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

[Count Dacula]

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 tweeter and weak mid driver on one side. I have no interest in fixing them, but I had an idea....convert to modest bass bins. A respectable repurpose after 35 years....still in use...?

The Q7s are for a big room, with a midrange amp, they still might need some help in the sub department. Ill buy new if necessary, but. These compact sealed C75 cabinets are a useful size, both already with ONE good 8" bass driver. I had a purchase set up for two more KEF replacement woofers to install in place of the 8" mid-tweeters, but the deal fell through for $125. Not a disappointment, and that's why I'm here, for advice.

I have two brand new Parts Express 8" woofers I bought for something, and have been innabox for >5 years. How cool would it be to not even buy any hardware, instead install the new Parts Express woofers, and harvest the crossovers and the good side Mid-Tweeter just to make a point. So, my question is about feasibility and wiring. The C75 with one woofer hits 57Hz, could it go lower with another woofer in the cabinet? I know "it depends" on the new driver. I don't have them in front of me, so idk the response and ohms, bringing me to the heart of it.

Should I wire the different woofers in parallel no matter what the ohms and let the amp deal with the goofy loads? I could maybe buy those replacement C75 wooferes to keep it "genuine" lol, or "balanced" is a better word??? One more config to consider would be, two old in one cabinet and two new in the other....? I originally considered separate wiring and terminals for each woofer, in case I wanted to run a stereo amp on one cabinet, but I think that's fallen off. Ill use both at the same time, in stereo and in parallel. I think I want to go with two mixed woofers wired together and let the amp figure it out. I have a ADCOM 545-II for it, so it might get spicy.

Input? Ideas?

4ohm in orig form.... https://audio-database.com/KEF/speaker/c75.html

The Parts Express woofers might be....Altec Lansing ...??? (for home use iirc)

BAD experience with my last powered sub, a MONOPRICE, so I am open minded with HOMEBREW

 

[Count Dacula]

I "repurposed" this post onto a more correct forum.

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

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...

www.audiosciencereview.com

Which was essentially pointless, considering the subject is within A"S"R perview. The plan is still a go. AI thinks Homebrew is cool.

 

[kemmler3D]

Should I wire the different woofers in parallel no matter what the ohms and let the amp deal with the goofy loads?

No, it matters a lot if the speakers are 4, 8, or otherwise. If they are 4ohm you end up with a 2ohm load which will tend to stress or most amps, especially if you're using them (effectively) as subs. The Adcom is not a toy but it's not magic, either...

I think I want to go with two mixed woofers wired together and let the amp figure it out. I have a ADCOM 545-II for it, so it might get spicy.

Why not do a quick modeling exercise in WinISD or one of the new apps that's been posted around here lately? They are nice cabinets so I feel like it's worth getting these right so you want to keep them around long-term.

The C75 with one woofer hits 57Hz, could it go lower with another woofer in the cabinet? I know "it depends" on the new driver.

Yep, it depends... on:
*characteristics of the woofer
*how loud you want to go, and how low
*what you do with crossover and/or EQ

I think you can probably get more bass out of it (at least 3dB more at any given frequency?) but maybe more.

All that said, since you have the drivers and cabinet selected already, I guess you might as well just wire everything up and then beat it into shape with DSP, but you should definitely take the time to make the right choice on series vs. parallel, as that could have a big impact on how well the amp can handle it. Very likely there is one correct answer to that.

 

[Flaesh]

Only 29 l. Use speaker parameters for modeling. Consider thermal power and excursion. This will limit the lower limit and amplitude.
I'll refer to myself: https://www.audiosciencereview.com/...quasi-plain-wave-radiation.24396/post-2504475

 

[Count Dacula]

I may need to tune the PRs just below the B200 freq of 521Hz to 38-40Hz with weights. AI says the PR I'm looking at is 70g and it'll basically need 30G extra to make 110g total mass.

I just got a Fosi MO-3 today, which can drive bother orig KEF B200 in their own cabinets with PR replacing the UniQ, and knocking the response down to 40Hz.

So, two org drivers and cabinets with the moderst sub amp, with all the controls already built in for $80. The PR are expensive, imo at $30ea Not a bad sub set for $150!

That all sounds good. Ill catch up later on. Thanks for the replies.

 

[Count Dacula]

It actually says 1cu-ft volume for C75 making the passive radiator perfect without tuning. I dunno if I am giving too much or too little content, just summarizing. We thought the volume was .75ft, but it is not, so that changed the parameters. Total expense of $60 for the units.

 

[Count Dacula]

View attachment 544049
Only 29 l. Use speaker parameters for modeling. Consider thermal power and excursion. This will limit the lower limit and amplitude.
I'll refer to myself: https://www.audiosciencereview.com/...quasi-plain-wave-radiation.24396/post-2504475

I'm still paying attention. The whole C55 speaker array is 4 ohm, the woofer driver is 8 ohms. Imma run both cabinets together, with just one woofer and a passive radiator. A nice simple system that can be driven easily by a TPA 3255, like the Fosi.

I have so many smallish systems, I will def use these someplace with bookshelves and a TV

 

[Count Dacula]

Maybe I can place small full range speakers atop them?

Somewhat short and stout, based on 8" drivers

 

[Count Dacula]

Im considering which location for the woofer and PR. I think I should move the woofer up top to keep floor interaction minimal, but they are nearly centered now...grr

If some says "how they would do it" I wont' beg for a reason. All I had to do was hack the UniQ / crossover out and now reconnect the leads. Red and black In and purple and white (I think) going to the woofer. I guess Ill need a look at the unit and just mate the red lead to the + side. I can snap a pic inside with cell phone, not even taking it apart.

 

[Count Dacula]

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

 

[Flaesh]

Two arbitrary drivers in parallel and one of them as a PR, all other things being equal (including box volume and voltage).

 

[Count Dacula]

Thanks, hopefully it pans out. 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.


My old KEF B30 amp blew up, so I'm converting it to passive w/a TPA 3255 power.




I'm also working on two old JBL Eon 15" powered subs for the new room. They have fixed crossovers at 150hz, ugh. Also they are XLR only, so Imma drive them off a DAC headphone, with a crossover inbetween with RCA to the mains,

 

[Count Dacula]

Thanks, hopefully it pans out. 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.


My old KEF B30 amp blew up, so I'm converting it to passive w/a TPA 3255 power.

View attachment 546214View attachment 546215


I'm also working on two old JBL Eon 15" powered subs for the new room. They have fixed crossovers at 150hz, ugh. Also they are XLR only, so Imma drive them off a DAC headphone, with a crossover inbetween with RCA to the mains,

For the JBL Eons, I bought a pro crossover with sub control and delay. It has XLR like my DAC, so we'll see how they work with the eq shifted down. Fixed at 150Hz seems crazy, but I guess they're Rock & Roll focused, or they were intended for the end user to select a low range for them.

I have the corresponding JBL 2 way PA speakers from my buddy as well. They sound presentable on a decent amp, but DJ/MCs don't tend to have nice amps, so they never do the PA speakers justice. My friend who gave them to me had some old Crown he used, but with his 100ft cables etc I suppose he never "nailed" the sound, lol. He just liked playing MC, more of a wedding DJ, which is not my normal gig. Wow.