Inductor Lead Length in Crossover Networks - Does it Matter

[MCM_Fan]

How critical is inductor lead length in a crossover network? I've only seen three pairs of JBL crossovers (L36 Decades, 4301B Control Monitors and L19s) in the flesh and all had what appeared to be excessively long leads on the inductors. Certainly much longer than necessary to make the connections to the other components and terminal strips.

I'm currently recapping the crossover networks for my 4301B Control Monitors with new caps, but borrowing the inductors and L-pads from a pair of L19 crossovers. My first instinct was to trim the leads to tidy things up. This would shave about an inch and a half off the leads of the inductor in the tweeter circuit and 2.5" - 3" off the leads of the inductor in the woofer circuit.

But, before doing so, I thought I'd ask. Will shortening these leads have any negative impact on the performance? It seems weird to me that they were all so excessively long. Any conductor with current flowing through it acts as both a radiator and an antenna. I've always been told it's best to keep lead wires as short as possible to keep them from generating/picking up noise. But, will shortening them alter the DCR or inductance of the inductor enough to audibly impact performance?

I measured the length of all of the inductor leads in my L19 and 4301B crossovers and all (8 inductors = 16 lead wires) were between 6" and 6 1/4". Knowing that JBL used the same components in many different designs, it makes me wonder if they just speced all inductor lead wires as 6" so they would be long enough for all their different models. In other words, for assembly/cost reasons, not electrical performance.

I look forward to the responses. I'm a little OCD when it comes to clean looking crossovers. So, as said, my impulse is to trim the leads, but sound quality always comes first. Once they are re-installed in the cabinets, no one else will see them. So I'm ultimately more concerned about the potential for picking up noise vs. changing the DCR of the inductors.

What say you? Trim them or leave them long? Why/why not?

 

[MCM_Fan]

BTW, here's why I am borrowing the inductors and L-pads from the L19 crossovers to use in my 4301Bs.

Original 4301B crossover showing obvious damage from improper storage (excessive heat and humidity):

These were stored under conditions hot enough to melt the wax in the capacitors, cause the glue holding the inductors to fail and humid enough for the outside of the L-pads to be covered with corrosion/mildew.

You can't really tell how long the inductor leads are in these photos. So, here's a photo of the L19 crossovers, that are physically over twice as large as the 4301B crossovers:

Mounted on this much larger board, you can kind of see why woofer inductor leads (green on the board on the left, blue on the board on the right) are 6" long. But, you can also see that the tweeter inductor leads are about twice as long as they need to be. Same for the purple capacitor lead wires - much longer than necessary.

In fact, in my limited experience with JBL crossovers, most capacitor and inductor leads are longer than necessary. This creates a real rats nest of wires in the more compact crossover networks. Here's what my L36 Decade original crossovers looked like:

Here's what they look like after I recapped them (and shortened the inductor leads):

 

[bowtie427ss]

FWIW, the corrosion of the plating on the L-pads is caused over time by the off-gassing of formaldehyde from the manufactured wood composition of the cabinet material. The plating on the L-pads is impervious to moisture(H2O), that's why they plate them in the first place.

 

[MCM_Fan]

FWIW, the corrosion of the plating on the L-pads is caused over time by the off-gassing of formaldehyde from the manufactured wood composition of the cabinet material. The plating on the L-pads is impervious to moisture(H2O), that's why they plate them in the first place.

So why is there so much corrosion on the 4301B L-pads, but absolutely none on the L36 and L19 L-pads?

Is this also a function of excessive heat during storage. Would that cause increased out gassing of formaldehyde?

Any recommendations for removing the corrosion?

I haven't opened up the L-Pads yet, but since they aren't sealed, would out gassing sufficient to cause that level of corrosion on the exterior likely cause similar problems to the windings inside the L-pads?

 

[bowtie427ss]

So why is there so much corrosion on the 4301B L-pads, but absolutely none on the L36 and L19 L-pads?

Particular chemistry(of both the plating and the bonding adhesive in the wood) would be my guess. If the cabinet is painted or finished inside this greatly reduces any effect. Formaldehyde is just one popular chemical used in the resins in manufactured wood products, it's the one that popped off the top of my head.

Beyond the melted potting wax i would blame little on heat alone. In most cases the role heat plays is to speed up processes that are already occurring. Time is the enemy.

A damp cloth to remove the loose corrosion will usually suffice. Sometimes the plating contains cadmium, and the corrosion is largely cadmium oxide, not good stuff to breathe.

Where the internals of the L-pads are concerned you generally just have to worry about the natural oxidation of the resistive winding and the wiper that occurs over time in nearly any conditions.

