L55 Inductor Info??

[zipplok]

I am replacing the inductors from a pair of L55's. It has 4 leads coming from it. Does anyone know what the additional two leads are? Are there just two wires off of the inner lead and two off of the outer? Would both measure the same? Since this inductor is still attached to the crossover, I can not get an accurate Mh measurement and do not know what value of air core to replace it with. Thanks in advance, Zipplok.

[Earl K]

Hi

- If that inductor has any more than two leads coming out of it ( and the excess leads are attached to something ) , you can be assured that it is a tapped autoformer ( autotransformer ) .

- You can not easily buy a replacement aircore-type autoformer . Typically, one would need to be hand-wound by a specialty transformer-winding shop / & therefore, very expensive .

- You can purchase a standard aircore (two lead ) inductor that will fulfill the second pole "hipass" function of the autoformer. ( the two other leadouts are taps which give the other autoformer function, more or less the same as an Lpad, ie; this function offers some attenuation ).

- A NOTE : Air Core can be inductors are senstitive to placement. This sensitivity includes being placed in metal boxes ( including the typical Hammond / Budd Industries type ) . The surrounding metal forms a return circuit around the coil altering it's inductance. I have experienced this effect first-hand while playing about with aircore coils. I haven't played about with iron-core to have any insights to them being in metal boxes .

- I don't have the tech sheet for the L55. Does it have a le20 tweeter ?. If by chance it's the same as the S99 then theautoformer has a 3.0 mH value. Buy or build an 8 ohm Lpad and insert it inline , after the inductor (meaning, closer in the circuit to the tweeter ) but amp-side of any LCR resonance filter that might be present .

[Robh3606]

They should have been attached to the switch. They are taps and are used with the resistors to attenuate the tweeter. They are custom and not available. I would leave them be unless you want to modify the crossover and come up with the correct resistance pads to replace the original attenuators.


Rob

[Don C]

Here's the tech sheet, thanks to Giskard for the pointer.

http://manuals.harman.com/JBL/HOM/Te...ancer%20ts.pdf

[zipplok]

Thanks for all of the information. Maybe it would be better if I left the inductors alone and just repaced the capacitors and wire. I plan on using Kimber Metallized Polys. I am sure the cap and wire upgrade will improve the sound dramatically. Thanks again, Zipplok

[rek50]

Zipplock seems to have some direction from his inquiry, So, Not to jack this thread , but, "Buy or build an 8 ohm Lpad and insert it inline , after the inductor (meaning, closer in the circuit to the tweeter ) but amp-side of any LCR resonance filter that might be present ." While this is "Fresh". Earl, would you elaborate on inductor placement relative to Lpad placement. I've seen BOTH (inductor AFTER Lpad-closest to driver) and inductor BEFORE Lpad.... on numerous JBL network schematics.

[Earl K]

"Buy or build an 8 ohm Lpad and insert it inline , after the inductor (meaning, closer in the circuit to the tweeter ) but amp-side of any LCR resonance filter that might be present ."

- I should have advised to make the Lpad of appropriate impedance / since I don't know the actual AC impedance of the circuit at that point . I assumed 8 ohms was close enough but I don't have an AC impedance curve of the le20 to work from.
- This ( 8 ohm ) assumption might be wrong / given the largish size of that inductor ( 3 mH ). Typically, in an 8 ohm circuit, a 2500 hz crossover point might ask for an inductor in the 0.7 to 1 mH range. Increasing impedance results in needing larger sized coils for the same Fc .
- OTOH , this inductor might have a high DCR making it a form of lower frequency resonance control for the le20 .

Earl, would you elaborate on inductor placement relative to Lpad placement. I've seen BOTH (inductor AFTER Lpad-closest to driver) and inductor BEFORE Lpad.... on numerous JBL network schematics.

- I also seen them in both positions .

- If its' found ( upon removal ) that the 3 mH autoformer has a very high DCR then I would be inclined to put any other standin close to the driver ( this is opposite from what I posted above when I assumed the autoformer was an active inductor until I checked the math ) . In this case I would put the padding "amp side" of the inductor.

- If the inductor has minimal DCR ( like, less than 1 ohm ) I would place it amp side of any new padding .

[rek50]

Thanks Earl, now that you mention it (high/low DCR value of the inductor), that's how all, well the ones I've looked at, seem to be. The 3.5 mh (higher DCR) is after the Lpad (closest to Mid driver) in one schematic, where as the lower DCR inductors are placed before the Lpad, for the High range driver. It's a lot to catch/learn, sometimes

[Earl K]

Said more succinctly :

- If the inductor is acting as an "Impedance Equalizer" ( high DCR / overly large mH value ) , I'd locate it closer to the driver than any padding . At least that would be where I would first listen to it .

- OTOH, if the inductor has the regular function of a coil ( along with typical size & DCr ) then I would keep it close to its associate / the capacitor .

EDIT:

Giskard did touch on this subject this year sometime . It's worth searching for that thread ( look for Impedance Equalizer along with his name as a poster ).
NOTE : a resistor , in line with a low DCR coil will allow the combo to act in this role of an "Impedance Equalizer". Giskard also demonstrated this contention to be true, with a side by side comparison to a high DCR coil . It's all back there in the archives somewhere .