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250timan
10-26-2004, 05:47 PM
Please explain to me what biasing a 250TI crossover means.
Ihave owned JBL Alphas, Century 100's, L112's, 150A's, a mint Paragon and now vintage 1998 250TI's Classics.
I have read the threads pertaining to the 250's in this forum and saw your parts picture.
Who does this modification? Would Greg do it if I paid him? Anyone at JBL? Your simple detailed explanation of biasing would be much appreciated.

4313B
10-27-2004, 05:29 AM
"Please explain to me what biasing a 250TI crossover means."

I'm going to ask Earl to do that because he's done several excellent posts on the subject already.

"I have read the threads pertaining to the 250's in this forum and saw your parts picture."

That thread is actually going to be populated with a few more pictures. The pair I'm doing is a one shot deal. No way in hell would I ever do another pair.

Well... ok... if someone traded me a cherry 1966 Chevelle with a fresh, blown 427 rat then maybe...

"Who does this modification?"

You do! :p

"Would Greg do it if I paid him?"

No comment.

"Anyone at JBL?"

I can't imagine, but that doesn't mean anything.



Link to schematic of charge-coupled (biased) N250Ti (http://www.audioheritage.org/vbulletin/attachment.php?s=&postid=2306)

Earl K
10-27-2004, 06:13 AM
Hi 250timan :D

I'll respond to your questions and concerns sometime later today. :yes:

regards < Earl K :)

Robh3606
10-27-2004, 07:06 AM
Hello 250timan

Do a search for Charge Coupled

References can be found on the site in the K2 brochures. That's where I got the idea to try it on my 2344 horns.

http://lansingheritage.org/images/jbl/specs/home-speakers/1993-k2-s5500/page11.jpg

Check this out as an overview Earl can give you the rest. All I can say is it works! My horns are set-up with an active crossover but the compensation is passive. With charge coupled caps they are nice and smooth lots of detail.

Rob:)

Earl K
10-27-2004, 05:39 PM
Hi 250timan

Once you read the info at the end of the link that Rob posted - you'll see that DC Biasing or "Charge Coupling" capacitors is a way of turning a pair of capacitors into a passive device that operates in a Class A mode.

Usually, a single nonpolarized capacitor with audio signals coursing through it, will do its' filtering in a way that is closer to a Class B elecrtrical model. Class B is a switch-mode of sorts. The "switching" within the capacitor leads to a form of crossover distortion. While this type of distortion is very slight in most film caps , it's very high in electrolytic type caps .


Originally posted by 250timan
Please explain to me what biasing a 250TI crossover means.
Well technically and mechanically, it requires replacing each capacitor with 2 capacitors that are each, double the original value.
At the junction of these two capacitors, one will find a trickle resistor. This resistor delivers a positive DC voltage into these 2 caps . This DC voltage polarizes this pair of caps. Typically, the value of this resistor is 2 to 3 Mohms. Usual DC voltage applied is 9 volts . I'll post a picture of a pair of DC biased caps tomorrow .

A fourfold increase in capacitance costs more - on average the cost seems to average out to be @ 3 times the cost of a single value cap.



I have read the threads pertaining to the 250's in this forum and saw your parts picture.
Your simple detailed explanation of biasing would be much appreciated.

Here's a link to a web site that has some actual oscilloscope shots of nonlinearities of various types of capacitor dielectrics. Navigate to the area on DC biasing the electrolytics to see the visual improvement in performance. Go to The Sound of Capacitors (http://members.aol.com/sbench102/caps.html) to see these instructive visuals .

Hopefully I've answered your initial question directly - though you probably have lots more. When these questions arise just ask them .
I'm happy to talk about this subject over the next few days.

- You may not realize it - but once anyone posts to a thread , the thread "pops" to the top of the queue.

regards <> Earl K

Earl K
10-28-2004, 05:29 PM
As promised ;

Here's a picture of a 2 cap setup, that's is ready to be DC biased ( Charge Coupled ) .

<> Earl K

250timan
10-29-2004, 07:44 AM
Wow you guys are good.
Thank you very much for the information.
So there is no way I could do this as I am a layman when it comes to schematics etc.

Please estimate the cost of parts and the estimated labor cost to have this done for me.

The 250ti speakers are incredible sounding as is. What improvements would be heard with this change? I have a pair or Dali MS4's and the two are an interesting contrast to listen to.

To improve the 250ti I would like more three dimensional sound outside of the center position and a little more excitment to the sound. My amp is a BAT 300XSE.

Thank you Giskard and Earl.

Earl K
10-29-2004, 12:16 PM
Hi


Please estimate the cost of parts and the estimated labor cost to have this done for me.

NOTE - I'm not aware of anyone here who wants to take on this type of custom project . But, maybe there is someone , if so they can jump in at any point .

BUT - So that you can get an idea of the parts' necessary, take a look at the info in the 250 Jubilee thread (http://audioheritage.csdco.com/vbulletin/forumdisplay.php?s=&forumid=4). These wouldn't be exactly the same parts since the Classic and the Jubilee appear to use different tweeters and slightly different crossover points. Go to Solen Canada (http://www.solen.ca/) and download the International Price Lists for capacitors and then for inductors. By cross-referencing the info that CLASS A posted to these price lists you'll get an idea of a basic costing using pretty decent parts. It's a tedious chore - but - this is your project .

- You'll also find in the info that CLASS A posted , a part number for the crossovers' pcb board . You should try to seek out that particular part numbers availabilty . Where are you located ?

- Another thought; a general upgrade of parts to your existing crossover ( excluding DC biasing it ). If you have a digital camera - remove one crossover and take some pics of it and post them here. That way we can get a general idea of the quality of the existing parts. You could be better off in the long run to pull the crossover and take it around to different HiEnd audio shops, to solicit a quote for an upgrade to whatever HiEnd parts they offer.

The benefit here is at least two-fold;
(A) With a shop technician , actually seeing the crossover they can more accurately access just what it is going to take to complete the job. This way they will deliver a reasonably accurate quote.
(B) Having you establish a relationship with some local audiophile outfit also means you can get across to them, a more accurate description of the changes you seek to the speakers overall voicing. Changing/swapping partswill change how the speaker sounds . You can do this easier through a show & tell method by comparisons to their present floor models .

Now, the fact that you're obviously not comfortable with "heating up a soldering iron" will ultimately cost you quite a bit of money .
Oh Well "C'est Le Vie"

regards <. Earl K