Micro AMT – A small box sounds from full breast.
The Micro AMT: A two-way system in miniature format, small but mighty!
The shape of the Micro AMT is strongly reminiscent of the former Kolibri from 1993, a two-way miniature speaker with appealing sound. The placement was already quite diverse at that time: free on the sideboard, as a shelf speaker, or as a wall speaker by simply hanging it on the wall.
So where do you use such a compact speaker today? The areas of application are manifold: mini and micro systems, PC speakers, gaming, second speakers for the vacation home - the creative speaker construction community can certainly think of many ways to replace the small accessories of home systems with the Micro AMT and thus improve the sound.
The addition of a subwoofer is possible and makes sense if the bass is to reach down below 60 Hz, because there the Micro AMT runs out of air, then the "lung volume" of the woofer is at its capacity. Nevertheless, the Micro AMT comes across more powerful than expected. I tried it out at home in the party room. My neighbor said affectionately: "The little devil really gives gas, I know others ...", but let's leave that alone. At the latest with a subwoofer, the little big one can also play a bit louder.
A two-way system in miniature format
Small, good loudspeakers are always in demand. "Please not always black or white", we often get in our mailbox. Here you go: a tiny speaker in purple-red makes a big impression.
So where do you use such a compact speaker? The areas of application are manifold: mini and micro systems, PC speakers, gaming, second speakers for the holiday home - the creative loudspeaker building community can certainly think of many ways to replace the small accessories of home systems with the Micro AMT and thus improve the sound. The addition of a subwoofer is possible and makes sense if the bass is to reach down below 60 Hz, because that's where the Micro AMT runs out of air, then the "lung volume" of the woofer is at the end of its capacity. Nevertheless, the Micro AMT comes across more powerful than expected. I tried it out at home in the party room. My neighbour affectionately said: "This little devil really gives power, I know others ...", but let's leave that alone. With a subwoofer at the latest, the little big one can definitely play a little louder.
At that time, the cost of the Kolibri was quite reasonable at under old German 100 DM. With the current currency conversion, we remain at one hundred today. If you have never built a loudspeaker before, you will find lots of advice and useful tips on building loudspeakers here in our community. The shape of the Micro AMT is strongly reminiscent of the Kolibri from 1993, a two-way miniature loudspeaker with an appealing sound. At that time, the set-up was already quite versatile: free-standing on the sideboard, as a shelf speaker, or as a wall-mounted speaker by simply hanging it on the wall. The Kolibri from 1993:
Even back then, the Kolibri could be admired here and there as a playing partner in surround systems. It could also have been used on a PC, but hardly anyone did this and the drivers would have needed magnetic shielding.
Technology
Large woofers are basically ruled out and the old Gradient TPC 80 RW-4 from the Kolibri can only be admired in the ancestral gallery. And yet we have found the ideal chassis: From our long-standing partner from the land of unlimited possibilities, we fell back on the Dayton ND 91-4. A full-range driver that we use here as a low-midrange driver and give it a bit of a boost in the treble range.
Dayton ND91-4
The ND91-4 shines with its voice coil of only 4 mm in a 14 mm deep air gap. This underhung design allows a linear excursion of 5 mm, and this in a homogeneous magnetic field. Fs 68 Hz, QT 0.4 and Vas 2.2L result in a clean tuning in a reflex cabinet with 2.5 litres net volume and a BR tuning to 65 Hz. Up to 5 kHz the ND91 performs perfectly, making it an ideal partner for the AMT Mini-8, which should definitely not be used below 5 kHz. The Dayton ND 91-4 is perfectly suited for the Micro AMT.
Frequency response:
Original measurements by Dayton with Omnimic:
Note: 1/24th octave smoothing - nearfield response included in graph below 450 Hz.
Black = 0 degrees
Red = 15 degrees
Green = 30 degrees
Blue = 45 degrees
Dayton AMT Mini-8
The Dayton AMT Mini-8 is supplied as a modular kit with an additional front plate. This means that it can be used for a wide range of applications, from self-built coaxial systems to use as a super tweeter. A pack of screws is also included. Considering that the Gradient HK 10 Uf-8 from the Kolibri could show its full potential from 500 HZ, the Dayton AMT Mini-8 needs a much higher crossover frequency: it definitely should not be used below 5 kHz.
Frequency response:
Original measurements by Dayton with Omnimic:
Note: 1/24th octave smoothing - nearfield response included in graph below 450 Hz.
Black = 0 degrees
Red = 15 degrees
Green = 30 degrees
Blue = 45 degrees
Frequency Switch
The frequency switch is very simple and intentionally low-priced components of small size are used. They fit on a circuit board of just 6 x 6.5 cm. A 2nd order low pass filter makes even the small resonance peak of the ND91's inverse dome aluminium diaphragm disappear. A 1µF capacitor is sufficient to couple the tiny AMT reliably. In combination with the resistor, this capacitor also provides the level adjustment. This way the speaker plays linear from 65 Hz to 20 kHz.
With regard to the phase, the following was found during the measurements: If the AMT is at ear level or below, it must be connected in phase opposition to the ND91. This is probably the case in most applications. However, if the speaker is positioned above the head, it should be turned by 180 degrees or the AMT should be connected in phase. Otherwise there will be a
noticeable drop in the crossover frequency range.
