Interesting field coil drivers in Germany

[Guido]

Looks good

http://cgi.ebay.de/WE594-successor-JBL-375-pair-field-coil-drivers_W0QQitemZ260336870901

[Jan Daugaard]

The seller has an informative picture in another auction:
http://cgi.ebay.de/Altec-Lansing-287-successor-pair-field-coil-drivers_W0QQitemZ260336875753QQcmdZViewItemQQptZDE _Lautsprecher_Martin?hash=item260336875753&_trksid=p3911.c0.m14&_trkparms=72%3A1229%7C66%3A2%7C65%3A12%7C39%3A1%7C 240%3A1318
but it's impossible to form an opinion without elaborate technical specifications, and the seller hasn't published any.

[Allanvh5150]

And one would pay 5000 euros for these because?:blink:

[Beowulf57]

And one would pay 5000 euros for these because?:blink:

To put them on display...they look so cool, surely no one would ever intend to use them! LOL :D

[Steve Schell]

This fellow has displayed good craftsmanship in converting some older JBL drivers to field coil. Unfortunately he has made the same mistake that several others have made by using an oversized center pole and undersized field coil. The old field coil compression drivers generally used a center pole the same diameter as the inner pole piece, which allowed plenty of room in the pot for a nice large cross section field coil. The common mistake of guys who have never seen the early drivers apart is that they look at the widened short skirt on the center pole of the permanent magnet drivers (done to accomodate the ring magnet) and think that this is the proper diameter for the center pole. This results in a skinny, pipe-looking field coil which lacks adequate cross section to properly energize the voice coil gap without excessive heating to the field coil.

I've attached an illustration of the work necessary to convert an Altec 288 to field coil (over a sectional drawing of an Altec Lansing 287 field coil driver). The new center pole section is machined to complete the circuit and the widened skirt of the 288 center pole section is machined off. The 288 magnet and die cast throat section are discarded. The process would be similar for the large format JBL driver.

[JoMoCo]

I wonder if GPA (Great Plains Audio) would consider remanufacturing the 287 field coil, for those who may be interested, with a 12vdc coil...like the Fertin.

[Steve Schell]

JoMoCo, you'd have to call Bill Hanuschak and ask him if he would consider such a project. I believe he is familiar with the early Lansing field coil drivers. Twelve volts is a good choice for the field voltage in my opinion. My past experiments have shown that the ratio of gap flux density to field dissipation is increased as the coil is designed for lower supply voltages. The allowable current of the larger wire sizes increases faster than the number of turns decreases with a given coil cross section, permitting more ampere turns. Actually 14 VDC may be an ideal design target as there are 13.8 VDC switching supplies available (such as Radio Shack) that perform very well at minimal cost.

In my opinion the special sound of the Lansing drivers is partly due to their field coil operation, and partly because of the differences in the diaphragm. The Lansing drivers used a diaphragm with a half roll outer compliance, not the tangential compliance used by Altec beginning with the 288. The simpler half roll compliance does not provide as extended a high frequency response, but does not misbehave as much either and sounds very smooth.

[wolfvonlanga]

Somehow interesting how speculative forum discussions are some times...
The choosen diameter of my pole pieces are calculated to take cross section of every iron part in the magnetic way into account. They are not too big and not too small. And material matters! Space for coils in my drivers is big enough to handle more flux than needed as the coils calculated from a well known transformer company and not only guessed. About the comments for price: I know that DIYs always looking for getting Gold for less than nothing but my drivers are not built for those people. I do want an equivalent value for the used materials and investments. So please don't argue on price. I wish people could hear what I'm talking about first. (I tried several filed coil drivers from different manufacturers - and mine are excellent, believe me). There are some field coil drivers out there for far more. After several reports confirmed their quality there is not even a word about prices. So be patient please - or go for a pair and hear! Kind regards, Wolf von Langa

[Mr. Widget]

Hi Wolf,

I'd love to hear more about your history, why you decided to go this route, how long you have been at it, etc. I have zero experience with field coil drivers, but from a purely technical standpoint their potential is quite interesting to me.

Welcome to our site... good luck with your venture.


Widget

[Steve Schell]

Hi Wolf,

Welcome to Lansing Heritage! I am happy to see you posting here, and I am sorry if my initial post seemed critical of your efforts. Long timers here know that I am enthusiastic about field coils, and try to encourage others to venture "back to the future." I have however consulted with several fellows whose efforts closely resembled yours with the large diameter center pole and thin field coil compared to the vintage units. They had based their choice of center pole diameter on the diameter of the widened skirt adopted when the field coil driver ancestors were converted to an interior ring alnico magnet. One fellow had reported excessive heating in the field coil and was rather down on the technology as a result.

