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The TangBand W4-1720 is a remarkable driver for its size and cost. It has a huge ferrite magnet and is an under hung design with low distortion and great performance for a 4 inch class driver. Holding one of these woofers tells you that this 4.1 pound, 4” woofer is a serious contender in the small woofer class.
Several DIY’ers have used this driver and their efforts have been rewarded with good results. I heard Rory Buszka’s small Neutrino bookshelf speaker last year at one of the DIY speaker events and was impressed by the sound of his design that uses the W4-1720 and SEAS 22TAF/G tweeter. Other designers have utilized the W4-1720 with equally impressive results. Paul Carmody chose this driver for his Speedster MT bookshelf design (crossed to the Fountek NeoCD1.0 tweeter) which has been widely duplicated by many DIY builders. While these speakers yield satisfying results for bookshelf use, they lack that extra octave or half octave on the low end of their frequency response to convince the listener that a subwoofer would not be needed.
Now I seldom miss an opportunity to adapt outstanding small enclosure designs to a mass loaded transmission line configuration. In most cases a MLTL version produces at least 10-25 Hz additional low end sound output so that a fuller frequency range can be achieved. Hence, music becomes truly full range while the footprint is no larger than a small enclosure speaker placed on a stand.
At the recent Kentucky DIY event I demonstrated a mass loaded transmission line version of Paul Carmody’s Speedster design. I chose his speaker as a candidate for a MLTL conversion based upon its potential to be full range and because I’m a pushover for ribbon tweeters like the little Fountek unit he uses. I call my creation the Speedster Towers. Now Carmody described a MLTL version of the Speedster this summer, but his design departs from the original Speedster MT bookshelf by adding a second W4-1720 driver and makes crossover network changes for a MMT arrangement. You can read about Paul’s design on his website. I’m reluctant to fully recommend his MMT MLTL as it does place the two 4 ohms drivers in parallel for low frequencies (2.6 ohms load impedance with 18 awg inductors) which may present issues for less than robust amplifiers. My MLTL version is true to the Speedster bookshelf design as it retains the original driver spacing and crossover while maintaining a 4 ohms impedance load.
I used Martin J. King’s worksheets to derive the MLTL enclosure for the Speedster Towers. After several iterations, my design efforts yielded an enclosure that has outside dimensions (I’m using 0.75 inch thick material) of 39.5 inches tall by 6.5 inches wide by 6.0 inches deep. The internal dimensions of the pipe are 38 x 5 x 4.5 inches. Hence, the box is small and very petite. But that small size yields a low frequency response that is 3 dB down below 35 Hz in these simulations. I’ve attached a set of simulation plots (system SPL response, port response, impedance, and woofer displacement) that resulted from my modelling. The width of this speaker is 6.5 inches vs. 6 inches for the bookshelf version which will slightly impact the baffle step correction that Paul used in the original crossover. But the tradeoff to a thicker wall speaker for a MLTL is an acceptable change, in my opinion, as you gain a more rigid structure for the heavy driver and it better copes with higher SPLs for the lower frequency coverage. The enclosures for the towers are straight (not tapered like some transmission line boxes) so they are as easy to construct as bookshelf speaker cabinets.
The port tube on the Speedster Towers is nominally 1.75 inches diameter by 4.5 inches long round tube centered two inches above the bottom of the box. For my prototypes I equated this round port area into a rectangular slot port and constructed a tunnel that maintains this cross-sectional area for 3.5 inches inside the enclosure near the bottom of the box. Thus the slotted port creates a straight line length of 5 inches. The port tube or slot port tunnel can be located on either the front baffle or the rear panel of the enclosure as chosen by the builder.
The upper-half (down to 3 inches or so below the woofer cutout) of the Speedster Towers is filled with 0.75 lb/ft3 stuffing. You can use loose damping material or consider that Meniscus Audio sells bonded Dacron in 1 inch thickness sheets that allow you to easily fill the volume. You will have to trim a few sheets to fit around the large rear magnet housing of the W4-1720. To permit access during assembly to the low half of the enclosure, I use a bi-amp terminal plate (such as the Madisound TD-cup) so that I can place the crossover networks inside the box and have room for wire routing.
