Field Coil conversion for JBL, Altec, and Western - diyAudio
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Wow, diyAudio is a nice place to visit but I wouldn't want to live there! Makes me appreciate this Altec User's Board! Pano's job moderating there is like herding cats!
I'll copy a comment about the DIY thread I made yesterday on the High Efficiency Speaker Asylum:
Thank you for the interesting thread link, Deon.
I won't get into the merits of field coil conversion except to say that IMO the results are worthwhile and not subtle. One thing I'll mention is that a mistake is often made when converting Altec or JBL compression drivers to field coil, and I see it happening again in this thread. The original Lansing 1930s field coil compression drivers used a constant diameter on the center pole from the voice coil gap to the back (actually front) plate. A sectional view can be seen at the bottom of this page:
http://www.lansingheritage.org/image...943/page07.jpg
When the conversion to Alnico V magnet was made in 1944, the center pole was shortened so that it could sit atop the interior ring magnet. Also, a skirt was added below the voice coil gap to widen the center pole to the diameter of the top of the magnet. This occurred with both small and large format drivers, as can be seen in the drawing of a small format driver here:
http://www.lansingheritage.org/image...804a/page1.jpg
Most folks who convert these drivers to field coil have never seen an original Lansing apart, so they machine a new lower center pole section to replace the magnet using the incorrect wide diameter of the skirt. Then a rather anemic field coil is wound to fit over this wide diameter, which will have inadequate cross section resulting in low gap flux density and/or possible coil overheating. When making the conversion, the skirt should be machined off the existing phasing plug/center pole and a new section of the same diameter machined to replace the magnet. Then a field coil of correct dimensions can be wound and the motor will work as well as the priceless 1930s Lansings.
I don't understand why field coils would sound any better than an Ainco or ceramic magnet, Isn't Magnetism just that? Or is the field coil a stronger magnet? or less stronger magnet?
I am sure there are more than one reason.
One I can think of is the ability to change the field strength to suit taste.
I'd like to hear this stuff, but probably never will.
i have limited experience, but the WE 555s I have call for 7 volts input, no idea on the current. I also had an old Jensen which would scream with a 10 watt amplifier, and sadly let out the magic smoke after a while. But I had no idea of the voltage or current I supplied it with. Made lots of smoke and did it stink.
some magnet materials and configurations, maintain their flux better than others.Quote:
Originally Posted by cradeldorf
Learn some of the terms associated with magnetism such as hysteresis, coercivity, and saturation, and you will automatically gain an understanding of why different magnetic sources can behave differently, especially in regard to the effects inductive pulses(an audio signal) have on them.Quote:
Originally Posted by cradeldorf
With nearly a 100,000 members that could be a daunting task.Quote:
Originally Posted by artlab
Whew, tough crowd over there in that Multi Way forum! Never got hate mail PMs before that thread.
It is very hard to have a conversation with all the topic cops and Parts Express catalog hit men lurking in the shadows. Pano deserves battle pay.
To think I went over there because I was interested in Altec field coil conversions...wound up fighting off idiots who think they are scientists but they are more like the Taliban.
Best way to look like a moron is to argue with morons, so I am guilty myself.
Definitely makes me appreciate the civil and friendly attitude on this board all the more!!!
Steve, appreciate your comment on the mechanical aspects. Where I am trying to get to is Lansing territory. That is a good place to be. Back in the day, I did compare Lansing 415s and 287s with the original Altec PM versions and it was a night and day difference.
Lansing was my main system for a few years. Now my parts are in the Silbatone museum. I'm trying to push to set this stuff up and reevaluate it, maybe take it out to a few shows so people can hear the glorious Lansing sound. That was some really great sounding theater gear.
Granted there are other differences in the drivers besides the motor, including those beautiful roll surround Lansing diaphragms, but they do share strong family resemblances.
So, maybe it wasn't a tightly controlled lab experiment but it was enough to teach me to think for myself and trust my listening experiences. At least I did the experiment. Thankfully, I was lucky enough to have that opportunity in the pre-ebay good scavenging days. It would be difficult and crazy expensive to assemble all that ancient stuff now.
The most compelling technical argument I saw for field coils discussed overload conditions and recovery characteristics of various magnetic materials. Most well-behaved were electromagnets, followed by AlNiCo, ferrite, and neodym in that order.
Now if only I could find that study again....does this sound familiar to anybody?
I tend to speculate that the presence of an external power source for the magnet forces the system back into line, but this is just a mental picture that I like.
Steve, thanks so much for the info and the links. That's the sort of real, practical info I'd love to see on the thread over at DIY. I despair that may never happen. :(
This was a good read.
High Efficiency Speaker Asylum
My take on ALNICO & PM drivers; The VC AC flux modulates the PM maginetic field which causes back EMF and higher distortion levels...non-linear maginetic flux...
My take on Field coil drivers; Back EMF in the field coil due to modulation of the Field coil magnet would be minimized as these currents would be shunted through the Field coil power supply....
