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Old 12th September 2006, 09:12 AM   #1
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Default Passive sub for Fostex BLH

Some time ago, I built this wonderful Fostex FE208EZ Nagaoka BLH. (D-3MKII.1) which is a smaller (and hence somewhat WAF-friendlier) BLH.

The result is really great across the frequency spectrum, including a superb mid-bass. Sadly however, deep bass is virtually non-existent.

To overcome this, I hooked up an active sub to the system, but got into trouble with hum (other post in Tube section).

Would it be possible to hook up a passive sub ? If so, could someone make a suggestion on what driver to use ? Also, what would be needed as crossover ?

Another possibility might be changing to another speaker enclosure with more bass extension, but I can't really find a non-BLH suggestion using the FE208EZ. Any ideas or will I need to stick to using horns.

I understand that the logical way out is building a bigger BLH, but there's always the chance that my wife decorates those with the chainsaw, so I'd rather try something else first.

Thanks,

Gert
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Old 12th September 2006, 10:05 AM   #2
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No reason why you couldn't though the crossover point will depend on exactly where the D3s roll off. You might need two subs, depending on how high it is -you don't want to be running a single sub over 75Hz if possible.

How about an MLTL? Here's a rough one. Internal dims
42in tall, CSA 131.25in^2, Zdriver 8in, port 3in diameter, 2in long. 4ohms series resistance applied. Not as sensitive as the horns, and might not have the dynamics, but it should still sound quite nice. The roll-off is useful to counter room-gain.
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Old 12th September 2006, 11:29 AM   #3
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Thanks,

I searched a bit on the MLTL enclosure and found this thread.

You talk about baffle-step compensation there. Could you elaborate ?

Gert
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Old 12th September 2006, 02:00 PM   #4
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Baffle step diffraction occurs at a frequency that is determined by the width of the baffle -the wider the baffle, the lower the frequency. Below this point the response drops by about 3db, meaning you either need to use active Eq to boost everthing below that point, or reduce everything above that point, either with Eq or a simple passive circuit, to flatten the response curve out. That's two to play with! I forget how many sims I've done...
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Old 13th September 2006, 08:05 AM   #5
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Right.

Actively equalising this is out of the question for now.

From the 208 data sheets I find that Re = 6.4 and Lvc ( = Le ?) = 0.0517 mH

If I calculate using the formulae from the BSC_Sizing article of Martin :

Given the baffle width of slightly over 11 inch and a required (guess, guess) attenuation of 3 to 4 dB I get to :

Rzobel = 8 Ohm
Czobel = 0.52 uF
Rpar = 3.2 Ohm
Lbsc = 1.23 mH

Questions :
- is Lvc indeed equal to the published Le ?
- how do I know what attenuation to go for ?
- am I on the right track here ?
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Old 13th September 2006, 10:00 AM   #6
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Yup, right track. About 3db is a good starting point -you can alter the attenuation by changing the value of the parallel resistor. And yes, Lvc is equivalent to Le.

Scott
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Old 14th September 2006, 11:04 PM   #7
joval1 is offline joval1  New Zealand
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Does any body know what the Le value is for the 206? The specs docs that goes with it says "n/a". Am also building a bsc and am using Le=0.4 at the moment.
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Old 15th September 2006, 07:54 AM   #8
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Scott,

In another thread (FE208EZ enclosure?) you state the dimensions of the enclosure to be :

Quote:
This enclosure is 42" tall, 9.5" wide, 11" deep. The centre of the driver is 11.25" down from the internal top. The port is 4" wide, and 2" long, and is located 4" up from the internal bottom. 0.25lbs ft^3 stuffing is assumed from the top of the enclosure to 12" from the base.
This is rather different from the dimensions stated above, although the exact same driver is used.

Could you please verify which of those are correct ?

I'd really like to try one of these.

Thanks,

Gert
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Old 15th September 2006, 08:21 AM   #9
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Both. They're different cabinets like I mentioned above. I forget how many I've simmed so I sometimes end up doing another for the same driver. Of the two, I'd go for the eariler one, mentioned on the other thread.
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