As you can see from the name, we made it our mission to create a box that was audible – and you can take that literally. Since the main contribution of the Power 17, the next-to-last box presented in the magazine, was its strong output rather than ideal audiophile-friendly quality, the GAM-100 Airmotion transformer created especially for the AXP series gives us another chance to take it up a notch. The fact that we had to give up the last little bit of volume didn’t bother us too much. Two AXP 06’s were put to work here, and the average volume of just under 97 dB/ 2.83 V would be plenty for us. Unlike the Power series, which was invented for anything that can be loud, the Audible series is designed for fans of small tube amps who, until now, only found noisemakers and obsolete wide-walled concepts on their quest for listening pleasure.
We hoped that the Airmotion transformer, with its huge membrane surface compared to calottes, would give us the necessary detail and that the AXPs would give us the corresponding dynamics that one expects from a high-output chassis. Naturally, we do not automatically require low-wattage tubes for the Audible – the revolution in today’s amplifier market is taking place more in the area of digital amps. Nowadays those can provide plenty of output, of course, but two sets of 10 to 15 watts from a circuit board the thickness of your thumb and a 12-volt power supply are the rule.
Reports have already been written about the data for the chassis being used here. If you flip back a few pages in the magazine, you will find them in other issues.
That brings us straight to the important cabinet-building part, without which even the best assembly kit is useless. We used SketchUp to make a drawing, which can be downloaded here. We used 19-mm MDF because it is easier to find.However, we actually built the boxes out of 34-mm Jerusalem pine, which is met with a whiff of disapproval from “real” cabinetmakers. Normally, they would just throw away such cheap packing wood after transport. However, they are overlooking the fact that it also has some desirable qualities for building boxes. We paid about 65 euros for a board measuring roughly 3 m², and like multiplex it came equipped with an attractive surface, too. The cutouts and routing were done using the Basis 1500 from CNC-Concept, and it even followed the programmer’s instructions and made a groove where we wanted to set in the plates. Gluing the boards together was fairly routine, but good for a couple of nice pictures.
Thanks to the sturdy building material, we were able to leave out the two reinforcements that we always use for the thinner MDF. Thanks to the grooves, the boards fit almost perfectly – we only had to sand them because of the joint glue smeared on the surface. Two coats of wood oil tinted the surface a light golden color, making the dark grain shine.
And now we had reached the point where our construction team’s work needed to be paired with the corresponding sound quality – building the crossover. We didn’t make a big fuss about the lower bass in the two-and-a-half-way system. Its only task is to compensate for the baffle step, so it was preceded by a coil to gently take it out of commission after 400 Hz.
The wiring for the upper AXP 06 was a little more complicated because it voluntarily increases slightly up to 8 kHz, almost like a broadband speaker. In order to give it a 6 dB limit at 3.5 kHz, it needs an 18 dB filter. We put additional brakes on its second coil by including a small overlaid capacitor. The tweeter would have done well with a second-order filter too, if we had only been looking at its own frequency curve. But it didn’t work for the addition of the branches, neither with the same nor a phase-shifted polarity. Since the components for the tweeter branch are small and thus cost-effective, we decided not to cut costs in the wrong place; we took another capacitor from our supply cabinet, and it added the two branches in a satisfactory way. The fact that we also weakened a little hitch in the curve at 8 kHz using a suction circuit was more of a cosmetic procedure, just to keep a few people who are experts in all the theoretical aspects from bothering us about it in various forums. A couple of components were left over once the crossover was set up; now they can be used to smooth out the impedance for tube freaks.
The crossovers were built onto small boards. If needed, the impedance correction is simply glued onto the back of the terminal and soldered in between the positive and negative poles. The tweeter has a phase-shifted connection to the six-inch units.
Four bags of Sonofil are loosely rolled up and distributed evenly in the box, through the bass cutouts, before the chassis elements are soldered on and screwed into the pre-drilled holes. The marking for the positive pole on the AMT still needs to be fixed. It is still missing, but generally it is on the left side when you are looking at the tweeter from the perspective shown in the middle picture. Four RS 07/ 22 rubber feet from the Intertechnik accessory line are a very cost-effective way to bring the Audible 34 up off the floor.
|Frequency under 0/ 30/ 60°|
|Distortion for 90 dB||Step response||Waterfall|
The final listening test started with the still-unpublished tube amplifier that had already been tested in the Power 17; its 6 watts with triode circuitry were plenty, even at the nine o’clock position, to reproduce Mozart’s Divertimento for violin, viola and violoncello at a pleasant chamber-music level. That was not the only positive result; the slightly rounder sound of the amp completely removed the PA character of the Audible 34 and turned it into an audiophile’s loudspeaker, one that wonderfully reproduced the differences between the three string instruments. The stage, locatability and detail left nothing to be desired – this speaker easily ties the listener raptly to the sofa. In the quiet passages, there was no change in the tonal color, which is an unfortunate and common problem with many “loud”speakers. There is not too much to say about the dynamics for slower music, but in any case even the smallest changes in pressure of the bow on the string were not suppressed. The warm tonal character was by no means disruptive when Patricia Kaas sang about “Autumn Leaves,” in keeping with the current season. An electric bass with clean notes and reverberation underlined her voice, contrasted with a short, dry bass drum; the whole production sounded round and of a single piece. There was not even any exaggerated hissing or annoying sharpness to complain about when we had the famous 2009 Bravo Hits spinning in the player. Well, okay, we still liked the first discs better, but that has more to do with our own particular musical taste.
When it was time to move away from “playing nice” in the jazz department, we switched to the pentode, since 12 watts could provide a little stronger stimulus here. Our gaze landed on Keith Jarrett’s “Köln Concert,” which we haven’t heard for much too long, and it didn’t disappoint. He sat there at his grand piano, hummed a few bars, coughed, tapped his foot on the floor of the stage, and then clearly sang along with the odd sequence of notes as he played – all of it much quieter than the forte hammering on the strings of his piano. Some good old completely uncompromising rock – hard to imagine these days – came from the Doors. “LA Woman” caused the listener’s legs to start bouncing rhythmically and made his head wobble. Of course it was loud, it’s not supposed to be quiet! More highlights from the ’60s and ’70s followed over the next few hours; sometimes there are more important things than putting words to paper.
Once we returned to real life, we connected the DTA-1 to the Audible 34; it also would have been a good idea to tie it to a smartphone, or what we used to call a telephone. But since the age of the evolving mobile phone has completely passed me by so far, I was forced to lament my lack of such a device. Now, I’m no longer young, and I don’t know any of the popular songs today except for whatever is being played five times a day on the radio. Well, I do have some Toten Hosen, from the time before they only played chart-toppers. “Hier Kommt Alex,” with the logarithmic half of the maximum available 10 watts per channel, promises that multimedia PCs, streaming music and other modern music sources will benefit from the nearly 97 dB/ 2.83V even after all the noise-sensitive spoilsports have fled the area.
|Loudspeaker drivers||Gradient 2 x AXP 06||Wood list in 19 mm MDF|
|Gradient 1 x GAM 100||per box in millimeters:|
||Intertechnik||1000 x 300 (2x) sides|
|Construction||190 x 300 (2x) top / button|
|190 x 962 (1x) back wall|
|Function principle||Bass reflex||190 x 914 (1x) front (baffle)|
|Nominal impedance||4 Ohm||190 x 102 (3x) reflex board / reinforcement|
|Connection terminal||T105 MS/ AU|
|Damping / insulation||4 mats Sonofil|