This behaviour is typical with a non-coaxial arrangement of two or more drivers, but is exacerbated here by the following circumstances: the crossover frequency is very high at 5 kHz and the horizontal, relatively deep slots in front of the AMT's folded diaphragm act as a kind of acoustic lens. They favour the horizontal dispersion behaviour, which is evident in the lateral measurements, but reduce the vertical one.
Measurements:
The Assembly
You should allow 30 minutes for gluing the boxes together. A tube of glue and a tape called Tesssss... or from another manufacturer. Packing tape works just as well. If you don't build loudspeakers all the time, you won't necessarily need clamps, but once you've had a taste of it, the right tools will wander into your tool cupboard all by themselves.
A dry assembly like the one above helps when gluing later, because once the glue has stuck to the good piece, it is difficult to change the parts again. Besides, the glue sticks so nicely to your fingers Here is a nice way to see how a possible assembly procedure might look like.
Packing tape or adhesive tape replaces the screw clamps if you don't have them available.
Wet everything well with glue, rather too much than too little. It should not whistle through the cracks, but through the bass reflex channel as the only opening besides our American friends, the Dayton Duet.
Always wipe off excess glue residues well right away: a small bucket of water and an old rag are enough to get rid of the sticky mass and save a lot of work afterwards, because glue residues can be quite stubborn to remove and require time-consuming sanding.
Inside, excess glue increases stability.
Now it's getting exciting: The second side panel can be put on. Cover all speaker edges with tape and pull the roll of tape over the speaker cabinet.
Do not skimp on the tape, it is better to use too much than too little.
Clean work is necessary so that the box does not become warped.
Important: Apply as much pressure as possible to the tape.
The main thing is stability: Well packed is half glued.
Hellooooooo, here comes the post: The package is ready - let it rest to dry.
With the current temperatures of 30 degrees, work can be continued after two hours. In winter, it is better to wait until the next day.
If there are glue residues, please remove them immediately with a damp cloth, because then, as mentioned above, less sanding is necessary and the sanding block can be moved over the wood more easily. Don't forget to break the edges with the sanding block so that the primer or varnish has a good hold afterwards.
You can remove the entire box with sandpaper before priming, depending on the wood you have used. You can ask questions in the forum, the guys (and gals) are happy about a lively forum participation.
Remove any excess glue with a chisel or knife before sanding.
Do not forget: Break the edges with the sanding block to make the paint stick.
We used white primer, which comes in different shades.
I will try out the paint-primer combination more often in the next construction reports. So, now it's off to the DIY store to do some shopping. For Lacquer Express and 2-to-1 coloured lacquer, a lambskin roller small and a brush, and some painter's masking film to cover the table before I can start.
Clean business: Now you're ready to go!
First apply the Adhesive Primer Express well with a roller and / or brush. It dries very quickly and can be applied several times a day.
Since most brushes and rollers today are made in the Far East, they should be washed thoroughly under warm water, as they have a protective coating. Depending on which lacquer colour is used, one can choose from white to light grey to coloured primers, which increases the overall coverage.
If you like, you can dose the primer or paint with a brush and then spread it cleanly with the roller.
The primer and paint application on the front side is a little tricky.
A small brush helps to paint the speaker openings cleanly.
Before applying the purple colour, sand again and dust well.
Purple-red was the choice this time. Apply with a roller and brush and leave to dry overnight. A tip from us: It is difficult to apply the paint with a spray can on such a small surface - so it is better to use a brush and/or roller.
Please do not leave this colour open if you have vampires in the house.
Once the first coat of varnish has been applied really cleanly, it's time for the final finish.
It's getting good: the second coat of paint was due the next evening.
Here, too, it is much cleaner to work with the roller.
A small brush helps to apply paint to the bass reflex port. The final assembly is not difficult: Help, someone is drilling into the almost finished speaker.
Mount the loudspeaker connection terminals.
Pre-drill the pole terminals, screw them in and solder them to the turnout. Assembling the crossover is anything but a difficult task due to the number of components and the pre-milled circuit board.
Connect the cables to the chassis and the crossover, paying attention to the polarity.
Put a little sonofil in the cabinet and tighten the screws of the chassis.
Don't slip when screwing down the drivers - DONE: Off to the sound test!
Sound & Space
Such small dwarfs are often underestimated in terms of sound and live a shadowy existence among their larger colleagues who are in the limelight of the DIY community. They are quickly sought out and remembered when the urge to build something oneself (here in the positive sense) won't leave one alone: "Build something again!", it sounds from within. But where to put the big boxes? Then the small ones come into play. All the more pleasing when they also have a solid bass foundation.
As one of the smallest of its kind, the AMT Mini-8 supports the Dayton ND91-4, which was born as a full-range driver. The latter masters its new role as a bass-midrange driver with flying colours in all registers of the music and, in teamwork with its colleague for the top scale, performs at its best - in every musical genre, although I have to admit that I left out the heavy metal concert. If you think the Böxchen are up to it, feel free to catch up and report back in the forum. I'm all ears.
Otherwise, the dwarfs can fill entire concert halls with sound - or turn the living room into a concert hall. Of course, the dwarfs don't produce abysmally deep basses, but they do a lot right above that. Whatever physics allows. The Micro AMTs cut a fine figure in our study, where they do not shy away from a good room volume.
Voila: It's Micro Time!