Film-Tech Forum ARCHIVE: Altec crossover freq question

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Posted by Ari Nordström (Member # 288) on 10-23-2000, 03:28 PM:

I've made some sound measurements at a friend's theatre and get an ugly dip around 1 kHz, pretty much regardless of what I do with the EQs. All three screen channels show this phenomenon to some extent, and while the auditorium is a bit strange, with a balcony and various sound traps, I'm suspecting that a lot of this could be corrected by getting new crossover filters to the speakers.

Unfortunately, I have no crossover data available so I was hoping that somebody here knows more. The speakers are old Altecs and I think this particular HF/LF combination is called Altec 608-16X.

Thanks for any help.

Posted by Steve Guttag (Member # 268) on 10-23-2000, 05:06 PM:

>>"Altec 608-16X."<<

That model number isn't jumping out at me.

The closest I can imaging is the Atlec 604 and that merely refers to the driver itself. The 604 is a coax (Altec called it a duplex) driver where the HF compression driver is mounted to the back of the 15" cone driver.

If you could give me the specific Altec drivers in the system (including your existing crossover), I can probably help you.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Ari Nordström (Member # 288) on 10-25-2000, 03:30 PM:

I got the speaker name wrong, according to my projectionist friend. It's indeed a 604. The full name, according to him, is "604-168X". The drivers are Altec originals, but the filters and the boxes are not; a shop in town used Altec blueprints and (probably) their own filters. The filters, of course, have no markings on them at all, except "High Power Two Way by Sam & Jimmy"...

But the story gets more interesting . My friend says that he chose the 604s because of good past experiences with 604s, then with crossovers called "N809-8A", at a single-screen theatre now long gone. He also has a spare 604 speaker with the original filter in; that filter, according to him, is called "904-8A".

So I guess my question really is, do you know what the crossover frequency for the above two filters is? Or what the recommended crossover should be for a 604?

Posted by Steve Guttag (Member # 268) on 10-25-2000, 06:48 PM:

The 604 is generally crossed over at 1.5KHz. The last crossover Altec manufactured for it crossed it over at 1.5KHz. How the 604 performs really depends on the box it is installed. The Altec Model 620 was the best for it.

As to the crossover or "filter" as you term it. Altec calls all of their crossovers "networks" so all Altec "networks" begin with the letter "N". An N809-8A would be an 800Hz crossover with an 8-Ohms impedance as presented to the amplifier.

Now as to your 604 you need to determine just what the impedance of BOTH the cone and the diaphram are to ensure the crossover your connect will be correct. The impednace of the driver will interact with the reactive components of the crossover and affect the actual crossover frequency.

Since your crossover is an 800Hz based on it's model number...the horn of the 604 is completely unloaded below 1.2KHz and as such, you would have miserable response below there until the bass driver rolls in around 800Hz. Now, the 604 had a "multi-cell" type horn (6-cells) up and until the most recent model, the "K" version which has the Mantaray. In either case, I would keep the crossover at 1.5KHz. If you go too much higher, the 15 inch driver will start to lose response (a bit too massive for those higher frequncy).

Steve

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"Old projectionists never die, they just changeover!"

Posted by Ari Nordström (Member # 288) on 10-25-2000, 07:50 PM:

Steve,

Thanks a lot for your help. It certainly sounds like (no pun intended) you've located the cause of that ugly dip. I'm going to find out more on Sunday, hopefully, as I'm set for another round against that auditorium...