Transistor noise cancelling audio preamp

DickCappels

Joined Aug 21, 2008
6,005
One microphone is acoustically set up to preferentially receive sound from one direction, a second microphone is acoustically set up to preferentially receive sound from every other direction. Each microphone has a preamp but one inverts the signal and the other one does not. The two signals are mixed in the collector load of the two out put transistors such that any noise that is in the same phase in both microphones tend to cancel.

I remember about 10 years ago Nation Semiconductor brought out an analog integrated circuit for cell phones that performed this function. They used analog because they could do the inversion and summing of the signals with much less power than it would have taken a digital circuit to do at that time.

This method is also done acoustically and has been for a long time. The front of the microphone diaphragm points to the preferred direction and the back of the diaphragm is exposed to sound from every other direction and since the air "pushing" on the back of the diaphragm produced the opposite polarity of output from the microphone from that of air "pushing" on the front, an signal that is common is cancelled.Its probably more complicated than that but that was how it was explained to me.
 

Audioguru

Joined Dec 20, 2007
11,251
Hi Willen, we have not talked together for too many years.
A single opamp can replace all 4 transistors because it has a non-inverting input and an inverting input.
Make it a differential amplifier.
If both mics pickup your voice then it will be cancelled like the noise is cancelled. Have one mic close to your mouth but not far from the other mic.

The 1k resistors powering the electret mics should be no less than 4.5V/0.5mA= 10k and fed from a filtered voltage.
 

Thread Starter

Willen

Joined Nov 13, 2015
288
Hi Dick,
Then do I need a directional mic for my voice receiving and another normal mic for receiving noise (without my voice) and signal received by 2nd mic will be cancelled?
Hi Audioguru, I hope you are fine. I used to talk with you another forum, my account was partially locked due to lack of upgraded email. I remember I used to irritate you in beginning days. :) Circuit posted here maybe useful in room recording like TV news show?
 

DickCappels

Joined Aug 21, 2008
6,005
I don't know whether a direction mike for the noise would be of much help, but it seems that if you don't use matched microphones (similar sensitivity and frequency response) you would have to work even harder than otherwise to get a useful amount of cancellation.

The circuit looks pretty simple so do not expect too much out of it when you first switch it on.
 

Audioguru

Joined Dec 20, 2007
11,251
Hiw do we know the amplitude of the noise is the same as the one picked up by the second mic?
Because the two mics must be the same kind and be close to each other. Then each mic picks up the same noise and as much noise as the other mic.
1) Cheap electret mics pickup sounds all around (Omni-directional). Then both mics pickup your voice and cancel it, but noise cancellation is good.
2) Expensive electret mics are directional (cardioid) with vents holes at the rear of the diaphragm. Then only the mic you are talking to picks up your voice but the two mics pickup different noise from their two directions so the noise cancellation might be poor.

I think one good cardioid mic will work better than this Mickey Mouse circuit.
 
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