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Alembic Club » Alembic Basses & Guitars » Archive through October 10, 2004 » Archive: 2003 » Archive through December 31, 2003 » Alembic F-1X phase problem « Previous Next »

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jrbrown
Junior
Username: jrbrown

Post Number: 38
Registered: 6-2002
Posted on Saturday, December 13, 2003 - 9:30 am:   Edit Post

I'm using the full range and low-pass outputs of my Alembic F-1X to feed two amps and two cabs. Through testing, I discovered that the full range and low-pass outputs are out of phase with each other. This is confirmed by a reduction in bass response when both amps/cabs are on. Bass is restored be reversing one of the speaker leads. I've tested the system using just the full range output to feed both amps; everything works fine. I've also tested the amp, cables, and speakers with a multimeter; all checked fine. The problem is in the F-1X. Can anyone assist?

Edit: I also tested everything with "Y" adapters. The problem is in the preamp.

(Message edited by jrbrown on December 13, 2003)
davehouck
Advanced Member
Username: davehouck

Post Number: 239
Registered: 5-2002
Posted on Saturday, December 13, 2003 - 9:51 am:   Edit Post

James; other members have reported that the full range and low-pass outputs are out of phase with each other on their F-1X preamps. I can not recall reading, and was unable to find searching, where anyone did anything about it.

(I just reread that and it's not worded very well. I guess what I was trying to say was that of the two people who reported the out of phase situation, neither reported having changed anything. That doesn't help much, but perhaps it's a little better. <g> I'm giving up and going back to watch more of my new Grateful Dead DVD!!)

(Message edited by davehouck on December 13, 2003)
jrbrown
Junior
Username: jrbrown

Post Number: 39
Registered: 6-2002
Posted on Saturday, December 13, 2003 - 12:57 pm:   Edit Post

WOW! The F-1X manual makes no mention of a phase shift in the crossover. However, it does illustrate using the low-pass outout to feed a low frequency; not much good if the "bass" is cancelled out. Maybe we can get some answers.
jrbrown
Junior
Username: jrbrown

Post Number: 40
Registered: 6-2002
Posted on Thursday, December 18, 2003 - 5:12 pm:   Edit Post

Does anyone else have the same problem?
valvil
Moderator
Username: valvil

Post Number: 287
Registered: 7-2002
Posted on Thursday, December 18, 2003 - 6:25 pm:   Edit Post

James,

I wasn't able to get the whole story from Ron because (as it often happens) he was taken away by an urgent matter, but when Mica & I asked about this today he started off with : " Yes, they should be out of phase..." and we got interrupted. Mica was guessing that our manual might be incomplete or inaccurate in that section.

I'll let you know something as soon as I can get to Ron again.


Valentino
bob
Member
Username: bob

Post Number: 93
Registered: 11-2002
Posted on Thursday, December 18, 2003 - 11:20 pm:   Edit Post

"Yes, they should be out of phase..."

That's the answer I was expecting to hear. I don't have an F1-X, but I've been curiously watching for the outcome here.

As many of you know, given his brilliant work on ELF and so forth, Ron is really quite a fanatic about phase, so this couldn't possibly be something like a design oversight. And though I suppose it could happen, given their obsession with quality, it's not likely to be an intermittent manufacturing problem either.

Probably just an unfortunate documentation problem, and it's great that a few people have been listening carefully enough to notice, and care enough to raise the question and help get it straightened out.

What I'm really curious about is, "why?". My best guess is that this particular design - assuming that you *know* that the outputs are out of phase, and that you therefore appropriately adjust for it elsewhere in the chain - actually results in better phase and/or time alignment, than you would get otherwise. It might also keep the circuit simpler and cleaner, but trust me on this: he didn't do it just because it was easier, or cheaper, or a shortcut, or even because he needed some sleep. There's a good reason.

