JBL L Pad Question

[Chas]

I notice that some of the JBL crossover schematics, like the 3133, have a strange looking L pad drawn in for the 2420 level control. My assumption is that it is actually a stereo pad with the wipers connected together.

I guess that this was done to present a somewhat constant DCR to the preceeding components.

I haven't actually seen one of these crossovers, could someone confirm that this is indeed the case?

Charles.

[4313B]

The 3133 uses a standard L-Pad on the 2420. It's 16 ohm but doesn't present a true 16 ohm load to the components in front of it since it doesn't have a 16 ohm load behind it ("16 ohm" 2420 paralleled with a 20 ohm resistor doesn't present the required 16 ohms for a 16 ohm L-Pad to present a constant 16 ohm load).

The "trick" part would be the 1.55 mH tapped autotransformer which in this particular case is center-tapped (-6 dB). Once you know the required DCR of this component it is easy to recreate.

[Gioxtream]

Could you please let me know the size of the gauge of the air coil inductors on the Crossover board? Just twe tweeter size and the mid range please.

Thanks in advance.
Wallace.

The 3133 uses a standard L-Pad on the 2420. It's 16 ohm but doesn't present a true 16 ohm load to the components in front of it since it doesn't have a 16 ohm load behind it ("16 ohm" 2420 paralleled with a 20 ohm resistor doesn't present the required 16 ohms for a 16 ohm L-Pad to present a constant 16 ohm load).

The "trick" part would be the 1.55 mH tapped autotransformer which in this particular case is center-tapped (-6 dB). Once you know the required DCR of this component it is easy to recreate.

[Chas]

Thanks Giskard, for the info. I thought that I may be able to get the center tapped inductor/autotransfomer fom JBL. I wonder if they still support parts for the 4133/L300? Probably not.

I do have a something similar center tapped from a pair of Altec 3000 Hz crossovers (from 601D's) although I can't measure the inductance, I have looked at the frequncy response and they extend well below 1 Khz into a purly resistive load.

Worst case, I guess I could lower myself to using something like the 3122 resistance voltage dividers.
Charles.

[4313B]

Yeah, try ordering the tapped autotransformers. I think JBL Pro recently "found" a bunch of those 1.55mH for the L300/4333. If you can't get any from JBL Pro let me know and I'll dig mine out and measure them for you.

[Chas]

Or, maybe I can try something like two 0.775 mH in series!
Charles.

[4313B]

An actual stock 1.55mH measures as follows:

input to ground = 1.51 mH @ 0.79 ohms

input to -3 dB tap = 0.15 mH @ 0.42 ohms
input to -4.5 dB tap = 0.26 mH @ 0.46 ohms
input to -6 dB tap = 0.39 mH @ 0.52 ohms

-3 dB tap to ground = 0.76 mH @ 0.60 ohms
-4.5 dB tap to ground = 0.52 mH @ 0.54 ohms
-6 dB tap to ground = 0.35 mH @ 0.47 ohms

[Chas]

Giskard, I am confused. The crossover schematic I have for the 3133 and 3133A shows a simple inductor that is (center)tapped. Where are the taps you are describing?
Charles.

[4313B]

I have the actual core that that particular inductor is made from so it has various taps on it. If you take apart a stock L300/4333 you may notice that the superfluous taps have been snipped off. When one ordered the particular N333/3133 part number in the past, JBL would often send it out with the -3 and -4.5 taps already snipped off. I asked that this particular one be sent with all the taps intact.

[4313B]

Oh, I should probably point out that this tapped autotransformer was also used in the LX80 and LX80A.

[Earl K]

Nice Pic

And I remember you lamenting about your photo skills .

What changed ??

<> EarlK

Say, what's the @ physical size of that inductor ?

[4313B]

I used sunlight and I sacrificed a chicken to the God of Kodachrome.

The size as you are looking at it in the picture is 1-7/8" wide x 2-5/16" high x 1-3/4" deep.

[Earl K]

Hi Chas

Re; Your 3133 crossover mockup;

You might consider substituting a 1.5 or 1.6 mh regular-style inductor (coil) in place of that center tapped 1.55mh inductor . The center tapped coil that JBL uses, does offer a 6 db padding for the circuit - which BTW, you will still need the padding from somewhere in your circuit. But you might as well do what JBL did in the 3133A Circuit - which in "Biamp" mode - forgoes the 6 db of attentuation from the center tapped inductor and instead employs a regular fixed pad a little further upstream. In the 3133a , JBL makes a fixed Lpad from a 4ohm series resistor and a 5ohm shunt resistor. Dependant on the rotational setting of the 16 ohm Lpad - I figure this fixed Lpad offers about 7.5 db of attentuation - if it is "seeing" a 7.2 ohm load ( which it will - in certain areas of that 16 ohm Lpads "rotation".

Do as JBL did and omit the 7.5 ohm resistor that occurs in front of the 1.55 mh inductor - if you go with a fixed Lpad instead of the centertapped inductor. The 3133A does this when it switches the crossover into biamp mode - (study the schematic and the effect of the switch positions ) .

Solen "On Line Parts" sell a 1.5 mh inductor , 18 gauge wire, air core, for about $ 7.25 cdn. This coil has a dcr of .6 ohms - meaning - to satisfy the 3133 circuit requirement for DCR as posted above by Giskard - you should put a .2 ohm resistor in series with the coil.

To get wirewound resistors , when in the Toronto area, I'll go "Supremetronics Inc." if I'm in downtown . They are located at the 300 block Queen St West area. They have a decent selection of 10 & 20 watt values - mostly 10% tolerance - 40¢ for the 10 watters - I believe .

regards <> Earl K

[Chas]

Earl, you provide a very simple, elegant solution. I had completely discounted the 3133A because I had intended to go pasive for now.

I should have looked a little closer at it!

Okay, now I am ready to order some parts and get moving on this thing!
Cheers,
Charles.

[4313B]

I don't believe it is that simple.

Do a search on this forum for my non-tapped autotransformer solution for the N333/3133A.