Strange problem - Phase Cancellation?

[stevem]

I have been switching back and forth between a 2-way and a 3-way speaker system configuration. All the drivers each have their own power amp channel, and are actively crossed over.

I noticed that when using the 3-way configuration, my 1/3 octave spectrum analyzer indicates a drop in response of about 9 db right at the upper crossover frequency (1.23khz). If I invert the phase of the mid driver, the notch disappears, but another notch appears around the lower crossover frequency (297Hz). I do not see this phenomena when using the system as a 2-way with the crossover at 1khz.

The tweeter and the woofers are positive to the black terminals, while the midrange is positive to the red terminals. I have compensated for this in the wiring to the divers from the power amps.

Does anyone have an idea what may be causing this and how to correct it?

[Zilch]

It's your tweeter that's out of phase, not the mid.

When you invert the mid, you put it in phase with the tweeter, but out of phase with the woofer.

Invert the tweeter, instead.

Does that work?

The phase cancellations are not only electrical, but also acoustical, from the physical alignment of the drivers....

[stevem]

I thought of that, but if the tweeter is out of phase, then wouldn't it also be out of phase with the woofer when used as a 2-way? I don't notice any cancellations that way at all.

I use a digital crossover that has delay functions, so the drivers should be acoustically aligned. That shouldn't be the problem either.

[scott fitlin]

What slopes are you using?

[stevem]

What slopes are you using?

48 db/octave Linkwitz-Reilly.

[boputnam]

Get or borrow a Galaxy "Cricket" and individually test each transducer for what it's "seeing", phase wise.

http://www.galaxyaudio.com/galaxy/Products4.html

Sweetwater's got 'em - http://www.fullcompass.com/Products/...726/index.html.
Call 800 356-5844 x1114 "George". He is my rep, and does me right!

I think I like Zilch's guesstimate, but personally I stopped such "guesstimates" a long time ago, and now take measurements, instead. Measurements are unambiguous and focus your trouble-shooting.

The Cricket takes no prisoners...

[boputnam]

I thought of that, but if the tweeter is out of phase (with the LF), then wouldn't it also be out of phase with the woofer when used as a 2-way?

Not necessarily. But, we need more info on how you run this as a "two-way" (only LF and HF, only LF and MF - what!!??). Until you provide that, I'll move out on this limb, here...

Just "guessing" , but if the MF is out-of-phase with the HF (thus presenting the cancellation), maybe the LF is in-phase with the HF, and therefore when in two-way mode (LF and HF only) creates a summing in the area of frequency overlap between the LF and HF.

Your observation:

I don't notice any cancellations that way at all.

... suggests the LF and HF are in-phase.

It is common in a 3-way (BUT NOT THE RULE) that the MF is alone out-of-phase.

Do you know?

[stevem]

Not necessarily. But, we need more info on how you run this as a "two-way" (only LF and HF, only LF and MF - what!!??). Until you provide that, I'll move out on this limb, here...

The 2-way is the LF and HF. The 3-way adds the mid-range. Thanks for the heads-up on the Cricket. It's inexpensive enough, and at least I'll know for sure how everything is phased. Something is not quite right with my system, and it does sound like a phase anomaly. When I invert the phase on the mids, it sounds a little different but not much. Can driver proximity effect phase? My drivers are almost touching each other (from the bottom, WWMT).

[baldrick]

Am I way of when I say that the phase changes 180' every 6db on a filter? It means, i.e. if you have a 2 way 12 db/ocate filter the phase is correct, if 18 db/ocate than you should change the phase on either tweeter or woofer, if 24 db/ocate than OK and so on... could this be the reason why you have to revert the phase on your 3 way system?

I've dealt with very much caraudio earlier (mainly JBL of course) and thats when I found out that when i changed the slope from i.e. 18 to 24db/ocate I also had to change the phase. The same experinse I've had with surround amplifiers with sub-preout and the small/large speakers button. When the amp have a 6/18 db filter you need to change the phase on either the sub or your woofer when running speakers in "large", otherwise the sub and fronts will phase out

[stevem]

I thought that the 48 db/octave Linkwitz-Reilly slopes were supposed to sum flat and in-phase. Is this correct?

[boputnam]

:dont-know

You've wired the MF differently than the rest, so I'm getting confused on what the -48dB slopes are doing across the spectrum.

If it were me - and while awaiting the Cricket - I'd experiment by cross-wiring the HF and measure the response.
Maybe yes, maybe no...

[stevem]

:dont-know

You've wired the MF differently than the rest, so I'm getting confused on what the -48dB slopes are doing across the spectrum.

If it were me - and while awaiting the Cricket - I'd experiment by cross-wiring the HF and measure the response.
Maybe yes, maybe no...

I'll try reversing the phase on the tweeters. Should I check the diaphragms to see if they're wired correctly? I ordered the Cricket this afternoon. Full Compass had the best price too.

[boputnam]

1. I'll try reversing the phase on the tweeters.
2. Should I check the diaphragms to see if they're wired correctly?
3. I ordered the Cricket this afternoon. Full Compass had the best price too.

1. Let us know...
2. Do you think they have been refurbished? Or are you just reasonably thorough to want to anyway (me too... ). It couldn't hurt, so long as you are comfortable tearing into them. The markings on the diaphragms are not always intuitive...

btw - what are all the transducers in this system? You've not posted that, far as I can see...

3. Good to know, and thanks for the feedback. I really like those guys, and George in particular has provided me, for one, with outsanding support.

[stevem]

I checked the system again. When I reverse the polarity on the midrange, the dip at the tweeter/midrange crossover frequency disappears, but a dip at the woofer/midrange crossover frequency appears. When I just reverse the polarity on the tweeter, the dip at the tweeter/mid crossover disappears. It's looking like you an Zilch are correct.

I bought the compression drivers used, so I guess anything is possible. I did peek inside once just to confirm that the diaphragms were the correct ones. They appeared so.

The drivers in the system are as follows:

Woofers - 1401Nd (one pair per side)
Midrange - 2012H
Tweeter - 2450SL with PT-F1010HF waveguide

[boputnam]

Often we find here that ears are the most important guide. If the reversed polarity on the HF is more pleasing - and removes the cut (cancellation...?) in the frequency response - go with it. Double-check when the Cricket arrives and let us know. Hell - we should form a rental pool for the Cricket!

When you get done with the triage, post some pics of these - they "sound" really intriguing...