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.

[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 :D

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 (http://www.jblproservice.com/pdf/Network%20Schematics/3133A%20Network.pdf) - 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" (http://www.solen.ca) 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.:banghead:

[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.

[Earl K]

I just did a quick look around - & couldn't find a rework of that circuit .

Might have sunk, along with the last forum to the bottom of the Dot-Com shoals.

& Speaking of "forcing a submarine to the surface" - how about answering your PMs .

<. Earl K :smthsail:

[4313B]

4333/S300/L300 equivalent bandbass circuit (http://audioheritage.csdco.com/vbulletin/showthread.php?s=&threadid=166)

[Mr. Widget]

Hey Giskard,

Since you have mentioned an aversion to the 2312, I doubt you have built and tested the charge coupled version shown in that thread. Do you know if anyone has?

While I share your feelings about the little megaphones I am considering that crossover for a different project. Actually I should get over to Bo's house and see if his 4345s can change my mind about the little guys.

Widget

[4313B]

No I haven't built and tested the charge coupled version and can't imagine ever doing so. I posted it as an example of something to try with the S300, L300, or 4333.

No, I don't personally use the 2307 or 2312 anymore. Plenty of people do though. I think the 4344 and 4345 are "best of breed". I think G.T. did an excellent job with those filters. Like I've stated before, if I still had my 4343's they'd be running 4344/4345 filters. No doubt about that.

[Earl K]

Hi Chas

Just so you don't think I abandoned you.

Here's the data upon which I made my suggestion for keeping the 1.5mh coil in the more or less original circuit . I reworked the data today . I had already done a partial mockup of this circuit with in-house parts .

- I figure to mate a 2216 and a 2307/2308/2420 horn combo you need to get the horn padded down about 12 to 14 db . As you stated in your first post to this forum - you're after something in the 1000hz range. A few posts later, you mentioned a 2307/2308 combo .

- As I said in my last post here , the 4 & 5 ohm fixed Lpad does give @ 7 db of attenuation. The variable 16 ohm Lpad was turned down to give another 6 db of attenuation . Now, when that "16" ohm Lpad is within that area of its rotation / looking into a 20 ohm shunt resistor with a 2410/2307/2308 as a load - the working circuit AC impedance - at 1000HZ is 6.7 ohms .

HiPass Crossover Frequency = 1021.17 hz
159000 divided by 110.55 = 1438.26 times a ButterWorth Transform scaling factor of .71 = 1021.17 hz ( 110.55 is the product of 6.7 ohms times the known capacitance of 16.5uf )

LoPass Crossover Frequency = 1001.38 hz
159 times 6.7ohms = 1065.3 divided by 1.5 mh = 710.20 hz times a ButterWorth Transform weighting of 1.41 = 1001.38 hz

- Here's a picture of Pink Noise on a RTA for this circuit . As you can see this HiPass portion of the circuit works very well with a 2410. This "curve" is as good a reason as any as to why most of the 2410/le175s' from eBay are disappearing into the orient . A comparison made between 500 hz and 1000 hz shows about a 10.5 db slope . 1000 hz is about 2 db lower than 1200 hz.

- Now your 2420 likely has a bit more lower -mid "bloom" that needs to be dealt with. You may need to start "steering" the crossover curve from a little earlier point ( say by a 1/3 to 1/2 on octave ) to deal with any "bumpy" lower mid response. Reducing your capacitor value to about 11 or 12 uf will be a start for this "corrective" treatment .

The "Equivalent 3133a Circuit" that Giskard has posted will likely give you similar results - though it was designed for a different ( longer ) horn and I believe a slightly lower ( 800hz ? ) crossover point . Its' bass ( lowmid ) section could work for your 2216 - though it was designed around a 136a /2235 woofers response . I don't know the response curve for the 2216 ( doesn't seem many do ) - but that woofer may need some extra attention in attenuating a "himid bloom" ( spikey response between 500 to 1000 hz ) . That's always a possibility when dealing with a deep gap woofer such as that .

regards <> Earl K

[Chas]

Wow, you guys are great, putting so much personal thought in on this. I really appreciate it. I have very limited experience with passive crossovers, so this is very educational for me.

Earl, I have ordered some parts based on your suggestions and hopefully, over Christmas, will do some bread boarding and tweaking. It sounds like fun!

What's the deal with the charge coupled arrangement? I used to do tricks like that for audio amplifier coupling caps (usually wretched tantalum types) in the old days. Nowadays, there are good caps easily available, or am I missing the point somewhere?

