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Old 23rd January 2005, 07:03 PM   #1
dhenryp is online now dhenryp  United States
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Default Another DIY Ribbon thread

I'm starting this thread because I don't want to hijack the other long running "DIY'ing ribbon" thread.

I've built a prototype ribbon tweeter using (relatively) simple and inexpensive materials. My end goal is to build ~36" multi segment ribbon tweeters for a line array. My prototype represents one of these segments and the powered portion of the ribbon is 5.25" long. It's open back so I can run it either as a dipole or absorb the rear wave without having a lot of problems with cavity resonances.

I don't have acces to a machine shop and I'm too cheap to pay for services so I ddid not want a design that required complicated machining of huge hinks of steel. I bought mine from Online Metals but McMaster Carr probably also has it. I've used standard 5/8" square 10018 steel bar bolted together with simple 1/4" machine screws.

The ribbon is made of thin alumiinium foil available and cheap from McMaster Carr. I've used .25 x .5 x1.75" N40 Neodymium magnets. These cost a few bucks +- each. These are very powerful magnets and a little tricky to handle. I wanted to avoid designs that required huge, limb threatening blocks of Neodymium. Femm shows me getting ~ .5 tesla across a half inch gap - not Raven class but not bad. Doubling to 1 tesla would only buy me 6 db but would make the driver much more complicated and expensive. Since my eventual goal is to run 36" inches of these ribbon, I don't think volume will be a problem.

Still in the works is a matching transformer. I've odered a couple types of ferrite torroids and will try winding my own. Until then, I've played around with the driver putting a four ohm resister in series with the foil and driving it from a 100 w amplifier.

Total cost of this single driver is ~$35. How does it sound? I just finished it a few hours ago so I've got very little time on it. Running it full range, it sounds surprisingly good. I was able to stretch the ribbon by turning up the volume too high while playing it full range. I've also tried it with 20uf capacitor in series to role off the low end. This reduces excursion a lot.
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File Type: jpg ribbon tweet front.jpg (24.9 KB, 13510 views)
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Old 23rd January 2005, 07:04 PM   #2
dhenryp is online now dhenryp  United States
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Default Femm simulation

Here is the output from femm:
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File Type: jpg femm ribbon tweet.jpg (87.9 KB, 11555 views)
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Old 23rd January 2005, 09:50 PM   #3
gl is offline gl  United States
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Outstanding effort dhenryp! I too have been slowly building a similar unit out of hardware store stuff. Where did get the magnets? Keep up the great work and please keep posting your results.

Graeme
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Old 24th January 2005, 12:36 AM   #4
dhenryp is online now dhenryp  United States
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I got the magnets at :

http://www.engconcepts.net

Good prices and good service.

FYI: Here is one more picture, of the back.
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File Type: jpg tweet three quarter back.jpg (26.4 KB, 11770 views)
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Old 24th January 2005, 01:10 AM   #5
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Hi dhenryp,

Have you considered adding a couple rear iron cross braces in an effort to make the magnetic field across the ribbon more uniform? The metal on top and bottom gives a non-uniform transverse magnetic field, which is not ideal. The rear cross braces can be of modest width, and could be covered with a thin layer of wool or fiberglass to absorb some of the rear reflections. Otherwise, you may want to use aluminum rear cross braces on the top and bottom to keep the transverse magnetic field more uniform over the entire ribbon, although this will come at the expense of lower efficiency.

You may also want to reduce some of the front diffraction created by the deep magnetic cavity and sharp front edges by adding an angled or rounded front baffle plate. Most ribbons do this. You will also probably measure a SPL bump due to the cavity resonance, but this should be near 20Khz, and probably not worth filtering, which is a nice benefit of a narrow 0.5" ribbon.
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Old 24th January 2005, 01:31 AM   #6
dhenryp is online now dhenryp  United States
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linesource - Thanks for the comments. I'm anxious for feedback since everything I know on the subject I've picked up in bits in pieces on the web or forums like this.

I know the flux density in the vertical pole pieces is much denser at the ends than in the center but it does not appear to affect the linearity of the field in the gap. I've run femm with the top and bottom cross pieces removed and it does not seem to improve gap linearity and, as you said, reduces field strength by ~ 40%. What is the benefit of distributing the field in the poles more evenly?

