Old compact tower speakers = New compact bass speakers...?

[Count Dacula]

Trying again to find the right forum.

Any advice on repurposing my beloved 2x8" sealed 3-way KEF C75s into 2x8" mini subs for my various minisystems?

These guys owe me nothing, but it would be cool to STILL use them after 35 years. However, this would be a tricky rebuild, or barely worthwhile, to replace one side mid-hi with a junkyard part for $100, only to yield two old obsolete units that still need new ferrofluid, et al. I have plenty of other newer speakers, all with varying sentimental value.

So, since I already have 2 good 8" KEF woofers, can I just brew up a unit with a small bass amp? like...

Fosi Audio M03 200 Watt TPA3255​

Right? Just enough power for 2x8" drivers in parallel. I'll just attach the thing like a wart, lol. The low/fat tower cabinets are such a nice form factor too. It's ideal, imo.

My question is about the #2 unit. Since I already have two brand new Parts Express 8" woofers, already here (somewhere) from another failed project, exactly how should I use them?

Ohms are unknown, I suppose the KEF B200s are 8 ohms each. So, should I make two different cabinet loads, one KEF and one with the new Parts, or Homebrew it with one of each, and let the amp figure it out? I don't plan on using them together, at once. It's possible, but not an objective. Ill see if one unit is better than the other too...

I think it's a good question, and hopefully meets the criteria for a post.

If those Parts Express are 4ohm, I better not put them together. I think they are "home" 8 ohm though.

Same thing for the Sealed cabinets. I assume the KEF drivers are ideal for Sealed, but will the new ones be ok if they are "for" Bass Reflex? How can I tell from volumetric specs??? Does it really matter if I put B-R in a Sealed cabinet? Should I mix them up, one and one, for any of these reasons?

 

[CDMC]

Read this, then you will have your answers.

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[Count Dacula]

Imma wing it. Just if someone saw any glaring issues, or had a preference with wiring or load. I could just ask AI to read the book on my behalf...

 

[technodanvan]

That's a lot of questions in one post! While the book might help it's really not going to give a terribly straightforward answer.

If you REALLY like the look and form factor of these cabinets then...yeah, MAYBE worth repurposing them if you plan ahead and know your way around a multimeter and a couple of power tools. But honestly for the couple hundred bucks you'd spend doing it you'd probably get more out of just making a smallish subwoofer that is purpose built. This GRS comes to mind.

https://www.parts-express.com/GRS-12PR-4-12-Paper-Cone-Rubber-Surround-Subwoofer-4-Ohm-292-820?quantity=1

What 8" woofers do you have?

 

[Count Dacula]

I understand, and am familiar with those reference books.

As it is, it'll be really easy, literally to slap in one woofer from the other speaker, chunk out the wiring and it's ready....,to test.

I ordered the TPA 3255 sub amp, which I think might attach on the back top, with just the knobs extending (?)

While it arrives from China, I have a Fosi V3 here, and a micro sub controller, that I could "test" it in stereo mode.

With the current cabinet config being 2x8, I feel like homebrew has a decent chance, without a ton of calculations or fiddling. Considering the first two drives are the actual KEF woofers. I considered buying two more, which is realistic, price and effort wise...but maybe unnecessary. I do need to dig out those Parts Express drivers and check the specs, for sure.

The thing with the reference manual is, I'm not gonna change a bunch of parameters anyway, I already have the equipment, it's just a matter of doing the best install possible with what I have, with the placement and wiring.

I figured this project deserved a thread, to get tips, then show the results. Im definitely doing it, ordered the Fosi sub amp last night, and can get the prototype started today. The cost is gonna be awesome for what amounts to a nice unique unit, in the right system. I have a bunch of C grade bookshelf systems so they wont' go to waste.

Thanks for the feedback.

 

[Count Dacula]

I am working on it. A good tip from AI was that dissimilar speakers together can interfere with each other physically as they are responding, making both distorted. Good tip, but they further said, if I had separate chambers in my cabinet, I can run different drivers.I think this is the case with mine. Not sure yet about impedance missmatch, but the KEF B300 is 8ohms. AI suggested a woofer from Parts Express that was "identical", but Imma look for the drivers I already have . If they are generic and 8ohm, Ill put them together in a separate speaker, but if I order "identical" ones Ill consider splitting them up one and one in each cabinet, with the chamber divider.

