Can an Lpad replacement change sound that much?

[Thocom]

I recently changed out the l-pad on one of my JBL C38 speakers with the recommended, 15W, 8ohm Dayton Audio Lpad from Parts Express.

While it is working (the old one had to be fiddled with to make work and pass the highs) the speaker seems to have lost its life. Side by side with the other speaker, the old one sound MUCH better.
Originally, I had taken the LPads apart and cleaned them, but only really one side. They are two monos that are sandwiched together, so I could only get access to one side. I also removed much of the grease inside there.
I'm going to try again, and this time try to take them completely apart, and re-grease them. What would be the best grease/lubricant to use? I have the deOxit, and following up with their green Faderlube product. What else?

Thanks in advance.

 

[bowtie427ss]

I'd think the grease should be non-conductive in the event that it migrates, although i really have no specific suggestions.

It is very difficult to find exact replacements for vintage JBL and Altec L-pads, their design and construction was specific to their application and i know of no replacements made today.

I suspect the use of a more generic L-pad requires the addition of a couple fixed components(resistors) to get it's values closer to those of the original.

Best bet is to rehab the originals whenever you can.

 

[doctorolds]

I personally use dielectric grease from the auto parts store. After I disassemble and clean them thoroughly I apply a medium coat, not to thin or thick, use your judgement. I have had excellent results, just my 2 cents.

 

[onplane]

I recently changed out the l-pad on one of my JBL C38 speakers with the recommended, 15W, 8ohm Dayton Audio Lpad from Parts Express.

While it is working (the old one had to be fiddled with to make work and pass the highs) the speaker seems to have lost its life. Side by side with the other speaker, the old one sound MUCH better.
Originally, I had taken the LPads apart and cleaned them, but only really one side. They are two monos that are sandwiched together, so I could only get access to one side. I also removed much of the grease inside there.
I'm going to try again, and this time try to take them completely apart, and re-grease them. What would be the best grease/lubricant to use? I have the deOxit, and following up with their green Faderlube product. What else?

Thanks in advance.

Any chance you could post the original schematic for the xovers in your JBL's?

Regards,
Jerry

 

[Thocom]

Any chance you could post the original schematic for the xovers in your JBL's?

Regards,
Jerry

Sure, it's the N2600

http://www.jblproservice.com/pdf/Network Schematics/N2600.pdf

I completely took the Lpad apart today, one side was easy, just removing the cap, but the other side...

Had to remove the c-clip, it was too involved for what it was, but I got it all cleaned out with Propanol.
I just used deOxit, then faderlube when it was all back together. Letting it sit then will test it on a meter.

 

[onplane]

From the schematic it looks like JBL used a standard L-pad. I don't know. I've completely given up on "controls" put inside a speaker.

They all eventually fail, and prior to failure, they corrode and add resistance where there should be none.

My solution is to by-pass and bi-amp. This way you move the controls outside the speakers to volume controls on amplifiers.

Regards,
Jerry

 

[Thocom]

From the schematic it looks like JBL used a standard L-pad. I don't know. I've completely given up on "controls" put inside a speaker.

They all eventually fail, and prior to failure, they corrode and add resistance where there should be none.

My solution is to by-pass and bi-amp. This way you move the controls outside the speakers to volume controls on amplifiers.

Regards,
Jerry

That's probably the way to go, although, I've seen mods on these that just install a fixed resistor. I think nothing there would be like driving a spike through my head with the 075 tweeter.

 

[WaynerN]

First of all, you didn't show how you wired the new L pad in.

Second of all, its called an L pad because it puts resistance in series and in parallel with the driver. In one extreme, it puts all of the resistance in series with the driver, in the other extreme it put all the resistance across the coils, and in between, splits the resistance between series and parallel.

 

[onplane]

That's probably the way to go, although, I've seen mods on these that just install a fixed resistor. I think nothing there would be like driving a spike through my head with the 075 tweeter.

If you are serious about bi-amping, you don't need any resistor at all. The purpose of the l-pad is to absorb energy such that the tweeter is starved and puts out less SPL. When you bi-amp you control power in the tweeter not by power absorbtion, but through power output of the amplifier.

In short, you make the speakers sound as you like, and you can adjust "voicing" on the fly.

Regards,
Jerry

 

[I LIKE MUSIC]

WaynerN is correct.

If one uses just a series resistor to attenuate the tweeter, it can result in a shifting of the crossover point. The point of an L-Pad is to present a constant impedance that is not dependent on the amount of attenuation. This helps to minimize any shift in the crossover point with changes in the attenuation.

