Interesting field coil drivers in Germany

[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_W0QQi...%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.