 

[w1jim]

Regarding your original question - I don't think the lead length of the inductors wlll have any effect on the speakers performance.
The leads themselves will have negligible impact on the inductance - so leave them as is or shorten them if you just want to clean the board up a bit.

 

[MCM_Fan]

Particular chemistry(of both the plating and the bonding adhesive in the wood) would be my guess. If the cabinet is painted or finished inside this greatly reduces any effect. Formaldehyde is just one popular chemical used in the resins in manufactured wood products, it's the one that popped off the top of my head.

Beyond the melted potting wax i would blame little on heat alone. In most cases the role heat plays is to speed up processes that are already occurring. Time is the enemy.

A damp cloth to remove the loose corrosion will usually suffice. Sometimes the plating contains cadmium, and the corrosion is largely cadmium oxide, not good stuff to breathe.

Where the internals of the L-pads are concerned you generally just have to worry about the natural oxidation of the resistive winding and the wiper that occurs over time in nearly any conditions.

Thanks for the additional details. I picked up the L19 crossovers locally to use the L-pads and inductors in my 4301B crossovers. So, I'm not too concerned about salvaging the original L-pads.

However, I have seen similar corrosion on the magnet structures of woofers, including one of the 116A woofers from this pair of 4301Bs. Is it generally best to just leave this corrosion alone? In another thread, it was suggested that white vinegar could be used to remove the corrosion from the L-pad covers. In that case, the L-pad covers could be left to soak in the vinegar outdoors, while I work on other things and then be brushed off and rinsed with water while wearing a mask. I'm less inclined to try this with a woofer for fear of saturating the cloth spider and/or the vinegar weakening the adhesive used to attach the cone to the spider.

 

[MCM_Fan]

Regarding your original question - I don't think the lead length of the inductors wlll have any effect on the speakers performance.
The leads themselves will have negligible impact on the inductance - so leave them as is or shorten them if you just want to clean the board up a bit.

Thanks, I'll probably shorten them to tidy things up a bit. I found online calculators for self inductance and resistance of wire based on diameter and length. Based on some quick calculations, it looks like shortening the inductor leads by an inch or two won't have any significant impact on the inductor parameters.

 

[ivandezande]

Pretty sure components came to the factory for crossover assembly with long leads. It's easy for us to trim them and make things look good when redoing a crossover, but when longer leads doesn't make a sound difference, nobody cares what they look like, and you have to make thousands, making the crossovers neat, clean, and efficient probably wasn't a big deal.

 

[MCM_Fan]

Pretty sure components came to the factory for crossover assembly with long leads. It's easy for us to trim them and make things look good when redoing a crossover, but when longer leads doesn't make a sound difference, nobody cares what they look like, and you have to make thousands, making the crossovers neat, clean, and efficient probably wasn't a big deal.

I also think it was an inventory issue. JBL used the same inductors and capacitors in multiple designs. So, just order them all with leads long enough for the "worst case" rather than lead lengths "optimized" for each design. That allowed them to get maximum quantity discounts by aggregating quantities across multiple designs and eliminated the need to inventory multiple versions of the same part.

And, as you say, if the leads are longer than necessary in some designs, it's no big deal. Trimming and stripping leads takes time. So, it's more cost effective to just leave them at the factory supplied length when building crossovers by the thousands.

I only rebuild a handful of crossovers a year. So, I'll take the time to shorten them and clean things up a bit.

 

[CopperWizard]

You can always remove length but adding more is going to be work. Say you wanted to redo the crossover later than you have more flexibility. Something to consider.

 

[MCM_Fan]

You can always remove length but adding more is going to be work. Say you wanted to redo the crossover later than you have more flexibility. Something to consider.

I'm not really worried about that. The physical dimensions of the crossover boards for the 4301B are very compact, which limits the placement options. Once rebuilt, I don't plan on touching them for another 30 years.

If I ever did want to make significant changes in the future, it would either require new, larger crossover boards, or moving the inductors off board. In that case, adding longer leads would be simple compared to the rest of the work involved.

 

[teal'c]

FWIW - , My JBL Lancer 66's longest leads were on the caps. More than twice the length of any other connection.

 

[MCM_Fan]

I just refinished rebuilding the first 4301B crossover network. Here's what it looks like:

I oriented the inductors at 90 degrees to minimize field coupling and mutual inductance. I don't have a photo, but on the original crossovers, they were physically touching and placed parallel to each other. Not a lot of room to physically separate them, but at least now they aren't actually touching each other and oriented at the desired 90 degrees.

As you can see, I trimmed the inductor lead length to clean things up a bit. Thanks to all who responded for your input. I appreciate your assistance.