Nelson Pass L300 Xover concerns … ?

[paul79]

Hi all,

Pondering doing the crossovers in a set of L300's to the Pass design.

Some things are not clear to me:

The reverse phase of the midrange. I believe a lot of JBL woofers are reverse phase from standard, not sure of the high frequency drivers being in proper phase either. There is no mention from Pass regarding this, and I have some concerns about that being taken into account. Because if not, then the woofer and mid may actually be wired in phase with each other. Curios if Nelson could answer this?

Also, he mentions we can use the original Lpads, but the revised NP schematic shows using 8 ohm Lpads for the mid, where the original midrange Lpad in the cab is 16 ohm. So another point of confusion for me.

Anyone have any experience with the Pass crossover in the L300 that can shed some light? @Nelson Pass ?

Thanks very much
Paul

 

[AllenB]

Perhaps this would help..
https://audioxpress.com/article/JBL-L300-Modification

 

[paul79]

That is the article in question. No mention of how JBL woofers are typically reverse phase from standard, and if that was not mentioned, might not have been taken into account, and the mid and woofer may actually be in phase with each other instead of reversed. Just something I don't see clarified.

Also the question of using the original 16 ohm Lpad for the mid vs. the 8 ohm written on the new revised schematic. This is a difference that should require the use of different components in the mid circuit depending on the Lpad used, no?

I feel these are good questions.

Thanks
Paul

 

[AllenB]

At least in this version of the article the mid has a 16 ohm Lpad. The schematic also shows the wire colours. It looks as though only the tweeter is reversed.

 

[paul79]

On the original JBL schematic shown, yes. On the NP schematic, it shows 8 ohm Lpad, but he does not say that is what he used, just that he left the Lpads in there. That is the part that confuses me, in that did he leave the original 16 ohm Lpad in there and the schematic is typo? Or?

 

[6sX7]

why not try it and find out?

 

[paul79]

Because I will always question it no matter, and if it is shared for DIY, it should be clear I think. Just hoping I am not the only one concerned with this, and the only one without measurement equipment to verify this, so it can be solidified. @Nelson Pass

 

[UserAbuser]

If your questioning it, then you must be able to work out a better way. Yes.

 

[WhiteDragon]

If the phase was incorrect
their would be a obvious null in the response.

So would assume schematic shows correct wiring.

 

[Island boy]

Hopefully Nelson pass could shed some light on this, as I’m thinking about doing the same to my 4333a’s

 

[Island boy]

Is there anyone who has used Nelson’s design with success?

 

[AllenB]

There is a way to continue using a 16 ohm Lpad and have it behave as an 8 ohm Lpad. That is to use a 16 ohm resistor in parallel with the entire 16 ohm Lpad (just to the left of it on the schematic). The reason this makes sense is because an Lpad tends to make the driver appear resistive anyway, varying depending on how much attenuation you select.. so there's no point being too concerned about precision within reason. If you want to do that then you can try at least beginning with it this way, which should still more or less demonstrate the intended result within the expected range, and then tweak it. If you use fixed resistors after taking measurements or simulating then you can create a true 16 ohm Lpad easily enough.

Here is the original article if I'm not mistaken. It's been a while since I read it but would be happy to answer any questions about it until the man himself arrives.

 

[Island boy]

I have just completed Nelson’s crossover for my 4333a’s I’ve left the original crossover and L pads inside, but just disconnected, and made the new crossovers external with new 8 ohm L pads as per Nelson pass design, the sound is superior compared to the originals, but I need to turn the mid range 8ohm L pad right down, as it’s to bright ,
Could I place a 16 ohm resistor in parallel in front of the L pad to help fix this ?

Dean

 

[AllenB]

When you turn down the 8 ohm midrange Lpad, can you get it to a good level? If so, what is it you're trying to fix?

The next thing to say would be that you want to keep it near 8 ohms so the crossover before it filters the response as it was intended. A resistor in parallel at that location may not be what you're looking for. I'll wait for you to respond to the first question before making a suggestion.

 

[Island boy]

Hi Allen

Thanks for your response mate ,

Responding to your first question, I do get it to a good level , but more trying to have the L pad control more centre, I think it’s a OCD thing with me 😳
What about I source 16 ohm L pads, would they work with Nelson’s design?
Or just leave good enough alone, as they do sound fantastic?
Thanks again
Dean

 

[AllenB]

but more trying to have the L pad control more centre,

Great. The thing is that an Lpad can in theory attenuate between 0 and infinity. A practical Lpad may not attenuate that much but there's nothing wrong with the need to go closer to the maximum it allows.

In some cases a control may be more or less sensitive to use near the edges but if this isn't a problem for you then it's all good.

On the other hand if you want precision, or if you can't match left and right, or if the controls have flat spots in their rotation then you can measure their series and parallel parts after removing them from the circuit, and replace them with fixed resistors.