[el`Ol]

I was looking through the Selenium product range looking for suitable 18"ers and found those two with the highest Qms have the best top end:
http://www.seleniumloudspeakers.com/site2004/catalogo/pdf/pro_woofer_wpu1809-wpu1809-slf_new.pdf
http://www.seleniumloudspeakers.com/site2004/catalogo/pdf/18ws600.pdf
I calculated the resulting Qts with a series resistance of 2 Ohm and I only got 0.56, 0.58 respectively, though the Qts of both drivers is well beyond 0.4.
Are my calculations correct?

[D OB G]

Hi el'Ol,

I get those results too, but when I use woofers I always run two in parallel, therefore starting with a lower resistance of usually something like a nominal 3.4 ohms or thereabouts.  Your first woofer has a higher than usual minimum impedance.  Using two in parallel with 2 ohms resistance seems to give a Qts of about 0.7, which I consider to be at the highest end of the optimum range (0.56- 0.7). In practice, with an inductor in series, 2 ohms is a little bit lower than is achievable, even with very large gauge wire, for optimum low frequency extension, so the resulting Qts would be somewhat higher than 0.7 with two of these drivers in parallel. i.e the Qts of the driver is too high for the normal series inductor approach. However, given a resistance higher than 2 ohms, the Qts for one driver turns out OK, with the proviso that using the series inductor method, you would need twice the inductance for a single driver than for a pair, maybe 45-60 mH for this driver, depending on how low you want them to go, which really obviously becomes unworkable. (I'm assuming air-cored inductors).  I've always stayed clear of iron-cored inductors, but using them is the only way this would work for one driver.

David

[el`Ol]

Thanks, David.

For me this would be the way to go. But over 300 Euro just for the coils, I have to consider hard. If you are interested in 18" experiments I have found somebody who has some used Ciare HW450 to give away.
http://www.sas.co.at/verkauf.htm

Greets,
Oliver

[D OB G]

An update to the start of this thread.
I have just completed a new speaker, which is at the testing stage.
I have found Basta by Tolvan Data, which I learnt about on this forum, absolutely invaluable.
Where I previously used my variable inductor to determine the series inductance, and trial and error to learn the compensating resistance, to be verified by "Imp", the theoretical values offered by Basta seem to go straight to the correct result.
I am using two Eminence EMI 1550 15" bass drivers.
They are magnet mounted to a box 200 mm wide by 500 mm deep by 750 mm high, filled with sand.
A trolley helps to move them!!
The baffle follows my vestigial baffle theory as outlined on DIY forum.
It consists of two Heuga carpet tiles bolted to the rim of the chassis.
The baffle is 500 mm wide by 850 mm high, which gives an upper cutoff frequency of 300-400 Hz.
(The carpet tiles are 500 mm square).
They are made by "casting" the carpet pile, a 100% nylon loop pile, into a backing of bitumen, ending up 5 mm thick.
Very versatile, especially for offices.
But for open-baffle loudspeakers they are ideal.
Using the magnet mounting technique, vibrations are damped by the massive sand loaded support.
The baffle does not sustain any vibrations due to the self-damping carpet/bitumen sandwich.
It is easy to cut to any shape, and easy to unbolt.
A separate sub-baffle holds the open-baffle midrange (currently the Jordan 2" wide range), also operating by the series inductance method.
This means that the baffle size again determines the upper cutoff frequency, in this case about 2.5 kHz.
This is then crossed over to a Dynaudio Esotar tweeter.
All at 6 dB per octave.
Optimising for a system Q of 0.7, I'm getting -3 dB at 29 Hz (alright, call it 30 Hz!), for a driver that is supposed to have an Fs of 27 Hz.
I've tried system Qs of between 0.56 and 0.7, and although the lower Q is supposed to be "faster", I don't notice a difference, except for the expected roll-off in the lower frequencies.
To put these speakers into context, I suspect that I am making design decisions that may be at odds with the general approach on this forum.
Obviously the series inductance technique is not orthodox.
I've tried various full-range drivers (although not the B 200), and have never been satisfied by what I consider a beaming, peaky treble.
The Esotar has a smooth, fast, extended, wide-dipsersion, very low distortion response.
I know that many on this forum don't like dome tweeters.
Most contributors seem to be using valve amps.
I'm using two Naim NAP 250 amps.
But we're probably looking for the same general things:
"Open" sound- obviously!!!
Freedom from box induced resonances- non-linear compression/rarefaction, re-transmission through the cone (or port) etc. etc.
Natural sound stage.
Smooth, and in my case, fast transient response, i.e easy to listen to, but very much alive.
I hope this offering may be of some use in the continuing fun to be found in mucking around with loudspeakers!

David

[kyrill]

ah David this demands (!)  for some pictures. Can you make any?
 

[D OB G]

Hi kyrill,

Sorry about the delay.
Computer problems.
I'm on a borrowed computer.
I have some photos for when I get the computer back.
How do I post them?
How do I scale them?

David

[Gordy]

This should help 

http://www.audiocircle.com/index.php?topic=36654.0

[D OB G]

Thanks Gordy.
Photos when computer is back

[D OB G]

[/img]http://www.audiocircle.com/image.php?id=11224[/img]

Gordy, I can get this far, but no image.
What am I missing?