I had the privilege of spending some time with Ron in person, just yesterday (talking about the amazing work he had done on my custom electronics), and I'd just like to say that he is a wonderful human being - not "merely" a brilliant engineer. The really cool thing is that no matter how "techie" he gets, he always comes back to the real point: what is the sound that you need, to express yourself as a musician, and how can I help you achieve that?

So, what I would like to know is, how does the opposite phase output help (those of you fortunate to have one of these things), and how do you best take advantage of it?
mattheus
Member
Username: mattheus

Post Number: 80
Registered: 5-2003
Posted on Friday, December 19, 2003 - 4:16 am:   Edit Post

I don't have a F1-X, so don't ask me all kinds of technical difficult questions. I'm also not a 'techhead', you know. But what I do know, is that phase problems occur a lot with large PA systems. It can be a wiring problem, a processor problem, all sorts of problems. As soon as people start to build a large PA-system with all kinds of amps, processors etc, they will most likely stumble over this problem.
But now the F1-X! I think that Ron has done a very smart move.
Supposely you use the biamped outputs for your amps and speakers on stage. You could say that yoy use a 'big' backline. It often happens, that the volume onstage is too much, so a sound-engineer who has to make a nice mix in the venue has a problem. In this case, with the F1-X, you use the normal output for a signal to the FOH mix-desk. In that case the signal onstage is out-of-phase with the sound on the PA system, and they won't add up to eachother. You, the bassplayer, won't have any disturbeing noises from the PA system ('Where is that boomng sound coming from!!!') and your FOH sound engineer won't have that loud bass coming offstage.

I don't know if this is the reason, it's just a wild guess. But it looks pretty tech, don't you think.... (and that for just a bassplayer... ;) )


Mattheus
811952
Junior
Username: 811952

Post Number: 39
Registered: 10-2003
Posted on Friday, December 19, 2003 - 6:02 am:   Edit Post

I know of more than one FOH engineer who delay the mains to match the backline. It also sounds really good to delay the mains slightly more than the backline, because the apparent effect is that people hear the sound coming from the backline and not the mains (a psychological matter of attributing the source of a given sound to the place from where you first heard it, even if the mains are much louder). Still another option some players use is to delay their feed to the board based upon how far back their amp is from the mains. I forget the specifics on how much delay per foot, but I can get that info if anybody wants it. I really hate it when I'm forced to stand in the spot where the mains and my amp are out of phase. I usually just turn the amp way down in those cases and listen to the house..
john
jrbrown
Junior
Username: jrbrown

Post Number: 41
Registered: 6-2002
Posted on Friday, December 19, 2003 - 6:52 pm:   Edit Post

I'm baaack! My co-worker and I have over 50 years of combined experience in communications electronics. When we narrowed the phase shift to the crossover network, our immediate response was: "There is supposed to be a phase shift."

I just got off the telephone with Ron Wickersham and we discussed the characteristics of crossover networks [past and present] for over an hour. Conclusion: "There is supposed to be a phase shift."

Why? In laymen's terms: When ever a signal goes through a crossover network, the laws of physics dictate that there will be a phase shift. The amount and direction (lead or lag) of shift is dependent on the frequency measured and cross over design.

What to do? Experiment with your system to determine what sounds best for your application. As in my case, this means inverting the polarity to one of my speakers.

Note: The phase shift is only applicable if you are using the "full range" AND a "cross over" output at the same time.
My rig (at one point):


(Message edited by jrbrown on December 19, 2003)
davehouck
Advanced Member
Username: davehouck

Post Number: 249
Registered: 5-2002
Posted on Friday, December 19, 2003 - 7:12 pm:   Edit Post

Thanks James, and thanks to Ron too!
bob
Member
Username: bob

Post Number: 94
Registered: 11-2002
Posted on Friday, December 19, 2003 - 11:34 pm:   Edit Post

James, perhaps you could pass on a few more details for us here? I don't want to go too far beyond layman's terms, but if you spent over an hour talking about this, then I sort of think you're obligated (to the group, and to Ron for that matter) to pass on a few more details.

