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Rhodium - A Tiny Ribbon Monitor

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  • Rhodium - A Tiny Ribbon Monitor

    Project Overview:

    ​The InDIYana contest theme this year was nanotech, which challenged designers to build the best sounding speaker possible with an internal volume of 3 liters or less. There were a number of very innovative approaches to generating the most bass output possible in that volume, and a number of those designs sounded very good. My previous small designs, Lithium and Helium, had been built around the Dayton ND series woofers and small, budget tweeters. For this project, I really wanted to try a ribbon tweeter and I wanted to pair that with a metal cone woofer in the hope that a stiffer cone would help provide a clean detailed sound and something clearly different than Paul Carmody's very popular Speedster design.

    Driver Selection:

    The Fountek NeoCD1.0 is a relatively affordable ribbon tweeter that has a few advantages. First, the ribbon is reinforced, so it is a little more rugged than other ribbon tweeters. Second, it has a round faceplate about the same size as a standard tweeter. Given that this enclosure had to have an internal volume 3 liters or less, every inch of baffle space counts. Fountek recommends a crossover point at or above 3.5 kHz with at least a 3rd order slope. This along with the size restriction narrows the field of possible woofers. Other considerations were bass extension and physical size. I wanted to have bass extension down to or below 70 Hz (f3) in a 3 liter enclosure and a driver frame size not smaller than the tweeter. This ruled out most woofers, but the SB Acoustics SB12PAC25-4 fit all the requirements and has a published x-max of 5mm. At $28, it is also affordable.

    Enclosure Design:

    ​The overall cabinet size is 5" wide by 9.5" tall by 6" deep. The width and height are about as small as possible given the size of the drivers. These speakers are approximately the same size as the Dayton B452 and noticeably smaller than designs such as the Overnight Sensations or Lithium.

    ​The last commercial speakers I owned before I went fully into DIY were Platinum Audio Solos. I liked the look of the natural wood sides, black baffle and black top and bottom end caps. For this project, I decided to employ a similar construction approach. I recently picked up a bunch of bamboo cutting boards at a clearance price to use as hobby wood and thought they would work well here as this design is small enough that the 3/8" thickness is appropriate. So, the side panels were constructed using 3/8" bamboo with 1/2" Baltic birch covered with black Duratex for the front, rear, top and bottom. The Parts Express 1" diameter by 4" long flared port (part number 260-470) tunes this cabinet to about 65 Hz.

    ​I was concerned about the bamboo panels being too resonant, so I applied roof repair sheet to the interior surfaces of the side panels and then wedged a couple of plywood strips between the side panels I also added a chunk of R-13 Ultratouch denim fiber insulation to the rear of the enclosure. I decided to make the rear panel removable, which I did by using plywood strips at the top and bottom of the rear of the cabinet and EZ-LOK threaded inserts. The interior picture should give an idea of these features.

    ​Crossover Design:

    ​Based on the published driver specs and recommendations, I set a target crossover point of 4 kHz with 4th order acoustic slopes. I measured the driver frequency responses in the cabinet and the impedances, then began to simulate the crossover. I found that I could achieve a relatively flat response with a crossover frequency of 4.25 kHz and approximately 4th order acoustic slopes using 2nd order electrical filters on both the woofer and tweeter. A series and a parallel resistor were added to the tweeter circuit to bring the SPL down to match the level of the woofer. The final measured response is flat within +/- 2dB from the lower limits of the gated measurement to 20 kHz. The royal blue curve was taken with the tweeter polarity reversed and shows a 35 dB dip at the crossover frequency, which indicates that the drivers phases are in very good alignment when the polarity isn't reversed. Horizontal Off axis measurements show that on axis peaks are at least partially compensated by off axis dips and on axis dips likewise correspond to off axis peaks. The other distinctive feature in the off axis response is the extended response of the ribbon tweeter at high frequencies. The response is less than 10 dB down from the on axis response 60 degrees off axis at 20 kHz. The impedance measurement (see next post) shows a box tuning frequency of approximately 66 Hz and a minimum impedance of 3.7 Ohms at 250 Hz, which isn't bad considering the design uses a 4 ohm woofer. This design should be a manageable load for most amplifiers.

    ​Conclusions:

    ​I designed and built these speakers - and I like them, so I can hardly be considered unbiased. I think they have good detail resolution in the mids and highs, without being fatiguing. They have enough bass extension to be enjoyable to listen to on their own or to allow a low (down to around 80 Hz) crossover to a subwoofer. Others who heard them at InDIYana this year can probably offer more unbiased opinions regarding their strengths and shortcomings. These speakers are not the lowest cost around, but drivers and crossovers still come in for around $200 per pair, which is pretty reasonable for a speaker that includes a ribbon tweeter. A parts list is included in the next post.


    Rhodium - A tiny ribbon monitor
    Keep an open mind, but don't let your brain fall out.

    Argon | Helium | Lithium | LithMTM | Hafnium
    Mercury | Matrix | Shrubbery

  • #2
    The impedance plot, described in the previous post, is shown here. Also shown is a bill of materials that show that the $160 driver cost for a pair is at least somewhat alleviated by a very inexpensive $35 per pair crossover. All parts except the SB Acoustics woofer can be procured at Parts Express.
    Keep an open mind, but don't let your brain fall out.

    Argon | Helium | Lithium | LithMTM | Hafnium
    Mercury | Matrix | Shrubbery

    Comment


    • #3
      I really liked these at InDIYana, Scott!
      Wolf
      "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
      "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
      "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
      "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

      *InDIYana event website*

      Photobucket pages:
      http://photobucket.com/Wolf-Speakers_and_more

      My blog/writeups/thoughts here at PE:
      http://techtalk.parts-express.com/blog.php?u=4102

      Comment


      • #4
        I did too. Really very nice overall presentation. Crisp, great detail, and imaged very well.
        https://www.facebook.com/Mosaic-Audi...7373763888294/

        Comment


        • #5
          I also thought these were a great design. They seemed to have more bass extension than the tuning suggests.
          -Kerry

          www.pursuitofperfectsound.com

          Comment


          • #6
            To me this was one of the most impressive nano designs I heard at InDIYana. Fantastic sound with a much more solid bass extension that you expect. For me I probably would have preferred the tweeter to be 1dB hotter. Otherwise I was genuinely surprised how good it sounded. I thought it must have no bass to sound this good in the midrange, but the bass is there.

            Comment


            • #7
              Originally posted by bcodemz View Post
              To me this was one of the most impressive nano designs I heard at InDIYana. Fantastic sound with a much more solid bass extension that you expect. For me I probably would have preferred the tweeter to be 1dB hotter. Otherwise I was genuinely surprised how good it sounded. I thought it must have no bass to sound this good in the midrange, but the bass is there.
              To bump up the tweeter output a little, R2 can be changed from 8 ohms to 10 ohms. Which will sound better depends on source material, the rest of your system, and the room. In my listening room, I found the 10 ohm R2 sounded great on about 90%-95% of recordings, but was a little hot on the other 5%-10%. Moving R2 down to 8 ohms brought the rest into line - but maybe at the expense of a little sparkle on the rest.
              Keep an open mind, but don't let your brain fall out.

              Argon | Helium | Lithium | LithMTM | Hafnium
              Mercury | Matrix | Shrubbery

              Comment


              • #8
                👍👍
                Kenny

                http://www.diy-ny.com/
                DIY NY/NJ 2014 https://www.youtube.com/watch?v=FGwA...ature=youtu.be
                Man does not live by measurements alone, a little music helps.

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