After the good result obtain with ceramic drivers in my Monitor 4 the idea was to explore the possibility of using an AMT tweeter to increase the transient response and decrease the distortion in order to have more the details.

The Air Motion Transformer (AMT) is a type of electroacoustic transducer invented by the physicist and scientist Dr. Oskar Heil (1908–1994).

At the begin this transducer was licensed to ESS, which has manufactured a range of loudspeakers based on the Heil-element since 1972.

Now many company produce tweeter based on Heil’s principle but among all Mundorf offers a wide range of products from dipoles to closed back models.

I have used the AMT in other two projects but both were particular applications so I'm curious to see how they perform in a reference speaker going to replace a ceramic tweeter.

This project born to exceed the performance of all my previous loudspeakers using AMT, Ceramic drivers and an active integrated subwoofer.

 

 

TWEETER

Mundorf  AMT U110W1.1 Nominal Impedance: 6 Ohm Sensitivity at 2.83V/1m: 93 dB Application (@12/6dB.Oct): 1.8/2.7-27 Kilohertz Dimensions: L55xH125xZ45 mm   Data sheet AMT U110W1.1 Data sheet AMT U110W1.1-R

WOOFER-MIDRANGE

Accuton C158-8-085 6.25 inch bass-midrange for small bookshelf monitors or slim floorstanders small and light voice coil with 25mm titanium voice coil former for high midrange resolution long excursion design for high SPL capabilities   datasheet

SUBWOOFER

Wavecor SW263WA03 SW263WA03 is a 4 ohm woofer designed for dedicated subwoofer applications. It works equally well for high-end hi-fi, high quality home theater systems, and top multimedia setups. The extremely stiff aluminium cone, the long linear throw, exaggerated ventilation, and the special dynamic linearization short circuiting rings together makes up for an unusually linear and low-distortion woofer. Even at very high signal levels, the unit behaves controlled and without distortion. Due to low mechanical losses SW263WA03 masters equally well low level details and extreme dynamic requirements. datasheet sealed box simulation

SUBWOOFER AMPLIFIER

See info on previous project Monitor 4.

 

AMT TWEETER

The Mundorf  AMT U110W1.1 is avalable with an optional faceplate which forms a small horn that shifts the offset and slightly modifies the frequency setting.

So as a first check I had milled panels made with the two configurations to verify this difference.

These pannels has been produced on my specification by Costruire Audio.

 

Here I am in my living room where the reflections on the walls are controlled with 10 panels model 3D by Acustica Italia.

I will use a miniDSP UMIK-1 Omni-Directional USB Measurement Calibrated Microphone and the Dayton DATS V3 Computer Based Audio Component Test System for the impedance measurement .

The box is an old Philips about 27lt used only for this first measurements.

In this first plot the configuration without since to be the best but if we change the limits, the difference after the 14KHz is due to the left and right drivers.

Now is necessary compensate the trend of frequency response with a choke and after we come back to faceplate choice.

Here the first test with only one choke in series.

Here as been add a resistor in parallel to the choke in order to get more high frequency.

If we add a 4.7uF in series to this cell we obtain the first complete filter for this tweeter

 

BOX

To get the best transient response the my choice is a sealed enclosure in a large box with a Qtc near to 0.6 for both woofer drivers.    

I decided to tilt the front panel of the speaker system to eliminate the offset between the 2 acoustic centers.

 

 

CROSSOVER

The mid-woofer and tweeter using a passive crossover of the highest quality centered at a frequency higher than the human voice.

Will be implemented a network for impedance compensation to facilitate the work of vacuum tube amplifiers.

This crossover is the result of a long series of measurements of the frequency response and several listening sessions with a my friend who makes live recordings.

The ceramic drivers need a notch cell in parallel to the driver to eliminate all the effects of the natural peak of this material.

Here a first filter with Accuton, mic at 40cm

3.3uF + 0.33mH // 15ohm

Follows the photo of the first comparison between the original ceramic tweeter and the new AMT.

The last filter.

The new frequency response of the AMT with the filter and the comparison with the original result with the ceramic twweter.

The microphone was placed at 50 cm and as a height between tweeter and midrange but in the case of the AMT has been raised a few inches because the vertical dispersion is limited. 

Obviously those who do not have enough space to accommodate these large speakers can use only the mid-woofeer and the AMT driver to create a reference little monitor with the same filter.
 

Closed Box design parameters

Vb: 8.5L, -11dB @ 40Hz, F-3dB: 70Hz, Q: 0.71 (typical)
Vb: 22L, -8.5dB @ 40Hz, F-3dB: 70Hz, Q: 0.50 (extended bass)

Vented Box design parameters

Vb: 15L, Port diameter: 50mm, Length: 190mm, Fres: 41Hz, F-3dB: 45Hz, Q: 0.58 (typical)
Vb: 22L, Port diameter: 50mm, Length: 130mm, Fres: 40Hz, F-3dB: 40Hz, Q: 0.50 (extended bass)
Vb: 12L, Port diameter: 50mm, Length: 200mm, Fres: 45Hz, F-3dB: 49Hz, Q: 0.61 (0.7dB Ripple @ 100Hz)  

 

 

LISTENING SESSION

Also in the listening session a direct comparison was made between the ceramic tweeter and the new AMT driver .

The difference between the 2 speakers is big, the ceramic tweeter does not seem to reproduce all the highest part of the frequencies as if it stopped at 5-7KHz.

The reproduction of high frequency harmonics makes many wind instruments more realistic with AMT.

The only problem with the AMTs is the limitation of the vertical dispersion which forces us to hear on axis but this is not a big problem.

In conclusion we can say that the transition from a ceramic tweeter to an AMT is an upgrade.