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Electret mic preamp to connect to Raspberry Pi
Electret mic preamp to connect to Raspberry Pi
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Old 26th October 2019, 11:41 PM   #1
inkl is offline inkl  Russian Federation
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Arrow Electret mic preamp to connect to Raspberry Pi

This is a follow up to this thread here - https://www.diyaudio.com/forums/anal...-new-post.html.


Basically, I'd like to use an omni electret mic and make a preamp, so I could connect it to Raspberry Pi. The whole package needs to be able to record a very quiet sounds.



As for the making an ADC, I do have one already and it works on RPi. But I thought I could make it as a single PCB so to keep the whole package as small as possible. But it was suggested it'd not be an easy task.


There is a detailed info in that thread on what needs to be done. But at the moment I still need to read more on the topic as I still don't understand everything yet. But I'll start with just a preamp (or two )
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Old 27th October 2019, 01:13 AM   #2
rayma is online now rayma  United States
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If you really need to pick up very soft sounds, an omni mic is the worst possible choice.
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Old 27th October 2019, 01:40 AM   #3
inkl is offline inkl  Russian Federation
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The sound may come from any direction, so I don't really know it upfront.
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Old 27th October 2019, 01:41 AM   #4
mchambin is offline mchambin  France
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A reference document.
Texas instruments publication on electret microphone and electret microphone preamplifier.
http://www.ti.com/lit/ug/tidu765/tidu765.pdf
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Old 27th October 2019, 02:03 AM   #5
PRR is offline PRR  United States
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> needs to be able to record a very quiet sounds.

You don't need a ton of gain into a low-volt ADC.

A 16-bit ADC has a 96dB range. The max input may be a couple of Volts. 96dB below that is 10 microVolts.

A $2 electret has a self-noise of several microVolts. A gain of 10 will put the smallest sounds comfortably above the ADC's lowest bits.

The self-noise of the low-price electret is commonly below 30dB SPL but never below 20dB SPL. The ambient sound in my livingroom is 19dB SPL; this is exceptionally low (I live in the woods). Good recording studios are rarely below 25dB SPL. Most more-urban livingrooms run 30dB-40dB SPL.

A typical sensitivity for an electret is -45dB (re:1V) per Pa (94dB SPL), or 5mV at 94dBspl. So an ambient noise level 60dB lower, 34dBspl, is 5uV (similar to self-noise). Again a gain of 10 is a good start.

Do you ever get loud sounds? My birds can exceed 80dBspl in my driveway. This is 1mV out of the mike. A gain of much over 100 will just overload the ADC input.

When I recorded various events with electrets I used gain of 50, gain-pot/limiter, and gain of 2, to feed tape decks with nominal overload of 1.5v. I almost *never* used the top half of the gain pot even for very quiet presentations (dance movement class with whispered instructions). Any more than you need is "empty gain" which does no good.

A '5532 on 12V supply can easily give gains like these and lower hiss than the capsule's self-hiss. At 9V the '5532 is a little gaspy, maybe not so you'd notice, but there are newer types made for low voltage.
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Old 27th October 2019, 02:36 AM   #6
PRR is offline PRR  United States
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It's not 1999 no more. THE obvious way to put sound to a digital system is now a Digital Microphone which outputs a bit-stream. Early ones sucked but modern ones beat the old P-Sonic electrets.

Analog sold their line to InvenSense (a branch of TDK):
Digital Microphones | TDK
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Old 27th October 2019, 02:49 AM   #7
sgrossklass is online now sgrossklass  Germany
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Quote:
Originally Posted by PRR View Post
A 16-bit ADC has a 96dB range. The max input may be a couple of Volts. 96dB below that is 10 microVolts.
Mind you, practical 16-bit ADCs might only reach 90 dB, or 87 (a pretty realistic range for what you find inside typical 16/48 USB microphones), sometimes little over 80 when they're in a basic little USB single chip device.

But it does show why typical electret mic preamps tend to be around 40 dB.

Basically you just did what I suggested over there.
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Old 27th October 2019, 09:58 AM   #8
inkl is offline inkl  Russian Federation
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Quote:
Originally Posted by PRR View Post
It's not 1999 no more. THE obvious way to put sound to a digital system is now a Digital Microphone which outputs a bit-stream. Early ones sucked but modern ones beat the old P-Sonic electrets.

Analog sold their line to InvenSense (a branch of TDK):
Digital Microphones | TDK

never heard of these!


Guys, thank you for the information overload! Gone reading for now and ordered some parts. I will still try to build a preamp and also I'll try to figure out how to use digital microphones. Very interesting!
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