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Old 25th January 2007, 12:30 AM   #1
Theli is offline Theli  Canada
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Default crossover L-pad modification

Hi all,

I'd like to add a second identical woofer (a 2 way with an Adire Extremis 6.8 and a Seas 27TDFC) to a design I just built . But I need to find the values of the replacement resistors needed to replace the current resistors with in the L-pad and how much less attenuation I need (3dB or 6dB?) ?

I have the loudspeaker design cookbook and looked around online but can't confidently make a good decision.

Soooo, is there any rule of thumb when trying increase/decrease attenuation with a L-pad? And I don't want to bother the designer just yet...

hopefully this makes sense..

Mike
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Old 26th January 2007, 09:47 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I don't think you should be considering padding down the bass units.

I see two alternatives.

run the two bass drivers in true parallel and alter the crossover to suit the halved impedance that the crossover sees.
or
run the second bass driver only at the lower frequencies and use a single pole or two pole low pass filter to roll it in at your chosen frequency.

Yes, it will raise the bass content of the speaker and yes the other frequency ranges may need to be INCREASED to compensate and make the speaker sound balanced. The tweeter may already be padded down.
However, if you roll in the extra bass driver very low down (say about 200Hz) then the effect may be quite pleasant and very little correction may be required.

Something to think about. The extra driver will lower the speaker impedance in the bass region. The amplifier will have to drive that lower impedance. A solution would be an extra amp just to drive the extra bass unit attached to it's own speaker terminals.

To answer your first question is it 3db or 6db. Depends.
At high freqencies the two drivers destructively interfere with each other and create lobing of the soundfield.
At middle frequencies the sensitivity increases by a sloping curve from +3db to +6db (+0db to +3db from the speaker and +3db from the halved impedance).
At the lowest frequencies you get +6db from the pair IF THE AMPLIFIER CAN DRIVE EXACTLY +3db POWER INTO HALF IMPEDANCE. See that previous paragraph.

The definition of low, middle & high depends on the spacing of the two drivers.
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Old 26th January 2007, 04:00 PM   #3
Theli is offline Theli  Canada
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Quote:
Originally posted by AndrewT
Hi,
I don't think you should be considering padding down the bass units.



Sorry, but you misunderstand. I want to "un"pad the tweeter to bring it back to the same relative level of the woofer(s). In other reduce the amount of attenuation of the tweeter.


Quote:

run the two bass drivers in true parallel and alter the crossover to suit the halved impedance that the crossover sees.
This is what I'm trying to do.


Quote:
Yes, it will raise the bass content of the speaker and yes the other frequency ranges may need to be INCREASED to compensate and make the speaker sound balanced. The tweeter may already be padded down.
Yup, the tweeter has been padded a lot. The original design has an 80dB effiency


Quote:
However, if you roll in the extra bass driver very low down (say about 200Hz) then the effect may be quite pleasant and very little correction may be required.

Something to think about. The extra driver will lower the speaker impedance in the bass region. The amplifier will have to drive that lower impedance. A solution would be an extra amp just to drive the extra bass unit attached to it's own speaker terminals.
Adding a second quality amp requires space/money. Adding a second woofer in parallel seemed a better solution for me.

Quote:
To answer your first question is it 3db or 6db. Depends.
At high freqencies the two drivers destructively interfere with each other and create lobing of the soundfield.
At middle frequencies the sensitivity increases by a sloping curve from +3db to +6db (+0db to +3db from the speaker and +3db from the halved impedance).
At the lowest frequencies you get +6db from the pair IF THE AMPLIFIER CAN DRIVE EXACTLY +3db POWER INTO HALF IMPEDANCE. See that previous paragraph.

The definition of low, middle & high depends on the spacing of the two drivers.
I think I'm starting to understand. To determine the right crossover mods (with a parallel setup), would require some measurements to get it right.

It would seem more powerful amp would be a better solution if I wanted more volume and something seperate for more bass.

thanks for the input, its given me something to think about.
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Old 26th January 2007, 04:16 PM   #4
sreten is offline sreten  United Kingdom
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http://www.rjbaudio.com/Extremis/extremis.html

Hi,

Is presumably the design in question.

Adding another bass driver will not extend bass, will just go louder.
So are you giving it enough juice ? More power is the easy option.

Modifying the crossover has plently of pitfalls if armed with a cookbook.

/sreten.

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Old 26th January 2007, 04:17 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
starting at the other end, I was suggesting an extra bass driver AND and extra amplifier, but only if the budget can stretch. But check your existing amp can drive two bass units. Your budget would be awol if you blow that one up.

Back to the front end of the enquiry.

The mid range will be completely messed up if you add an extra bass driver AND it extends up into that region.
Quote:
...At high freqencies the two drivers destructively interfere with each other and create lobing of the soundfield.
what frequency band does the single driver cover?
This may answer whether you parallel two bass drivers or go for the other option i.e. to roll in the extra bass driver at low frequencies only.

If the mid frequencies don't get boosted then you won't need to increase the treble either. But stuck with that low sensitivity.
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Old 26th January 2007, 05:52 PM   #6
Theli is offline Theli  Canada
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sreten, you are correct , that is the design. I'm happy with the bass that I get from it, I was just hoping to increase the sensitivity. But I think I need to learn more before I go that route, or a different design. Ces't la vie...

Maybe I'll try adding a couple of TB w6-1139's to the mix as Andrew suggested and see what happens. Thanks for the feedback.
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Old 26th January 2007, 06:00 PM   #7
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I can't see why you would combine the TB's with the Adire. Both are good drivers in their own right and they have a similar LF response but the Adire goes higher and can act as a mid bass. The TB cannot and it's extension is no better the Adire. What was your plan?

I don't see where Andrew suggested this.
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Old 26th January 2007, 06:31 PM   #8
Theli is offline Theli  Canada
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Quote:
However, if you roll in the extra bass driver very low down (say about 200Hz) then the effect may be quite pleasant and very little correction may be required.

It was just a thought, not necessarily something I'd implement. Musing out loud , I suppose. And because I have a pair of the TB laying around.
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