DIY 802 field coil drivers may be a reality!!

[Steve Schell]

As to the permanent magnet vs. field coil discussion... I advise everyone to listen and to trust your perceptions. For me, f.c. drivers take the best vintage p.m. drivers a step further toward true transparency, but then I am a manufacturer so there you go. I do a lot of late night listening and the best f.c.s take me to places the p.m.s never did, but your mileage may vary.

In designing a driver, the cross sectional area of the rest of the circuit must be four or more times larger than the voice coil gap to keep the magnetic reluctance low everywhere outside the voice coil gap. I calculate the voice coil gap by multiplying the diameter of the inner pole piece times the height of the (usually tapered) outer pole piece. The rest of the circuit is calculated by multiplying the height times diameter (horizontal sections) and the area of the outer diameter minus the area of the inner diameter (vertical sections). This all makes sense once you draw out a circuit and begin calculating.

Calculating field coils is a real bag of worms. Basically you want the most ampere turns with a given coil cross section without exceeding the maximum current carrying capacity of the wire at a given supply voltage. I can go into this further if you like, though I will have to revive a few brain cells.

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Further thoughts... the original Lansing drivers achieved gap concentricity via stepped machining of the top plate edge and pot edge, and by the center pole being rigidly affixed to the front (exit) plate. In an 802 retrofit one needs to retain the stamped ring that maintains concentricity between top plate and center pole, and some sort of mechanical connection between the newly created center pole beneath the phasing plug and the front (exit) plate. This can either be via screws (as Lansing did), or with machining of the center pole that fits into the 1" exit hole in the front plate. The newly created f.c. driver does not retain the cast exit flare piece; instead the new iron center pole piece incorporates the tapered exit bore below the phasing plug, closely mirroring the geometry of the original Lansing driver.

[mah]

Jeff'

Your experience with FC loudspeakers does not mean ALL FC loudspeakers are deficient - that argument is called rash generalisation.

The tap dancing phenomena was due to horn time-alignment, not field coil drivers. Impeccable source: http://www.aes.org/aeshc/docs/afternoon_hilliard.pdf - see page 2.

The car wreck analogy is meaningless in explaining electromagnetic bucking in electrical circuits.

The interaction of magnetic fields has been known at least as far back as Michael Faraday(1791-1867). One type of interaction can cause dominant flux modulation in loudspeakers which can be measured: https://www.klippel.de/fileadmin/kli...Modulation.pdf


IMO the electro-dynamic/electro-mechanical components are the most critical in the sound reproduction chain.

[BillWojo]

Steve, any info on the coil design would be most interesting.
Your thoughts on FC performance has been mirrored by most folks that have tried them, hence my interest.
As far as the comment (Recall the original " two taps for one with a tap dancer"... using Western Electric field coil gear.), I'm rather surprised to have that attributed to field coils. Makes me laugh.

BillWojo

[mah]

Humbuck is relevant to this thread. Humbug is not.

[LowOhms]

Jeff'

Your experience with FC loudspeakers does not mean ALL FC loudspeakers are deficient - that argument is called rash generalisation.

The tap dancing phenomena was due to horn time-alignment, not field coil drivers. Impeccable source: http://www.aes.org/aeshc/docs/afternoon_hilliard.pdf - see page 2.

The car wreck analogy is meaningless in explaining electromagnetic bucking in electrical circuits.

The interaction of magnetic fields has been known at least as far back as Michael Faraday(1791-1867). One type of interaction can cause dominant flux modulation in loudspeakers which can be measured: https://www.klippel.de/fileadmin/kli...Modulation.pdf

IMO the electro-dynamic/electro-mechanical components are the most critical in the sound reproduction chain.

hi mah,

The RCA MI-1428B and Mi-1443 I owned and used are among the better of field coil industrial designs ever done. So I would disagree about your term "rash generalization", from what I heard.

Have you ever heard a field coil driver of HIGH quality, such as a Mi-1428B or a MI-1443? Or are you just theorizing with no experience???? When and what direct experiences can you, and do you, lay claim to ??

