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Old 9th November 2009, 05:04 PM   #1
thadman is offline thadman  United States
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Default Direct Drive amplifier for DIY ribbon?

I'm interested in building a DIY ribbon loudspeaker in my free time over Christmas break and would like to drive it directly. Following the resolution of some meetings with faculty in the engineering department, I might be able to gain access to the Universities Laser Vibrometer and Anechoic chamber for measurements (and comparison with the RAAL 140-15D)

I do not wish to use a transformer, as I believe it may contribute excessive distortion. As a result, I will need a special amplifier to drive the ribbon. The impedance of the ribbon will be almost purely resistive, but very, very, very low. Most of my knowledge in the field of loudspeakers is related to acoustics and the physics of transducers...I haven't yet been able to extend it to amplifier design.

My primary goal for the project is reproducing a Dirac Pulse/Square Wave, so impulse response will be weighted with the highest significance.

Are there any designs available? Can anyone recommend any texts?

How difficult would it be to build a direct drive 10 watt amplifier with negligible distortion (linear and non-linear distortion <20khz)?

Thanks,
Thadman
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Old 9th November 2009, 06:44 PM   #2
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I would be interested in attempting this design. Do you have some idea what the resistance of the ribbon will be? Are there other restrictions I should be aware of?

You should know that I cannot build the amp. You will have to take care of that.
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Old 9th November 2009, 06:53 PM   #3
rdf is offline rdf  Canada
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Not in any meant to discourage but my experience measuring line level transformers is they can have phenomenally low distortion, often lower than the electronics and certainly well below typical transducers. In this application, who knows?
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Old 9th November 2009, 07:10 PM   #4
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I would hardly consider this line level.
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Old 9th November 2009, 07:11 PM   #5
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Originally Posted by rdf View Post
Not in any meant to discourage but my experience measuring line level transformers is they can have phenomenally low distortion, often lower than the electronics and certainly well below typical transducers. In this application, who knows?
I think he's looking at a very low impedance power amplifier, to get around the typical power level transformer used in these types of speakers. I always wanted to try this, too, so I'll be interested in the result. Clearly it will be important to have a good idea what the actual speaker driver impedance will be. Possibly all you need is current gain if the preamp signal voltage level is large enough to drive full power.
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Old 9th November 2009, 07:14 PM   #6
thadman is offline thadman  United States
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Originally Posted by Steve Dunlap View Post
I would be interested in attempting this design. Do you have some idea what the resistance of the ribbon will be? Are there other restrictions I should be aware of?

You should know that I cannot build the amp. You will have to take care of that.
That would be fantastic! I would thoroughly enjoy building the amplifier, as it would afford me the opportunity to apply the theory.

The design is not finalized as of yet. It will depend on the foil I'm able to source. However for what it's worth, I expect the element to be a few microns thick, 4-8mm wide, and a few inches long. The element material will be aluminum.
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Last edited by thadman; 9th November 2009 at 07:16 PM.
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Old 9th November 2009, 07:16 PM   #7
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Originally Posted by thadman View Post
That would be fantastic! I would thoroughly enjoy building the amplifier, as it would afford me the opportunity for some real world experience.

The design is not finalized as of yet. It will depend on the foil I'm able to source. However, I expect the element to be a few microns thick, 4-8mm wide, and a few inches long. The element material will be aluminum.
If you're looking at a tweeter (driver of your dimensions) I would start by working toward a heavy duty unity gain buffer amplifier.
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Old 9th November 2009, 07:33 PM   #8
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The ribbon in the Maggies is one ohm. Anything close to that would be easy. A very short ribbon would be less, but doable.

The output stage I have posted here already is a power buffer that does not need a front end, or feedback.
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Old 9th November 2009, 07:39 PM   #9
Baldin is offline Baldin  Denmark
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Default Elektor Amp for Ribbons

As menitioned in the Planar forum, there was a construction in Elekto for such an amp.
The articles was published in November and December 1992.
I'm sure you can get a back issue at Elektor, but otherwise, I can maybe make a PDF, and send it on a mail.

It's not a beginners project at all, and I think the construction might be a bit overkill, being able to diliver 160W in 0.4 Ohm.

Actually I think a small Class-d amp would be perfect for the job ..... at the very low voltages needed you could probably make one with a rather high switching frequency if wanted!

See attached schematic of the Elektor amp
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File Type: jpg Elektor Ribbon Amp 001.jpg (382.9 KB, 616 views)
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Old 9th November 2009, 07:52 PM   #10
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Leave it to Elektor to offer up a circuit with more transistors than anyone dreamed necessary!
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