[jk@home]

I have some of these drivers to eventually use on my OB, just found this fairly recent review from Joseph Crowe on his web page. He likes them, pretty much said they can be driven by any amplifier. Maybe someday he will design a beautiful wood horn for them. 

https://josephcrowe.com/blogs/news/original-ess-heil-air-motion-transformer%E2%84%A2-amt-tweeter

Anyway, for those who are interested, ESS has these back on sale again.

[Bodhi]

Pure Audio Project use that AMT driver in their Trio 10's. It's a nice driver, though a bit limited in top end extension. Personally I prefer pure ribbon or planar magnetic tweeters to AMT's.

[jk@home]

Yeah the biggest negative to these drivers are the weight, like 14 pounds each! But still impressive technology, considering they were first designed in the 70s. PAP uses a nice baffle mounting bracket, they show it on one of their web pages as an accessory. I tried to inquire a purchase of a pair but received no reply. From what I gathered on another forum, the price is pretty steep.

I plan on mounting mine naked on top of my Butcher Block baffles, which will support the weight. Mounted on a detached round aluminum disc, resting on a "cup" of more aluminum angle. That way I can experiment with toe-in of the AMT, not the entire baffle. The weight will then be an advantage in this case.

[Bodhi]

Yes the full driver incl: waveguide is chunky. Your speaker project sounds interesting. I do have one suggestion. You might want to experiment with placing the AMT tweeter on two small, square AV Room Service EVP's which are very effective at reducing resonances. And the version with the rubber top and bottom layer should hold the tweeter solidly in place, as well as protect the cabinets from any scratches (see pic below). Here is a link fyi - https://avroomservice.com/evp-2/ Though you'd have to weight up the aesthetics of raising up the tweeter by one inch on a pair of iso feet. In any case, I look forward to reading your report when they're finished .

[jk@home]

Thanks for that, the large round one would fit under the disc perfectly. But I'm trying to keep the distance between the AMT and the mid-woofer as short as possible, so probably will just go for some thin rubber gasket material, and then later blue tack for the permanent position.

https://www.amazon.com/dp/B0CXJBFWYQ?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

[Jon L]

I don't recall seeing such extensive measurements of Heil AMT before, and these have VERY clean cumulative spectral decay and low distortion 

I have a bunch of them and would love to find something like Altec 414 12" woofers (99dB sensitive) and place a double-stack Heil on top!

[jk@home]

I guess the one other negative that needs to be discussed is the possibility of receiving units that are wired out of phase. I read your posts on diyaudio about the problem. When I received my pair, had the same issue. Ordered another pair with the thought of identifying the mis-wiring unit, they too came mismatched! So I just  "matched" up one pair, and will keep the other set as spares.

I never corresponded with the seller about this issue (my bad). Unless I'm mistaken, the proper way to check this out is with a magnet (I used a magnetic tipped screwdriver).

[Jon L]

I guess the one other negative that needs to be discussed is the possibility of receiving units that are wired out of phase.

I did communicate with ESS about the out of phase issue, and the excuse I got from them at the time was that they had just hired some new employees who are not as experienced.  Hopefully, they have fixed this issue by now, but I would definitely check for phase before (semi)permanent mounting. 
Luckily, once the issue is identified, the fix is easy.  Instead of messing with taking out the diagphragm, I just change the color-code of the external wires and flip around the Heil 180 degrees since they look identical both sides.  Perfectionists may need to move the ESS sticker to the opposite side 

[jk@home]

 

[jk@home]

This posted recently on the Joseph Crowe website:

https://josephcrowe.com/blogs/news/tweeter-ball-no-3181-for-ess-heil-amt-tweeter

Looks like it was very labor intensive ( = expensive  ) Nice looking though.

[Jon L]

Looks like it was very labor intensive ( = expensive  ) Nice looking though.

Looks nice and premium but labor and cost-intensive 
It does smooth out the spectral decay, but stock Heil already has good CSD, and the downside is narrowed directivity with the ball.



What I would personally love is to be able to buy something like this