The OFFICIAL JBL Owners Thread - Page 520 - AVS Forum
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post #15571 of 15576 Old 11-20-2014, 02:19 PM
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Originally Posted by gferrell View Post
So what are the principals of a bias XO? How do you go about doing it?
Go over to www.lansingheritage.org
Search for Charged-Couple Networks.

What it involves is changing the caps. Lets say the tweeter circuit has an 8uf cap and a 4uf cap.
The 8uf is replaced with two 16uf caps, wired in series.
The 4uf is replaced with two 8uf caps, also wired in series.

You do the same for all caps.

The common points of each pair of caps are connected to 2 megohm resistors.
The other ends of the resistors connects to the + side of a 9v battery.

This does two things, the 9vdc keeps the layers of the caps tight.
And it also keeps the audio signal above the 0 volt line.
W/O the 9v charge, the layers tighten, loosen, tighten, loosen as the frequency and amplitude change. And by keeping the signal above the 0 volt line the caps are always on, eliminating a slight phase sift as the signal changes direction.

As you can see, both the doubling of size and number of caps will require a lot more space. The original L212 XO fit on one 4.5 x 7 card. The Biased XO required 3 cards, one for each driver.
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post #15572 of 15576 Old 11-20-2014, 05:42 PM
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I will study that. How did you time align the L212? Those sure are fine looking speakers. I think the best looking JBL ever made.

Wishing I could post in the Ultra Hi-End HT Gear ($20,000+)

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post #15573 of 15576 Old 11-20-2014, 06:44 PM
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I will study that. How did you time align the L212? Those sure are fine looking speakers. I think the best looking JBL ever made.
I added two strips of 3/16" ply to the front edge of the original 1/2" spacer. That tiled back the box 1 1/4". Then cut a tapered doughnut for the tweeter to bring back to vertical position.
Notice, I replaced the old 066 tweeter with 052Ti.
And decided to really go reto with blue grille cloth. Bought a pair of L55s in '73 with the same shade of blue. Also going to use it on all the PT800s, subs. Really tired of cafe, black grille cloth.
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post #15574 of 15576 Old Today, 10:26 AM
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Finally was able to obtain an L890 to complete the system
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post #15575 of 15576 Old Today, 12:37 PM
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Finally was able to obtain an L890 to complete the system
Congrats! How tall are they?
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post #15576 of 15576 Old Today, 02:56 PM
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Congrats! How tall are they?
Wow never realized how big they were compared to the 880s... You're using the 880 for a center? I use them for front L/R but have been wanting to upgrade for a while.

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