Hello

I want to try to play around with noise cancellation primarily for my computer case fan noise and maybe even dorm corridor noise .

What I am currently looking at is a way to phase shift an audio signal from 0 to 180 degrees .
I Found something like this (first circuit on the page) but something tells me that this is designed for a single variable frequency and not a complex audio signal due to the capacitative reactance changing with frequency. Are my assumptions correct or do you guys think that something like that would do the job of phase shifting an audio signal ?

 

No one?

 

To shift the phase by 180° use an inverting amplifier.

 

To shift the phase by 180° use an inverting amplifier.

Thanks for the reply , but not what i'm looking for. I want to shift it in a continuous fashion between 0 and 180 degrees. Not descreetly 0 and 180.

 

Bucked brigade analog shift register. The problem with that approach is that it will only give accurate phase shift for any given frequency. Each frequency will require a different amount of delay for any given phase shift. For example, to shift 1kc 180 degrees will require 1/2 millisecond delay. For 500 hz, it would require 1 millisecond delay. Have you considered that? Look into how a noise cancelling headset works for more ideas, but I'm pretty sure they use only inversion and attenuation.

 

Thanks for the reply , but not what i'm looking for. I want to shift it in a continuous fashion between 0 and 180 degrees. Not descreetly 0 and 180.

You could look at designs for guitar phasers, they use several cascaded stages of variable C/R networks with JFETs used as the variable R - there are various websites with effect pedal schematics, but don't expect much on the copy quality.

Maybe its worth a look at the Gilbert cell, they're often used for frequency shifting (mixing) but you might be able to fine tune it for merely a phase shift. I haven't really done much with Gilbert cells, you can probably get the basics from Ham radio sites.

 

Bucked brigade analog shift register.

A digital approach would be to digitize the signal with an A/D, store the digital words, and them play them back using a D/A after an adjustable delay period. Some micros have built-in A/D's and D/A's so the device could consist of one-micro with memory.

 

Bucked brigade analog shift register. The problem with that approach is that it will only give accurate phase shift for any given frequency. Each frequency will require a different amount of delay for any given phase shift. For example, to shift 1kc 180 degrees will require 1/2 millisecond delay. For 500 hz, it would require 1 millisecond delay. Have you considered that? Look into how a noise cancelling headset works for more ideas, but I'm pretty sure they use only inversion and attenuation.

Thanks for the replies .
Yes , the reason I made this thread is the issue that you mention.
I linked a page which contains a circuit which I think has that problem (not a uniform phase shift through the whole spectrum).
Yes I know that noise cancelling headphones are a thing but my problem is that my distances from the noise source to the antiphase source can be a few meters , that's why I need the phase shifter.

You could look at designs for guitar phasers,

Sounds interesting , I'll take a look at the Bucked brigade analog shift register and the Gilbert cell.

A digital approach would be to digitize the signal with an A/D, store the digital words, and them play them back using a D/A after an adjustable delay period. Some micros have built-in A/D's and D/A's so the device could consist of one-micro with memory.

Well I initially wanted to keep this project Analog but... now that you mention it It wouldn't take me too long to implement a phase shifter + r/2r dac with an AVR so I guess I'll try that first and see from there . Thanks for the advice.

 

It would seem that a delay circuit, such as the bucket-brigade or digital delay would be want you want. You shift the signal by a delay equal to the difference in distance between the noise source and the anti-noise source. Then, if you invert the delayed signal, it should subtract all the noise frequencies with the correct anti-phase since the delayed-and-inverted signal should equal the inverted image of the noise signal at that point.

 

a real simple phase shifter, if you can find one is a synchro control transformer. a single phase rotor, with a two phase stator. the angle of the rotor shaft determines the phase of the stator signal.

 

a real simple phase shifter, if you can find one is a synchro control transformer. a single phase rotor, with a two phase stator. the angle of the rotor shaft determines the phase of the stator signal.

Unless you know someone at an aircraft breakers yard, they're not that easy to come by.

Also might make a pair of noise cancelling headphones a bit top heavy!

 

Unless you know someone at an aircraft breakers yard, they're not that easy to come by.

Also might make a pair of noise cancelling headphones a bit top heavy!

I'm not making headphones , but thanks for the advice