High sensitivity microphone circuit for ambient noises - Page 1
Forum section: electronics / projects

author: Faranight
author_rank: Unknown
timestamp: April 17, 2022, 01:14:22 pm
content: Hello!I'd like to build ahigh sensitivity low noise circuitwith anelectret microphoneto record ambient sounds. It should be able to pick up faint sounds in a quiet room i.e. a fridge hum, a cricket or a bird singing outside, footsteps and floor creaking, a dog barking in the distance, etc. Looking at the noise scale I believe this sums up at about 20 to 40 dB SPL. I've already experimented on a breadboard and managed to come up with a circuit that could pick up these noises quite nicely. They were recorded on a laptop via the mic line-in port. Yes, I used a spare external USB sound card to do the recording just in case so that I didn't fry my laptop's line-in port by accident. Although I was powering this circuit from a lab-grade linear PSU, the final PCB will be powered from a more noisy 5VDC switcher or a USB even.Anyway, I'd like to ask someone to take a closer look at my circuit and see, if everything is assembled correctly and maybe answer some of my questions.I'm using using this operational amplifier:https://www.ti.com/lit/ds/symlink/tlv6742.pdfI have two electret microphones available:https://eu.mouser.com/datasheet/2/683/MO093803-1-359263.pdfhttps://www.cuidevices.com/product/resource/cmc-6027-24t.pdfQuestions:1)Since I plan to power this thing on 5VDC from a USB port, is the power supply is filtered enough? Originally, the circuit was powered directly from a 5VDC rail, but I added a 3V3 linear LDO, some extra bypass caps and ferrite beads. The mic, opamp and the bias are now all on the 3V3 line, decoupled and with separate ferrites. Should I add some extra filtering on the 5V line before the LDO... i.e. a ferrite bead? A simple LC filter? A butterworth filter? A capacitance multiplier? Something else?2)How many stages should an electret amplifier have? Mine only has one with high gain and it seems to be enough to amplify the mic signal to audiable levels. Should I redesign it with two opamps with lower gain instead for lower noise?3)Which opamp topology to use for low-noise applications? When asked, some people said it doesn't really matter noise-wise, yet others claimed to use non-inverting topology because the inverting one "hisses". I think I should also change RV1 and R8 to lower values to reduce the resistor thermal noise.4)Should I replace the voltage divider and buffer with a dedicated voltage reference chip? for cleaner bias voltage?5)Am I missing anything else in this circuit?Thanks.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 17, 2022, 02:00:21 pm
content: There's a general rule that the signal to noise ratio is set in the very first stage of amplification. It can only get worse as you move beyond that stage.3.3V bias tells me that this is a little electret with a built-in FET. The noise performance of those is usually so-so at best, so the circuit is probably overkill. To do better you should look for a better capsule with no built-in FET. The problem is they tend to be expensive. OTOH, a good 1/2" mic and preamp can get you down near 20 dBA SPL noise levels. You'll also do better with a large diameter capsule and probably a high voltage phantom circuit. There's a group you might find useful dedicated to this stuff athttps://groups.io/g/MicBuilders/messagesSome people there are even making their own capsules from scratch!

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2022, 02:08:26 pm
content: You may also want to look for MEMS microphone instead of electret. MEMS one are better at capturing ambient sounds than the electret ones. MEMS mics have their own preamplifier in the same metal can, so no noise problems. They might be found for free in defective or no longer used mobile phones.For electret mikes the signal is usually big, so 1-2 amplif stages are enough. The main damaging cause is the high temperature during soldering. The electret inside is very, very sensitive to high temperature, and will lose its property if heated too much (which will lead to permanent loss of sensitivity).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 17, 2022, 02:32:32 pm
content: Those MEMs mics can be quite good. A huge number are made by Knowles-https://www.knowles.com/department-details/dpt-microphonesand brand new ones aren't even that expensive.

author: cdev
author_rank: Super Contributor
timestamp: April 17, 2022, 04:10:00 pm
content: Thank you!I am also interested in home-building of microphones. (For speech and ham radio use)Quote from: Conrad Hoffman on April 17, 2022, 02:00:21 pmMy experience with electret capsules has been that the sensitivity is good, noise level is adequately low, but the resulting microphones I made lacked a low end response in voice that I wanted more of. I experimented with different kinds of enclosures for the capsules. The best result I got was mounting the capsule in one of those thick straws they use for tapioca drinks. Yes, they are pretty flimsy. I didnt use a pre-amp, it didnt seem to need it. I tried to use the tapioca straw capsule for recording outdoor sounds and it was sensitive but picked up a lot of wind. So I tried wrapping it in a thin open cell foam sheet, which helped a lot. Then it was acceptable. I was trying to monitor an outdoor noise source that was driving me nuts in order to figure out if iot was within legal limits. Luckily its much reduced now.I have two Onkyo USB-audio units that provide low voltage for a condenser mike if its needed, via a three conductor miniplug. That worked okay but I feel as if I could probably do much better. I have one high quality condenser mike that I got at an estate sale but it requires phantom power (and uses a balanced XLR audio connector) So as I dont have a phantom power source I cant use it. Whats the best not too expensive way to provide that on my own? I'd like to try that better mike out.

author: cdev
author_rank: Super Contributor
timestamp: April 17, 2022, 04:21:26 pm
content: Behringer, I think, makes a high quality mike which is calibrated which is not too super expensive. It can be used for sound measurements.I just found this thread which I saved because it looked like a practical approach, with modest costs.https://www.diyaudio.com/community/threads/behringer-umc-202hd-for-measurements.341309Don't know how authoritative as far as quality it is but it looks credible.

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2022, 04:57:36 pm
content: What does R6 do? Nothing but reduce the output of the microphone slightly, by the looks of it. I recommend removing it.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2022, 05:13:49 pm
content: My guess is R6 might be there to make a low pass filter together with C4 (against RF, so the mic+wires won't catch FM radio or HF noises from PCs).

author: Faranight
author_rank: Unknown
timestamp: April 17, 2022, 06:48:03 pm
content: Correct, R6 and C4 form an EMI filter. See this app note:https://www.st.com/resource/en/application_note/an1534-ts971-based-electret-condenser-microphone-amplifier-stmicroelectronics.pdf

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2022, 08:17:34 pm
content: Quote from: Faranight on April 17, 2022, 06:48:03 pmThat makes sense and is a good idea if the microphone is connected to a long cable, but wouldn't it be better to put it on the op-amp's input instead?

author: cdev
author_rank: Super Contributor
timestamp: April 17, 2022, 09:25:12 pm
content: A bunch of open ended tubes of different lengths can be used to make a very directional "bazooka" microphone good for recording bird vocalizations. It works really quite well. The different lengths are there quell the resonance peaks.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 17, 2022, 09:53:28 pm
content: It wouldn't be crazy to make up a quick 48V phantom supply by plugging 5 nine-volt batteries together. They're usually about ten volts. I'm not up on these things but it might need a big series resistor as well. Maybe only loosely related, but I recently put this page on my site-https://www.conradhoffman.com//Audacity%20SPL.htmthat has some mic info.