If you stay with a passive crossover and want to eliminate the variable L-Pad you can use to resistors to make an L-Pad. See the picture below.



For 6 dB of attenuation with an 8 Ohm speaker R1 would be 4 Ohms and R2 would be 8 Ohms. You could adjust the L-Pad for the sound that you want, remove it from the crossover and measure the series resistance and the parallel resistance and use resistors of the same value.

Bi-amping with an active crossover is not a bad idea either.

 

[onplane]

For 6 dB of attenuation with an 8 Ohm speaker R1 would be 4 Ohms and R2 would be 8 Ohms. You could adjust the L-Pad for the sound that you want, remove it from the crossover and measure the series resistance and the parallel resistance and use resistors of the same value.

Bi-amping with an active crossover is not a bad idea either.

Actually, what I was suggesting was passive bi-amping and a complete by-pass of the l-pad. Notice that this is no different than the l-pad being full on. The 8 ohm driver will still be seen as an 8 ohm load by the xover.

Regards,
Jerry

 

[I LIKE MUSIC]

I was responding the comment about using a single resistor in place of the L-Pad.

I fully understand bi-amping as applied to full active bi-amping and bi-amping using a speaker's built in crossover and removing any existing attenuation other than the frequency sensitive attenuation supplied by the crossover.

It is just information for those that might be interested in how things work.

 

[onplane]

I was responding the comment about using a single resistor in place of the L-Pad.

I fully understand bi-amping as applied to full active bi-amping and bi-amping using a speaker's built in crossover and removing any existing attenuation other than the frequency sensitive attenuation supplied by the crossover.

It is just information for those that might be interested in how things work.

Ahhh, now I understand what you are saying. Further, it's a very valid point. If you plan to attenuate, you must use two resistors! A single resistor will alter the xover frequency!

Thanks for the clarification, as this is an important nuance for those not bi-amping, yet still giving up on the internal controls.

Regards,
Jerry

 

[Thocom]

First of all, you didn't show how you wired the new L pad in.

Second of all, its called an L pad because it puts resistance in series and in parallel with the driver. In one extreme, it puts all of the resistance in series with the driver, in the other extreme it put all the resistance across the coils, and in between, splits the resistance between series and parallel.

I directly replaced the JBL Lpad in the crossover, I didn't just add an Lpad. I wired it just like the old one was wired in (yes, I triple checked I had the wiper in the correct location).

 

[Thocom]

WaynerN is correct.

If one uses just a series resistor to attenuate the tweeter, it can result in a shifting of the crossover point. The point of an L-Pad is to present a constant impedance that is not dependent on the amount of attenuation. This helps to minimize any shift in the crossover point with changes in the attenuation.

If you stay with a passive crossover and want to eliminate the variable L-Pad you can use to resistors to make an L-Pad. See the picture below.

View attachment 893450

For 6 dB of attenuation with an 8 Ohm speaker R1 would be 4 Ohms and R2 would be 8 Ohms. You could adjust the L-Pad for the sound that you want, remove it from the crossover and measure the series resistance and the parallel resistance and use resistors of the same value.

Bi-amping with an active crossover is not a bad idea either.

This may be what I try. But, I completely took apart the old one (that appeared to have a few dead spots) and cleaned it thoroughly. It doesn't have any location in the dial that looses connection now.

 

[totem]

I can't see how an equal rated Lpad would sound different than the original.

 

[Thocom]

I can't see how an equal rated Lpad would sound different than the original.

Yeah, me either. Trust me, I'd be skeptical too, but hand to God, it is.

 

[bowtie427ss]

I can't see how an equal rated Lpad would sound different than the original.

It wouldn't.

The new L-pad isn't equal.

You won't find a new L-pad that is equal to an original JBL or Altec. You must preserve the originals if you want to preserve the original sound.

A simple 8 ohm(or 16ohm) one size fits all L-pad is not a direct replacement for the ones used in vintage Altec and JBL systems, particularly those used to attenuate compression drivers. You will have to defer to one of the respected elders of Altec or JBL inclination if there are any left who will post here.

 

[toxcrusadr]

So you've got it wired the same, and it does attenuate when you turn the knob, I assume. It's just that it's missing some sparkle in the treble? Or what does "lifeless" mean?

Did you check the new L-pad and verify it is really 0 to 8 ohms on both sides?

 

[toxcrusadr]

It wouldn't.

The new L-pad isn't equal.

You won't find a new L-pad that is equal to an original JBL or Altec. You must preserve the originals if you want to preserve the original sound.

Did they use some kind of unobtanium resistive wire in those things, or what?