please help a newb with a noize cancling headphone circut

[dstich]

**broken link removed**
here is the text to go with

The circuit shows one channel of the system. The whole circuit requires just one quad op-amp, a TL084 type. It is very simple to understand. The signal from the microphone is amplified by the first op-amp and this signal is passed to the second op-amp via a preset labeled 'null'. The final signal is fed to the headphone via a passive mixer (2x100 ohm resistors) which allows the headphones to be connected to a 'walkman' stereo cassette at the same time, or the headphone socket in an aircraft entertainment system.
The 1uF capacitor in parallel with one of the 100 ohm resistors compensates for a small loss of high frequency response due to the inclusion of the 100 ohm resistor in the first place!

it it says that its only 1 chanel of the circut but i dont know how to convert it to both chanels i hope that you know what i mean if u can please help o one more Q what does the null mean

 

[evandude]

well stereo headphone jacks have 3 conductors... ground, left channel, right channel... just build two, and connect them both to the ground ring, and one each to the left and right audio conductors!

i am not familiar with that circuit but i assume "null" is just a way to adjust the amplitude of the output signal so it properly cancels out the noise.

 

[audioguru]

Null means to adjust for best cancellation.
Because this circuit is wideband and due to the short wavelength of high frequencies, this circuit must be duplicated and have two mics, one for each earphone and mounted directly on each one. Even then, the highest frequencies will have some of them actually added to the ambient noise. High frequency cut might be needed to avoid it. Use separate null pots.
The TL084 is noisy (ironically), you should use a TL074 which is low-noise and costs just a little more.
Using a dual TL072 for each channel would be more convenient. Design only one small pcb and duplicate it.

Why use a Mickey Mouse passive mixer? A real mixer can be made by feeding the input through a coupling cap and series 2.7K resistor connected to the - input of the 2nd opamp. :lol:

 

[dstich]

Thanks for the help now that you pointed that out it makes more sense in the text it says something about a quad op amp does it mater if I just use 4 individual op amps thanks for the help o and the null is that just a variable resistor and does it matter if on the stereo which signal goes to which ear

 

[audioguru]

You can use four TL071 single low noise opamps if you want.
The null is a 10K potentiometer.
You should wire the channels correctly, use a switch to swap them if you are not certain. When someone in a movie goes to the left side of the screen, you don't want the sound to go to the right earphone.

Maybe the circuit is designed to cancel very loud noise from a piston-engine war airplane and the cockpit is in the open. It doesn't have much mic gain.

 

[dstich]

k thanks for the help but are you saying that I should amplify the mic more than the original circuit if it is to cancel quieter noises like from the inside of a car or inside a commercial airplane

 

[dstich]

**broken link removed**
if I am reading the circuit right the input from the mic comes in through the first opamp and is amplified then it goes through the second op amp where it is inverted then it is mixed with the audio source if there is one then its sent to the headphone and if I want to increase the gain all I need to do is decrease the resistance on the 22k resistor before the cap on he - terminal of the opamp

 

[audioguru]

The gain of the left opamp would double if its 22K input resistor was halfed only if its source impedance is very low, which it is not. The source impedance is the 22K mic load/supply resistor in parallel with the high impedance of the current-sink FET inside the mic.
You could short the opamp's 22K input resistor to increase the gain of the opamp to only about 3. If you also decrease the value of the mic's 22K load/supply resistor to 10K, the gain would be about 7 or 8.

The passive mixer at the output makes me sick to look at it. Even the 100 ohm resistor feeding the earphone from the opamp worries me. I hope your earphone is high impedance. What is it? :lol:

 

[dstich]

it would be just a standard headphone earphone I found another circuit it looks a bit more complicated but here it is **broken link removed** maybe it just looks that way because its both halves of the circuit or maybe its just me here is the link to the page where i got the circuit **broken link removed**

 

[dstich]

i just reread your posts and i saw the part about the mixer with the opamp thats just what the other 2 op amps are for right??? thanks for all the help

 

[audioguru]

The Headwize circuit is much better:
1) The mics get a gain of 340, 34 from IC1 and 10 times more from IC3. Your original circuit has a mic gain of only about 6.
2) The mics have load/supply resistors of 2.2K (kinda low) while yours has 22K (kinda high).
3) IC1 has a very high input impedance so it doesn't load-down the mic and its load/supply resistor. Your circuit has them equal.
4) The program input is a true mixer. Yours is a Mickey Mouse passive mixer.
5) It uses a very low noise opamp that can drive a 600 ohm load to full output. Yours has a noisy opamp that can drive no less than 2K to full output.
A huge difference between the circuits.

 

[audioguru]

S1 in the Headwize circuit allows you to choose the phase from the mics to be out-of-phase for noise cancellation when IC2 is bypassed or for use to hear things far away and around you when IC2 is switched into the circuit.

In my other post I recommended adding a 2.7K resistor to the input of your 2nd opamp as a true mixer, that the Headwize circuit uses R17 and R18 for.

 

[dstich]

thanks for the help i think i have it figured out and il use the headwise circut instead of the other one and if i need anymore help il pm u or make another post apreaceate it

 

[bogdanfirst]

hello!
it's been quite a long time since i had been here.
well, i want to build the noise cancelling headphones too. i stopped at the Headwise project. and was wondering if there would be any difference in using TL074(or TL072) instead of the NE5532.
i know that the NE5532 has lower noise, but i cannot tell how low, and if it would make much of a difference. the problem would not be the cost, but i have to order it online, and probably the shipping would be more than 3 of those IC-s. but i have a couple of TL074 around and a TL072.
a friend of mine suggested using a TCA5500 for the headphone op-amp, wich also has a little equalizer. do you think that that would be better?
i was wondering if anyone around here has build a headphone noise cancelling circuit, and what the results were. i am not going to use expensive headphones, and want do do the project for my computer, to have teh headphones around. the idea is that sometimes there is too much noise and i have to turn the phones louder to hear it well, so this is not a good idea for my health.

i have build the final stage from that circuit using half of a TL074, and with the input shorted, i couldn't hear anyting in the headphones, but if i connected an audio cable(and the other end was loose) then much noise was picked up. i can also say that i tested it with a cd-player, and the noise of the playes itsself was increased significally.(when using the player directly, if there was no sound and no noise around i could hardly hear the noise, but when i added that amplifier then the noise was easilly hearable). the headphones i have used are 32 ohm, and i have to say that the distorsion seemed quite big at high volume. maybe this could be because the TL074 cannot supply enough current. for powering the amp, ia have used +/-9V for powering up the circuit.

 

[David Bridgen]

..... does it matter if on the stereo which signal goes to which ear

It does if you don't want the violins and cellos arse-about-face on the sound stage.

 

[audioguru]

Hi Bogdan,
The Headwize circuit uses NE5532 opamps that have enough output current to drive the 600 ohm headphones used. You are using lower current TL074 opamps to drive your 32 ohm headphones. You need a power amp IC like the LM386 to drive your low-impedance headphones.