Passive 100hz Xover build help

I would like to add a sub to my main speakers. The system sub I have, collecting dust, is part of the Fried model H . Its a 2ch dual transmission line woofer and because of its size, its affectionately called the Coffin.
The Xover for it and the 2 satellites are located in an external Xover box. The woofers are 8ohm Kefs B200's.
That sub goes way down and the LF is robust, articulate and very smooth. I've always missed their bass performance. You can find a full description here:

My 4ohm main speakers here:

The monster coils form a 100hz Xover. I would like to try and make a first order 100hz xover and see what it sounds like when paired with my mains: Salk/Fried commemorative tower speakers. My amp is 550w, so plenty of juice there and all speakers will be fed from it. I know there will be a lot of overlap and efficiency concerns but would like to try this inexpensive option without damaging the mains or the amp. Perhaps an L-pad?
I'd appreciate all the help offered.

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Dave is on top of things for sure. He has offered me several options on how to integrate my Fried woofers with my main speakers.
I thought placement of this particular issue here, would attract a specific knowledge base necessary to handle this one option of 4 I'm exploring. Thanks in advance!
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Overlap is a good thing for modal diversity (the mains aren't or are sparsely crossed).. except where the reason for using subs is to reduce the excursion of smaller speakers, which is often done commercially for cost saving purposes. In DIY there can be many reasons. I'd consider what you need, then how to do it.
if I understand you correctly, I'm not sure if I want to go into that territory that you call modal diversity just quite now. if you have a 100 Hertz signal emanating from four different locations in the room, who knows what the acoustical effect is really going to be. It would be predicated on the rooms unique dimensions and whatever type of sound reinforcement you currently have or don't have.
I would hope that what I hear will be manageable as I'm aware of my rooms frequency response and the specific frequency issues along each wall surface.
Being a relatively low cost option in which to supplement my lower end I see it as a no brainer to try it and see how it sounds.
Sorry about that, somehow I accidently posted it b4 I was done..

Yes, Id like to use only the Fried subwoofer for more bass while playing my main speakers.

I'd like to pull the huge coils from the Fried Xover to create a simple bandpass filter for those subs. An online Xover calculator shows that coil at 15.1mH is good up to 84Hz. Is it correct to say that only that coil in series with the subwoofer driver of 8ohms is all that's necessary to play music from 20hz to 84Hz? If so, then that's done. If not, what would it be.

Secondly the hook up of now, two sets of speakers from my main amp. Will that connection hurt my amp? Will it be stable?

The sub is 8ohms and main speakers are 4ohms. Each amp channel will have 2 wires each on every pos and neg output terminal. Is that a parallel hookup and what would be the combined impedance now to determine if my amp can safely handle it.

Thirdly, efficiency concerns. If the subwoofers output is not loud enough, its a lost cause I imagine but if the output is too high while playing the mains simultaneously, would a simple L-pad in series with the inductor coil work to decrease its volume?
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for those subs
Are there two subs? Is it a single sub with dual voice coils? or just one sub? This is necessary to know before answering.

If the subwoofers output is not loud enough, its a lost cause I imagine but if the output is too high while playing the mains simultaneously, would a simple L-pad in series with the inductor coil work to decrease its volume?
At the low end either a change in inductance or adding resistance may work.

Is it correct to say that only that coil in series with the subwoofer driver of 8ohms is all that's necessary to play music from 20hz to 84Hz?
Broadly speaking, yes. Each case has it's own exceptions but at least you have a place to start.
The "sub", as you call it, is actually a single cabinet split lengthwise to form 2 separate TL woofer channels, each with a single driver, Kef B200. I guess i;m being confusing by calling it a sub since in reality it will be functioning as one to my main speaker system when im done. No single driver with dual voice coils. 2 drivers total, one for each channel. here ya go:

Kef B200 specs:

If what you say is correct, that the low pass would be just the coil in series with the B200,If I were to add an 8ohm resistor in parallel, tje amp would to lock it in?? see it as 4ohm....8+8/2=4 Would that resistor change the Xover freq?

The reason I ask is that each sub or woofer drivers is 8ohms and the mains system Xover are 4ohms. Off the bat, it sounds like the sub wont be loud enough especially since the sensitivities are 96 spl db @ 8v @8Ohm for the B200 and 88db @3v @ 4ohm for my main speakers.
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Ok. If I were in your shoes I'd connect the subs through the inductors and test them for level. Don't turn them up too far since it's unclear whether they would show a low impedance at the lower frequencies. You'll be interested in high passing the mains at some stage to take care of that.

The next step depends on the result of that. Changing the levels, either way, or even high passing the mains requires some thought. If it turns out the best option is to get another amp, that might save some trouble in other ways.
I can do that this coming week, thanks. BTW Allen, I;m not able, just yet to get my hands on those plate amps i thought i had but I do see one and I saw an old SubWoofer i built about 15 years ago i forgot i had. The spare basement room became an unorganized and massively stuffed dumping ground.... I doubt they're identical bc i built 2 at first being identical and gave one away to a friend who really wanted it. So, there are at least 2 subamp plates I have to at least try the other option you came up with if this doesnt work out.
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Out of curiosity, to save some wire, what about a different hook up to the speakers from my amp:

Amp outputs to my mains as they currently are but then hook up to the subwoofer TL's. positive on the main speaker to positive on the subwoofer speaker and so on since they're only 5 feet apart instead of a 20ft run. Thats still the same parallel hookup as before, right?
Heres that behemoth that's just too big and heavy to put somewheres else, that I figure it's time to deal with. Even though my main are flat down to 40 these should give me what I've been missing. the coffin tests flat down to 16hz and Ive been trying to make my room get better to hear down to 20.
Thanks again for all your help. be in touch next week.


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I know of it. Ive been doing woodworking as a hobby for 30 yrs i guess. Those B139 are wonderful drivers. I wanted to build a set with those but could never find a reasonable pair to buy under 400. Ive built a few with the kef b110 and k27 drivers,,,theyre really nice as well. The next upscale coffin went with 10" dalesford drivers (the model T) which are more robust but i never had a problem with my B200.s. I have a spare pair just in case bc they get hard to source especially when you buy them just to get a match and then ya gotta dump the others.