In stereo systems, you find speaker designers who are adamant about using only first-order crossovers (arguing that these tend to be better at phase and time), and others who prefer fourth-order (possibly for more even frequency response and/or dispersion characteristics, but I'm stretching here). Maybe some thirds, not many seconds, I forget.

The concerns here, for hi-fi speakers, seem mostly related to the fact that you typically have multiple drivers in each enclosure, covering different frequency ranges, and therefore get some unfortunate phase cancellations around the crossover frequencies. Much of this seems to be based on distance between drivers, both vertically and horizontally, plus you have to think about the dispersion and interactions of two speakers in the room, and the resulting reflections and cancellations - something Ron dealt with on a much grander scale with the "wall of sound". (There's a link to a great article on this, somewhere in here.)

Tied into this somehow, is the crossover slope - the rate at which output drops from one output, as it rises to the next. I'm pretty sure this is closely tied to the order of the crossover design, and typical slopes are 12 dB per octave, sometimes 6, and maybe 18 or 24.

if the crossover is steep enough (probably more than 12), then the range in which cancellation occurs might be small enough to mostly ignore. But if it's more like 6, then you could easily have an audible cancelling problem over more than an octave, which could be troubling.

There is another consideration, though I don't think this applies in most bass applications, because most cabinets either have a uniform set of drivers (e.g. 4x10) or they don't bother to include a crossover. But you probably still have to factor in the natural rolloff of the drivers, e.g. a good 2x10 is probably going to roll off below 40-50 Hz, and above 5-6 kHz. I think most drivers, in this range, tend to naturally roll off at about 12 dB per octave, but I'm also not sure about that. But somehow you have to think about whether the crossover slope is greater or less than the rolloff of the cabinets you are driving, because only one will really matter.

What we need here is some practical advice. Again, I don't have an F-1X or a manual, but my guesses would be something like this:

- If you are simply routing the outputs to two vertically stacked cabinets, you should reverse the leads to one of the speaker cabinets.

- If the outputs are routed to cabinets which are significantly spaced, either laterally or "fore and aft" (sorry, I'm a sailor, not a performing musician), then you might have to just listen, and figure out what sounds best.

- In this latter case, you can save a lot of time if you know what the crossover slope is, because it might (for example) tell you that you only have to worry about the range from 100-200 Hz, so you could focus your listening around a specific octave, or possibly less.

So James, based on your discussion, how would you expand on (or correct) these suggestions? Hopefully, the manual can be improved in the future.
thebass
Member
Username: thebass

Post Number: 80
Registered: 3-2003
Posted on Saturday, December 20, 2003 - 1:52 pm:   Edit Post

@jrbrown:

I just read this thread and its good to know that someone else has the phaseshift problem which I mentioned here about 4 months ago. I want to use the same setup as you described. After a few emails back and forth - and I hate to say that: but no email feedback from Alembics customer hotline - I gave up and are driving both stereo channels with the full range signal. I also tried to invert one channel at the output but it was also not the sound I expected. Without having measured it with a scope I guess the phase shift between full range and low pass is not exactly 180 (inverted). Following the theory it has to vary over frequency, especially near the cross over frequency. Of course the sound can only be stable if the phase shift between full range and low pass is either 180 or 0 and does not change much over frequency below the cross over frequency. In all other cases you will hear dips or peaks.

My next plan is to use a SF-2 instead and use the two outputs to drive the stereo channels. Does anyone else use this setup ?
dnburgess
Intermediate Member
Username: dnburgess

Post Number: 153
Registered: 1-2003
Posted on Saturday, December 20, 2003 - 3:16 pm:   Edit Post

I used the outputs from a stereo chorus into an SF2 driving a stereo power amp and 2 Acme Low B1s. Sounded pretty nice.

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