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Planar Mid/tweeter design page
This page is meant to keep interested people up to date on the progress of the ribbon driver. Comments and suggestsions, of course, are more than welcome. Send to firstname.lastname@example.org.
The first prototype is up and running. I hit several snags along the way and did a bit of redesign on the fly. Here are a few notes:
- The idea of laying down adhesive transfer tape of the correct width, laying Al with excess on top and trimming to the width of the tape won't work. The Kapton is cut far too easily to make a cut just outside the tape and the tape holds far too well to cut just inside the tape. I tried this a few times with flat & curved Xacto blades and with a rotary cutter and had poor results. The idea of making 8 4' long cuts this way was a no-go. I opted to cut the Al foil to the correct width ahead of time and to use the 1/2" tape to hold the two conductors in the gap. This worked fairly well, but:
- While laying down the conductors, I broke the last one about a foot from the end. Through creative use of glue, pressure to kink the conductor, and pot-metal solder I "repaired" the broken conductor. By the time this happened, I was viewing this as a prototype diaphram anyway.
- The tensioning system works fairly well, but can lead to uneven tension at the top and bottom if gross adjustments are made. The first attempt to center the diaphram was less than perfect and I ended up using the tensioning mechanism to help center it. Unfortunately, this left me unable to maintain a constant tension across the diaphram and the top is quite loose. The next diaphram will be marked with vertical lines where it should meet the edge of the frame so that it can be centered and evenly tensioned from the start. The static cling of the Kapton, the excess adhesive on the diaphram, and the displacement induced by the foam all conspired to make the task of placing the diaphram on the frame impossible without a good guide.
- All mounting holes to keep the two plates and the frames together must be made before the magnets are placed on the plates. It's a real bummer if you don't.
- Crazy glue and Weldbond don't work worth a darn. Epoxy is the best solution I've found as it is more reliable and doesn't break with minor flexing.
- The frames can be assembled by one person, though two would be better (I did it by myself by using standoffs to keep one of the plates away from the diaphram frames and then dropping screws through this plate into holes on the frames). There is a remarkable amount of force at play during final assembly. Even with both ends fixed, there was a sizeable deflection of the steel plates in the middle of the driver.
- Initial rough measurements showed a falloff at the upper end and low efficiency. I'm not going to read much into this until I get a better diaphram in place. The low end was in the 100Hz range. More will follow on this. FWIW, I clocked in at 6 ohms. I've yet to weigh the diaphram and right now don't have a good way to do so.
- The use of a PCB for the top and bottom connections to the foil worked quite well. In this version, I used WD-40 to try to prevent oxidation and help make good contact. It's what I had on hand.
- The corrugator worked well enough. I put several strips of the ribbed rubber on a piece of wood, layed the diaphram on this and used another piece of the ribbed rubber to press the diaphram into the "corrugator". The Al was nicely rippled after this.
- The gap present between the magnets and the diaphram is probably too large. I'm loosing efficiency here and trading it off for excursion. I seriously doubt I need the 0.1" I have on each side and should probably try to trim each frame by 1/16" or so.
- Apart from possibly hearing the DAC I designed work on the first powerup, watching and hearing the ribbon built from scratch playing the Brahms violin concerto was the biggest thrill of my DIY career. Sure, the physics says it should work, but there's still the thrill of actually seeing it work. Far too cool.
This project has its roots almost a decade ago when I read an article in Speaker Builder (Ref 1) on building a ribbon tweeter from scratch. I toyed with the idea for a while, but the project was put on the indefinite back-burner. Recently, Speaker Builder had another article (Ref 2) on the same basic topic. Daniel Patten produced a push-pull mid/tweeter planar that seemed to perform well at a moderate cost. Thinking the cost could come down and the performance could go up with some work, I posted my interest to the Bass & DIY Loudspeaker list and began to do some research.
Have you checked out the DIYAudio page?
- Basic design, Target specs, Materials & FAQ:
- Details of design / construction
- Calculations (or things passing as calculations)
Planar mid/tweeter - 15 OCT 1996
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