Bypassing a passive crossover

[BobHealey]

I was working with some 1989 vintage JBL speakers last night, trying to figure out why there was no low end in them. Got most of the full range back by bypassing the passive crossover on the woofer, but I have concerns about the amp loading now.

The highs are JBL 2245 drivers on horns and the woofers 4507 cabinets. The horns have a 16 ohm impedance, and the woofer impedance is unknown. The amps are Ashly FET2000s (L/R) and an FTX2002 (C). Assuming a properly configured active crossover before the amps, can I get away with a 16 ohm load? The amps are rated for 300W into 8 ohms.

 

[bishopthomas]

A higher impedance is no problem, it's going lower than 4 that you need to be wary about.

 

[Aman121]

Should be fine. Most amps will take a 16 ohm load, it just usually isisnt stated because these days few speakers are made with 16 ohm coils. This was done back in the day to allow multiple speakers to be used with one amp that couldn't do less than an 8 ohm load.

Where you run into trouble with amp loading is when the impedance is to low, that's when they work harder, and clip.


I bet the jbls sound nice, I'm an altec man myself but both companies were definitly in the same league in those days.


Edit: double post with bishop!

 

[BobHealey]

Thanks.

My audio expertise is more getting the noise from point A to point B and less component level repair. This is a 25 year old installation used for movies (until recently, 35mm Dolby SR/DTS). Wanted confirmation my "get it working now" fix wasn't going to be worse than the problem. The plan was to eventually convert them to bi-amp as the woofer mount is suboptimal. Each is pulled back about 3 feet from the screen so that its time aligned with the horn above it, and the sound travels through a cabinet sized chipboard tunnel to reach the screen.

 

[Aman121]

One more word of warning- be very careful when setting crossover points. Those horn drivers will not take kindly to any signal below 600hz or so. Even a light ground hum can stress crack the diaphragms, and those 70 dollar replacements you see on eBay are cheap knockoffs that wouldn't work well at all. You'd probably have to order more from jbl and be out a few hundred bucks.

 

[BobHealey]

Based on this data sheet, I've got the cross point set to 514 with 24 dB/octave roll off. Should I go up 100 Hz and make it 48 dB/octave just to be safe?
http://www.jblpro.com/pub/components/2445j.pdf

 

[Aman121]

Based on this data sheet, I've got the cross point set to 514 with 24 dB/octave roll off. Should I go up 100 Hz and make it 48 dB/octave just to be safe?
http://www.jblpro.com/pub/components/2445j.pdf

No that's right, just a typo on me! Meant to say 500. 514/24 sounds about right. Sounds like a fun project, those old cinima speakers really preform well. Jbl was shoving altec out of the cinima market by this time, and it's still the standard today.

 

[TimmyP1955]

I'd think it more likely that alignment would require the woofers to be even with or in front of the horns (owing to the "phase delay" in the woofer and the filters).

 

[museav]

I'd think it more likely that alignment would require the woofers to be even with or in front of the horns (owing to the "phase delay" in the woofer and the filters).

The 4507 is ported, direct radiating design rather than a horn loaded design where internal path length can be a factor. And if the low pass and high pass filters of the crossover have the same slope then the related delay should be the same for both the LF and HF.

It sounds like you may have a 4673 (http://www.jblpro.com/pub/obsolete/Theater_Systems1.pdf). 4507 LF enclosure with 2225H LF driver and 2380HF horn with 2445J compression driver. The standard crossover for that speaker was the 3115A, a 500Hz, 12dB/octave crossover with a three position HF gain switch and a three position CD compensation switch (note that if you go to an external active crossover you will have to provide the CD horn compensation in some other manner). According to Note 2, for cinema application they recommend a 500Hz crossover but for "high power sound reinforcement" applications they recommend a 800Hz crossover, which would be the 3110A (http://www.jblpro.com/pages/pub/obsolete/3105-3115A.pdf).

 

[BobHealey]

It was definitely a 3115 or a 3110. I'm not sure what you mean by CD compensation though. The system just got upgrading yesterday from 35mm/Dolby CP-65 to DLP/Dolby CP-750. The signal off the 750 is definitely pushing the amps harder than the 65 ever did. I've got a 2:1 compressor set to -3 dB threshold with a 500ms attack set as some basic protection. Trying to balance over agressiveness with protecting the speakers.

 

[museav]

It was definitely a 3115 or a 3110. I'm not sure what you mean by CD compensation though.

Constant Directivity horn compensation or CD horn EQ is high frequency compensation to account for the natural roll-off in the very high frequency response that occurs with compression drivers on CD horns. This equalization is typically a 6dB/octave boost starting around 3kHz to 4kHz.

 

[BobHealey]

Ah. The projector tech spent about 5-10 minutes with the software on the new cinema processor to get a "flat" response. Only major mistake I made was making L/C/R full range in the crossover (regardless of actual speaker ranges, L/C/R must be high passed at no lower than 80 Hz, sub must be low passed no higher than 80 Hz).