Heil AMT Review
Since I’m taking a breather from the 18 liter boxes, I decided to take another look at this Heil Air Motion Transformer (AMT) tweeter that has been bugging me for a long time. This particular AMT was used in the ESS PS-820, a vintage speaker that was introduced in the late 80s. According to their brochure, the Heil AMT was crossed at 2kHz.
Fig 1 – With Plastic Frame
The plot in Fig 1 was with the AMT mounted in the speaker with the plastic frame as shown in the ESS PS-820 (top left). I couldn’t quite believe the horrendous plot. Is this what the highly regarded Heil AMT looks like? It’s certainly not something I want to waste my time on. I set aside the AMT and moved on to more pressing drivers. That was in early 2019.
Fig 2 – Blue plot=Surface Mount (No Frame) • Red plot=Flush Mount (No Frame)
After working on the G1, it occurred to me that perhaps the frame was affecting the response. I removed the plastic frame (top center) and did another measurement (Blue plot in Fig 2). I was right, the response looks better but there’s still that depression at 2kHz~3kHz.
I then did another sweep, this time with the AMT flush mounted, also without the frame. The result (Red plot) is a huge improvement but it’s still not good enough.
Fig 3 – Top of Cabinet
As a last resort, I sat the AMT on top of the box (top right) like in the ESS AMT-1. Incredibly, it gave me the response I was looking for (Brown plot in Fig 3). I was shocked, to say the least. I never expected this Heil AMT to be ultra-sensitive to baffle diffractions.
What I really like about this new response is she’s virtually flat from 2.5kHz to 6kHz. I have no issues with the two peaks at 8kHz and 10kHz. There’s not much information up there anyway but I can EQ them if need be. At the lower end, the dip at 2kHz and the bump at 1.4kHz spoilt an otherwise beautiful response but I’m quite confident I can handle them.
Now that I’ve found a way to use the Heil AMT, I will work her again with the Audax HM130CO. I think the sound will be fabulous.