Coils for field coil drivers.

[TerryG]

I am curious if anyone knows anything about field coil drivers, basically an electromagnet. Any formula's anyone knows of that indicate how strong of a magnetic field a coil produces per how many windings with a given amount of voltage and current?

 

[gyraf]

https://xyproblem.info/
Other than that - have you asked wikipedia? Electromagnetism - Wikipedia

 

[TerryG]

I have no problem I want to know of the math formulas for winding a core/choke for a field coil speaker. Usually you start with the magnetic strength needed then go from there to make the coil.

I want to have the ability to vary the voltage and current through the coil in order to vary the magnetic flux in the gap of the voice coil. If you do not know that is fine, just ignore my asking. If however, you know of any book I can look to purchase that has this information that would be very helpful. There is little if any information on any of this stuff online, much of this although not new technology seems to be guarded knowledge for some reason.

 

[gyraf]

So you are planning on from-scratch designing a field-coil speaker?

Did you actually read the wiki on the subject? I think it sums it up pretty good - unless you're looking for the readymade field-coil-speaker recipe - in which case I think you're perhaps asking a bit much..?

There are some pretty good magnetic simulation software packages popping up the recent years, if that helps

/Jakob E.

 

[TerryG]

Well after reading as much as I can find online, I think I will go the trial and error route. I will wind a coil with as much 18awg magnet wire as I can fit in the prescribed coil space, and then run up the current until get the magnetic field I want. Might just unwittingly make my own MRI machine. LOL

So my plan is to cannibalise some of these cast aluminum speakers I have and install a field oil in the available space of the permanent magnet. I have a pair of EV SP12 drivers, that have whizzer cones.

 

[Brian Roth]

I've collected antique radios...USA-made from the 1930's such as Philco, RCA, etc... for many years but never bothered to learn about the field coils. They were usually used as a filter inductor for the PS as well as generating magnetism for the speaker. I have no clue about the gauge of the wire in the coil, but gut instinct says it was smaller than 18 AWG. The current through the field coil was MAYBE a few 100 mA max.

Bri

 

[TerryG]

Yes because it was double duty they spec'd to operate as a choke for the PS. This worked but was not ideal for the field coil speaker.

 

[abbey road d enfer]

I am curious if anyone knows anything about field coil drivers, basically an electromagnet. Any formula's anyone knows of that indicate how strong of a magnetic field a coil produces per how many windings with a given amount of voltage and current?

The main factor there is the reluctance of the magnetic circuit, which is dominated by the gap. Once you have determined that, you know how much A.t you need for the targetted B.
Once you have that, you must determine, by trial and error the correct combination. 100A.t can be one turn of 100A, 100 turns of 1 A, or any combination. Then you must calculate the resistance, which will yield the necessary voltage.
Check this:
https://www.diyaudio.com/forums/multi-way/212953-project-ryu-diy-field-coil-loudspeaker.html

 

[TerryG]

Well this is more experimental at this stage than well defined. I want the ability to vary the amount of current so I can vary the amount of magnetism in the gap. So the ability to have a lot of current to create a lot of magnetism is good. I may not need that much current or magnetism but to see how it effects the usefulness of the driver and its frequency response is what I am after.

Once I come to some conclusions based on this initial experiment then I can go about designing a better more defined field coil and driver. It would seem that there would have to be a delicate balance for a given amount of room for a field coil, and the wire's size and winding count to fit a given area. Because the current in a coil produces some heat, and you can only pass so much current through any specific gauge wire before it can no longer dissipate the heat enough to not destroy the insulation causing a catastrophic effect.

One thing I forgot to consider before choosing a driver to canibalise for this little experiment was whether I want the voice coil underhung or overhung in the magnetic gap.

 

[abbey road d enfer]

One thing I forgot to consider before choosing a driver to canibalise for this little experiment was whether I want the voice coil underhung or overhung in the magnetic gap.

This is a very important strategic decision.
Overhung has become the most common setup, because it provides the flattest response and minimum distortion, at the expense of efficiency.
Underhung is more efficient, has a higher Qts, but suffers distortion at high cone excursion and requires equalization.
Traditional loudspeaker designers have vigorously rejected underhung setups for woofers, because optimizing them requires electronic means; most do not like it because it puts them out of the final performance control.
A good example is the ill-fated 18Sound 18iD and 21iD, where the conflicting convictions of the speaker manufacturer and the amp manufacturer resulted in a half-assed product.
I don't know which way you wanna go, but you must establish your goal and your methodology from the beginning.
And remember, whatever you do, it will be very difficult to top 2 Tesla (20 000 Gauss).