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Old 12th February 2013, 09:29 AM   #1
hozone is offline hozone  Italy
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Default electret mic preamp for micro ADC

I would like to build a preamplifier for an electrect mic, actually i've some WM61-A.

I would like to get SPL values.
My SPL measure could event be low accurancy (+-6db is accettable) even not weighted, and it should be at least from 40/50db to 90/100.
Micro side i'm using simple RMS, or radix-4 fft, depends on the signal precision and on how much resources i've got on micro.

Because i would like to post this project, i prefer NOT to do Linkwitz.
Said this, my electronics skills are not as good as my it skills, so i ask you expert if you've a working circuit, i've try some as you can see, but nothing as good as i need

zx-sound: http://www.inexglobal.com/downloads/ZX-sound_e.pdf
transistor pre: http://cdn.head-fi.org/b/bf/bfb6b58e_simplepreamp.gif
opamp pre: http://www.zen22142.zen.co.uk/Circui.../tl071_mic.png

Last edited by hozone; 12th February 2013 at 09:37 AM.
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Old 13th March 2013, 04:52 PM   #2
hozone is offline hozone  Italy
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solved using a schematics base on this http://www.johncon.com/john/wm61a/gr...chematic-6.jpg

see attached if interested
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File Type: png wm61a_pre_rev01.png (9.2 KB, 70 views)
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Old 13th March 2013, 07:32 PM   #3
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Wire directly the output of the IC to the output of the circuit, all these resistors and capacitors are useless as DC at both points are equal.
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Old 14th March 2013, 09:14 AM   #4
hozone is offline hozone  Italy
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thank you.

i add the decoupling cap C6 and biasing res R7 / R8, cause i need signal biased at 2.5v, (given 5v supply).
C3, R6, R4 i do not know what are there for they are there because john use those.
note that i need this circuit for a basic SPL meter, so i use john schematics even because i know the RMS voltage at a given SPL (es 315uV ~ 32db), and so if i get the RMS voltage i can calculate output SPL: SPL db = 20*log10(newRMSv/referenceRMSv).
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Old 14th March 2013, 11:59 AM   #5
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The amp biased as you did, is actually 2.5VDC at its output, no need for the output network, save components, cost, weight and space in the PCB.
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Old 14th March 2013, 12:08 PM   #6
hozone is offline hozone  Italy
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so, straight out from the output 1 of IC. right?
i will try.
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Old 14th March 2013, 12:20 PM   #7
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Yes! Perhaps a 1K resistor in series to protect the opamp's output about short circuits, although internal protection is readily found inside the chip.
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Old 14th March 2013, 12:47 PM   #8
hozone is offline hozone  Italy
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works perfect from ~55..100db SPL, better than before, it seems it has more dinamic on low SPL values.
attached the new schematic, R4 optional.

if you have time to reply me i've some more questions, otherway thanks for the above suggestion.
1) C5, C2, R10 are filters, right?
2) do you see any other improvement?
3) john write as the reference value 315uV ~32db SPL, is it this value take out from experimenting with AC coupled scope and emitting a sound @32db, or there is a way to calculate this value based on the IC, supply voltage, mic, and components used?
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Old 14th March 2013, 01:05 PM   #9
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1) All network is a filter any way, some are simpler and some too complex.

2) I have no experience in microphones, but the gain in the amplifier is the value of the resistor from output to -input(R3), divided by the value of -input to ground (R9) (Ignore the capacitor in series), plus 1. This case gain is 11.

3) Improvements, no, there are lots, but as Murphy says: any equipment working is untouchable.
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Last edited by Osvaldo de Banfield; 14th March 2013 at 01:10 PM.
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Old 14th March 2013, 01:16 PM   #10
hozone is offline hozone  Italy
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thank you!
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