Author Topic: Reducing noise in microphone pre-amp.  (Read 6319 times)

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Offline DanKalcTopic starter

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Reducing noise in microphone pre-amp.
« on: December 25, 2013, 03:04:15 pm »
Hi,
I'm trying to build a simple microphone circuit, but my results aren't so great. I have very high noise level on the output, is it possible to reduce it?
I'm using OP07 with has a low noise level about 10nV/rHz. Supply voltage for this amp is +-3V. I made it on a breadboard and tried to lock it in a faraday cage from tin foild, but noise is still very high.

datasheets:
OP07 (http://www.analog.com/static/imported-files/data_sheets/OP07.pdf)
 

Online Zero999

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Re: Reducing noise in microphone pre-amp.
« Reply #1 on: December 25, 2013, 03:29:17 pm »
What power supply are you using?

Is the microphone cable screened?
 

Offline Sander

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Re: Reducing noise in microphone pre-amp.
« Reply #2 on: December 25, 2013, 04:22:45 pm »
The higher the values of the resistors get, the higher their noise will be. I will suggest you lower the value on the input filter (and as a result, increase the value of the cap). Also your gain is a bit high as far as I can see. Lower the feedback resistor, 1Meg is really high! You might want to increase the voltage on the microphone “pickup element” (only if it is a capacitor microphone configuration!) to about 50 volts to get more signal on the input of the amplifier.

Oh, and ditch the breadboard. Solder it point to point on a piece of blank PCB.
 

Offline w2aew

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Re: Reducing noise in microphone pre-amp.
« Reply #3 on: December 25, 2013, 04:42:57 pm »
...and, don't forget the power supply bypass capacitors!
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Offline fcb

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Re: Reducing noise in microphone pre-amp.
« Reply #4 on: December 25, 2013, 09:00:54 pm »
You need to put a BIG filter on the 4k7 biasing resistor.  Every uV of noise on the Vcc is going to be 'audio'.

Measure the noise with (1) the input shorted to gnd (2) with a 4k7 to gnd (3) with no microphone [noise from the 4k7 bias resistor].

OP07 is good, but not worth the extra c.f. NE5534/32
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Offline KJDS

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Re: Reducing noise in microphone pre-amp.
« Reply #5 on: December 25, 2013, 09:07:32 pm »
First, work out how much gain you need then reduce the 1M0 down to something sensible. If you drop the gain by 10 then the noise will also drop by that amount.

Then add some supply decoupling. That 4k7 may be better off going to a well decoupled linear regulator powered of Vcc. That would remove a lot of power supply noise. Is the mic cable properly screened?

Offline mhzghz

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Re: Reducing noise in microphone pre-amp.
« Reply #6 on: December 26, 2013, 05:37:02 am »
Hi,
I'm trying to build a simple microphone circuit, but my results aren't so great. I have very high noise level on the output, is it possible to reduce it?
I'm using OP07 with has a low noise level about 10nV/rHz. Supply voltage for this amp is +-3V. I made it on a breadboard and tried to lock it in a faraday cage from tin foild, but noise is still very high.

datasheets:
OP07 (http://www.analog.com/static/imported-files/data_sheets/OP07.pdf)

R4 should be 10x your source impedance.

Adjust C1 according, but you really want C1 as large as possible to reduce your source impedance noise.

Reduce R3 to something more reasonable, say 10k, with a 1000 pF cap in parallel.

Replace R2 with a series 2.2 uF / 100 ohm resistor for your filtering.

Add series 10 ohm resistors in the +/- Vcc power lines and large caps as close to +Vcc and -Vcc as possible.





 


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