OB or TL - Help me decide

[jim rose]

This is my first post... I could really use some help.

I just wired to France a substantial sum for a pair of Fertin EX20 drivers. There goes my tax refund.

The Fertins 20ex is a field coil driver with many features, including a cryo-treated silver wire voice coil, carbon fiber spider, and brass phase plug. According to the specs, the range is 20KHz to 27 Hz. If these are accurate, who knows.

Before I state my issue, I should provide a qualifier. I know next to nothing about speaker design. About a month ago, I heard the Auditorium 23 Solovox speaker, which is a modest sized OB using the PHY driver. I loved it and that is what got me started on this project. Auditorium 23 claims the speaker goes down to the 50-60 range. Check out this site and note the back of the cabinet in the photos.

http://www.6moons.com/audioreviews/a...2/solovox.html

Here is my dilemma. Ideally these drivers work best in a large OB (i.e 5x4 feet or so). The problem is I just don't have the room. I understand that the driver works well in a TL, although the OB is better.

I have been speaking to Bob Brines about building a QWTL. I have all the confidence in the world that Bob would design the ideal TL for these drivers.

or

Go with a small OB. For those with expertise with OB designs, what kind of base can you get from with this driver using a OB maybe 12" to 15" wide and 36"-40" high. I would include some side wings as well, maybe 6" to 12" inches deep.

Adding a base driver is an option in the future.

My questions are:

1. what kind of bass could I expect to get from the above OB.

2. How much do the side wings help in lowering the bass? Significant or not much?

3. Would using curved panels like the Solovox help or hurt? Not really sure what effect these would have.

Any comments or design suggestions would be appreciated.

Thank to all who contribute.






Impedance: 8 ohms
power capacity: 50 W rms
sensitivity: 100db
flux density: 1.4 Tesla
total flux: 200,000Mx
moving mass: 18 Mms.gr
mecanical compliance: 0.00085 Cms.m/s
resonance frequency 27Fs Hz
mechanical efficiency 15.813 Qms average
electrical efficiency factor 0.428 Qe
total efficiency factor 0.416 Qt
BL factor 8.26
inductance 0.25 mh
equivalent air volume 48Vas
displacement of diaphragm +/-3
voice coil resistance 6.4 Re ohms
effective diaphragm area 0.02 Sd m2
overall diameter 230/210 mm
bolt circle diameter 216 mm
depth 150 mm
net weight 8 Kg
power supply 12, 5/1, 5
consumption 20 W
GAP height 12 mm
voice coil height 6 mm
surround neoprene
cone paper
voice coil material cryo silver

[felixx]

I made TL,MLTL,hibrid horn,closed and bass reflex enclosure for Supravox 215rtf bicone.
None of these satisfied me.In the end I tryed ob's.
From the first attempt the sound is different.
I experimented with 60cm wide baffle.
Because the space....and design...I was motivated to try something else... like Bert quasar design.So...I bought 4 Eminence Alpha 15 A drivers.
If I was have enough money...the choice will be KM30 ...ob.
With Alpha's I have something close to 30hz.The detail of Supra is stunning and I'm sure Fertin will do the job same well...maybe more.
Keep that in mind-every single cm on the depth(U baffle) is equal with 4cm wide baffle.
From the 60cm wide ....after some weeks of experiments....I remain with this:
-42cm wide
-15cm depth
-120cm high
This will be (at list for me) the best compromise.
All these dimms is more than enough for outstanding sound.
The bass is amaizing ,very clean,soft,warm...and full....but not boomy.
The instruments and voices- sublime.

But if you want to experiment only with Fertin...you should try at list
120cm wide...
But from my experiments....the bass responce is not going well only with 8" driver...no matter what is the name of that driver.

[lrntglls]

But from my experiments....the bass responce is not going well only with 8" driver...no matter what is the name of that driver.



I guess you haven't heard the new Fertin 20EX with the

new cone design and features: carbone spider- silver v.c/cryog.

and the treated cones.

What did the carbone spider do?

