depending on your selected woofer and the resultant enclosure dimensions, the aforementioned hypotenuse may not be sufficient to accommodate many of the current plate amps, and I'd prefer to avoid putting the amp in the same enclosure anyway
you'll note the terminals immediately under the driver - "we" generally suffer no shortage of power amps to power woofers - one very nice budget candidate that I think we've tried is the Dayton APA150
Dayton Audio APA150 150W Power Amplifier 300-812
for more power and elaborate controls, including line level HP output, there's the SA1000
Dayton Audio SA1000 Subwoofer Amplifier Rack Mountable 300-811
you'll note the terminals immediately under the driver - "we" generally suffer no shortage of power amps to power woofers - one very nice budget candidate that I think we've tried is the Dayton APA150
Dayton Audio APA150 150W Power Amplifier 300-812
for more power and elaborate controls, including line level HP output, there's the SA1000
Dayton Audio SA1000 Subwoofer Amplifier Rack Mountable 300-811
I'd say there are two options here.
You could try to stay with one driver per side, and hope you'll get the bass you want from them. You'd get to keep your FH3 cabinets, and (with the MA drivers or maybe one of the new Fostexes) there's a fair chance it'll be better.
Alternatively, you could introduce more drivers and amplifiers.
This would involve new (potentially smaller) cabinets, and improved performance.
For example, the speakers I'm using at the moment have a Fostex FE126 (fully treated) and a 6.5" Tang Band mini-sub per side.
Bass goes down to 40Hz nicely, and cabinet volume is around 14L.
I took the sledgehammer approach to the difference in driver inefficiencies: the TBs have ~120w/ch to play with, the Fostexes around 10w/ch.
You can really crank this set-up, even in big rooms. Cone excursion on the woofers can get really silly, but the midrange/treble range (because the Fostexes are covering that) stays clean. IMO, this is where single-driver systems fall over - you can't ask a single cone to swing around at 40Hz while reproducing the rest of the range cleanly.
The above uses a 500Hz crossover. This is to automatically add BSC as well as relieve the Fostex of LF duties.
A third option would be a single subwoofer and plate-amp. You could hide this in the room, while re-inforcing the LF. That'd improve system performance, without having to rebuild the cabinets for the Fostexes.
I'd say it depends on your music tastes and how loud you like to listen. Many people would get away with the FH3s and be happy. I like something with more oomph.
Chris
You could try to stay with one driver per side, and hope you'll get the bass you want from them. You'd get to keep your FH3 cabinets, and (with the MA drivers or maybe one of the new Fostexes) there's a fair chance it'll be better.
Alternatively, you could introduce more drivers and amplifiers.
This would involve new (potentially smaller) cabinets, and improved performance.
For example, the speakers I'm using at the moment have a Fostex FE126 (fully treated) and a 6.5" Tang Band mini-sub per side.
Bass goes down to 40Hz nicely, and cabinet volume is around 14L.
I took the sledgehammer approach to the difference in driver inefficiencies: the TBs have ~120w/ch to play with, the Fostexes around 10w/ch.
You can really crank this set-up, even in big rooms. Cone excursion on the woofers can get really silly, but the midrange/treble range (because the Fostexes are covering that) stays clean. IMO, this is where single-driver systems fall over - you can't ask a single cone to swing around at 40Hz while reproducing the rest of the range cleanly.
The above uses a 500Hz crossover. This is to automatically add BSC as well as relieve the Fostex of LF duties.
A third option would be a single subwoofer and plate-amp. You could hide this in the room, while re-inforcing the LF. That'd improve system performance, without having to rebuild the cabinets for the Fostexes.
I'd say it depends on your music tastes and how loud you like to listen. Many people would get away with the FH3s and be happy. I like something with more oomph.
Chris
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Thank you all for taking the time to comment 
I'm going to replace my drivers first with some Alpair 7's. Thanks to uni I'm not going to get this amplifier done anytime soon and I want to enjoy my music between now and then so I thought this the first step.
I'll get the amp done asap also and see how things sound after that. Failing that, I'll look at some LF assistance.
