building speakers with kef b139

[freo]

Hello
I have been given a pair of b139 oval drivers a pair of tweeters apparently pulled from b&w, I would like to use these plus a mid range driver something like a peerless 6 1/2 inch to build some speakers, anyone any ideas of size of cabinets or crossover design I would need for such a project, I am new to this. I have built a set of ed frias diy from audio review which I am really pleased with, but that is my total experience in speaker building
Regards Roger

[heater]

In the early 80's I built a pair of transmission line speakers from a kit from Wilmslow Audio based on a design by Chris Rogers and described in Wireless World magazine.

These used the B139 and a Peerless mid. If you squint very hard you can read the scanned article here:

http://www.tech-diy.com/pro9.htm

I don't know how this compares to modern designs but at the time my friends and I loved those speakers. Wish I still had them.

There was an earlier design by Baily in the same magazine which looks simpler to build. There is a scan of that article on the net some where but I can not find it just now.

Having said that I would worry about undertaking such a big project with such old drivers. They may be physically deteriorating now. Check them carefully and maybe measure their parameters for yourself.

[Damosan]

Roger,
Send me an email to damosan_72 at hotmail.com so we can get in contact. I live in Perth too so may be able to help you.
Damo

[Scottmoose]

Design an enclosure in Martin King's worksheets. Forget using ye olde rule of thumb -you won't get anything like the same quality of results as you would from using the software. Anyone got the T/S parameters of the 139? I might be able to come up with an enclosure for you. If you don't want that, I suppose you could try going for one of the 'Alpha TLs', which seem to be the spiritual successor to Bailey's original 'Transmission-Line' misnomer. Again, I don't think these are in the least optimised, but a baboon could build them, they are so simple

You'll either need a mid, or a large tweeter that can go very low: these are bass, not mid-bass units, and they won't thank you for pushing them up too far. I would say definitely worth pursuing though. If memory serves, they'll need a hefty notch-filter somewhere as they have a break-up mode: can't remember exactly where I read that though.

[sreten]

Hi,

SM is correct in that the B139 has a really bad break up mode
(around 1K I recollect) which makes it difficult to use in a 3-way
design with a simple crossover crossing over to a mid unit.

Consequently its more sensible (if they are in good condition)
to use the B139s for a powered subwoofer or stereo subs.

From what I remember the TS parameters aren't that bass
alignment friendly and a sealed box is probably the best bet.
If you go for this a sub amp with bass boost option might help.

(reflex alignment - ~ 120 litres tuned to ~ 25Hz. Sealed ~ 60L.)

This leaves selection of the midbass unit. Probably best to take
an existing design and shoehorn the B&W tweeters in there
with an L-pad arrangement for impedance and level matching.

The tweeters should also be carefully checked for signs of
distortion - they can make odd noises on their way out.

/sreten.

edit : parameters for one version of the B139

Fs 25 hz
Mmd 43.5 grams
VAS 164 litres
Rscc 6.2 Ohms
Vl 0.56 Mh
Bl 12.3
Qms 5.5
Qes .4
Qts .37

Thse agree with the values given in Win ISD which adds

SPL 84dB
Pe 100W

[rjb]

B139 SP1044 (B version seems to have very sililar parameters)
From KEF spec sheet
Nom Impedance 8 ohms
Freq range 25-500Hz
Sensitiviy 87db
Total flux 1.1mWb
R(E) 7.2
R(e) 4.87
L(E) 1.04mH
L(e) 0.97
Sd 349 sq cm
Xmax 6mm pk-pk (B=7p-p)
X(limit) 12mm pk-pk
Moving Mass 43.5g (B=54g)
R(MS) 1.43 mech ohm
C(MS) 7.43x10e-4m/N
V(AS) 127 litresBl 12.5N?A
F(s) 25 Hz +/- 5Hz
Qm 6
Qe 0.39
Qt ).37
Closed box vol 30-60l
Reflex box vol 60-140 l
Power handling program 100w.

These units very robust and age does not seem to be a problem.
Roll surround lasts well.

[Scottmoose]

Cheers guys. Assuming it's the SP1044, this thing is a doddle to use in a Quarter-wave resonator (TL)

Two options here: a Mass Loaded Tapered Quarter Wave Tube, and the next post will have a straight, Mass-Loaded Transmission Line. For more on these geometries Freo, if you haven't seen them before, look at the Projects section on Martin King's site: www.quarter-wave.com

The Project 2 ML TQWT shows the basic cabinet shape. Looks good in-room. If it's too tall for you, you can simply fold it in half, the top to the rear, and it will perform exactly the same.

ML TQWT first.
Line-length: 60"
Width: 16" internal at the base, 4" internal at the top.
Depth: 13.75" internal
Driver centered in the cabinet at 30" from the internal base and 30" from the internal top.
Use 0.25lbs ft^3 of stuffing from the top to just below the driver.
Circular, round port 3" wide by 2" long 3" up from the base.

Here's the projected frequency response:

You can cure the slight rise with either a touch of series resistance or a 1 long port instead. However, I deliberately aimed for a slightly falling response or this thing will become very bass-heavy in-room.

[pinkmouse]

Scott, I have a pair as well for a project that got put on the back burner. I was thinking more for stand alone subs, and I got good results with an eight foot straight line with a 4-1 taper, but never got round to building anything. The full spec sheet is on P10's website, here, and my measured T/S parameters matched well with the published ones.

To be honest, I wouldn't run these at more than about 200Hz, but that does give a wide range of options for decent mids.

[Scottmoose]

Thank you Sir, you are a Scholar and a Gentleman!

And here's the response of a straight, Mass Loaded Transmission Line. For more on these, Look at Martin's FE208Sigma and Lowther MLTL projects. They look like bass-reflex cabinets, but they don't behave like them!

45" internal height.
Internal width 11.5"
Internal depth: 14.35" (So & Sm=3Sd)
Driver at 33.75" up from internal base. (0.25 of line length from the top)
3" wide by 2" long port 3" up from the internal base.
0.25lbs ft^3 of stuffing from the top to just below the driver.

Note: with both of these designs, you can put the assumed midrange and tweeter wherever you like, within reason. They shouldn't affect the cabinet etc. In the case of the MLTL, I'd place them above the B139. Actually, if you did that, and were using a midrange unit, you could lower the B139 to 22.5" from the internal base of the cabinet, which would remove the touch of ripple you can see. Again, I was aiming deliberately for a slightly drooping response to compensate for room-gain.

In the ML TQWT, unfolded, I'd probably place the mid and tweeter below the B139. Folded -your call. These MathCad sheets of Martin's are staggeringly accurate by the way. This is exactly how these designs will perform in anechoic conditions, assuming they are built correctly and the drivers are working as they should. However, as our friend has above mentioned, the bit you want to be paying attention to in both cabinets and graphs is below around 200Hz: above that, you'll need a decent midrange unit. That's fine: place it in a location like those I suggested above where there is some stuffing and that should flatten its behaviour nicely!

All the best & I hope some of this helps a bit
Scott

[freo]

Thank you all for your incredible responses, this will give me a lot of ideas to work with, thanks again
Roger
(Great new cd harry manx mantras for madmen)