An information roundup for larger AMTs

This thread is intended to be an information resource for the larger AMT drivers suitable for use in HT applications. Measurements for all brands and models are welcome.

First, the test conditions I use. To provide a low-diffraction environment, all drivers are placed on a carpeted floor with a 3/8” thick wool felt pad between the driver and a small piece of MDF; the driver faceplate is faired to the surrounding carpet with additional wool felt pads. Measurement distance is 1M with a calibrated ECM8000 and SoundEasy, 10ms Kaiser-Bessel window, no smoothing.

First up, the Aurum Cantus AST30130:


At more than 7” tall by 4” wide, the 30130 certainly qualifies as a large HF driver. Overall build quality is excellent. One demerit for slightly inconsistent pleat spacing, which may reduce ultimate xmax (before pleats contact each other).

This is an open-back driver with only a thin felt damping pad across the rear of the motor, so LF output may increase with a user-supplied enclosure/back-cup.

AST30130 FR:

The 30130 is remarkably smooth; making for an extremely easy crossover.

CSD:

Smooth response = very quick spectral decay…~35 db down in 1ms.

Distortion:

Non-linear distortion is a little higher than I’d like, but a WG/horn will reduce LF distortion levels.

Next, the ESS AMT-1 diaphragm in a DIY motor:

First, a bit about the motor. I built this (full-dipole) prototype motor to evaluate the ESS as well as AC diaphragms (diaphragm dimensions and materials are somewhat different). The proto motor has only half the neobdymium of the AC motor so sensitivity is about 6db lower…but 92db is still quite useful. With less neo, cost is lower than the AC. Like the AC driver, LF output should increase with a back-cup and horn.

ESS FR:

Despite reading past references to poor quality, the ESS diaphragms I tested were very nicely assembled. I had high hopes for the ESS because it is nearly as large as the AC, pleat spacing is nearly perfect, and it costs less. However, as seen in the FR plot, the ESS suffers from resonances at 7k and 11.5k.

CSD:

Spectral decay is "okay"…except for those pesky resonances.

Distortion:

Non-linear distortion is simply too high. Even more troublesome, is sub-harmonic output at several frequencies, including clusters of frequencies generated from 1-2k when driven from 2-4k. While visible on the sweep screen, sub-harmonics are not plotted as typical 2nd-5th, but they are clearly audible when running the HD sweep…they sound far worse than typical HD. Two samples of this diaphragm were tested with substantially similar results.

Aurum Cantus 30130 diaphragm in the same DIY motor, no changes but the diaphragm:


FR:

Ahhh…that’s better! FR very closely tracks the 30130…no visible break-up at any frequency.

CSD:

Also very similar to the excellent performance of the 30130.

Distortion:

Drive signal for the lower powered motor was increased to match AC30130 output, but distortion but remains closely aligned with the AC30130. No sub-harmonics were observed.

Reserved for the Beyma TPL 150

(You send one, I’ll test and return it.)

Reserved for the Beyma TPL 150H

(You send one, I’ll test and return it.)

reserved

The thread was actually your idea

Possibly an MTM of horns with a pair of BG Neo 10s or SEAS W15CH above and below an AMT. The idea would be to achieve greater LF vertical directivity than a single horn in the space available. Prolly roll off the upper driver a little early so the dual mids don't "pinch" directivity below the AMT.


PS: Don't forget these open-back drivers do not have a back cup to raise Q or completely seal front/rear cancellation through the felt sheets.

Here's the AC 30130 with the OEM felt pad placed directly on MDF (blue) vs the thick felt pad over MDF in the original measurements (black). Not a proper back cup, but the possibilities are quite evident. I was a little surprised by the slight improvement in the little dip at 14k.



Here is my Proto 2 motor. A little higher gap strength and lower % open area.


Proto 1 (blue) vs Proto 2 (black). As expected, sensitivity increased. The rise around 8k is now higher Q, but I found it extremely sensitive to back treatment (proto 1 was spaced 1/4" further off the thick felt+MDF backer). Since tweeters are very sensitive to back treatment, I won't get serious about back-cups until I have horns to play with (expect horns and back-cups to interact).

Sorry for the late response...been on an Internet-free vacation in northern California wine country.

Very nice SEAS drivers, but I'd use a conventional vertical layout with the AMT. With the AMT horizontal, the polar pattern would "flip" above and below xo...the mid pattern relatively wide horizontally, narrow vertical; the horizontal AMT would be relatively narrow horizontal, wide vertical.

Hey Brandon,
The proto motors are dimensioned so they will accept both diaphragms, but the ESS and AC are a bit different. I kept the ACs but returned the ESS diaphragms, so this is from memory...
-Thickness, nearly identical
-Width, very close
-Height, 30130 is definitely taller.
The AC is much better damped so, unless about 0.5" height is problem, they would be worth a try in the ESS motors.

(FWIW, I think Tom still has a pair of the ESS he isn't using.)



Hi Paul,
Yes, I'm still open for testing Beymas...if I ever see one!