My conclusion is that a Field coil driver should sound better..... But JBL came up with a
mod for PM drivers, A shorted turn to shunt the back EMF and reduce non linear flux ....This would be an interesting research project.....................My 2 cents......GC
Yeah, cradeldorf, that thread was interesting.
One great point was that field coil does not automatically make a speaker good! So true. Most of them are junk, or at best nothing special.
But some real good drivers only came out in FC versions, so how can you know if it is the FC making them good?
The comment about the negatives of humbucking coils was interesting also. I recently dismantled a Jensen 18" Auditorium speaker and a WE TA-4181 18" driver. Since the WE unit was OEM Jensen or at least Jensen parts, the metal parts and construction were identical. The same replacement cones fit both units. However, the Jensen had a humbucking coil and the WE did not. That was the only visible difference.
Well gotta get off the computer and finish packing. Leaving for the Munich High End show in the morning, where I will be hearing field coils for a week, 10 hours a day...
PS: Here's some pics of the inside of a TA-4181. Scary looking ancient stuff!
Great info and read Craddledorf, but now I am a little perturbed, there was a company in europe selling new repros of 16" 604's in a field coil design. At a price tag of around 7000 a pair I thought it was a little extreme, but maybe I was wrong!!!! Now I cant find the company.........
I remember seeing something on Ebay about totally rebuilt 604's. It was a company/Person in Germany. They had pictures of one completely stripped down and sandblasted. I remember they were priced way up the scale from the normal prices but I don't recall anything about it being converted to a field coil.Quote:
Originally Posted by juniper
A604 | Kilimanjaro-Series | Wolf von Langa
Maybe Joe will run into this fella at the Munich show, and can give us some insight.
Yup I was going to suggest this too. Trumped by that Chevy guy again ! :Big smile:
I'm particularly interested in the A284 & the A416 . Hope you get to hear the Wolf Von Langa FC's.
http://www.kilimanjaro-series.com/En...ries/a284.html
http://www.kilimanjaro-series.com/En...ries/a416.html
Cradeldorf, I have intended to conduct experiments to test for the superior quality of field coil magnetic circuits, but to date I have failed to do so. The prevailing theory is that field coil motors resist flux modulation by voice coil currents better than permanent magnet motors do, providing better fidelity to the driving signal. A permanent magnet structure is provided a magnetic charge from a momentary infusion of energy. This charge can be momentarily or permanantly altered by voice coil currents. An elecromagnet is connected to a DC power supply which is capable of providing current to maintain the magnetic circuit under dynamic conditions. The subjective results of field coil operation are of control of the moving mass, providing great fidelity to the audio signal. The results are similar to a good Alnico motor, but a bit better yet.
This all begins to be a religion after a while, like triodes vs. pentodes, tubes vs. transistors or you-name-it. Most field coil advocates have reached their preference after numerous convincing listening sessions. We are all allowed our preferences, and field coil advocates enjoy the sound of their drivers. A field coil cannot make a flawed speaker great, like a 6" cone trying to operate full range. Electromagnets do seem to provide benefits though, at least to their adherents. "Nuff said.
Some people allready have a business in FC conversions. On the bottom of this page //www.ch-audio.com/CH288287.html there is a measurement comparison between a stock alnico and a fieldcoil converted 288 driver. Clearly the field coil is smoother and more extended, surprisingly (or not?) most at the bottom of the FR. I would expect more difference at the top end if the differences where only due to the FC having a stronger field.
As least as interesting is that the driver has a very straight FR from 150-15kHz! Very nice for any full range driver. Now you just only need a suitable horn, like an WE-15A ;).
Logic should suggest this conclusion as lower frequencies require larger pulses of AC current to maintain a given acoustic output. It makes sense to me that it is in these critical low midrange frequencies that we would most likely sense(hear) differences that could be directly attributed to the reluctance and saturation characteristics of various motor types.Quote:
Clearly the field coil is smoother and more extended, surprisingly (or not?) most at the bottom of the FR.
Beyond that, i just think it'd be cool to have a nice rack of glass cased wet cells to energize my speaker motors.
Like these replicas.Quote:
Originally Posted by Coendert2011
home
JBL Technical Notes Volume 1 Number 9
This is the app note ; ALNICO Vs. Ferrite maginets and the shorted
turn mod to prevent modulation of the maginetic field. Notice the reduction in distortion.
No data is given for field coil drivers but I presume the field coil power supply is the
shorted turn.............GC
AB my friend we have 219,331 members and yes, it is a daunting task. You have to spend 15-20 minutes moderating stuff before you even get to post anything of your own, even with around 10 moderators. The upside of it is that you attract a lot of talent. The downside is that not even the brightest minds are necessarily social aficionados.Quote:
Originally Posted by Altec Best
I hear you brutha. I'm with you on that Cal (Social Aficionados)! But my gosh 220K members now,wow the last time I checked was quite awhile ago.Can't believe DIY has more than doubled since then.Quote:
Originally Posted by Cal Weldon
And I thought I had it tough. ;)
Quote:
Originally Posted by Steve Schell
Thanks for the info and pictures, Steve.