Regarding the diffraction effects - I had planned to put 3/4" quarter round, or maybe just straight 30 degree angle pieces, on the front and the back. In the back, I might add 1/4" felt over top of the quarter round so that its surface is even with the surface of the magnets. How does that sound? This is just a prototype so I may not fiddle with this stuff until I build the final tweets.

Thanks again,

Denis
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Old 24th January 2005, 06:09 PM   #7
Joules is offline Joules  United States
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This is very interesting Denis. They look very well made. I'd like to know more about the design you used. How do they sound? Do you know the resistance of the ribbon? with 36" of ribbon do you really need a transformer? ... Good work !
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Old 24th January 2005, 06:37 PM   #8
tiroth is offline tiroth  United States
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Very practical design for DIY. It looks like the bulk of the cost is in the magnets...the cost for the frame should be very low in bulk.

Are the magnets simply stuck to the frame?
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Old 24th January 2005, 07:25 PM   #9
dhenryp is online now dhenryp  United States
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The design is not an exact copy of anything else I've seen but ribbons are so simple I can't take too much credit for original work. The biggest contributor to the design is the femm magnetic simulation tool. I've played around with it a lot over the last several months trying to come up with a simple efficient magnetic circuit.

As I said at the top, I wanted something that had the potential for high performance but did not require a machine shop to produce. The "machine shop" I used to build this consisted of a sawsall, a bench grinder to clean up the sawsall cuts, a drill (I have a drill press but a hand drill wood work as well), and a tap to thread the holes in the vertical pole sections.

I crazy glued the magnets on. Without glue, they tend to repell each other and leave ~ 1/8" space between each of them. I may go to 5 minute epoxy. Crazy glue dries too quickly and these magnets are strong enough to have a mind of their own when you are trying to force them into position.

I've upped the capcitor to 40 uf to give me a first order filter (with the 4 ohm resistor) of 1k. This may turn out to be too low, but it's been OK so far. All I have set up in my basement right now is an old receiver, so I've only listened to FM radio so far. I've turned the (100w into 8 ohms) receiver all the way up and given my resistance assumption, I was probably applying less than 1 watt to the ribbon. It was too loud to listen to comfortably at a distance of ~ 3 feet - so this thing is at least moderately efficient. I've got lspcad/justmls but I'm not set up for measurements yet. I may take out my Radio Shack meter to get a rough idea. As I said at the top, it sounds pretty clean with no buzzes or obvious distortion (as long as I filter out the really low end - I did get some interesting noises when I stretched out the ribbon pleats by playing full power full range).

I made my own ribbon pleating "tool". Home Depot sells a flexible rubber "cord" that is used to keep screens in wooden screen doors/windows. It's ~ 3/16 inch diamter, hollow with, with 8-10 ridges running down the length. I stuck some finish nails in the center of two pieces cut to ~ 3/4 inch, stuck them in the crudest possible jig so they will roll against each other like gears. I get pretty good ribbons with this. It takes about 20 seconds to pleat a 6 inch ribbon. This may get refined as I go along, but maybe not.

I don't have a milliohm meter, but using what I have (.1 ohm lsb), it is definitely less than .1 ohm. I've read somewhere (the Raven site?) that a similar sized commercial ribbon is ~ .02 ohm so I'm guessing mine is probably in the range 0f .02 - .04. I should be able to get a better estimate by comparing the voltage drop across the four ohm resistor to the drop across the ribbon for a given frequency, say 3k. I have to dig out my signal generator and hook it up. Even six of these ribbons would probably only get me something on the order of .1 - .2 ohms - still too low for direct drive.

The next step is the Xformer. I should receive my ferrite torroid cores tomorrow. I'm going to start with an 8-10 turns ratio (assuming I can fit all the wire on the relativey small cores I bought). I've read that the resulting impedance would be = turns ratio **2 * ribbon impedance. This should get me into the single unit ohms range, the exact value depends on the ribbon resistance.

Stay tuned.

Denis
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Old 24th January 2005, 07:31 PM   #10
tiroth is offline tiroth  United States
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Great work and great documentation -- often the missing link in terms of being able to replicate a design. Thanks very much for your contribution + will definitely look forward to hearing more.
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