The Neil Davis Heil AMT

[kenpeter]

Instead of Neodymiums with health warnings like "will break fingers",
and assembly procedures described "might swing around explosively"
Have you guys considered a simple field coil???

[paulfx]

Well I wish I had joined this excellent forum before almost breaking my fingers putting this thing together!?

How much current would be needed to get the same kind of output? More generally could a short electromagnet be trial-built that would, say be almost impossible to get off the front of a refrigerator when energized?

[kenpeter]

Have you ever tried to force open a maglocked door???
I think thats less than an amp...

[paulfx]

As a matter of fact, I work at an airport, we have them all over!
Do maglocks use DC? If so, how many volts? (I'm thinking power supply)

[paulfx]

I just asked our technician here at the airport, and he told me maglocks typically use 24VDC. Probably too noisy to use the power supply, but that could be constructed. One or two of those...

[cuibono]

Its a cool idea - I wonder if there would be an even flux density through the diaphragm area, and if not, what effect that would have. I guess the next question is what current, resistance and wire gauge. It should be pretty easy to setup...

[kenpeter]

I think you might want a moderately high impedance current source?
Remember that any field coil might also act like a secondary winding.
I'm not sure how well leakage inductance moderates that effect???

Voltage protect (transorbs or crowbar) to handle any sudden collapse
or short across the magnetic gap... We still speak of crushing forces!
Just forces with an on/off switch.

I'm no expert on field coils, totally blind leading the blind here.
Maybe missing something critical? Definitely seek a second opinion
from someone who's actually done it before building anything field
coil supply-wise for audio. I am not that person.

[paulfx]

Cuibono -
Uniformity of flux over time might be a deal-breaker if a huge delta in current spikes through the coil. Maybe we could get an EE to opine on this?
As for consistency over the plane of the diaphragm, that's no problem. These AMT's are very tolerant of imprecision. The Neo magnet I used is a bit stronger on one area of the flat pole surface, and this does not seem to have affected the sound. Even the ESS models had a good deal of manufacturing variance.

[kenpeter]

Maybe you also want one big shorted turn on the pole? That would effect only AC fluxes,
and not the DC flux that is present continuously... Sorta like JBL has a shorting ring???

I'm not sure shorting a turn, thats any diff than a low impedance style voltage supply???

But I'm pretty sure we don't want said power supply to look like 8 ohms, seen through
a transformer. Else your amp is gonna work into that load maybe instead of the parts
that should be moving.

Somebody has done this voice coil thing before. Its gotta be documented somewhere...
Why am I not finding it? Other than my Hammond organ...

[cuibono]

kenpeter, I had a different picture in my head (if I understand you) - I had thought we could wrap the edge of the diaphragm in wire, then run dc through it to create a magnetic field through the diaphragm - basically an 'active' tweeter. Of course, the diaphragm's wires would be connected to an amp as usual.