The Neil Davis Heil AMT

[paulfx]

Hello from PaulFX!
I've recently finished my single Heil AMT driver based on Neil Davis' magnet and pole design and a Great Heil diaphragm:

100_0163.jpg

I was dying to hear it, but I'm out of stuffing so I put rags in and sealed the top and bottom. There is a dip of about 6 dB around 4K, roughly 1/4 wavelengh of which is about the inner diameter, so I'll want to fill it right. Then I hung it on the HF output of my crossover and played some jazz drums. Quick and loud! In its day this must have been unmatchable. Of course in its day they didn't have monster NIB magnets, either.

This is going to go on a center channel MTM speaker. What do you think, Mr. Davis?

[paulfx]

Here is where Neil Davis shows how he built his Heil:

http://home.comcast.net/~neilandbarb...DIY1/index.htm

I copped his design but using the large "great" ESS Heil diaphragm so I would have plenty of overlap for the crossover.

I got the metal online from Metals Depot, 2 feet each for the 2 X 3 rectangular tubing and the 1" square rod, the minimum order. Be sure to get the abolute softest steel, "for cutting, grinding, punching." I used a 10" chop saw to cut it, fitted with a metal cutting blade. I practiced on the one end and set up wooden spacers to get the cuts centered, then used the other end of each for the pole pieces.

I went with half metal, half air in front, sanded and smoothed to minimize diffraction, or at least shift it above what I can hear. The flanges I won't move so I can get the midranges closer.

[paulfx]

Okay, so that was the outer pole piece, the inner pole piece I sliced and diced using the chop saw with metal cutoff blade. The magnet is a monster Neodymium-Iron-Boron from Forcefield Magnets, 3" X 1 1/2" X 3/4". I found it too dangerous to trial fit, so I cut a block of wood just a millimeter bigger in order to grind down the backside of the pole piece. I used a couple drops of epoxy to bond them. I used 1 hour epoxy, not 5 minute or superglue, in case I had to desperately pry them apart if they snapped together wrong.

[paulfx]

So by now, I have a new appreciation for Mr. Davis' ironic comments about the excitement getting to me - - This NIB magnet is so ridiculously strong that I had to cut wooden test jigs for the magnet and the ESS diaphragm to avoid damage and broken fingers. Once everything was ground, filed, fitted, sanded, smoothed, and cleaned, then came pole assembly. Used heavy leather gloves and another couple drops of 1-hour epoxy, then tried to guide the inner pole into the outer. Like hand feeding an alligator!

[paulfx]

Having a ball here posting to myself - - this time I'm moving up to three thumnails - - baby steps, as the movie character Bob would say.

I made a a mounting bezel for the diaphragm out of 3M/Scotch VHB two-sided tape. I went from the front plastic frame of the diaphagm outwards, enough to stick to the front inside of the outer pole piece and to cover up the various lengths of hand-cut ridge openings to the left and right. To keep that two-sided tape covering from sticking to anything inside like stuffing, I replaced the red handling film, which is what you can see in the pictures. To keep the assembly from sticking to the inside front before you want it to, place a piece of wax paper inside before sliding in the diaphragm and tape. Sorry, I was so frazelled I forgot to take a picture of the mounting bezel when it was outside!

Concerns:
1) Sounds a little brash - but I still have to seal it, stuff it, and damp it before really listening.
2) The diaphragm came a little warped on one corner, and it has separated from the VHB tape bezel. I'm going to carve 4 dowels that will fit front-to-back as hold-downs, then apply a drop of some kind of contact cement so I can pull everything apart if I fry the diaphragm and have to replace it.
3) Which is the positive lead? I think purple was + from ESS, but I couldn't test this beast of a magnet for polarity with another one (I'd never get them apart!). Besides, you got pleats, vectors, the left-hand-rule (or is it right-hand?). I think I can take a stock Peerless tweeter, play a 2KHz tone into both and pick the loudest sound while reversing polarity on the Heil and then mark the leads.
4) Costs got a little out-of-hand. Metal, blades, magnet, shipping, masks, wrong hardware, & broken stuff came to $250 or so.

Sure I could have gotten a great driver for that and saved a bunch of time, but where would be that 1970's hi-fi exotica mojo? Hope diffraction will be less than with those classic Great Heil AMT tweeters. You wouldn't put a dome tweeter underneath iron prongs and then in a square hole. This should be less diffraction and what there is hopefully is higher in frequency. The penalty, as Davis points out in his Audio Amateur articles, is less output due to less gap flux compared to the Great Heil. I've gotten around that by just bombing it with this wide faced north-south NIB magnet. I think the Great Heils had Alnico's, maybe someone can verify that.

