Altering the phase of an audio output. (in degrees, not simply 0 or 180)

Thread Starter

dansteely

Joined Feb 26, 2017
37
Hi All,

I am working on a home cinema sub bass speaker project and am using a (pre-made) low pass filter board and a LM3886 amplifier.

What I'd like to add is a variable phase adjustment using a pot so the phase can set from anywhere from 0 to 180 degrees.

I'm not sure quite how to search what i'm after and therefore finding a suitable circuit has drawn a blank.

Can anyone point me at a simple circuit I can make on Vero board to achieve this?

Many thanks.
 

Papabravo

Joined Feb 24, 2006
19,617
Wow. That looks a bit beyond my skill level to say the least!!.
I guess that I'm not sure what else you might have had in mind. It is certainly the case that you can use a variable resistor (potentiometer) to create an analog input that can be used to control something else, like delay. How you do the delay is the problem. The only other thing I can think of that might work would be a tapped delay line, but I have very little idea what that would do to the fidelity of the audio source.
 

Papabravo

Joined Feb 24, 2006
19,617
Perhaps you are looking for an all-pass filter.

www.analog.com/media/en/training-seminars/tutorials/MT-202.pdf

Don't be put off by the math, just look at the graphs and schematics.

View attachment 122236
The problem I see with this approach is that instead of a uniform delay for all frequencies across the audio spectrum, each frequency is delayed by a different amount. The effect on say piano chords, would sound like hitting the keys one finger at a time.
 

DickCappels

Joined Aug 21, 2008
9,524
Nice way to illustrate dispersion, note the fingers depressions will be very close together. Whether this is a problem or an advantage will depend upon the details of the application.
 

crutschow

Joined Mar 14, 2008
31,156
I assume you want to have a variable phase shift so as to adjust the subwoofer speaker output waveform to the output of your main speakers at the crossover frequency so they are in phase.
That could likely be best handled by an all-pass filter, such a Dick posted (but a single-pole circuit should be sufficient since it can be adjusted for 0° to 180° phase-shift at any one frequency).
 

MrAl

Joined Jun 17, 2014
9,640
Hi All,

I am working on a home cinema sub bass speaker project and am using a (pre-made) low pass filter board and a LM3886 amplifier.

What I'd like to add is a variable phase adjustment using a pot so the phase can set from anywhere from 0 to 180 degrees.

I'm not sure quite how to search what i'm after and therefore finding a suitable circuit has drawn a blank.

Can anyone point me at a simple circuit I can make on Vero board to achieve this?

Many thanks.
Hi,

Do they still make phase shifter special effects pedals?

Way back i remember you could get one for your electrical guitar. it would shift the phase of the signal and that would make an interesting audio effect.
 

AnalogKid

Joined Aug 1, 2013
10,178
"Phase" shifting audio is ambiguous because of the 1000:1 frequency spectrum. My guess is that he wants an adjustable time delay that is constant for all frequencies. This can compensate for both physical placement issues and crossover effects. As the post #2 schematic indicates, this is not a beginners project. Commercial rack-mount devices popped up in the late 80's so TV stations could compensate for time differences between the video and audio signals that accumulate with digital video processing. Fast forward 10 years, high end VCRs had an audio delay to compensate for the video delay in the digital timebase corrector. Fast forward another 20 years, one of my TV's has an adjustable delay built in.

ak
 

crutschow

Joined Mar 14, 2008
31,156
Folks, you are going off the deep end here. :rolleyes:
He's connecting a subwoofer and wants to adjust its phase.
All he needs is an adjustable phase-shift for the sub at the crossover frequency between the sub and the main speaker to take into account the distance between the two speakers.
A delay-line type of circuit is complete overkill.
 

nsaspook

Joined Aug 27, 2009
10,715
http://sound.whsites.net/subcon.htm
One of the easiest ways to position a sub is to place it in the listening position - in the chair. Play material that has significant low frequency material, and then crawl around the room, placing your head in the most desirable potential locations. Listen carefully to the bass - it should be smooth and extended, with a minimum of large peaks or dips. The optimum position for the sub is now the location where you heard the best response.

It is very likely that the position of best response is completely undesirable for other reasons, so be prepared to spend a fair bit of time moving around, and listening carefully. There are always compromises, but with care you can still find a location that is acceptable aesthetically, is not inconvenient (e.g. the middle of the lounge room doorway), and does not cause howls of protest from SWMBO (She Who Must Be Obeyed).

It is a given that the other members of your family will naturally assume that you have finally lost it completely during this exercise, but it may be possible to get their assistance - or at least a second opinion. Involvement in the process could make it a lot easier to explain why the china cabinet really should be moved - preferably to another room if you have a powerful subwoofer
http://sound.whsites.net/pcmm.htm
http://sound.whsites.net/project103.htm

I use room treatments (The most important ingredient for good sound) , DTA (Digital Time Alignment) , external bass Parametric EQ and four carefully placed sub-woofers for uniform bass in a home theater space. Don't expect a simple phase-shift to do very much for sound quality.


Electronics
 
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Thread Starter

dansteely

Joined Feb 26, 2017
37
Hi All,
Many thanks for all responses to my question.
As has been said, I'm looking for a simple type circuit that can be knocked up on Vero. Digital circuits for this task are beyond my skill level..
Regarding the quality of the sound, the sub content will be below 120Hz so may not be an issue.
A friend has a Jamo 2.1 system. The sub has the phase control i'm looking for.
I've been searching for a schematic/circuit diagram that details the control but have not found one.

Again, sincere thanks for all your help.
 

nsaspook

Joined Aug 27, 2009
10,715
If your acoustics room are good you won't need phase adjustment with good woofer placement. If the room acoustics are bad then it's a just a band-aid on a stab wound.
 

Thread Starter

dansteely

Joined Feb 26, 2017
37
Hi nsaspook,

Room placement in my world is always a compromise/slam dunk...

I have stock of some TL062 dual channel op amps. The pinout is the same as the TL072. Could these be substituted?

Thanks
 
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