It sounds as though you have good reasons for your dimensions, and seem to be aware that you have apportioned the space inside the pot rather differently than the original designs. Actually I have only seen pictures of your Altec 288 conversion, so I do not know what you have done with your JBL conversion. The production Western Electric 594A upon which the JBL large format drivers were based used a center pole the of same diameter as the inner pole piece, just under 4". Edward Wente's U.S. Patent drawings of the prototype driver that became the 594 actually indicate a center pole shaft of smaller diameter than the inner pole piece. These early drivers all used a solid iron center pole with the conical throat exit bored through the center; the cast throat inserts began with the permanent magnet drivers.

[JoMoCo]

I share an interest in the recent expanding availible choices in field coil offerings. It would be interesting to hear from those who may own them and would be willing to share their considered listening observations.

I just noticed Classic Audio finally updated their web site and are showing field coil components including a 15" woofer, 3 different mid drivers & SHF driver... all 12vdc motor coils!...

Anybody going to CES this year?

[jcrobso]

In 1949 my dad bought a Hi-Fi it had a 15" field coil speaker. It worked sounded good for the time period.
My gut tells me that the voice coil cannot tell the difference between the flux created by a PM speaker vs a field coil speaker.
Do they list the power required for the field coil in watts?

[louped garouv]

I think in Volts...

[jcrobso]

A magnetic filed is created when current flows through the wire.
The larger the current the larger the magnetic field.

[Horn Savant]

....without excessive heating to the field coil...

WE 594's tend to start heating @24+v ! -- whether they're designed w/ this in mind or not, I'm not sure. I for one would not be comfortable w/ that. ~22 v seems much better for temp stab (just marginal warming)

[Horn Savant]

In my opinion the special sound of the Lansing drivers is partly due to their field coil operation, and partly because of the differences in the diaphragm. The Lansing drivers used a diaphragm with a half roll outer compliance, not the tangential compliance used by Altec beginning with the 288. The simpler half roll compliance does not provide as extended a high frequency response, but does not misbehave as much either and sounds very smooth.

Agreed--I like the sound of JBL 375/2440 (I have three - not being used - w/ early half roll aluminum -- two of them seem to need magnetic recharge--will have to get them zapped one day)(one advantage of Field Coils ;))

[jcrobso]

http://cgi.ebay.com/JBL-150-4C_W0QQitemZ260450296111QQcmdZViewItemQQptZVintage _Electronics_R2?hash=item3ca40c212f&_trksid=p3286.c0.m14&_trkparms=65%3A10|66%3A3|39%3A1|293%3A1|294%3A100

[pos]

Is it a E145 cone?

[ratitifb]

Probably yes from the pictures. Actually it's like the K145 cone with the earliest paper surround.

[altec288]

Hi Steve-thanks for the great posts. I am going to attempt to convert my 288c into a 287. I noticed people seem to try to use a 12-15v power supply rather than the original. Do you know what wire gauge is used? Do I still need to be at 2500 ohm resistance. The original 287 from pictures appeared to use a thin wire. I guess what I am asking is what is my goal on making the coil? Wire size? Gauss? Are thsese even the right questions? Thanks, Bobby




This fellow has displayed good craftsmanship in converting some older JBL drivers to field coil. Unfortunately he has made the same mistake that several others have made by using an oversized center pole and undersized field coil. The old field coil compression drivers generally used a center pole the same diameter as the inner pole piece, which allowed plenty of room in the pot for a nice large cross section field coil. The common mistake of guys who have never seen the early drivers apart is that they look at the widened short skirt on the center pole of the permanent magnet drivers (done to accomodate the ring magnet) and think that this is the proper diameter for the center pole. This results in a skinny, pipe-looking field coil which lacks adequate cross section to properly energize the voice coil gap without excessive heating to the field coil.

I've attached an illustration of the work necessary to convert an Altec 288 to field coil (over a sectional drawing of an Altec Lansing 287 field coil driver). The new center pole section is machined to complete the circuit and the widened skirt of the 288 center pole section is machined off. The 288 magnet and die cast throat section are discarded. The process would be similar for the large format JBL driver.