David

[Grumpy_Git]

shouldnt the first img ident not contain the / ?

that is the string should read: [ i m g ] blurb [ / i m g ]

or:



yup, that'll do it....

Nick

[D OB G]

The previous image showed the sand filled box with the square insets for the magnets of the Eminence woofers.
The insets are made slightly undersized, the magnets sqeezed in, and then the mdf is screwed tightly to prevent movement.

This image shows an example of a Heuga carpet tile (500 mm square). 

[D OB G]

[D OB G]

This image shows the "finished" speaker- the AUDIOSYNCRASY (Mk. something or other).

Since then the Jordan JXr 6HD has been replaced by a Focal 7N303, superseded many years ago, but still a favourite.

The baffle is just the two pieces of Heuga tile bolted to the rim of the woofers, and is, as mentioned earlier, self-damping.

David

[Gordy]

Thanks Grumpy     David, if you click on the picture icon (second from left on second row) it will give you the brackets automatically, just paste your photo between them.

[D OB G]

re: the AUDIOSYNCRASY, I find the virtues of the woofers being open-baffled with the series inductor method to be replicated if the same approach is taken with the midrange driver mounted on a separate baffle.

The upper cutoff frequency is again determined by the baffle size, and for a normally sized midrange driver will fall in the 2-3.5 kHz range (at 6 dB per octave).

The lower frequency is determined by the value of the inductor for the Re of the driver, and the excursion capabilities of an equalised driver.

If MJK is reading this, there is no issue with a rising impedance in the lower part of the operating range.

If the driver is magnet mounted, the basket and mounting flange usually provide the baffle, but a vestigial baffle can be attached, informed perhaps by the Edge, or by experimentation.  I find however that the free driver is normally the best.

Advantages in positioning and imaging.  Easier to align for step response. The Edge seems to show a (generalised) pattern of smoother response as the driver becomes a larger proportion of the baffle.
In this case we obvioulsy have a large proportion.  Round drivers benefit from a squarish vestigial baffle. And in any case, the part of the lumpy response demonstrated in the Edge above turnover frequency is precisely the part that is rolled off by the inductor.  The linear response below the turnover is the part used in this method.

It still seems that no-one has trialled the loading of a woofer with the series inductor method (it does require a large inductor), but a number of middle-sized inductors in series will give some idea. Only a small scrap baffle would be needed to show the results.

I would be interested to hear the results of any attempts.

David

[el`Ol]

Hello David!

I have found something that could be worth trying (price is very reasonable):
http://www.monacor.de/de/produktseite_carpower.php?artid=1873&spr=DE&typ=u
http://www.intertechnik.de/index.html/JTI2bmF2aWQlM0QxNzY4JTI2bGFuZyUzRGRlJTI2c2lkJTNEbjQ2YzMxZThlZWUwZDklMjZiJTNE.html?basis=75&detail=3567&suchwort=
Cone breakups are not as good as Selenium, but I have my doubts that your Eminence OEMs are better.
I am still searching for a 4 Ohm fullranger. I heard the little Jordan as midranger in a Japanese box and didn`t like it too much. Ciare`s 4 Ohm fullrangers all have whizzers, even the 4". The Supravox 165LB is a bit expensive. I have my doubts that DC Gold drivers are better than Jordan, they also have Aluminium diaphragms. Does anybody have experiences with them?


Oliver

[D OB G]

Hi Oliver,

That Monacor driver does look very good.
Good frequency response, good Fs, but the Qts is way too high for the series inductor technique.
The Eminence has a good Fs of 24 Hz, but also a good Qts for this method.
Results are good. I'm only using them to 300-400 Hz.

The Intertechnik inductor is interesting.
They are getting 30 mH with, have I understood it correctly?, 1.18 mm wire, and 0.9 ohms.

When I use the thicker 1.8 mm wire, or even 1.6 mm, to get 30 mH I am over 2 ohms.
(I wind them air-cored myself).

As they are transformer coils, are they ferrite cored?
If so, then I don't think they are appropriate for a crossover of this nature (after all, the dc resistance is part of the design).

Thanks for going to the trouble of checking out these details.

David

[D OB G]

P.S.  Ted Jordan specifically says that the JXr 6HDs CAN'T be used open-baffle!!!

They must be used infinite baffle, and the Qts is determined by the volume, partly because there is no spider.  i.e. the cone is held solely by the front suspension and the ferrofluid in the gap.

They were only an experiment (that failed), but I belive they may have a use (open-baffled !) if used only above 1 kHz (He uses them down to 150 Hz, or 120 Hz with his four driver line-source).  We'll see one day.

David

[el`Ol]

Hello David!

My calculations yield Qts 0.535 for the Monacor and the IT coil. Rather a bit low for OB, but as the Monacor is recommended for free air use with a Qts of 0.44 I think I can give it a try.
Strange what you tell about the Jordan. Why should a driver that is suitable for IB be not suitable for OB?

The values of the IT are as you mentioned, the core is an E-core made of silicon-iron.

Oliver

[D OB G]

Hi Oliver,

Apparentley the compression within the enclosure of the Jordan in an IB (and it is a small IB) "stabilizes" the travel of the cone, in the absense of a spider (obviously at odds with open-baffle advantages).  They do have remarkable travel (about +3mm/-3mm) for a two inch driver.

David