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|Yesterday, 06:58 PM||#1|
Join Date: May 2007
DIY ribbon dipole tweeter, reductio ad minimum
A ribbon dipole tweeter reduced to the bare essentials that works and measures surprisingly well.
This is my third attempt at building a dipole ribbon tweeter, the first two used steel frames for holding the magnets and closing the field. This time I decided to drop the steel frame in an attempt to reduce the width of the tweeter as much as possible.
Dimensions are in millimeters. Thickness is just 9mm. What looks like single magnets are actually two custom N52 4mmx10mmx40mm magnets glued on a 1mm thick frame of aircraft quality plywood.
Using two magnets separated by a non-magnetic material results in a much more even flux density across the gap.
FEMM was used to simulate the flux density across a 19mm wide ribbon in a 20mm gap. The red line represents a single N40 (standard) magnet, the blue line represents dual N52 magnets with a distance of 0.5mm and the green line represents dual N52 magnets with 1mm distance. With the single magnet the maximum is much higher and located on the outermost edge, if you wanted to flex the ribbon this would be ideal. With the green line the difference between maximum and minimum is much less and the maximum is also located about 1mm from the edge, less corrugation will be required to prevent flexing of the ribbon.
The ribbon itself is made from 10Ķm household aluminium foil etched down to about 5Ķm using a diluted sodium hydroxide solution.
The foil is cut to size using a roller cutter and corrugated with this simple contraption
Itís even called ribbon cable Duh. The trick is to have just the right amount of corrugation, too little and there will be serious distortion, too much and youíll get resonance problems.
How to drive this ribbon? Normally a transformer is used with this kind of (small) ribbon, I decided to try the brute force method with a series resistor and lots of power to see if this could work.
A Hypex NCore400 and four 0.5Ω 100W Caddock resistors. Not what youíd call an efficient approach but there are some advantages.
- The amp sees a nearly perfect resistive 2Ω load.
- Small variations in the contact resistance at the ribbon do not matter.
- Thermal compression is not an issue as the resistance and thus current is determined solely by the resistors.
So, how bad is it? Is it a space heater? No, it is not a space heater. Well, it will be if all you do is playing test tones at full power but with music the temperature of the heatsink barely rises above room temperature. Playing music very loud I measured peaks of 15A but the average was just over 3A.
Is it really a dipole? If so, up to which frequency? Next up, the measurements.
|Yesterday, 07:10 PM||#2|
Join Date: May 2007
All measurements were performed using a Clio FW system and an MBNM 550 EL microphone. The level was calibrated with an ISO-Tech SLC 1356 sound level calibrator so the dBSPL numbers actually mean something. To prevent reflections the stand beam has an XLR connector glued into the tube. Hereís a picture showing the setup for measuring vertical directivity.
In the following graphs the measurement parameters are always displayed below the graph. All frequency response measurements, directivity graphs and cumulative spectral decay plots are unsmoothed!
Letís start with the frequency response measured at a distance of 50cm, vertically centered.
The vertical scale is 50dB. The blue line shows the response for the unfiltered ribbon. As you can see by the 6dB/oct slope it is actually a dipole, I would say up to about 7kHz. The red line shows the response for the driver with DSP applied. A Hypex DLCP was used with the following settings:
Just for biquads are required. There is a slight drop-off above 16kHz but -2dB at 20kHz is good enough for me, thatís the price one has to pay when using a ribbon that is 19mm wide. I wonder if the little blip above 7kHz is actually the dipole peak, this would make it a candidate for EQ correction. If that blip is removed the response is flat within 0.5dB from 3kHz to 16kHz, not bad, not bad at all.
Here is the cumulative spectral decay for the ribbon with DSP:
The vertical scale is 40dB, the time window is 5ms. All in all a very quick and clean decay. My theory is that the mass of the ribbon is so low (with respect to the surface area) that the surrounding air provides the necessary damping, remember that because of the series resistor there will be no electrical damping to speak of.
Next up are the directivity measurements.
|Yesterday, 07:21 PM||#3|
Join Date: May 2007
I measured the directivity in 5 degree steps from 0 to 90 degrees, the plots are mirrored to create a 180 degree plot. Letís start with the bad news, the vertical directivity. The vertical directivity is, of course, the weakness of a ribbon driver. The question is whether it is acceptable for the intended purpose.
Vertical response for several angles:
Directivity and normalised directivity:
Normalised polar plots:
At 8000Hz the -6dB angle is just 30 degrees, at 16kHz itís not even 20. There is serious beaming but itís not too bad and at least itís pretty much textbook behaviour. The lobing is below -12dB, everything above is very smooth. The main consequence is that there is a minimum listening distance of about 3m, this clearly rules out any application in a near-field monitor but the ribbon certainly can be used in a larger multi-way system.
And now for the piece the resistance, the horizontal directivity.
The response for several angles:
Directivity and normalised directivity
Normalised polar plots:
Pretty much constant directivity up to about 7kHz, thereís a little widening around 10kHz and narrowing above that. I think youíll be hard pressed to find anything thatís better than this.
The last item on the measurement list is the harmonic distortion. The distortion was measured Ďin situí together with a B&G Neo10 magneplanar. The scale for the fundamental is in dBSPL on the left, the scale for the harmonics is linear in % on the right with 1% full scale.
The 2nd harmonic has a peak of 0.44% just below 7kHz but for the most part stays below 0.3%. The 3rd has a bit of a plateau of 0.2% from 1.5kHz to just above 3kHz. The higher harmonics start out at 0.1%, which is definitely on the high side but fortunately they drop to 0.02% at 3kHz.
There is room for improvement here and this will be the subject of further study.
On a practical note, this ribbon is very insensitive to draft which is very nice. Nevertheless itís wise to keep a set of spares handy just in case:
Although this open construction is inherently vulnerable the fact you can fit a new ribbon in under 15minutes removes most of the worries. There is also no need to worry about amplifier failure. There are only low cost and readily available materials involved in the making of the ribbon itself so itís no problem the make a couple more.
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