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Old 16th June 2012, 11:13 AM   #1
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Default Open Baffle with Field Coil speaker

Hi,
My question is if Jensen Field Coil speakers (12-15 inch) would do well in a open baffle.
These probably were used for Leslie guitars or Hammond organs, hence a dedicated purpose that might be far away from HiFi.

I have been triggered by some readings on the beautiful field coil speakers around, vintage (there seem to be a lot of Magnavox, Jensen Field Coil speakers around); new such as Supravox; and specialty shop products such as from Rullit.

I am contemplating to use the Jensen 12" as a bass/mid bass in an open baffle, supplemented with a proper fullrange for the top end. I was thinking Lowther, or what else.

The Jensens have a phenol spider, quite different from a normal pleated spider, that implies the dynamics go up. And dynamics, are what I am after, plus of course uncolored sound.

Loading the open baffle:

I think it was on Nelson Pass' site that I saw some arguments for a slot loading, a reasoning that made sense. The intent is to have a smaller surface on the front than on the back, this means the pressure is reduced on the back - if I recapitulate well this gives a nicer atmosphere. I would go for about half the surface on the front as on the back.

A 12 inch Jensen Field Coil will give about 95-98 dB of warm bass and lower midrange.
A smallish fullrange such as Lowther C45 can take the top, preferably from as low as possible, also providing 96 dB.

This could run happily on my Hiraga 8W, my F5 or my 300B (6-15 Watt).

My initial drawings for a baffle height of about 1 meter and a width of about 50-60 cm, the sides adding another 20 cm:

open baffle.jpg

Click the image to open in full size.

See f.e. JENSEN field coil speaker F15LL

I found this Jensen OB project that shows this frequency plot:

Click the image to open in full size.

Well, that is not a straight line!! But I recognize the effects of a too small baffle also.
  • Is this type of speaker useful?
  • Are these old trash speakers bound to start wobbling and rubbing the voice coil such that this would give a lot of pain? Better to use a modern speaker?
  • Should I go for full mounting or is this slot idea good?
  • Should I leave the fullrange wide open or roll it off a bit above its resonance?

any ideas appreciated

albert
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Last edited by triode_al; 16th June 2012 at 11:49 AM.
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Old 16th June 2012, 12:36 PM   #2
Bigun is offline Bigun  Canada
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Having read the thread about slot loaded OB's more than once I have become persuaded by the arguments made there by people who understand speakers well that it (the slot) doesn't provide any 'free lunch'. And given that you will use a sub, I would suggest that a simple 'traditional' OB design would be a better starting point, not least because if you are not sure about the results, you have one less variable to worry about. Where the slot loading seems to make sense is at lower frequencies, with the potential to reduce the amount of higher frequencies emitted (less distortion - a fact exploited by Tom Danley in his Synergy horn) and as you will use a sub I don't see that it makes sense.

I like the look of that driver. I've never heard a field-coil unit myself but they certainly have a good reputation for sound quality.
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Last edited by Bigun; 16th June 2012 at 12:49 PM.
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Old 19th June 2012, 01:33 PM   #3
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Default Jensen not fit

I have simulated the Jensen in the Open Baffle program and this does show the unit has high efficiency, but that the low end is lacking compared to other drivers. The Jensen is more fit for a reflex enclosure probably.

Here is a simulation I ran:

Jensen en eminence sim.jpg
  • It is clear that the low compliance of the Jensen FC is hindering performance. In fact (in theory) the 12" Jensen Field Coil performs is the 'worst' in this shoot-out in OB.
  • The Eminence Alpha is the winner in smooth low end. E Kappa 15 with SE amplifier - an amplifier with a low damping factor - is not bad at all and just needs a slight lift that it can easily accommodate in power.
In another sim I ran Lowther against Visaton B200, and the latter is very suited also for a real full range in the bottom end, better than the Lowthers.

SEAS, LOWTHER, B200.jpg

Re slot loading:
The slot loading can be done with the Eminence Kappa's and then the Q factor will rise to a very handsome level I expect, but I can't calculate just guestimate

albert
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Old 20th June 2012, 07:05 PM   #4
4krow is offline 4krow  United States
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Default Open Baffle Option

This may not directly apply, but I think that there is reason for consideration. Go to the Decware Audio site, and look up the Open baffle design there. You will find that Steve Deckart, the designer, has made use of the hemholtz resonator tuned to a low frequency. It would be cool if that concept could be used in your project. Obviously changes would have to be made for your driver. Just a thought.
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Old 21st June 2012, 05:08 PM   #5
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Default super point source?