Work safely! This means eye, ear, lung and hand protection. Unless you have a shop with vent ducting, do the cutting and grinding outside, preferably while your neighbors are also running power tools and lawn equipment. I looked up after grinding the vents and found 3 neighborhood kids and their Dad looking at me. And of course, when I explained what I was doing, he smiled and nodded.

Finally, the diaphragm came with a couple stock warning stickers from ESS, "WARNING don't you dare...," so I stuck one on the back.

[cuibono]

Wow, this is just what I wanted to see! I am very interested in AMTs, and am going to be thinking about this for a bit!

Thanks for sharing! Do you have any measurements to show?

[Neil Davis]

Originally Posted by paulfx

Hello from PaulFX!
What do you think, Mr. Davis?

Very nice. I'm sure you will be pleased with the results.

I went up in the attic to my retired Heil collection and found one that looks similar. It's got a lot of silicon glue all over it because it was mounted in the wall. This one was made about 20 years ago, back when I was still etching diaphragms. It was crossed over to the small Heil on the right.

Heils.jpg

Hmm...my wife still wants me to fix the hole in the wall where these speakers used to be, but I figure what's another 10 years?

I've got a set of designs for circular Heils that scale from about 1.5" to 4" in diameter, but I just haven't had time to work on them. I made the magnet structure for the 1.5" Heil--it's in the pictures on the left. I laid out the diaphragm and it's been ready to etch for almost a year now. I even used Frontpanelexpress to make a folding jig, so it shouldn't take that long to develop a prototype.
microtweeter.jpg
microtweeter2.jpg

I still think there is still a lot of untapped potential for the Heil design, and the new magnets open up some possibilities that didn't exist years ago.

It's kinda funny thinking about these drivers. I made my first DIY Heil over 35 years ago. I had just graduated with a degree in philosophy, and didn't know what to do next, so I tried writing two articles for Audio Amateur--one of which was on the DIY Heil. Both articles were accepted, and it was nice getting paid for them, but it became clear rather quickly that I was never going to support myself by writing articles.

--Neil

[kenpeter]

Thats cool, I been wanting Beyma AMTs, but always
choke on the price...

I've wondered if its possible to make a DIY ESL AMT?
An interdigitation of positive and negatively charged
thin plates, structured as two "combs" with insulated
tips. Hopefully mostly open at front and backbone of
the two combs. Pleated ribbon zigzagged between.

No magnets, just static.... Surely someone has done
this configuration before?

You gonna drive a horn lens with those round ones???

[cuibono]

I'm really interested in a dipole version - I assume that one could set up the diaphragm with magnets on the sides like a ribbon? I used to build ribbon mics commercially, but I don't know enough about magnet types/poles to reconfigure things - any hints?

[c2cthomas]

Originally Posted by kenpeter

Thats cool, I been wanting Beyma AMTs, but always
choke on the price...

I've wondered if its possible to make a DIY ESL AMT?
An interdigitation of positive and negatively charged
thin plates, structured as two "combs" with insulated
tips. Hopefully mostly open at front and backbone of
the two combs. Pleated ribbon zigzagged between.

No magnets, just static.... Surely someone has done
this configuration before?

You gonna drive a horn lens with those round ones???

Dr. Heil did propose that and ESL AMT could be made by subbing an electrostatic field for the electromagnetic field. I forget if I read that in the original patent or a later one tho.

paulfx - very nice project - congratulations!! I'm looking forward to seeing how this comes along.

Neil - I guess that I'm late to the party as I have always had thoughts about making my own AMT's using the already existing diaphragm (now) available as a replacement part and some big Neo's but never seem to get around to doing it. To many other irons in the fire (excuses, excuses - I know) plus I already have some old stock AMT1's that I'm pretty happy with. I'm with kenpeter's approach on this one - some big Neo's on the sides of the diaphragm and some thin plates running across - maybe some slotted metal - dipole configuration. I think that one of the mistakes being made is trying to use one diaphragm reproduce a broad rage of frequencies when a better approach might be to use larger ones for lower frequencies and small ones for the higher freq's - a mid range and high range AMT so to speak.