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Circuit design problems on building a noise cancelling headphone.

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crazydave2002

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I am currently building an active noise cancelling headphone. However, there are some circuit design problems that I am not sure on my design.
Basically, in my design, there is a mic capsule connecting a low pass filter pre-amp.Then the pre-amp is connecting to inverter, which can generate the inverted frequency. However, I don't know which circuit should I build to connect the mic capsule to the pre-amp.

Actually I am some more questions. If you are pleased to answer the questions would you mind leave your email/ message me via this forum. Thank you.
 

Audioguru

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I am currently building an active noise cancelling headphone. However, there are some circuit design problems that I am not sure on my design.
Basically, in my design, there is a mic capsule connecting a low pass filter pre-amp.Then the pre-amp is connecting to phase reverser, which can generate the reversed phase. However, I don't know which circuit should I build to connect the mic capsule to the pre-amp.
What does the preamp do? It amplifies the weak signal from the mic (and it has a lowpass filter) so simply connect the output of the mic to the input of the preamp.

Actually I have some more questions. If you are pleased to answer the questions would you mind leave your email/ message me via this forum. Thank you.
You started this thread so why not continue on this thread so that people reading this can learn more about a noise-cancelling headphones circuit.
 

Audioguru

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Your "phase reverser"?, connect the mic the other way around? have you a dummy head and ear cavity to test it with?
Frank
You cannot connect an electret mic the other way around because it is polarized and its metal case is its ground. Connect the headphone speakers the other way around.
 

crazydave2002

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Hello,

As I would like to capture the ambient noise (which generally in 50Hz - 1000Hz). So, I would like to build a lowpass filter to capture that noise from the 6mm mic capsule. However, I tried to connect the output of the mic to the input of the preamp and it doesn't work. I've checked the circuit using signal generator. There's nothing wrong with the preamp, inverter and the mixer circuit. I think the problem now is the input connection(mic cap) and the output connection(ear cavity).

Screen Shot 2015-03-26 at 02.05.21.png

Basically, the lowpass filter preamp received the ambient noise from the mic (signal generator) and invert the phase. After sound balance by the mixer, the inverted signal and music will be generated to the headphone. The inverted signal will cancel out the ambient noise.
 

Audioguru

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You do not say which type of microphone. Dynamic (coil and magnet like a little speaker) or electret (it has a Jfet inside that must be powered properly)?
The output of a microphone is 10mV if you speak 10cm away from it. Background noise might produce an output of less than 1mV (0.001V).

Your "preamp" has a gain of only 4 times when it probably should be hundreds of times. Your opamps have no part number so they might be too noisy to be a preamp.
Why does the third opamp have a 20k resistor?
Why do the second and third opamps have a capacitor to ground on their outputs which might cause them to oscillate?
 

crazydave2002

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Sorry that I missed the details.

The feature of the microphone: http://onecall.farnell.com/projects-unlimited/pom-3044p-r/microphone-omni-pcb-6mm/dp/1193018
As your said, what would be the problem of my preamp?
I chose NE5532AP to be the opamps.
The 20k resistor in the third opamp is because I found that there is no changes on the simulator if I use 50k/100k instead. Should I choose another resistors?
I found these circuits on the internet and group them together. There is no signal to be generated if I don't have the capacitors.

Thanks.
 

betwixt

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Those are electret mics with a built in FET amplifier, they need a supply to them before they produce any output. Add a resistor (~4.7K - ~10K) from the top end to 9V and capcitively couple their output signal. You also need a resistor across the 10nF capacitor to stabilize the DC point of the first amplifier.

Brian.
 

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