Phase shifting (BD-Design)

by GC, Thursday, February 17, 2005, 13:21 (4723 days ago)

Hi Bert

There seems to be 2 ways that one can experiment with matching the phase of the main (full range) driver and the sub (BD15).

Either 0/180deg (2 position) or 0-180deg (infinitely variable). Does infinitely variable have significant benefits over just a 2 position phase switch?

Cheers Chris

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Phase shifting

by GC, Thursday, February 17, 2005, 13:25 (4723 days ago) @ GC

Hi Chris,

Either 0/180deg (2 position) or 0-180deg (infinitely variable). Does
infinitely variable have significant benefits over just a 2 position phase
switch?

In some cases yes, especially with subwoofers because the room changes time/phase due to standing waves and such..

Ciao,

Bert

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Phase shifting

by GC, Friday, February 18, 2005, 12:58 (4722 days ago) @ GC


In some cases yes, especially with subwoofers because the room changes
time/phase due to standing waves and such..

To get fully adjustable phase control I think you have to use an active filter. I take it when using the passive filter, fine control of phase is achieved by adjustment of the main and sub speaker positions.

When reversing the phase 0/180deg it is easy to hear the correct position, but what do you listen out for when fine tuning.

Cheers Chris

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Phase shifting

by GC, Friday, February 18, 2005, 14:08 (4722 days ago) @ GC

Hi Chris,

When reversing the phase 0/180deg it is easy to hear the correct position,
but what do you listen out for when fine tuning.

With real low frequencies you could use an active crossover with variable phase control, that won't hurt much. With an Oris system the crossover frequency is much higher and more sensitive to the ear. Going active will reduce the integration as in listening to one speaker instead of hearing them all seperately, even if the phase might be better.

But, always give it a try because the gross result might bring a better sounding system.

Ciao,

Bert

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Phase shifting

by GC, Sunday, February 20, 2005, 11:23 (4720 days ago) @ GC

Hello Chris,

In my case reversing the polarity of the bass units gave only a small improvement so I decided to build a phase shifter. I finished it yesterday and gave it a first listening. The shifter inverts so the bass is put back to straight connection. When tuning the phase the lower midrange becomes fuller, male voices and instruments like tenor sax get more body. At the same time bass intruments get a faster respons and more overtones. The change is not dramatic but it is an improvement.

A shifter is easy to build as you can see on the image. I checked it on an oscilloscope and it works exactly as it should. The OPA27 is a high quality opamp, I don't hear a sound degradation or noise due to it. I do have a small hum now but I did not yet optimize all wiring. My filter now has three potmeters so it is not easy to tune it by ear to the optimum response. (I can tune it to my taste but that is not what HiFi is about.) I put the filter and its power supply in a separate box so it can easily be adapted. In case the filter is second or third order the first potmeter should be a fixed resistor. The shifter can also be used for additional gain if necessary by changing the ratio R1/R2.

[image]

Yours,
Eddie

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Phase shifting

by GC, Sunday, February 20, 2005, 23:24 (4719 days ago) @ GC

Thanks Eddie
Thats very interesting. For the time being I am going to stick with Berts passive 'speaker level' low pass filter and see what tuning and speaker positioning can achieve.
If this is not satisfactory then the active filter and phase adjuster remain an option
Cheers Chris

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Phase shifting

by GC, Monday, February 28, 2005, 21:50 (4711 days ago) @ GC

Hello Chris,

This weekend I put all parts of the filter and the phase shifter in one box. After that I wanted to measure the frequency response so that I know at which cross over frequency the sound is best.

To my regret I found out that the opamp spoils the function of the low pass filter.

One week ago I tested the phase shifter only separate. So I do not know what I heard when I included it in the loudspeaker, probably at the "optimum" setting it did almost nothing. I definitely know that a more sophisticated approach is required.

Anyway, because I took the filter out of the amplifier, I took the opportunity to improve the internal wiring and I changed the last stage of the power supplies from Elna Cerafines to Black Gates. Both brands are highly regarded and out of production as far as I know. In my situation the Black Gate is by all means the winner. A large improvement in clarity most notably in the high frequencies. Besides that my hum problem is reduced to completely inaudible at the listening position. So after all it was a good weekend for me.

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