[Steve Schell]

Hi Bobby,

You are on the right track and asking the right questions. A field coil can be designed to make use of most any supply field voltage. High voltage requires a field coil made of very fine wire and therefore high DC resistance, while low voltage requires a field coil of large wire and low resistance. The design process involves several steps: choose your desired supply voltage, determine your field coil cross sectional area in square inches, then start making calculations. A given size magnet wire will have a specified number of turns per square inch. Determine the average length of turn (I based my calculations on 2/3 the way from inner to outer coil diameter) and multiply this length by the total number of turns to determine the length of wire used in the coil. Do this for several possible wire sizes. Then find a table of magnet wire resistance in Ohms per thousand feet and find the coil resistance for each wire size. Then make an Ohm's Law calculation of the current at each resistance at your chosen supply voltage. Then find a chart of maximum permissible current in circular mils (whatever they are) and pick the wire size that allows the highest current from your supply voltage without exceeding the max permissible current. Simple, right?

The figure of merit of magnetic strength of a field coil is measured in ampere turns: the DC current flow in amperes times the number of turns. As one goes to larger wire sizes the number of turns decreases, but the permissible current increases faster. This results in higher ampere turns in low voltage coils. In my calculations this continues to well below 12 VDC, though this is a most practical voltage to design to. In making gap flux density measurements years ago, some Jensen field coil 12" drivers with 5000 ohm coils could not generate the gap flux density of identical drivers with low voltage coils. Never noticed much difference in listening, though.

I found my chart in an old 1947 ARRL Handbook, but there must be more modern and accurate info on the web these days. After all, my chart was so old it was for silk covered wire!

Give me a couple days to try and find my old notes on the 288/287 conversion. If I find them I can probably give you a few more hints.

[ivica]

Hi Bobby,

You are on the right track and asking the right questions. A field coil can be designed to make use of most any supply field voltage. High voltage requires a field coil made of very fine wire and therefore high DC resistance, while low voltage requires a field coil of large wire and low resistance.....

Hi Steve,
not to argue with You, but I would suggest to use active constant current amplifier, as to day semiconductors are not so expensive as it was the case in the early previously, but using today strong magnet (based on neodymium-boron or samarium-cobalt) would be much more reasonable in order to saturate magnetic circuit of the driver, and be much more reliable solution.

regards
ivica

[Steve Schell]

Hi ivica,

Thanks for your comments. Rare earth magnet structures are the norm these days, and have many advantages, but will not convey the same bragging rights as field coils to the truly fevered, LOL. Count me among those who have found field coils to have certain magical sonic properties, and the beautiful thing is that we are all entitled to our preferences!

[altec288]

Hi Steve

Cant wait for any hints. I think I am going to bypass the making of a 2500 ohm old school 287. Will just do newer version that other people do. Still would like to know wire size. Here is what I do know, the 288 was 18,500 gauss.
So if I did a 15 volt supply using 18 guage wire. 1000 feet is 200 ohm resistance. Am I trying to achieve the gauss rating of 18,500 with the field coil? It justs seems there should be an answer that is easy.






Hi Bobby,

You are on the right track and asking the right questions. A field coil can be designed to make use of most any supply field voltage. High voltage requires a field coil made of very fine wire and therefore high DC resistance, while low voltage requires a field coil of large wire and low resistance. The design process involves several steps: choose your desired supply voltage, determine your field coil cross sectional area in square inches, then start making calculations. A given size magnet wire will have a specified number of turns per square inch. Determine the average length of turn (I based my calculations on 2/3 the way from inner to outer coil diameter) and multiply this length by the total number of turns to determine the length of wire used in the coil. Do this for several possible wire sizes. Then find a table of magnet wire resistance in Ohms per thousand feet and find the coil resistance for each wire size. Then make an Ohm's Law calculation of the current at each resistance at your chosen supply voltage. Then find a chart of maximum permissible current in circular mils (whatever they are) and pick the wire size that allows the highest current from your supply voltage without exceeding the max permissible current. Simple, right?

The figure of merit of magnetic strength of a field coil is measured in ampere turns: the DC current flow in amperes times the number of turns. As one goes to larger wire sizes the number of turns decreases, but the permissible current increases faster. This results in higher ampere turns in low voltage coils. In my calculations this continues to well below 12 VDC, though this is a most practical voltage to design to. In making gap flux density measurements years ago, some Jensen field coil 12" drivers with 5000 ohm coils could not generate the gap flux density of identical drivers with low voltage coils. Never noticed much difference in listening, though.

I found my chart in an old 1947 ARRL Handbook, but there must be more modern and accurate info on the web these days. After all, my chart was so old it was for silk covered wire!

Give me a couple days to try and find my old notes on the 288/287 conversion. If I find them I can probably give you a few more hints.