In the beginning, one thing followed another – a new FirstTime was always hot on the heels of the last one. Our first issues featured Numbers 1 through 6, which enriched the do-it-yourselfer’s life with economical chassis elements in somewhat unusual cabinets. Things went a bit slower after that, although in the meantime we have already reached Number 12. Transmission lines (FT 2 TL), band-pass filters (FT 7, FT 10, FT 12) and a double-chamber reflex (FT 9), even a large pedestal box with a side bass (FT 8) and one with an enclosed cabinet (FT 11) are all represented. The response was not any more subdued when the last new release was introduced in 2011. All seven of the FTs still available are still among the most popular assembly plans – and those are not just the newer models, but also the old ones. They live in dorms, home offices, bedrooms and children’s rooms, and combinations with the Quickly series transform a living room into the perfect home theater. In fact, we already have everything the ear desires, and there were very few incentives to come up with something new. But when the new Coax SN 6 CX from P.Audio fell into our hands, it was immediately clear that it was time to do something else with the FirstTimes. Setting aside all superstition, we called the child by its logical name: the FT 13.
Once again, the occasion was a conversation between Udo and Andreas in Kerpen, during which Andreas held a data sheet under Udo’s nose. “You were looking for a coax with volume control, right? Can you do something with this?” Now, a #16 membrane, more than half of it covered by a large horn, and a basket that cannot be recessed are not the ideal conditions for a successful development. Still, we were excited about it, because it also said 91 dB/ 2.83V/ 1m. The fact that we couldn’t just stick the chassis into a little box, and the fact that the crossover would also pose certain challenges, made us even happier; it wouldn’t be a conventional 08/ 15, or rather 07/ 25. We couldn’t resist the temptation, so it was no surprise to hear “Let’s have it!” cross our lips.
Before using the SN6-150CX in an assembly kit, it first needed to go up on the measurement wall.
Data sheet SN6-150CX
Item number: 1411627
Bass mid-range speaker:
|Membrane:||card board||Air gap height:||6 mm|
|Surround:||rubber||Winding height:||13,5 mm|
|Pole piece hole:||yes||Mounting holes:||4|
|Centering:||raised pot spider||Outside diameter:||187/ 162 mm|
|magnetig shielding:||no||Installation opening:||148 mm|
|Voice coil size:||44,5 mm||Milling depth:||8 mm|
|Voice coil former:||Kapton||Installation depth:||135 mm|
|Frequency response under 0/ 30/ 60°|
|Distortion for 90 dB||Step response||Waterfall|
|Membrane:||Titan||Effektive Piston Area:||9 mm²|
|Voice coil size:||34 mm||Pole piece hole:||yes|
|Winding height:||3 mm||Mounting:||Schraub|
|Pole piece hole:||5 mm||Magnet:||Feritte|
|Frequency response under 0/ 30/ 60°°|
|Distortion for 90 dB||Step response||Waterfall|
After the usual tinkering around on the measurement wall, we fed the parameters of the SN6-BMT into our LspCAD. Just under 10 liters of volume, HP/ BR 50 shortened to 8 cm, and loose insulation filling produced a usable little PC box that reaches its -3 dB point at 80 Hz. Today it is very common to call something like this a near-field monitor, even if the technical world defines it very differently.
Using the FT 13 as a computer box? Well, that wasn’t really our plan. But what can you do with a chassis that sacrifices its deep resonant frequency for the efficiency factor? It’s very simple: you build something a little bigger and see what happens. With another six liters and the same reflex tube measuring 6 cm in length, we came up with 60 Hz/ -3 dB, according to the simulation; but the world of practice vehemently disagreed with the theory. Oh well, 16 liters isn’t big enough for a pedestal box anyway, with almost 20 cm of front width. And then we thought of the SB 12 ACL. No, we still don’t have a plausible explanation for the amazing bass capacity of these mini basses, but we decided to use their structure despite the gap in theory. When you don’t have a real plan, it’s easy to go ahead and use SketchUp to pull the walls of a functioning structure outward, which creates a new acoustic chamber reflex with four different-sized chambers. Almost by chance, we chose 26 liters for the overall volume; we left the pass-throughs in the intermediate boards at 90% of the membrane surface, and for the reflex balance LspCAD told us HP/ BR 70 with a length of 8 cm. The downloadable SketchUp plan was then quickly finished..
Communicating the outer contour of the SN 6 to the CNC router was no trivial matter. First, the PDF had to be converted to a JPG, which was then halfway decently transferred to a drawing program and finally to the router file. There must be easier ways to do it, but we didn’t come up with anything during a Google search. We can live with our solution, anyway, and from two meters away without our reading glasses, there were no serious defects visible. If you don’t trust yourself to get the cutouts accurate enough, you can also place the coax right on the front. There is no negative impact on the sound.
Our digital camera insisted on observing the progress of our work. However, we always had plenty of time to get out of the picture.
The front, rear wall and interior boards were cut out of black-stained MDF with a surrounding groove to fit them together. In addition, the interior boards were inset with a little extra space around them. This type of cutting makes assembly even easier than our originally preferred butt-joint miter.
The front intermediate board was glued onto the front and then set aside for later processing. The rest is faster to put together than it is to explain.
After assembly, the cabinets had a little time to dry before starting the sanding and painting process. In order to save ourselves two days of waiting around in the basement, though, we moved on right away to the job that intrigued us the most: developing the crossover.