Thanks 4crow,
nice link.
Incredible driver that they have 4 an Open Baffle, at least there is a nice form factor: MODEL FRX2


This formula might be what I was looking for: it allows a calculation of the cavity and output slot! A horn uses a Helmholz resonator too doesn't it for the horn throat adaptation?

I tried a calculator in a first attempt, but the numbers are very high.
What I expect is to find the low-pass frequency of the slot and the influence of the cavity size on Qt. In the calculator, for something reasonable in size the resonance frequency in the calculus leads to e.g. 180 Hz. I am not sure if this is a sane outcome.
What am I doing wrong?

Albert
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Old 29th June 2012, 01:13 PM   #6
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I got two drivers from a surplus. They are ghastly. Double magnets. > 94 dB.

XingHui car bass 2663.jpg

Not a field coil, agree, but I'll try them in an open baffle anyway. A FC will have to wait till later this year for a next round of experiments.
Outside 27 cm, cone 20 cm, 8" eff.
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Old 29th June 2012, 06:06 PM   #7
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Default ongoing project...

Good evening Albert,

(i could switch to Dutch now but that would not be very polite to English readers....)

I noted your ideas on a fieldcoil woofer in an open box design. It is just what i did......
Started this year with 2 amazing Fertin 38EX 15" fieldcoil woofers (see pictures). I decided to pair them with fullrange drivers in a FAST-configuration. For the fullrange-units i opted for Feastrex D9NF-units.

A great advantage of the fieldcoil-woofers is that you can change the Qts. That's a great asset when you want to match the woofer to the open baffle (or other enclosure). Of course you'll sacrifice efficiency but with these units you'll always stay on the healthy side of 90 dB.

I'm still tinkering with the crossover. Currently i use an autoformer to level out the efficiency between the Feastrex and the Fertin. At this moment it is in a very simple 18 mm plywood construction just to see if/how it works. The actual (large) open baffles are already ordered but the holiday-season will stall their delivery. The open baffles will be made from 30 mm plywood + 18 mm chipboard + 25 mm MDF in order to suppres resonances.

When the project is ready i'll post it on this forum......

Rest of my equipment: self build turntable + self build tonearm (18") + self build power amplifiers (TB 3/1000 triodes) + self build DAC (TDA1541A with step-up transformers) + Gryphon preamp.

I will follow your thread and look forward to the next building phase.

Regards, Reinout
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Old 29th June 2012, 08:39 PM   #8
tvrgeek is offline tvrgeek  United States
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Linkwitz has a lot on dealing with OB in his Orion project. Specifically, how to determine the eq needed.
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Old 30th June 2012, 07:05 AM   #9
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Quote:
Originally Posted by ReinoutdV View Post
Good evening Albert,

(i could switch to Dutch now but that would not be very polite to English readers....)

I noted your ideas on a fieldcoil woofer in an open box design. It is just what i did......

A great advantage of the fieldcoil-woofers is that you can change the Qts. That's a great asset when you want to match the woofer to the open baffle (or other enclosure). Of course you'll sacrifice efficiency but with these units you'll always stay on the healthy side of 90 dB.

Rest of my equipment: self build turntable + self build tonearm (18") + self build power amplifiers (TB 3/1000 triodes) + self build DAC (TDA1541A with step-up transformers) + Gryphon preamp.

Regards, Reinout
Hi Reinout, looks like a marvelous setting.
Funny you have the TT disjoint from the arm. I once made an arm with two crossed springs (scheermesblaadjes) 5 cm before the shell, with a rigid pivot (only horizontal). Was big fun. Until one day my wife was cleaning and thought the arm should be straight .

The variable Qts of the FC is indeed interesting, I will follow up that. This means the efficiency goes down but also the damping, hence the rendition of the lower (up?) and mid (down) bass changes. Only to be done on the ears.

albert
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Old 30th June 2012, 08:41 AM   #10
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Quote:
Originally Posted by tvrgeek View Post
Linkwitz has a lot on dealing with OB in his Orion project. Specifically, how to determine the eq needed.
Good link, Linkwitz has always been very good at the filters.

In an OB a baffle step of just 6 dB at a variable Fc will work well too. I like the simpleness.
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