Where Do I install an adjustable L-pad in this circuit?

[cdy21792179]

I just ordered a 4" sheet of 30 PPI foam trom Swisstropics. I plan on installing it in the horns of my 3677s to hopefully get rid of the little bit of HOM or Honk it has.

I know it'll attenuate the CD about 2-3db so I want to bypass the resistors in the Crossover and and use a parts Express adjustable L-pad (the same one used in the econowave Xover) to bring my Cd level back up level with the woofer.

I'm a Xover newbie and I've been reading up on it. I'm thinking I should remove R4, R5, R6 from the network and put my l-pad here. From what I gather these are the CD's attenuator resistors. R7 and R8 look like it may also be part of the attenuator circuit. I probably don't need to remove them all.

But anyways am I on the right path here?

The Xover schematic is linked here.

http://www.jblproservice.com/pdf/Theatre%20Series/3677.pdf

Thanks

[cdy21792179]

By the way...I just checked the Xover (now that I'm home and can look at it) and all resistors on the board are 10w resistors. So If what I read is right and every 10w of resistance about 3db I can just remove 1 resistor out of the L-pad. The speaker is 99db sens. as is the woofer itself. So I'm guessing the CD which is rated at 109db should have 3 10w resistors on it to knock the SPL down. Correct?

[billfitzmaurice]

But anyways am I on the right path here?
No. R4, 5 and 6 are part of a shaping filter in conjunction with C4. Install an LPad post high pass filter/pre driver. It must be the same impedance as the driver.

[cdy21792179]

Thanks Bill,

The only thing is I need to remove attenuation because the foam is going to add more attenuation so I need to increase the level of the CD. I should have about 10db of attenuation on the CD somewhere in that circuit.

I may be able to just remove one and not need a variable l-pad.

[Chris Roemer]

Thanks Bill,

The only thing is I need to remove attenuation because the foam is going to add more attenuation so I need to increase the level of the CD. I should have about 10db of attenuation on the CD somewhere in that circuit. All of the resistors are 10w. I think I may be able to just remove one if i can find out which ones are in the attenuator circuit.

Because they're all 10w resistors and everyone I remove should increase the CD about 3db , I may be able to just remove one and not need a variable l-pad.

Your understanding of attenuation resistors is wrong. "Watts" don't attenuate, ohms due. R4/5/6 is a parallel pack of 3 24ohm resistors, acting like a single 30w 8 ohm resistor. If you take one out, you end up with MORE attenuation and LESS power handling. You'd have a 20w 12n resistor which will cut the output more.

If you mess with R7/8, you'll screw up a "notch" filter (just like Bill said).

I'm not sure what the impedance (or resistance) of the 2 "lamps" are, but if they're higher than the 16n resistor (R1), then I think you could put a variable resistor or L-pad in place of R1. It appears to be acting like a regular "amp side" series attenuation resistor.

Chris

[cdy21792179]

Your understanding of attenuation resistors is wrong. "Watts" don't attenuate, ohms due.
Chris

That's what I meant to say above. I read that for every 10ohms of resistance ( a 10ohm resistor) it would attenuate about 3 db. I'm not sure how correct that is but thats what I meant. My earlier post was just a brain fart:D and I confused myself. Sorry for the confusion.

Thanks Chris your breakdown makes it all clear to me now as to what each section is doing. As for the Resistor R1, I see what you and Bill are saying. I'm still a little worried I may blow the fuses, but if I'm thinking right ,reducing the resistance of R1 to increase CD output should actually send less of the load throught B1 and B2 as more will flow thru R1 because it'll have less resistance, correct?

[Chris Roemer]

That's what I meant to say above. I read that for every 10ohms of resistance ( a 10ohm resistor) it would attenuate about 3 db. I'm not sure how correct that is but thats what I meant. My earlier post was just a brain fart:D and I confused myself. Sorry for the confusion.

Thanks Chris your breakdown makes it all clear to me now as to what each section is doing. As for the Resistor R1, I see what you and Bill are saying. I'm still a little worried I may blow the fuses, but if I'm thinking right ,reducing the resistance of R1 to increase CD output should actually send less of the load throught B1 and B2 as more will flow thru R1 because it'll have less resistance, correct?

I've SEEN bulbs in pro XOs before, AFAIK they're just there to shed power. Bill probably understands their exact function, but I don't. I think to model them I'd have to know their resistance. If you have this unit, maybe you could measure the DCR of the bulbs for us?

[cdy21792179]

I think to model them I'd have to know their resistance. If you have this unit, maybe you could measure the DCR of the bulbs for us?

I actually had my meter in my hand earlier to do just that. Then I wondered if sending the 9v through them from the meter may blow the fuses. It probably won't...I'll go check.:D

After looking at the schematic I'm not sure I'll get an accurate reading of them in the circuit, i'd probably have to remove them.

[billfitzmaurice]

I'm not sure what the impedance (or resistance) of the 2 "lamps" are, The resistance of the lamps increases with temperature, that's how they work as voltage limiters, and why they can't be placed post-filter.
BTW, speakers of this sort tend not to use LPads because they shed power as heat, and in pro-sound heat is the enemy. Response should be adjusted via EQ, and in pro-sound 31 band EQ isn't an option, it's a necessity.