The sight of all the peaks and valleys in the frequency range of the bass mid-range speaker and the tweeter did make us wonder whether it would have been better to skip the FT 13 and start with a simple FT 14. But who wants to throw in the towel and give up right away as soon as there’s a little challenge? After all, the amplitudes for coaxes can’t be linear; the horn positioned in the front prevents the mid-range from projecting through the membrane, and the decay for the horn can never be hidden, either. The first crossover draft consisted of a mass mid-range speaker filter made of a coil with a capacitor over it to smooth out the bump from 5 to 7 Hz; a parallel c to increase the edge steepness; and a CLP suction circuit that removed the exaggeration around 1 kHz. The tweeter needed to be tamed with a heavier hand. In addition to a third-order filter, the bump around 1.5 kHz needed to be removed using a suction circuit, and of course the volume level needed to be reduced by a good 12 dB using a voltage divider. Yes, we can also build crossovers with a lot of components! Now let’s throw in the most expensive components and call it high-end ;-)….
No, there must be some other way, even if a couple of spikes remain in the curve. A smooth-ironed line was never our thing – character requires edges. So on to something new! We used a coil and capacitor for the bass mid-range speaker and still 18 dB for the tweeter, but with very small capacitor values, as required by a somewhat unusual voltage divider. Instead of leaving the impedance at the 8 ohms of the tweeter, the volume regulator created an even 15 ohms and gave us even better opportunities to positively affect the curven.
At a volume of just over 90 dB, we naturally also had to consider the increasing number of listeners with tubes, and measure out an impedance correction.
Measurement diagrams FT 13
|Frequency under 0/ 30/ 60°|
|Distortion for 90 dB||Step response||Waterfall|
The best setup is if the boxes are very slightly folded in toward the listener. The frequency curve below 15 degrees remains within a narrow range of +/- 2 dB from 200 Hz to 20 kHz.
The crossover now fits easily onto the S3-2W circuit board, which we offer for those who prefer not to use a wooden board.
When we went to install the crossovers in the boxes, a small oversight in the assembly process came back to bite us. How are you supposed to pull a wire up into the box from the terminal if your arm isn’t jointed in four places? Well, you need two different tools.
We tied a screw to a cord, whose weight drew the band through the air holes in the upright box with a little wiggling back and forth. The earth’s natural gravity was helpful to us here, a physical value that Newton had already investigated in great detail after being hit on the head by an apple. Once the cord had reached the bottom, we tied the wire to it and were able to pull it up easily in order to solder it to the crossover.
We screwed the circuit board with the components onto the top intermediate board, and then loosely packed one mat of Sonofil (we didn’t happen to have any in white, but that would also have worked) into the upper chamber. Naturally we left the pass-through free.
The positive poles are marked in red on the SN 6 CX; please note that the tweeter needs to be connected out of phase with the bass mid-range speaker. Finally, the HP/ BR 70, shortened to 8 cm, is placed in its cutout.
Ready to go, the FT 13s marched out of our own measurement/assembly/photo studio straight into the listening room, which is familiar to many readers with its venerable old sofa. Here it is worth mentioning that we let our chassis elements wobble into place for 24 hours before measuring them, using the appropriate sine signal and volume level. That saves us the breaking-in phase that the bass in particular requires. Only after a couple of CDs did it achieve the parameters that were calculated for the cabinet. That explains the missing bass often noted during the first listening test, which goes away later on. In any case, we were able to get right to business because that part had already been taken care of.
For the first test, we put Pink Floyd’s “The Wall” into the CD slot. The music that met our ears was pleasant and lacking any harshness, but still full of details and dynamics. It was different from what we were used to with our SBs, Etons and Seas speakers – more direct, bouncy. Well, it does use chassis elements from the PA area, whose strength is less in carefully breaking down complex structures than it is in their rough dynamics. Although the chassis elements have to do without some bass depth thanks to the high effectiveness level, there was nothing missing from the strong drumbeats. We would love to know whether that is really due to the ACL principle. In any case, it gives us a project to try when we retire, shortly before our 80th birthday. This thought hit especially close to home in conjunction with the “Hotel California” live version of “Hell Freezes Over,” something that may be played just a bit too often around here. The bass drum was by no means too small, but the rest above it did not come across as thin, let alone annoying. Hissing is not a characteristic for which the tweeter is known, and for a loud horn with waves in the frequency curve and a low K2 distortion it is not necessarily expected. Compared to a wide-range speaker, the most similar chassis type, the sweet spot is much wider, but the stage representation is no worse. We were surprised by the recording that “Don” from the community recently sent us: the Fantastischen Vier “Unplugged” – it’s not for everyone, but still very listenable on the FT 13. Rammstein is somewhat harder when it comes to contemporary rock, so we put on “Herzeleid” for the big finish and bobbed our heads a little to the guitar rhythms like in the good old days. And that made it easy to choose the category: the fun class!
|Chassis||P.Audio SN 6 CX||Wood list in 19 mm|
|Construction||Udo Wohlgemuth||100,0 x 19,0 (2x) front/ back wall|
|100,0 x 21,5 (2x) sides|
|Function principle||ACL (Acoustic Chamber Line)||15,8 x 21,5 (2x) lid/ floor|
|Nominal impedance||8 Ohm||15,8 x 15,5 (3x) reinforcement|
|Terminal||T 105 MSAU||Milling depth: 8 mm|
|Damping / insulation||1 bag Sonofil|
|Reflex port||HP70 length shortend to 8 cm||
without wood: 150 EUR/USD