Re. the 2216 upper hump, maybe I can zobel it out. I notice the 4320 had such an arrangement.

BTW: I do have a pair of 175DLH, are they considered better than the 2420/2308/2307 by JBL connaiseurs?
Charles.

[4313B]

Originally posted by Chas
Nowadays, there are good caps easily available, or am I missing the point somewhere?Hovlands, AudioCaps, SonicCaps, etc don't necessarily require biasing/charge coupling.

[Earl K]

Chas

- Do a search for the thread called "Capacitors". Lots of info in there about capacitors . If you have questions ( which you should ), ask them there, the thread will reappear at the "top" of the reading queue.

- You'll find that quite a few of us have a bias against using Solen capacitors with ( especially ) JBL compression drivers . The best I can say about them is that purchasing them is " the beginning of an education in the sonics of different dielectrics" . All caps have a sonic signature & to my ears ( & others ) - that sonic signature is not complimentary to JBL drivers . If you were a SR sort - the easy explanation would be: " the 901s' compression ratios are set all wrong for a singers' voice on a 58". If your hip to that statement, great / if your not - sorry , I don't have the time necessary for a proper explanation. Short explanation ; Solens have way toooo much low mid compresson builtin to them . This "squeezes" out all warmth from the bottom 2 octaves from JBL drivers. Apparently, some of that loss can be "afforded" if you move your amplification type - to SET tubes .

- Giskard has pointed out for years that the easiest/cheapest "tried & true method" to getting the best from your JBL drivers is JBLs' own approach . This consists of using Metallized Mylar Caps ( for the Base cap value ) and then " Bypassing" that Base cap with a .01uf (Expensive) Film & Foil and then another Bypass of .005uf PolyStyrene (Film & Foil ). These bypass ratios , with these dielectric mixes have been determined to sound good & musical by the chief designers at JBL .

- All the caps that Giskard just mentioned would very likely make you very happy ( they are pricey ).

- Charge-Coupling or Class "A" biasing caps can give great sonic benifits in removing Class B distortion from the caps. Though , It won't change much of the Sonic Signature of a cap excepting the fact that you're filtering the signal through effectively four times the capacitance.

- I'd need to have your 2216 and that Zobel to really comment on it's effectiveness . I'd really need to see an impedance and actual response curve of a 2216 to make any guess .

- 175DLH are nice thing-a-ma-jigs. I wouldn't mate it with what you've mentioned for this project. It's a smaller magnet driver and ought to be used as such. It'll have less midrange "torque" than an equivalent diaphragm operating in a bigger magnet-motor ( like the 2420 ). For some people, this midband torque reduction, gives an impression of being more HiFi.

My suggestion would be to couple it to a high compliance woofer with well controlled mid .

Two "eBay regulars" are ;
The le14a, & a pair ( per horn driver ) of le10s ( an interesting project )

regards <> Earl K

[B&KMan]

Solens have way toooo much low mid compresson builtin to them . This "squeezes" out all warmth from the bottom 2 octaves from JBL drivers. Apparently, some of that loss can be "afforded" if you move your amplification type - to SET tubes .

regards <> Earl K

Hello

dear eye
I am about to finish a long modification of the network from my
4343.

The question of the condensers which you raise is pointed and, like earl it so intelligament summarized, several approach has...

For my part I initially installed film and foil Solen on my horn 2420
and my 2405 and I then to say to you that the metal effect of the
Solen condensers is true and False.

The time of burn-in is very long bus are of the metallized type... after 6 weeks ave the always opened system, I guaranteed to you that the signature metal disappears completely... I do not know for the other kinds... but I have a very pointed system and that did the work perfectly...

this is my 2 cents coments.

:cheers:

Jean

[B&KMan]

Dear chas,


for complementary informations,
I give a spec test impedance on 2420


http://www.audioheritage.org/vbulletin/showpost.php?p=63088&postcount=408

Actually I suspect the best result of this is addition of zobel circuit...

the impedance curve explain why is dangerous to go down frequency with 2420....

:cheers:


Jean.

[Chas]

Dear chas,


for complementary informations,
I give a spec test impedance on 2420


http://www.audioheritage.org/vbulletin/showpost.php?p=63088&postcount=408

Actually I suspect the best result of this is addition of zobel circuit...

the impedance curve explain why is dangerous to go down frequency with 2420....

:cheers:


Jean.

Salut Jean. You are really dredging up the old archives! Today, I'm happily using 2441's now. It turned out that the 2420's I have had phenolic diaphragms that I really didn't care for.

[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.