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Cs7 with BD139 passive radiator for deeper bass?

 
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eschenborn
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Joined: 25 Feb 2018
Posts: 11
Location: Berlin, DE

PostPosted: Fri Aug 16, 2019 9:01 am    Post subject: Cs7 with BD139 passive radiator for deeper bass? Reply with quote

dear forum members,

Originally I had a pair of B110's, B300B's and T33's, having in mind to create a speaker resembling the CS9, but with the tweeter section of the R105.4. Thanks so much for all the advice you have given me here.

This plan is now obsolete however, having found wonderful used R105's (see my thread below). Still, my curiosity is wide awake. And I still have the B110s and T33s lying around (I sold the B300Bs).

So I am wondering if I should try to build a CS7 instead. I'd need two B139's which seem to be relatively easily available (used or new, but expensive, at Falcon).

Looking at the CS7 construction leaflet, I see the bass rolloff begins already at 70Hz. So I wondered if I could expand the box volume and help the bass with a passive BD 139, put at the back. I have a pair of those lying around, too. Following common opinion I should use two per box. But maybe one would suffice?

I'd slightly raise the height of the box (to about 1 meter, making its appearance resemble more that of a C80, albeit bigger in its overall dimensions) and build it a bit deeper, so I should be able to reach a volume of 120 liters.

Would one BD139 per box work? Could I use the CS7 crossover? Is this a good idea at all, or should I opt for just building a CS7 and raising the lower bass by selectively enhancing the region below 50Hz, in order to get it more or less flat down to 30Hz?

I am in no need to replace my 105's and surely won't, but I am curious how the CS7 sounds, and I'd like to experiment a bit. and there is still some space left in the flat.

I am desperate for advice, so thanks in advance! does my plan sound like a good idea... or am I headed towards frustration county (vented design difficult to tune, wrong size of now slightly elongated baffle, two BD139 needed per box, better to put in a simpla port like in the Concerto)?

There could be a reason why KEF did not come up with the design I have in mind. I have heard of an obscure studio monitor by swiss builder ACR, with Fostex FT3 RP tweeter, Audax Norme C.E.L. mid driver, KEF B139 bass and B139D SP 1042 on the back. but I cannot confirm this information anywhere. And the design might not have sounded good anyway Smile

thanks so much for your input, andreas
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SaSi
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Joined: 24 Aug 2008
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PostPosted: Fri Aug 23, 2019 3:49 pm    Post subject: Reply with quote

You can get a rough idea of what to expect in terms of woofer response using one of the many simulation programs around. I am still using Winisd Pro (alpha version).

The CS7 is not rolling off bass response at 70Hz. The specs say 35Hz-20kHz -/+ 3dB, which is not bad at all for any kind of speaker. Actually, I am not sure you want a loudspeaker producing such low frequencies if you don't have a properly treated room. Otherwise you will excite the main modes of a typical room and have a hard time finding a location for the loudspeakers and one for you that provide adequate bass.

One shortcoming of the B139 is its relatively limited power handling, not being able to provide 110dB level output (in case you need that), but yet again, in a small room, it's kinda irrelevant.

If you want to experiment and don't mind a large, long enclosure, I suppose you could be looking into a transmission line for the B139. And since you don't have a B139, it's much better to get the Falcon one, as it provides longer excursion and higher power handling.

The CS7 sound should be just about what you get from the R105, lower SPL and less refined.
The TL sound is something to listen to to experience. I haven't and I do have a handful of B139 so I am planning to make two TL systems, just to listen to them and perhaps love them.
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Sat Aug 24, 2019 3:45 pm    Post subject: Reply with quote

SaSi wrote:
... since you don't have a B139, it's much better to get the Falcon one, as it provides longer excursion and higher power handling...


Not true; see https://www.falconacoustics.co.uk/drive-units-1/falcon-b139-8-ohm-kef-b139-sp1044-replacement-woofer.html where they only claim to have copied the KEF original.

