L-Pads, 8 or 16?

[dmtp]

As discussed many times, JBL's use of 16 or 8 ohm designation seems arbitrary at best. My question is this: I am building an XO for a LE85. This is designated a 16 driver by JBL and has a DCR of 6.4. JBL's XO show a 16 L-Pad (e.g. 200B). i am guessing that I should go ahead and use a 16 L-Pad even though it seems that 8 would match the driver better.
Any comments, suggestions? Thanks!

[Earl K]

Hi


Any comments, suggestions? Thanks!

- Giskard reported some time ago that JBLs' usage of 16 ohm variable Lpads, was most likely driven by the needed power handling ( for the device ) . ie; The inferral was; Back when a 2420/Le85 was current product / 50 or 100 watt , 8 ohm Lpads , were unavailable . That does seem a credible enough explanation to me .

- Personally, I'd use the 50 watt, 8 ohm variety . Though it really does depend on what sort of voltage drive curves you are trying to achieve . The 16 ohm type will offer greater impedance fluctuations ( within its' normal rotation ) when connected to an 12 ohm driver. This can be "helpful" as part of the design "tool-kit" / say for instance; if one wants to include a "bump-filter" that grows in magnitude when the variable Lpad is turned down .

- Whatever value Lpad you decide to go with / learn how to measure the AC impedance of the driver/horn combo ( with the Lpad in place ) / and then do your calcs. for your crossover design. Otherwise, you're just building on an unknown foundation.


:)

[dmtp]

I have the set-up Dickenson describes for measuring impedance on woofers (using a signal generator and a DVM with a reference resistor to calibrate). WIll this also work to measure impedance of the horn/L-pad or are you referring to a different set-up?
Thanks

[Mr. Widget]

I haven't compared the impedances of the LE85 (2420) to the 376 (2441) but I do know that when JBL did the 4355 they used a 16 ohm L-pad on the 2441 and an 8 ohm L-pad on the 2405. My inclination would be to follow Earl's advice about determining your combined impedance before designing your network, but I would go with the 16 ohm L-pad.


Widget

[Earl K]

I have the set-up Dickenson describes for measuring impedance on woofers (using a signal generator and a DVM with a reference resistor to calibrate). WIll this also work to measure impedance of the horn/L-pad or are you referring to a different set-up?

- Yes, that method will work well . You'll need to plot out the different values to see the curve .

- A more costly approach ( less than $ 200. ) is to buy "Woofer Tester 2" (http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=390-802&CFID=136133&CFTOKEN=51263407) from Parts Express . This software / hardware bundle ( PC only ) will do your impedance tests, as well as obtain TS parameters for your woofers .


:)