View Full Version : L65 Crossover rebuild .3mh Inductor

06-22-2007, 07:27 AM
I am rebuilding my L65 crossovers. I am having trouble locating the .3 mh 3 lead inductor. Is there a replacement available? If not what could be substuted?

06-22-2007, 07:49 AM
Reuse the tapped autotransformers you already have.

Measure the tapped autotransformers you already have and come up with a fixed resistor equivalent (often changes the values of the other filter elements as well and isn't trivial).

Otherwise design, or have someone design for you, a new network that doesn't employ tapped autotransformers (probably the best bet for that particular system).

Earl K
06-23-2007, 10:50 AM
I am rebuilding my L65 crossovers. I am having trouble locating the .3 mh 3 lead inductor. Is there a replacement available?

- There's no readily available ( off-the shelf ) substitute for this tapped coil ( autotransformer ) / that would also represent a step-up in component quality ( ignoring the fact that there are still companies who will wind custom autotransformers at "custom rates" ) . Sowter transformers in the UK is one example .

- An autotransformer has some builtin attenuation ( the amount of attenuation is dependant on where the leadout line is tapped into the coil ).

If not what could be substuted?

- You can substitute in a fixed Lpad ( inserted just before the variable 8 ohm Lpad ) and just after your new .3 mH coil ( ie; a coil of better quality ) . This is going to be ( at best ) a simplistic approximation of how the original autotransformer functions in the original circuit. As Giskard has stated, coil and cap values should be tweaked appropriately in an attempt to duplicate the filters' original curve ( also called a "voltage-drive" ) .

-----(i) The suggested combo will never sound "exactly the same" as the original ( all in one ) approach / even if you have someone duplicate the voltage drives to the Nth degree. The reason for this is "damping" / adding a pair of extra resistors to the circuit changes the damping of the circuit . Will you discern the difference ? I have no idea though the "extra" damping provided by the additional resistors should provide a constructive improvement to the overall sound of the 2405/077 / within this speakers' overall voicing .

-----(ii) What amount of attenuation does your fixed Lpad need to provide ?
- I imagine Giskard could find out / but then, so can you, & just as easily ( since you have the original part ). You'll just need to measure the voltage drop that occurs across your .3 mH inductor .
- Do you know how to do this ? It's quite easy, you need a DMM ( Digital MultiMeter, a 10K sine wave source, and an 8 ohm load resistor ) There are easily obtained, downloadable, software based function generators , found on the web . Mr. Widget just recently posted a link to one . Once you have voltage measurements to give us ( one, taken just before the coil & then the other taken just after the coil ) , it's a simple matter of converting the measurements into "db attenuation" .

-----(iii) What impedance of fixed Lpad should you build ?


- Above are 4 AC impedance curves for 077s and 2405 tweeters .

- In the absence of anyone stepping forward with an actual AC impedance curve of the 077 ( load side of a "partially" attenuated 8 ohm variable Lpad ),,,, I'd buy a bunch of resistors ( since they are quite cheap ) and try out 8 ohm to 10.5 ohm fixed pads ( in .5 ohm increments ) . I'd measure the results with an RTA to see which pad will give the best looking curve, as well as best fit for the 5" midrange below .


As Giskard has said, none of this is trivial / OTOH / it's not an insurmountable challenge to build something that could "float your boat" and represent an improvement over the originals' sound ( at least, to your ears ) .

06-23-2007, 02:15 PM
Hi Guys, I am doing some testing with these at the moment. I have wound a lot of inductors but never the autotransformers. I have always wondered why JBL used them but they must have had a good reason to do so as there are a few other "easier" ways to do the same job. Does any one have any of these laying around that they dont need? If so, could you send them to me and I will reverse engineer them so that we can keep our beloved JBL's as cose to original, albeit with upgraded components, as passible.

Cheers, Allan.:)

Earl K
06-23-2007, 02:36 PM

Read This Thread (http://audioheritage.csdco.com/vbulletin/showthread.php?t=5376) for some insights into how to construct a tapped autotransformer / and to view the response curve one can expect from these "imperfect" devices .

- As Giskard just said, the use of tapped autotransformers has fallen out of favour / one reason ( an obscure example ) ; one can nearly achieve the same "imperfect" FR response of the tapped autotransformer by burying a coil in series with the conjugate resistor ( as it goes to ground ) of the standard fixed two resistor, Lpad. One would use this aproach where one wants the HF attenuated somewhat less than the midrange .
- This "buried coil" arrangement mimics the frequency dependant padding ( of the tapped autotransformer ) that you'll see in the link that I just provided .


06-23-2007, 03:44 PM
Then I remembered RobH censoring Zilch for telling L100 owners things they didn't come here to hear / so I thought better of it

There's that. Which is a good reason to just reuse the stock tapped autotransformers with new caps and coils for everything else and be done with it. I will delete my previous post. I obviously wasn't paying a whole lot of attention. Thanks Earl.

Earl K
06-23-2007, 04:14 PM
There's that. Which is a good reason to just reuse the stock tapped autotransformers with new caps and coils for everything else and be done with it.

Pretty much everything I've said to this point, can be construed as just circling round the Mulberry Tree with the intent of helping one realize the wisdom of your original suggestion ( I just happen to have more time to type today ).

<> Earl K

06-23-2007, 04:52 PM
Would you like me to move it too DIY??


Earl K
06-23-2007, 05:18 PM
Would you like me to move it to DIY??

- Only if you're volunteering to spark up your copy of LinearX to produce an equivalent network ( using a standard coil and fixed Lpad ) to replace the tapped inductor.
- In the absence of such a like-minded commitment, theres' actually little to no "DIY" going on here ( merely motivational suggestions ) .

<. EarlK

Earl K
06-24-2007, 05:23 PM
Would you like me to move it to DIY??

Definately no need now Rob / see eBay sale item # 330139298381 to understand my comment / the Jubal is leaving ( the building ) and our efforts here have become superfluous .

- Let me be the first ( today ) to recommend that all L300s should their networks upgraded to a Giskard developed, DC Biased "Equivalent type .