Introduction: Noise Cancelling W/ 741 Inverting OP-AMP
This project is called the NC741 because Noise Cancelling with 741. Also NC741 sounds cool.
The purpose of this build is to show that Noise Cancellation is possible with basic parts like OP-Amp. And that application of fundamental physics of wave. Cool!
Step 1: Plan...
At the center of the noise cancelling is inverting wave.
Take a look at my "kids can understand too!" diagram. The diagram shows red waves and blue waves.
Red wave represents ambient noise. It does not change and it will be there always.
Blue wave represents noise captured and processed. The microphone will capture the ambient noise. Then it goes through a pre-amp to 741 inverting OP-Amp. At this phase, the wave is inverted . (bad pun intended.) Finally, inverted sound will be played at the earbud, effectively cancelling out the ambient noise.
The hard part is building 741 Inverting OP-Amp. This shouldn't too challenging, right?
Step 2: Build an Inverting OP-AMP
Build an inverting OP-Amp. For this particular build, an 741 was used. It is untested but any inverting OP-Amp should work.
For R(in), 4.7KOhm resistor was used and for R(f) a variable resistor was placed.
741 Takes -12V and +12V. Therefore, use a voltage divider to divide 24V and set ground to get -12V and +12V.
Step 3: Mic and Pre-Amp
Build a pre-amp for an electret mic.
https://www.instructables.com/id/Pre-amp-to-electr...
This instructables has a nice and simple circuit. Thanks mate!
Since it is all on the same breadboard, a voltage regulator is placed to supply +5V.
Replaced R2 from the circuit to a variable resistor(100K Ohm).
Step 4: Optimization
So, there are two variable resistor which will control amplification.
Obviously, if you amplify way too much, it will amplify noise, which means more noise. But if not enough will unable to cancel ambient noise.
The best way to do this is to use Bob the dummy head but I found using my own ears also works.
Simply good luck finding the sweet spot!
Do note, There are few factors that plays a role: earbud's acoustic properties, Acoustic properties of head, and acoustic properties of the mic.
The earbud I was working with was HiFiMan's RE-400. The reason why I chose this earbud is because the acoustic property was accurately measured by innerfidelity.com (http://www.innerfidelity.com/images/HiFiMANIE400.p... link to the pdf.
As shown, RE-400 does not change original sound source's phase up to 1kHz. Since we are aiming to cancel noise around 200Hz to 300Hz, this shouldn't be a problem.
When dealing with 200Hz and 300Hz, your head vibrates and creates interesting sound acoustics. This is a reason why fine tuning with your head is the best. Because ear canal is slightly angled, I found aiming microphone slightly angled towards back of my head worked better.
I mentioned acoustic properties of the mic but most mics won't change phase significantly. Although I found x10 amplification through pre-amp created significant distortion. Distortions are not good.
Step 5: Noise Is Your Enemy.
My circuit made various kinds of white noise from many different things and so noise killing journey is on.
Initially I had a giant breadboard with ground wire around it and it made strange white noise. Turns out, 741 was picking on 91.1FM. Assuming the ground wires acted as antenna, I built exactly same circuit on a smaller breadboard with shorter running wire. From recent testing, it was picking on random morse-code like sounds, which is likely to be transmitted by car remote. This meant the 741 was picking on radio signals around 800mHz.
I can't explain this strange phenomenon and I don't think anybody can but it happens randomly and rarely so I let it pass.
On the other hand, I found increase in white noise as human body touches running wire to the microphone. The wire to the microphone has to be long and because it has to be next to ear, the only way to solve this is to shield the wire. This solution kind of worked.
It seems deleting white noise completely is near impossible. To be fair, most noise cancelling headphones out in the market also suffers from white noise.
Step 6: Conclusion Time!
As a result, I ended up with a noise cancelling headphone. Not a big surprise. Although there are a few set backs.
Aiming microphone to get it to the optimal noise cancelling position is annoying and because I have to hold the mic next to my ear physically, this is not practical.
To resolve this, building a 3D printed housing to fit both mic and earbud will work perfectly. Although, that will again change acoustic properties so it will need more optimization.
Proving that this works is kind of a different story. If somebody were to tell you how it feels like to use Bose QC35, you;ll confused. Also there's limit to the testing method of mine, which is to use a microphone mounted piece of block. Although the Oscilloscope reading tell a bit of story. The amplitude of the sin wave does decrease when the noise cancelling is turned on.