A car wreck analogy may be meaningless to you, regarding bucking, but I understand it easily, and so will others.

Jeff Medwin....low Ohms

[mah]

Jeff, Seeing that you understand the car wreck analogy would you kindly relate it to electromagnetic 'bucking' in terms that this, apparently ignorant, Electrical Engineer can grasp?

[LowOhms]

Jeff, Seeing that you understand the car wreck analogy would you kindly relate it to electromagnetic 'bucking' in terms that this, apparently ignorant, Electrical Engineer can grasp?

(1) You never answered my Q. about field coil experience. Should we all assume zero????

(2) I am sure you are not ignorant mah, but there is no way I, or anyone else, could ever help you.

(3) Can we institute what I suggested several weeks ago, that you not respond to my posts, and me, vice versa. It will make for a more pleasant Forum experience for all.

You never ever addressed that. This is about the third request for self-regulation between us, with no response from you. Lets resolve this quickly and most efficiently. Thanks.

Jeff Medwin...... Low Ohms

[mah]

No, Jeff. Rather, you should give more consideration to the credibility of your advice and your justifications.

This forum has had a reputation for sound comment and advice and I, for one, would like it to continue. Members will stand corrected if better information comes along. You don't appear to be amenable to informed correction.

The ball is in your court, not mine.


Back to the thread's purpose: Field Coil project.

[LowOhms]

No, Jeff. Rather, you should give more consideration to the credibility of your advice and your justifications.

This forum has had a reputation for sound comment and advice and I, for one, would like it to continue. Members will stand corrected if better information comes along. You don't appear to be amenable to informed correction.

The ball is in your court, not mine.


Back to the thread's purpose: Field Coil project.

mah,

I am plenty credible. And I offered sound and credible advice. Who are you, who probably has zero field coil listening experience, to contend that my comments are ill advised?? Me, and my closest of audio friends have direct experience with hundreds of such field coil drivers, over many years and many venues.

I was very clear in what I stated, that the coils buck each other. Everyone hears it, but not everyone recognizes what they are hearing.

This is just my opinion. I am free to state it up here.

No rigid theoretical proof, ( which may or may not be correct ), is needed. The proof of the pudding ......is in the eating.

You, and others, are welcome to have a different opinion, but it would be much more credible to me and others, if you had field coil experience..

This of little consequence to me. I am happily using an 802D, remagnetized by GPA in 2015, as the top-end on A7-800s. My system's wiring, and amplification, is at a level that very much pleases me.

This is an open Forum where we can share our listening experiences, hopefully for the betterment of all. Each individual reader has the mental capability to discern who or what ideas they prefer to follow in any audio discussion, ....it is a right we all have.

Now, for a fifth time, lets get back to the idea that "we two" do not post after each other in the same thread. Its a waste of bandwidth. Anyone can see that is needed. Are you willing to address that ??

Jeff Medwin.....Low Ohms.

[Steve Schell]

To calculate the field coil, you start with the cross sectional area in square inches (one side only). Then find a chart of number of turns per square inch (mine was from a 1947 ARRL Handbook) to calculate the number of turns with several different wire sizes. Then figure the average length of turn (I went with 2/3 out between I.D. and O.D) to calculate the total length of turns. Then look at a chart of ohms per 1,000' (again ARRL) to calculate maximum current without exceeding the max rating. Eventually you will have a little chart of wire sizes vs. the maximum voltages with which they can be driven. Funny thing is that many of the old drivers used high supply voltages (120 VDC across the Lansing 801b field coil) while the calculations favor much lower supply voltages. The figure of merit of magnet strength is ampere-turns (amperes of current flow times number of turns in the coil). As one shifts to larger wire sizes in a given coil cross section, the number of turns decreases but the current carrying capacity of the wire increases faster. Thus the ampere-turns figure favors large wire sizes, low voltages and high currents, though I chose 12 to 14 VDC as a practical value for our commercial efforts.