I'm unsure about the magnet wire. My house has to be bengal cat proof and they like to eat, errrr anything ad this might be a hazard to them plus it'll constantly neeed repairing I think. I'll have a think about that.
I'm going to replace my drivers first with some Alpair 7's. Thanks to uni I'm not going to get this amplifier done anytime soon and I want to enjoy my music between now and then so I thought this the first step.
I'll get the amp done asap also and see how things sound after that. Failing that, I'll look at some LF assistance.
I'm unsure about the magnet wire. My house has to be bengal cat proof and they like to eat, errrr anything ad this might be a hazard to them plus it'll constantly neeed repairing I think. I'll have a think about that.
Alpair 7.3 drivers fitted and things are much improved. ish.
Sat in the central listening position bass is still a bit flat but standing up or nead the amp in the centre off to one side etc bass gets a lot fuller.
I think I'm going to try making some corner platforms for the horns to sit on. I can feel the bass in the carpet and vibrating the floorboards just not the air!
The A7's are superb though. No need for the tweeters anymore and I'm sure the bass is now available to find and exploit. Lovely sounding things despite the few hours on them.
Sat in the central listening position bass is still a bit flat but standing up or nead the amp in the centre off to one side etc bass gets a lot fuller.
I think I'm going to try making some corner platforms for the horns to sit on. I can feel the bass in the carpet and vibrating the floorboards just not the air!
The A7's are superb though. No need for the tweeters anymore and I'm sure the bass is now available to find and exploit. Lovely sounding things despite the few hours on them.
After some experimentation I've made some good progress.
I've put the speakers on some nice big and heavy slate tiles. This helped a bit but the bass became boomy.
Next, I tilted the speakers down at the fron edge tipping th ehorn forward, bit better.
Lastly, I've moved them away from the back wall but kept them tight to the outer walls and, yes. Very yes. Boom has gone, detailed bass has arrived and is present in the listening spot rather than everywhere else. Its is now very VERY good to listen to!
I'll report back some more once the drivers have bedded in some more. They're loosening up quickly which is good for my ears.
I've put the speakers on some nice big and heavy slate tiles. This helped a bit but the bass became boomy.
Next, I tilted the speakers down at the fron edge tipping th ehorn forward, bit better.
Lastly, I've moved them away from the back wall but kept them tight to the outer walls and, yes. Very yes. Boom has gone, detailed bass has arrived and is present in the listening spot rather than everywhere else. Its is now very VERY good to listen to!
I'll report back some more once the drivers have bedded in some more. They're loosening up quickly which is good for my ears.
The Compact backloaded horn might be the ticket (built by me and some measurements as well)
http://www.diyaudio.com/forums/full-range/227240-mjks-fostex-ff125wk-blh-project.html
Or the CT_164 horn
Single Driver Website
http://www.diyaudio.com/forums/full-range/227240-mjks-fostex-ff125wk-blh-project.html
Or the CT_164 horn
Single Driver Website
Its been a little while now and the drivers have now bedded in. The sound is overall, excellent but on certain notes and frequencies the horns can make you cringe. I don't know quite how to describe it. Its only on certain notes that it becomes prominent, mostly on acoustic guitar music and when it occurs makes you pull a displeasing face!
I tried packing some wadding into the horns opening which corrected it but the sound suffered everywhere else, is there a specific internal setup for the alpair 7 I should try?
I tried packing some wadding into the horns opening which corrected it but the sound suffered everywhere else, is there a specific internal setup for the alpair 7 I should try?
You mention adding / removing stuffing in the area below the driver and the effect it has on the sound. I found the same, the area below the driver is like changing a 2 - way speakers cross-over network. Huge effect and all one can do there is get the overall balance of low, mid and highs as good as possible. One cannot fix certain areas in the frequency range there without effecting other areas in a bad way. What I did to get rid of those "ear-hurting-shouting" frequencies was padding the area behind the driver. I found the problem I had was sound waves reflecting back through the driver cone. I solved that by adding about 20g of acousta stuff right behind the driver, the section between the V and the throat. My suggestion would be to try there. BTW, I have the CSS EL70en drivers.
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