It extended the frequency range of the 20EX. Yes , the 20ex

(carbone) has a fs:27 compare to 42 in the old model.

You may still buy the old model of cones but all of the

people who buy fertin 20Ex go with carbone. This means

much better bass in cabinet and in open-baffle.

And furthermore you have an extended high frequency as

well (20Khz+). So even in open-baffle, the bass is pretty

good and you may not need to add a woofer because the

bass is that good.

Fertin has become, with this ,the only true fullrange;

27hz-20khz+

Most use it in open-baffle not only for the dynamic, aliveliness,

but also for the quality of bass.

You must really experience the Fertin20EX(fieldcoil)

fullrange(no Xover) in open-baffle(no box coloration)

with a low power amp to know what it all mean.

Anything that is 32 wide with small one foot wings

is sufficient enough with the Fertin20EX.

Try it and you'll be very surprised.

[felixx]

From an open baffle design the driver need to have a major xmax. specialy an 8" driver.

But...you could try ...and if you are not satisfied ...go to something like Quasar open baffle.

[lrntglls]

From an open baffle design the driver need to have a major xmax. specialy an 8" driver.

Maybe that's what happened with the carbone spider compare

to the old fiberglass spider. The carbone spider being more

supple and at the same time more reactive allows to gain

some excursion in the cones; the direct effect being the

gain Fs42 to Fs27.

But here again the more excursion the cone has; the more

distortion you create. It is surely better to add an 15inch to

an 8 inch and let the Fetin20Ex works where it's more efficient

which would be 80hz and higher and let the woofer cover

the bottom. You would gain not only more robust bass,

buy increase clarity in the middle range where it's more

important (200hz to 3000hz).

But here you get other problem which goes with adding

a woofer: xover, phase and time alignment. Done right

it could be very rewarding.


It is also be pretty neet to have one driver cover all the frequencies,

with no crossover.

High excursion(Xmas) as I mention before can cause

distortion and you can get a musher bass than with

a low Xmas driver. Better had more woofers with a

short Xmass then having one push to the limit.

But in the case of the Fertin 20Ex, it seem to work and

they do get much better bass.

Open-baffle bass also depends on the size of the room,

amp use and room acoustic. These are factors much

neglected.

One thing is for sure, in open-baffle the bigger(wider) the

baffle the better bass you'll get. You can't have everything.

[planet10]

I've not seen the Frtin used in anything but an OB, and the (2) end users really liked them.

Shouldn't there be a range of parameters depending on how high you turn the wick on the power supply?

Specs -- on 1st glance -- look like they would support a TL ... it might even work in a Bruce variant.

dave

[jim rose]

I wish to thank all those who replied. This is a fantastic forum and is an excellent source of information for a fool like me.

I am going with the baffle that is illustrated in the link below
http://members.myactv.net/~je2a3/open.htm

Not sure of the exact dimensions yet, but I will build to the largest size possible.

Should be a fun and interesting experience getting these field coils up and running.

Thank again!

Jim

[nullspace]

Hi Jim --

Do you have a PS for the field coils yet? If not, I strongly urge you to get yourself a 26ah (or so) SLA battery. I found battery power to be far better than an industrial HP power supply I had lying around.

Regards,
John

[jim rose]

John:

Thanks for the advice re batteries. A few follow up questions.

1. how do you adjust the field voltage with batteries? Or you don't?

2. how often do you need to recharge?

Thanks

Jim

[nullspace]

Jim:

1) You could use a current regulator or add some series resistance, but I never have. The lack of ready adjustability is the downside to batteries, but as someone who doesn't resort to hyperbole often, it's night and day between batteries and anything else.
2) Well, if you run both field coils off a single 26ah battery like I do, after 6 - 7 hours of use. I'll admit to being lazy and running my batteries down pretty far, but you're not supposed to use more than half the amp hours if you're hoping for long life from your battery. My field coils do not match (one's 9.9ohms the other 11.0), and I encourage you to measure yours when they arrive. V=IR and then you'll know how many amps your drivers will draw.

Regards,
John