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TOPIC: Re: Low quality design or faulty mic ?

Re: Low quality design or faulty mic ? 7 years 7 months ago #8761

  • georgio
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Hi Michele, all.
Sorry for the delay, I was going to do a proper post with pictures and all, but got hooked up. And it seems that I won't make it today either.

So a quick description of two solutions, an easy one and the advanced that with the mic closed I got -90db to the top of the noise peaks.
I will make a post maybe tomorrow with more details.

Simple mod:
It will give you about 17 db of improvement and it only takes one capacitor:


More advanced mod:
The one I ended up with needs the following steps :
- Remove the resistor that gives power to the microphone feed from the 3.3 Volts.
- Use a LM4040 3V reference in parallel to the filtering capacitors of the mic power to get 3V
- Power it from USB 5 Volts through a choke (600 Ohm at 1Khz, Murata) and a 180 ohms resistor
- Added 100uF Very low ESR ceramic capacitor in parallel to the caps and LM4040
- Added 47uF very LESR ceramic cap just before the 180ohms and after the murata choke.

I also moved an 680ohm resistor from it's position on the pcb to a better place to improve noise pickup and let the differential input of the A2D work better (I do not know if this helped much though).
This is now giving me -90db with the microphone assembled.

I hope to find the time to make a full post tomorrow with more details and tips on how to make an initial measurement to keep for reference, open the mic, mod details etc.

Good luck

George
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Re: Low quality design or faulty mic ? 7 years 7 months ago #8790

  • devteam
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Good Stuff George! Thanks again for your time sharing..

- Here are few info worth for those of you trying to do a modification on a RevB microphone and trying to remove these spurious peaks close to the noise floor by basically switching the power to the capsule back to 3.3V.
NOTE that these changes are to be made by end users knowing how to rework an SMD. If you need help, feel free to get in touch with us. We are planning to do a new batch of SMD board that could be replaced with a simple soldering job. We'll soon give an update to the whole community. It just takes time to work a manufacturing batch last minute.

Please see the diagram below as a reference for location of pads.


1) R5 from 680 ohm change to 1K ohm
2) R6 from 680 ohm change to 1k ohm


3) R17 from 330 ohm change to 0 ohm
4) R18 from 330 ohm change to 0 ohm

5) R11 from nc change to 0 ohm
6) R12 from 0 ohm change to nc

- The trick recommended by George of adding a larger decoupling cap on the Mic power + LDO is also a great idea. That's something done already on revC.

Hoping this info helps.

Devteam
miniDSP, building a DSP community one board at a time.
Last Edit: 7 years 7 months ago by devteam.
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Re: Low quality design or faulty mic ? 7 years 7 months ago #8791

  • willshui
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My mic has serial number 0565, is it affected? Thanks.
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Re: Low quality design or faulty mic ? 7 years 7 months ago #8802

  • miki_ada
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Hi everybody!
I started to mod my mic. This is the first result with a generic 220uf electrolytic installed where George suggested:

noise floor without cap:


noise floor with cap:


So a great first result. No spikes at all!!

I'll now try with an LT1763 3.3V regulator to see if the noise floor can even go down. Indeed George has a noise floor of about -90dBFS while I achived about 53-133= -80dbFS.

Unfortunately I don't have a good selection of smd parts so I can't test the mod suggested from minidsp team :(

off topic: how can you switch from db to dbfs scale in REW? so we can compare better our results!

thanks!!

Michele
Last Edit: 7 years 7 months ago by miki_ada.
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Re: Low quality design or faulty mic ? 7 years 7 months ago #8805

  • miki_ada
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willshui wrote:
My mic has serial number 0565, is it affected? Thanks.

I think... It's affected. Mine is #0514 and has got the spikes from 1kHz as you can see from the graph. Open rew and do a measure with no sound in your room (measure the silence :) )
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Re: Low quality design or faulty mic ? 7 years 7 months ago #8809

  • willshui
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hi, your pictures are too small to see. Can you tell me how to measure the "silence"? Thanks.
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