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Old 23rd January 2008, 07:06 PM   #11
Jay_WJ is offline Jay_WJ  United States
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How about my ER18RNX/27TDFC MTM design?

http://www.geocities.com/woove99/Spk...X_MTM_2Way.htm

I can easily change its BSC amount since it uses a simple 2-way xover.

The ER18RNX has exceptionally good power handling as a 7" hi-fi midwoofer, and lower distortions than other Seas Prestige 7" (e.g., CA18RNX), thanks to its improved motor. I think using this in MTM can meet your need.
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Old 24th January 2008, 07:52 AM   #12
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Quote:
Originally posted by Jay_WJ
How about my ER18RNX/27TDFC MTM design?
That looks pretty much perfect! How would one tweak the BSC to compensate for placement? (I should point out that the speakers are not each in a corner, but the TV is in a corner and the speakers are either side, about 3' apart inside edge to inside edge, and about 6" from a wall each (but that wall is diagonally sloping inwards behind them)).

Alex
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Old 24th January 2008, 03:42 PM   #13
Jay_WJ is offline Jay_WJ  United States
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The best way is to adjust BSC in your room by your ear. In fact, this is what any sensible DIY speaker designers do to find a proper amount of BSC for each design, which is affected by multiple factors and cannot be quantifiable by measurement.

For this MTM design, it's very easy to adjust BSC yourself. Start with my default design which has pretty much full BSC. By listening, if you hear too much bass and/or too heavy tonal balance in midrange, you can reduce BSC by unwinding the inductor in the woofer network and adjusting the padding resistor in the tweeter network. Read my notes about voicing speakers:

http://www.geocities.com/woove99/Spk...esigningXO.htm ("Voicing your speakers" section)

If you know the coil spec (gauge and the coil's physical size), the number of turns you need to take out to reach a certain value can be computed. I can do the calculation if you want.
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Old 24th January 2008, 08:04 PM   #14
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Let me throw out something from left field. This is an idea at this point, no one has developed it and no XO exists yet (a couple of projects including Tim Forman's latest indicate it is feasible)

Probably the new bass champ in smallish woofers is the CSS SDX7 (think next gen Extremis). With the XBL2 motor it has low distortion and goes low. 4 of these in a room will produce BASS.

Then take my current favorite tweeter -- the Fostex FF85k -- i think of it as a very good 3/4" tweeter with an overlarge surround (i admit i've not listened to a stock one yet). Fostex claims 32 kHz out of this, even if that is boastful, it still gets up there. The big thing is that you can XO it as low as 250-300 Hz (you can actually get an xMax limited 100 Hz out of it, but i only recommend that for use as a computer speaker).

2 SDX7 + FF85k would allow an MTM with an XO at or near the bafflestep. Both Tim & Al Wooley in his Apex III use the SDX7 up to 700 Hz -- 8.5" has an F3 for BS just below this, so one could just juggle things to get the BSC into the XO point. And with an XO that low, many of the issues with MTM go away.

dave
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Old 24th January 2008, 09:06 PM   #15
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Quote:
Originally posted by planet10
Let me throw out something from left field.
Just looked up those drivers - you really did mean left field!

Alex
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Old 24th January 2008, 10:53 PM   #16
Jay_WJ is offline Jay_WJ  United States
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Quote:
Originally posted by planet10
2 SDX7 + FF85k would allow an MTM with an XO at or near the bafflestep. Both Tim & Al Wooley in his Apex III use the SDX7 up to 700 Hz -- 8.5" has an F3 for BS just below this, so one could just juggle things to get the BSC into the XO point. And with an XO that low, many of the issues with MTM go away.

dave
A good experimental design. The SDX7 is interesting. I hope our favorite testers measure it soon. If its linear/nonlinear distortions are (even slightly) better than the Extremis, it'll be a strong contender on the 7" market.

I don't know about the FF85K. Not sure if it can give cleaner sound than our favorite Seas tweeters above 1 kHz. Although it's a fullrange driver, I'd not cross it below 500 Hz LR4 or 700 Hz LR2 when used in a design with high xmax midwoofers.
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Old 25th January 2008, 08:42 AM   #17
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One more question - I have a CRT TV so magnetic effects from the speakers are an issue. How can one add shielding to a cabinet?

Alex
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Old 25th January 2008, 10:31 AM   #18
sreten is offline sreten  United Kingdom
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Hi,

Steel sheet on the relevant side of the cabinet will reduce stray
magnetic field, the other option is fitting bucking magnets or of
course shielded drivers.

3 ft apart ? That just too close except for a small room and close
listening / viewing distance. Probably for such an arrangement
subs / satellites would be better then largish fullrange speakers.

I'm not sure what would be best, I'd say possibly slim floorstanders
with good dispersion to reduce localisation, but subs are needed.

/sreten.
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Old 25th January 2008, 06:14 PM   #19
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Quote:
Originally posted by sreten
3 ft apart ? That just too close except for a small room and close
listening / viewing distance. Probably for such an arrangement
subs / satellites would be better then largish fullrange speakers.

I'm not sure what would be best, I'd say possibly slim floorstanders
with good dispersion to reduce localisation, but subs are needed.
Bit of an underestimate - more like 3'6" to 4' apart, inside cab edge to inside cab edge.

I'm in a bit of a dilemma - I'm realising the 7" MTM design will be possibly a little too visually imposing but it will have the dynamic capability and bottom I want, and presumably the larger cones will have less dispersion and thus fewer problems with side wall reflections than smaller woofers. It's also relatively simple, using my existing amp and a basic passive crossover.

On the other hand, maybe a very slim floorstander with TM design, using something like a 5" midbass would do the job, in a sealed box and designed to cross over to an active subwoofer. I was previously looking at adding a sub to my existing speakers using an XLS 830500 with a 200W plate amp.

Alternatively are there any full range designs that could work in a line array with such a sub?

Alex
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Old 25th January 2008, 07:57 PM   #20
Jay_WJ is offline Jay_WJ  United States
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Size of the midwoofer has nothing to do with the speaker's dispersion characteristic if the woofer is used with a xover frequency that is low enough to avoid its beaming effect.

If you want a seald design, 7" is a minimum size that can used with a subwoofer with 80 Hz xover. I wouldn't cross a sub higher than this.

You can build my ER18RNX/27TDFC MTM in a sealed cabinet that can be only 8" wide and less tall than my default design. It'll need a sand-filled bottom chamber to reduce the box volume, if you want a floorstander. Or you can build a standmounter. If you want to go this route, I can calculate an optimal volume for a sealed box for you.
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