 

[Count Dacula]

AI says two woofers require 2x the volume, and will raise the resonant freq. but a passive radiator would do the opposite, and lower the resonance. I may have to go that route.

 

[10PSPMemSticksForMiku2007]

You might get suboptimal but okayish results just slapping random drivers in an existing sealed cabinet. If you're doing a port, let alone a PR, you should really be doing the reading and simulations.

If this speaker has separate chambers and you install a PR in only one chamber, you also rotate the phase for only one chamber, resulting in destructive interference between the chambers at the tuning frequency. Very counterproductive to getting more low frequency output. Therefore, you must remove the chamber divider. Spending money on PRs in a project that you're unwilling to buy drivers for also makes no sense. Better to add a port on the back or bottom from a $5 pvc pipe, if you need to add anything.

Even with the sealed design, you should remove the chamber divider. Having different chamber volumes is bad for tuning. Put identical drivers in each cabinet, not mixing types. Impedance aside, you don't want one driver to reach xmax while the other can still move further. That lowers your clean bass output. One driver sounds terrible when it bottoms out, and the other driver's potential is wasted.

The two cabinets should be on separate amp channels that have individually adjustable volumes, again, so they reach their limits on the same (summed mono) input signal.

This an example of where asking AI won't set you on the right path. They might not have good understanding of niche technical topics, but even if they do, you may not know what information you need to give them, and there is the concern that it won't tell you you're asking the wrong question.

 

[Chrispy]

AI says two woofers require 2x the volume, and will raise the resonant freq. but a passive radiator would do the opposite, and lower the resonance. I may have to go that route.

Mostly your bass drivers may or may not make poor subs....you'd need to model the box and t/s parameters (if they even provide them). Probably better just to go to an actual sub. Passive radiator boxes are more akin to ported boxes, and you generally need two of the same size passive radiator per driver....altho some engineering and particular passive radiator you might get it down to two again, but....why?

 

[Count Dacula]

Yes, the 2x PR to driver area was mentioned, but some parameter made it not a problem, lol I forget. I think it's only a factor at high spl, and I got a long excursion PR, soIll take a chance on my $60 investment. It should hit 40Hz, and work great, according to some software modeling.

I'm gonna try to keep it in one thread.

 

[Count Dacula]

Repurposing old 8"+ 8" KEF C75 into bass bins

Alas, the end of an era, or is it??? My intense collection of KEF speakers has come full circle. As I await delivery of new Q7 Metas, I'm testing out old equipment to recycle and rematch them towards their destiny. Working backwards, my ancient KEF C75 that served me well have shown a blown...

www.audiosciencereview.com

This solution was offered... they have potential.

https://www.parts-express.com/Dayton-Audio-DS215-PR-8-Designer-Series-Passive-Radiator-295-492

I'm waiting for the PRs to arrive. I got some recommendation on the woofer location. Gemini says to keep it in the lower position, for motor reasons and for blending the highs and lows physically since I am definitely gonna use them as speaker stands, yup. I have :LS50M in my bedroom, which is tight on floorspace. Usually the LS sit on subs, but they can use more height, and I don't really need two fat subs, just something like this to fill out the lows. My "Main" listening is moving tot he new addition anyway. The den system is just steps away, and is getting upgraded to Q300s and Adcom 5400 and now a sub.

 

[Count Dacula]

Fitment is very good. Gaskets included.

 

[Count Dacula]

I just need to tune them. Like 5 washers, lol.

 

[Count Dacula]

I found the 8" JBLs, they are more like mid bass, so no loss there.

In my guestimation, my speakers with SP1237 will work fine, in proportion to their potential and cost. That is, reloading them with better 8" sub woofers is a Fool's Errand....right?

My PR investment of 2x$30 feels about right, with a $70 amp, I think they might outperform a janky $100 pwrd subwoofer. They're more like a sub booster, maybe just adding them to an AV system.