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Can this MIC circuit work?

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tony_lth

tony_lth

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Hi, ALL,

I designed a MIC circuit, the sound firstly goes through IC400, then IC401, and last feeds into the modem IC.
I am not sure if my design gain is enough or not?
And any other comments?

Best,
Tony Liu

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In the IC400 vendor datasheet, it is used as un-banlanced mode.
But my modem and IC401 are diff mode, so I changed IC400 in diff mode.
I doubt my change is ok or not?

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It seems in my design, the MIC can't be biased properly. So how can I do? How to link the MIC to diff input of audio-amp?
Or pls comment a diff MIC. Thanks.

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Hi,
I modified my design, just adding a GND in the circuit, so that the FET and MIC can be properly biased.
Pls comment.
Tony Liu
 

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It needs a ground return on the mic to power it's internal FET (assuming it IS an electret type). However a direct ground connection means half your design carries no signal.

You need to use an equal value resistor in the feed from +3.3V and the feed from ground but be careful that the wiring to the mic itself is screened and be aware that most electret mic bodies are connected to the negative (normally ground) side so it needs electrical isolation.

Brian.
 
Connecting an input of the PA to GND is same thing so you will make it connected as single ended.
But if you'd find a differential model of the MIC because differential audio connections are much better against noise.( consider S/N )
Instead of using an active microphone, it's possible to use a passive microphone with an pre-amplifier for differential connections.
 
Hi, betwixt and BigBoss,
Thanks for your reply.
I bought a Philip MIC, SHM1000, which I only know is capacitor MIC, and needs external power supply.
I used multi-meter to test it, and found MIC+ and MIC- is short, i.e. 0 ohm.
If using diode mode test, it is 0.4V and 1.2V vice versa.
SO I wonder:
1. If I can use post #1 circuit for SHM1000?
2. If I use 12V as power supply, what resistor should I series in?
Best,
Tony Liu

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And Philip designed it for computer external MIC.

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Its connector is 3.5mm, three sector. So it should be MIC+, MIC-, and GND.
 

The Philips mic is not a capacitor mic, it is an electret mic. A capacitor mic needs a 48V bias on it but an electret mic has the 48V bias permanently stored on its electret material. The mic jack on a PC feeds a few volts to the Jfet in an electret mic.
Circuit #1 does not power the Jfet in the electret mic so use circuit #2 but with a 750 ohms resistor from the mic positive wire to +3.3V and use another 750 ohms resistor from its negative wire to ground.
 
I'm not sure why it has to be differential but most microphone systems, for example on stage or in a recording studio use a standard single ended microphone and use a phase splitter to derive the two signals. They can then be carried along long cables differentially to minimize hum and interference pick up. At the destination, one signal is inverted and they are then added, this gives twice the wanted signal while common mode interference is canceled.

Brian.
 
What Brian is mentioning, is the low impedance (600 ohm), balanced microphone line which have been used in pro-audio for perhaps 70 years.
 
Most 600 ohms pro-audio dynamic (coil and magnet) mics have the low and high frequencies cutoff and have a peak in the upper mid frequencies (presence).
Most condenser and electret mics have a perfectly flat frequency response:
 

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