New Supertweeter - Progress Report

I have, for over a year, been secretly developing a new AudioSmile Supertweeter to replace the current version.

Part 1

The new design started for two reasons. Firstly, I often found the current model sounded best toed out from the main speaker to make it fire off-axis. In some cases it also sounded better 'upside-down' like the image below with Kensai. This way the sound comes to the listener more diffused and doesn't intrude on the balance on the main speaker. So I got thinking about a design with two ribbons at angles to each other. A trial with dual ribbons gave even wider and taller soundstage than the single unit.



I decided that the shape of the new supertweeter had to be able to work in both an upward and also forward firing position. It strongly depends on the speaker and room which position sounds best. I got stuck into AutoCAD and came up with this basic design. The drawing shows controls sticking out from the end-cheeks but in practice they will be recessed into the cheeks allowing the supertweeter to be stood on end and a channel in the cheeks and large face allow the cables to neatly hide.




Secondly, I got thinking about making a highly adjustable filter circuit that could be tuned to match almost any speaker. It's certianly not easy to do and I can see why many designs have limited options. Many days simulating and testing came up with a circuit that has 6 volume levels in combination with 3 filter settings - combining these it's possible to choose 18 different filters. A problem was hit when trying to implement level adjustment because a resistive divider would always present a low impedance to the amp. Eventually a method was found using resistors in parallel with different capacitor values such that lower level settings present a higher easier to drive impedance.


Each of the small groups needs switching in and out, both signal and ground to keep them out of the way when not in use. So it meant having a lot of PCB pads to be soldered to rotary switches.



PCB back from production. It's extremely satisfying when you populate a new PCB, and find it works first time when testing!



Stay tuned, more to come!

 

I still think it should be made from crisp, dark, chocolate and named the 'Audiosmile Toblerone'

 

Easy material to machine!

 

Part 2

Right well after making the 3D drawing it was a case of separating out all the parts and setting up the cutting lines and details to machine material.

Before I had a CNC machine I thought it was as easy as the computer figuring out how to cut the 3D part, but oh no you need to draw every path for the cutter to take and then tell it how deep to cut, at what speed and how many passes, what direction... for every little bit.



Machining the parts was actually one of the biggest challenges of this project. The plastic end pieces need to take into account the exact thickness of the tops so the controls and wood end-caps line up perfectly. The base part needs to have a perfect quality of cut finish because you can see the edge. The top pieces are the most difficult because they can not have any fixing holes or you would see them! They can't even be held down by a vacuum table because once cut-out the surface area is too small to give a good hold. Eventually and after wasting much material I found the method was to stick the sheet of acrylic down with strong double-sided tape on a sheet of wood which is then held perfectly flat on the vacuum table. Much to my surprise this method is strong enough!

First the base pieces are cut. Those lines are the channels for speaker cable. You may also see the 4 holes in each corner which are to fix the base on the supertweeter but conveniently double as fixing spots so I can run a final edge trim after the main cut-out work to get a really smooth cut.



The next stage is to cut-out the structural end pieces. Each one needs holes to fix it down. Thank god they are hidden by the wooden end caps or it would be impossible



Finally the top pieces are cut. These have a slight rebate for the ribbon driver and leaves a 3mm 'losenge' hole for the sound to come out. In that losenge I fit a another piece with a chamfered slot for smooth sound dispersion and use a frosted finish which I think nicely offsets the gloss of the main body.



The tricky part is adding the 45-degree chamfered edge. It needs to be exactly the right depth of cut because any change here will make the part smaller too given the cut angle. If the piece vibrates or flexes while cutting it's game over. I first tried clamping each piece through the centre cut-out but it just wasn't strong enough and the part often slipped or vibrated on the table. The vacuum table really helps here because it holds the entire sheet while cutting, not just a small clamp area. A bit of cutting lubrication does wonders too. The full depth cut removes loads of material hence cutting forces are quite significant.



The perimeter cut-out is made using a down-spiral mill because it helps put downward force on the piece and reduce movement on the final passes.



You can see here that the protective film is still on the top pieces which is really important because in a workshop environment it is easy to mark them during assembly or testing.



That is nearly all the acrylic parts produced! The only remaining one is the losenge made from frosted acrylic and I didn't photo that during production.

More to come

 

Part 3

The final parts that need producing are the solid wooden end-caps. The timber is first face-milled to an exact 18mm thickness.



Next the chamfered cuts are made for the perimeter and the controls to allow easy finger access.



Next the straight-cuts are made for the binding posts and the rotary controls and finally the outside perimeter cut. The outside cut is left with just a veneer of thickness remaining so the part doesn't fly away. It can then be removed with a knife. This is called 'onion skinning' a cut.



In need of clean-up!



Sanding is a time consuming business I clamp the sander to the table. First all surfaces need to be taken down to 120-grit. It's important not to sand too much or un-evenly or they will not match the supertweeter body shape

After that we go around again with 220grit. This shows up all spots that were not perfectly sanded initially, lol. When done well it looks nice and smooth. The circular cut-out gets sanded by hand as do the small squared corners.



A clean with white-spirit gets the dust off ready for sealing with lacquer.



Spray with acrylic lacquer. It needs about 3 coats to get a good seal on all faces. The end-grain absorbs a lot.



Now we need to de-nib the finish so it is smooth. This is done by hand-sanding with 600-grit paper. Wire-wool is sometimes better if there are stronger 'nibs' from coating too thickly, but if each coat is fine then the paper works a treat.



After another clean with white spirit these pieces can be waxed with tinted walnut paste wax. Left to dry and buffed. The tint is great for Walnut because it has deep pores that trap dust and look grey if the viewing angle is against it. Here I was using clear wax and later added the tint on top.





Eventually, with all the parts prepared assembly can begin! Yes, yet more to come

 

Ahh well nearly all the components are ready.. the frosted losenge insert for the ribbon to fire out through still needs to be cut and cleaned.



Much better!

So first we need to bond the two tops together along the 45-degree cut edges. The best glue for acrylic is Tensol, which chemically melts the two surfaces together. A small area of the protective film is removed from the pieces and glue is applied sparingly so as to minimise leakage onto the visible surface.







Next the internal triangular front and back and bonded in. These are the parts that the switches, binding posts and wooden end-cheeks attach to.









Small triangular pieces are them bonded in each bottom corner of the assembly. They are drilled and thread tapped to allow the base component to screw onto a flat surface. This needs to be done very precisely so the base will line up perfectly with the top assembly.





The base is then countersunk to keep the screw-heads from scratching the surface of the speaker below.



The frosted losenge sections are glued and inserted in the main assembly. They need to be precisely aligned so there is no visible gap and so they neither protrude nor lay below the main body.









The circuit, already wired to the switches, is then bonded to the base component on a layer of damping material. I know it's not very pretty. The sticky tape helps hold the base down during machining and is invisible once assembled.



Now the base and main assembly are together, the visible edge on the base is sanded multiple times to a very smooth finish.





The ribbons are installed, the switches are attached and the binding posts are inserted which locate and fix the wooden end-cheek.



The binding posts are wired up along with the ribbons and the front end-cheek attached. All we need to do now is test it and if working well, remove the protective film and clean the exposed areas!

 

Here it is on a speaker I had recently built for a family member.