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Old 31st March 2003, 03:37 PM   #1
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Default Pre-bias capacitors in crossover

I just read that JBL in their K2 uses a 9V PP9 battery to pre-bias the capacitors, improving their linearity and avoiding crossover distortion.

I don't know but it sounds intressting, anyone know how to do it and if it really works?



Magnus
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Old 31st March 2003, 08:36 PM   #2
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Unhappy Getting crazy here

JBL call it "Charge-Coupled Linear Definition Dividing Network"

I kind of know how ccd works but I can't apply it to passive crossovers.

Anyone tried to DC bias a passive crossover?

I thought is that it could be like a class A crossover, because the components wount have to "switch" between off and on.

Maybe it would be possible to save alot of money on high-end filter caps with using ceaper ones always charged?

Maybe it is crazy, but if it is, why would one of the oldest manufactors like JBL use it on their "high-end" speaker.

I guess I have to start frying drivers with DC brute force here, if I can't get any advice on the way.

Magnus
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Old 1st April 2003, 07:50 PM   #3
uli is offline uli  Austria
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Cool DC-bias

Hi Magnus,
DC biasing a passive Xover is not that new.
Its just getting expensive:
Take a cap say 10F. If you want it biased you need 2
caps in series each has 20F and apply some DC voltage
over a resistor (eg 1k) to the junction.
The bias voltage should be at least as high as the maximum
peak output of your amp.(better add some volts).
Those exotic Paperinoil or tinfoil caps are marked for
polarity -> very useful.

Uli

PS: and it works indeed!
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Old 1st April 2003, 09:44 PM   #4
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Default Thanks

It almost drove me crazy, I hate to have something like this bugging me.

Well I know it's not that new, but it had to be an improvment because they use it in the K2's that I think is the best sounding out there right now.

Thanks again, time for me to test, messurment, listen and see/hear for myself.

Strange if it works, why don't everyone with passive filters do it, the cost isn't that much more.



Have you played around with it alot, does the bias voltage effetct the sound.

Would a get some new problems with phase and so on?

Magnus

PS: a great SACD is Jorma Kaukonen "Blue country heart"
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Old 2nd April 2003, 07:41 AM   #5
uli is offline uli  Austria
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Magnus,
Yes I tried, but the advantages are hardly hearable.
It makes sense in systems where everything else is
state of the art. Every cap has different behaviour
when polarized in one or the other way. This leads
to some added distortion. Now if you bias any cap
it doesnt change polarity any more (kind of similarity
to Class A if you want).In woofer Xovers it makes no
sense as there you can hardly hear it. I use it for
my Focal tweeters and I think they sound cleaner but
thats subjective. I bias the caps with 25Volts DC.
IMHO it makes no sense to use a 9V battery. The so
achieved headroom is too lo. 9V / 8R = 5Watts!
If you drive that system higher you leave the bias
area!

Uli

PS: no it doesnt affect phase.
2 20F caps tin foil and a psu cost far more than one
10F cap and this multiplied by 2 and the count of caps!
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Old 2nd April 2003, 03:22 PM   #6
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OK, great and sometimes a theoretical improvment make me think it sounds better maybee enough. Well I'm not into magics and don't expect wonders, more querious to see if it work and how much improvment it is possible to get tweaking it back and forth.
Well strange that 9V wouldn't do it, is it just a decoration in the hughs K2's?
Well I havn't start to play with it yet but have plans for it tonight tho.

Well it do cost some extra, but not as much as hearing my wife complaining over active x-overs and extra amps in the livingroom. That makes me give her my visa so she could go shopping, getting happy again.

Dentist time


Magnus
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Old 2nd April 2003, 04:23 PM   #7
uli is offline uli  Austria
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Hi Magnus,
The point is that in a commercial device like those JBLs
its necessary to build it "foolproof". Hence the 9v battery.
Better but more complicated is to use accus (2x12v for example).
Maybe the reason is that when this method is only used for caps
in the path (hipass) of tweeters or mids there will hardly be a
situation where you need more than those 5 Watts (RMS!) in
a tweeter. Normally the powerdispersion is like 100 Watts for
Bass equals about 3-5 Watts for the tweeter.
You can calculate this by analysing a signal with a FF Analyser.

Uli
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Old 2nd April 2003, 08:25 PM   #8
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Default OK

Well I know, messured that my tweeter powerdispersion at normal use is just about 0,3W, mid around 3W and bass around 9W. Maybee it's me that is to careful with the volume..
My thinking were as long as the caps are bias a little they will be "on" and at hire volumes I won't even be able to hear a differns if they are under biased?
Just tried it on my monitors with 20V and right away I couldn't hear any differense at al, but no serious listening yet.
Will listen more serious tonight, and test it in some other filters and speaker, and have my wifes super ears listen to it, then I will know.

Thanks again for taking your time.

Magnus
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Old 3rd April 2003, 05:47 AM   #9
uli is offline uli  Austria
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Im happy "taking my time"

Uli
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Old 3rd April 2003, 03:50 PM   #10
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Cool It works great

It works, even I hear the difference.
My tweeters is accuton c-12 when I first tested it in 2st order filter both unbiased and biased with audiocap theta film caps, i didn't hear any difference at al, my wife said it was a little clearer. Maybe just a subjective difference. After that I tested it in a 1st order filter with audiocaps both ways, now we both heard a difference it got clearer and the sound stage got bigger but still no big difference, don't think it was worth the effort. At last same filter tested with ICW Clarity caps, big difference between the bias and unbiased caps, the unbias ones sounded flat muffled compaired with with the bias version. The testing was far away from blind testing, so I really don't know if it is worth it yet, sometimes my brain fouls me because I want to belive in something.
I will play around with it a while and have my wife blind test it.
If a really find a big difference, I might try to pre-bias some caps in the signal path in my amp, qurious


Magnus
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