that has AGC with 40/50/60 dB gain. Anyhow, I wired everything up, connected the preamp board output to an oscilloscope and turned on the PSU on 5VDC. Wah wah wah waaaah...

The scope was showing a clear and strong 50Hz sinusoidal wave on the amplifier output. It seems that besides the audio, this mic was also picking up mains AC noise for some reason. I tried replacing the PSU with another and also hooking the preamp board directly to a 9V battery, but to no avail. If I disconnected the mic, all noise disappeared and I could see a flat line on the scope. The wire connecting the mic to the board was like 2 cm long. Funny thing is I've also tested the preamp board with some other electret microphones that I've salvaged from various devices. They're all different and most of them had unshielded wires (even up to 30 cm long), but none of them caused any AC noise on the preamp output. It's just this specific mic that happens to be picking up AC, but otherwise works fine audio-wise (can see audio appear on the scope while speaking aloud).

Electret mic datasheet: https://www.cuidevices.com/product/resource/cmc-6027-24t.pdf
MAX9814 datasheet: https://datasheets.maximintegrated.com/en/ds/MAX9814.pdf
Preamp board: https://www.ebay.com/itm/114828779130


Any ideas why this is happening and how to solve it? I mean, I could just use one of the other electret mics, but I bought a bigger number of the noisy ones that I had planned to use.

Thanks in advance!

[Terry Bites]

check your earthing scheme for ground loops.

[Kim Christensen]

Funny thing is I've also tested the preamp board with some other electret microphones that I've salvaged from various devices. They're all different and most of them had unshielded wires (even up to 30 cm long), but none of them caused any AC noise on the preamp output. It's just this specific mic that happens to be picking up AC, but otherwise works fine audio-wise (can see audio appear on the scope while speaking aloud).

Those mics have an metal casing that is crimped/bonded to the ground/return lead. It is possible that the offending mic's shield can isn't making proper contact. The better mics might also have a metal screen over the opening and the offending mic doesn't.

[floobydust]

Usually the can is connected (with pcb traces) to the ground terminal, so you might have it backwards.

[Faranight]

I double checked the mic with visual inspection as well as a multimeter and the shield case seems to be firmly in contact with the ground wire. I was also afraid I might have borked the mic while soldering wires to it as I used a helping hand alligator clip to hold the mic still and might have squeezed the case too hard. So I used a second mic from the batch and carefully soldered the wires on the bench directly. Still no difference. All mics have some sort of black textile over the front hole. The polarity is definitely not backwards - I checked the datasheet and it was wired according to the example schematics.

I am starting to think this may really be a grounding issue. I noticed that the amplitude of the sine wave increases drastically as I move my palm really close to the mic (without touching it). If I touch the ground wire with my other hand in the mean time the sine wave disappears. Still, it's strange that it only happens to this mic, but not others. Could this have something to do with this mic sensitivity since it's a -24 dB mic?

Regards!

[Kim Christensen]

All mics have some sort of black textile over the front hole. The polarity is definitely not backwards - I checked the datasheet and it was wired according to the example schematics.

If it works, you don't have it backwards since these mics have an amplifier inside. That's why they need a bias voltage.

I noticed that the amplitude of the sine wave increases drastically as I move my palm really close to the mic (without touching it). If I touch the ground wire with my other hand in the mean time the sine wave disappears.

It sounds like capacitive coupling to the electret element inside. I don't know if there's much you could do about that except using a different one (Like the other ones you have). Or you could try placing a "grounded" metal screen/mesh in front of it.

[Faranight]

I think I solved it.

It appears that this mic is in fact very sensitive to the EMI. I put it in a small metal enclosure that was connected to the GND terminal and the 50Hz noise went away completely. Even though the mic was connected to the amp by pretty long wires, they didn't seem to pick up any noticeable noise. I'll have to keep in mind that if I want to use these mics I'll need some extra shielding.

Regards!

[magic]

Old problem and not unique to mics.
high impedance + high gain → electric field pickup, every time

The solution is, as you discovered, to surround the circuit with something which produces zero electric field with respect to local ground. Put everything in a metal box or use two boxes and coax between them.

[wizard69]

I think I solved it.

It appears that this mic is in fact very sensitive to the EMI. I put it in a small metal enclosure that was connected to the GND terminal and the 50Hz noise went away completely. Even though the mic was connected to the amp by pretty long wires, they didn't seem to pick up any noticeable noise. I'll have to keep in mind that if I want to use these mics I'll need some extra shielding.

Regards!

Microphones are an interesting beast and some a better built than others.   The thing is you have already proven to me that you don't want to use these microphones as you have already come up with better solutions.    Beyond that all microphones have their own characteristics (real and imagined), that is why musicians often have obsessions with specific microphones.   Personally I'd give the microphones that give you the least trouble and opportunity in your circuit.

[Wallace Gasiewicz]

I will remember this, sounds like something that will happen again.