Active crossovers - phase distortion?

[rjon17469]

Here is a link to a set of active low-pass, high-pass, and band-pass crossovers. My question is, for the low and high pass crossovers, would there be any phase distortion at the crossover point, similar to passive crossovers?

Crossovers

Thanks in advance!

Reece

[GRollins]

Assuming an analog circuit, there will always be phase shift at the crossover point. I don't know that I would go so far as to call it 'distortion' though.

Grey

[rjon17469]

Originally posted by GRollins
Assuming an analog circuit, there will always be phase shift at the crossover point. I don't know that I would go so far as to call it 'distortion' though.

Grey

Thanks for the info/correction.

What keeps getting me is the low pass crossover. The only thing in the signal path is resistors, and then the caps are going to ground. In this arrangement, would the caps still exhibit a phase shift on the signal?

Thanks!!

Reece

[carlosfm]

Yes.
Everything is in the signal path.

Green is gain, red is phase, black is group delay.

[GRollins]

There's a tendency to think only of series connections as representing "the signal path" but, as Carlos notes, parallel ones count, too.
Think of the resistor/cap combination as a voltage divider. The resistor is frequency-neutral, but the cap passes high frequencies. In low pass configuration, the cap passes those frequencies to ground, where they are lost. What remains is sort of a photographic negative of the high frequency 'image' that went through the cap, i.e. the lower frequencies. Whatever the cap removed leaves an impression--in the negative sense--on what remains. Thus the cap does have an effect on the sound quality, by virtue of what it subtracts. This includes phase relationships, except that they too are reversed.

Grey

[rjon17469]

Thanks so much you guys for your help.

That makes sense. So, I guess, there's no real way to avoid the phase shift besides moving into the digital domain, and that has it's whole fair share of problems.

It seemed too good to be true! (my motto for audio)

Thanks all.

Reece

[G4ME]

for me to get a grasp of it, i have to think of the characteristics of caps.

at DC caps are open
at AC caps are short

in the frequency (S) plane, DC is equivalent to 0Hz, so at low frequencies caps are open. So if a cap was in series with a driver it would block all low frequencies.

At high frequencies caps are shorts, so if one was placed in series with a driver only high frequencies get by.


But thats just for caps and the s plane, you are looking at how this affects the phase of the driver.

One thing that i would recommend you do is look at a 4th order Sallen-Key filters as there is no phase shift over all frequencies

http://www.linkwitzlab.com/filters.htm#13

check it out, if you aren't to adept with EE stuff the S plane and all the poles and zeros might mess you up a bit.

[rjon17469]

Thanks for the link. Yeah, the EE stuff is getting me a little.

On that same link though, if I read correctly, it says 4th order LR crossovers don't have phase shift?

Reece

[SY]

Any time that Nature gives you a non-flat frequency response, there's phase shift. Any time there's a time delay (e.g., between the time of recording and the time of playback), there's a phase shift. Getting rid of phase shift in a crossover involves doing some very unnatural manipulations to the signal.

I think that the real problem is that you don't really know what "phase shift" means but have read somewhere that it's a Bad Thing, probably written in an article by some audio writer who knows less than your cat about these issues. If you really want to understand them, it would be worthwhile to pick up a basic physics text like French's "Vibrations and Waves" and try to get the basic concepts down before deciding what to worry about and what not to.

[rjon17469]

Well, I understand that phase shifts correlate to the period of compression or rarefaction of the wave at a particular point in space as it moves.

The reason I'm trying to avoid phase shifts is to help in the transistion between drivers, and especially to avoid having two speakers 90 degrees out of phase with eachother at the crossover point.

Plus, and I guess it's just an issue I have, but at times it seems like to me that severe phase shifts detract from the realism of a speaker, although for me to actually make this claim would require a lot more hard data than what I have.

Reece