The B300 did have a longer excursion and higher power handling. I know because I developed the B300 motor from the B139 and designed it to have twice the B139 excursion; 1/2" as opposed to 1/4".
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eschenborn
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Joined: 25 Feb 2018
Posts: 11
Location: Berlin, DE

PostPosted: Sat Aug 24, 2019 10:59 pm    Post subject: Reply with quote

Dear SaSi, dear speakerguru,

thanks for your kind replies. I have indeed been thinking about trying a TL, but I am hesitating because of all the woodworking involved. I am tempted, though.

I found the picture of the CS7 frequency response in the construction manual. It makes me think that the rolloff starts at about 70Hz. It's on the KEF site (last page of the pdf file): http://www.kef.com/uploads/files/en/museum_pdf/80s/Constructor_Series_Model_CS7.pdf

Falcon Acoustics write that their new B139 has a chassis that is deeper by 25mm to allow better power handling. They don't say anything about a new, or different, motor:
https://www.falconacoustics.co.uk/drive-units-1/falcon-b110-t27-b139-drive-unit/falcon-b139-8-ohm-kef-b139-sp1044-replacement-woofer.html

Speakerguru, I'd love if you could comment on pairing the B139B with one B139D to get a bass response that is more linear for maybe an octave. Or should two passive B139 radiators be used?

maybe this idea of mine is nuts. In the meantime I acquired two vintage B139Bs, cheap-ishly, so I could think about starting to experiment. As I said, I am totally happy with my R105's, and I am not wanting to upgrade anything. It is only about curiosity. But that's so nice about it.

thanks so much for your interest!
eschenborn

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speakerguru
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Joined: 18 Nov 2005
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PostPosted: Tue Aug 27, 2019 4:14 pm    Post subject: Reply with quote

Please read http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=652&highlight=transmission
It will save you wasting a lot of time on unnecessary cabinet work. If you do like woodwork then the B & W matrix cross-bracing is a much better idea and well worth doing.

At audio frequencies, there is no such thing as a transmission line that will fit into an aircraft hanger, let alone a room in a house.

RE "pairing the B139B with one B139D to get a bass response that is more linear for maybe an octave", yes, a reflex design, be it port or PR, will reduce woofer excursion at the box frequency. The bandwidth of this region depends on the alignment and the ratio of box to driver compliances (see Fincham's paper on the coupled cavity here: http://www.hifiloudspeakers.info/Anatomy/Articles/ArticlesGallery/Fincham1/Fincham1.pdf
(where only the port/PR output is used and the woofer is enclosed in another box)


Last edited by speakerguru on Wed Aug 28, 2019 10:12 pm; edited 1 time in total
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SaSi
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PostPosted: Tue Aug 27, 2019 5:08 pm    Post subject: Reply with quote

Quote:
The Falcon B139 chassis dimensions are identical to the original later version KEF SP1044 with an extra 25mm chassis depth to allow for greater power handling ( now 150W), helped by the provision of a bump stop bottom plate and vented pole.

This is from the part description on the Falcon site.
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Tue Aug 27, 2019 8:29 pm    Post subject: Reply with quote

SaSi wrote:
Quote:
The Falcon B139 chassis dimensions are identical to the original later version KEF SP1044 with an extra 25mm chassis depth to allow for greater power handling ( now 150W), helped by the provision of a bump stop bottom plate and vented pole.

This is from the part description on the Falcon site.


Yes, I see. That just affects the gross overdrive damage limited excursion. The linear excursion is not changed. KEF went over to bumped backplate/pole forging for all B139 and B300s while I was there so not sure why Falcon are making a big deal of it.
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eschenborn
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Joined: 25 Feb 2018
Posts: 11
Location: Berlin, DE

PostPosted: Wed Aug 28, 2019 9:16 am    Post subject: Reply with quote

Thanks for all your answers. I have seen that there is another thread on this forum which deals with the question (see below), and there ColinR suggests it is possible to add a B139D for a volume of 80 litres. I'd love to hear of some firsthand experience.
Andreas

http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?p=832&sid=5c0dee478f06953fd3246fd79713ce53
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