Film-Tech Forum ARCHIVE: Question about JBL 4675's

This is topic Question about JBL 4675's in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=010371

Posted by Tom Wienholt (Member # 1510) on 03-09-2010, 09:43 PM:

Just purchased used JBL 4675C's. These came from a theatre that had a bi-amped system. The built in crossover has been removed. Is this the proper way to wire these speakers for a bi-amped system? I assume that the built in crossover was removed to have access to the low frequency cones for direct wiring purposes. Also, what is a good crossover to use with these speakers?

Posted by Tony Bandiera Jr (Member # 2365) on 03-10-2010, 01:26 PM:

Tom:

Yes, removing the built-in crossover is the right way to wire for a bi-amp system.

As for the second question, depends on if you are going to continue in bi-amp or want to go back to a passive (1-way) wiring. If passive, the original JBL crossover module is the ONLY safe bet.

If bi-amp, any active crossover such as the XTA's in your other post, QSC's crossover modules XC series, the crossovers built into the CP-500/CP-650, etc.

As I have advised you in the other thread, your best bet is to get an experienced technician involved before you get in over your head.

Posted by Tom Wienholt (Member # 1510) on 03-10-2010, 02:16 PM:

So having the "hole" in the side of the speaker cabinet where the built in crossover originally was is OK and will not affect sound quality?

Posted by Gordon McLeod (Member # 33) on 03-10-2010, 03:32 PM:

It should be sealed over with a piece of MDF

Posted by Tom Wienholt (Member # 1510) on 03-10-2010, 07:01 PM:

Ok, so if I seal the "hole" with MDF, how do I have access to the wires? Drill a small hole? Install some type of terminal?

Posted by Phil Ranucci (Member # 3820) on 03-11-2010, 12:07 AM:

Get a banana plug terminal at Radio Shack, connect it to the speakers and mount it to the filler plate. Then you can attach your wires from the amp.

Posted by Marco Giustini (Member # 4544) on 03-11-2010, 03:47 PM:

I too found JBL speakers with a hole in place of the crossover and, obviously, it has to be covered. Drill a hole in the cap to allow wires and seal everything.

Posted by Tony Bandiera Jr (Member # 2365) on 03-11-2010, 06:16 PM:

Either Phil's or Marcos' method will do for getting the wire out of the cabinet..and you're right, the hole where the crossover needs to be sealed. Forgot to mention that.

For the HF driver I usually run my speaker wire directly to the driver's terminals, no need to splice onto the existing HF wires really. But it won't hurt things if you do.

Posted by Tom Wienholt (Member # 1510) on 03-11-2010, 07:22 PM:

Ok, so here is another question relating to these speakers, or any speakers with a separately mounted horn. When "aiming" the left and right stage speakers to the center of the room 2/3rd's back, should the entire speaker (LF cabinet and HF horn) be angled? Or, should the LF be placed parallel and as close to the screen as possible, with the horn being "aimed" by rotating it on it's mount?

Posted by John Walsh (Member # 168) on 03-11-2010, 08:12 PM:

With bass frequencies being somewhat non-directional, you can have the bass cabinet flat to the screen. Just aim the HF.

Posted by Gordon McLeod (Member # 33) on 03-11-2010, 09:37 PM:

just remember to but a protection cap in serries with HF driver

Posted by Tom Wienholt (Member # 1510) on 03-11-2010, 10:00 PM:

What do you mean by a protection cap? And why is this necessary?

Posted by Gordon McLeod (Member # 33) on 03-12-2010, 06:10 AM:

There is an altec note on the value of capacitor that should be placed in serries with HF driver to protect it from things like turn on thump and the like

Posted by Tony Bandiera Jr (Member # 2365) on 03-12-2010, 11:36 AM:

Gordon, while in theory it is a good idea to put a protection capacitor on the HF driver, with virtually all of the modern amps in use today it is totally unnecessary. I have serviced literally hundreds of bi-amped screens and NONE of them had the caps or issues with blown drivers.

Most if not all modern amps are designed to not thump or make spurious noise when being turned on or off.

Tom, From the nature of the questions you keep asking, (And I'm not trying to be mean or insult you, but) you really need a qualified technician to assist you with this install before you get in way over your head. There are too many elements involved that cannot be taught or walked through via an endless stream of posts on a website. [Big Grin]

Trust me , the cost of the tech is going to be repaid by the speed of installation and lack of headaches involved. [Big Grin]

Posted by Gordon McLeod (Member # 33) on 03-12-2010, 04:23 PM:

Often good practice is not followed when it should be