As designed, the slim Speedster Towers will need stabilization to withstand usage in a typical home environment. A base plate with spikes is one solution if the intended application is on carpeted floors. With wood floors in my home I use an overlapping hardwood X stand on the bottom of each speaker box. The X stand raises the enclosure 1.5 inches which places the ribbon tweeter at the ideal listening height for seated listeners.
In the near future my prototype towers will be painted so that raw MDF look will disappear. But the sound that they produce will still be rewarding and pleasing to the listener.
Several DIY’ers have used this driver and their efforts have been rewarded with good results. I heard Rory Buszka’s small Neutrino bookshelf speaker last year at one of the DIY speaker events and was impressed by the sound of his design that uses the W4-1720 and SEAS 22TAF/G tweeter. Other designers have utilized the W4-1720 with equally impressive results. Paul Carmody chose this driver for his Speedster MT bookshelf design (crossed to the Fountek NeoCD1.0 tweeter) which has been widely duplicated by many DIY builders. While these speakers yield satisfying results for bookshelf use, they lack that extra octave or half octave on the low end of their frequency response to convince the listener that a subwoofer would not be needed.
Now I seldom miss an opportunity to adapt outstanding small enclosure designs to a mass loaded transmission line configuration. In most cases a MLTL version produces at least 10-25 Hz additional low end sound output so that a fuller frequency range can be achieved. Hence, music becomes truly full range while the footprint is no larger than a small enclosure speaker placed on a stand.
At the recent Kentucky DIY event I demonstrated a mass loaded transmission line version of Paul Carmody’s Speedster design. I chose his speaker as a candidate for a MLTL conversion based upon its potential to be full range and because I’m a pushover for ribbon tweeters like the little Fountek unit he uses. I call my creation the Speedster Towers. Now Carmody described a MLTL version of the Speedster this summer, but his design departs from the original Speedster MT bookshelf by adding a second W4-1720 driver and makes crossover network changes for a MMT arrangement. You can read about Paul’s design on his website. I’m reluctant to fully recommend his MMT MLTL as it does place the two 4 ohms drivers in parallel for low frequencies (2.6 ohms load impedance with 18 awg inductors) which may present issues for less than robust amplifiers. My MLTL version is true to the Speedster bookshelf design as it retains the original driver spacing and crossover while maintaining a 4 ohms impedance load.
I used Martin J. King’s worksheets to derive the MLTL enclosure for the Speedster Towers. After several iterations, my design efforts yielded an enclosure that has outside dimensions (I’m using 0.75 inch thick material) of 39.5 inches tall by 6.5 inches wide by 6.0 inches deep. The internal dimensions of the pipe are 38 x 5 x 4.5 inches. Hence, the box is small and very petite. But that small size yields a low frequency response that is 3 dB down below 35 Hz in these simulations. I’ve attached a set of simulation plots (system SPL response, port response, impedance, and woofer displacement) that resulted from my modelling. The width of this speaker is 6.5 inches vs. 6 inches for the bookshelf version which will slightly impact the baffle step correction that Paul used in the original crossover. But the tradeoff to a thicker wall speaker for a MLTL is an acceptable change, in my opinion, as you gain a more rigid structure for the heavy driver and it better copes with higher SPLs for the lower frequency coverage. The enclosures for the towers are straight (not tapered like some transmission line boxes) so they are as easy to construct as bookshelf speaker cabinets.
The port tube on the Speedster Towers is nominally 1.75 inches diameter by 4.5 inches long round tube centered two inches above the bottom of the box. For my prototypes I equated this round port area into a rectangular slot port and constructed a tunnel that maintains this cross-sectional area for 3.5 inches inside the enclosure near the bottom of the box. Thus the slotted port creates a straight line length of 5 inches. The port tube or slot port tunnel can be located on either the front baffle or the rear panel of the enclosure as chosen by the builder.
The upper-half (down to 3 inches or so below the woofer cutout) of the Speedster Towers is filled with 0.75 lb/ft3 stuffing. You can use loose damping material or consider that Meniscus Audio sells bonded Dacron in 1 inch thickness sheets that allow you to easily fill the volume. You will have to trim a few sheets to fit around the large rear magnet housing of the W4-1720. To permit access during assembly to the low half of the enclosure, I use a bi-amp terminal plate (such as the Madisound TD-cup) so that I can place the crossover networks inside the box and have room for wire routing.
As designed, the slim Speedster Towers will need stabilization to withstand usage in a typical home environment. A base plate with spikes is one solution if the intended application is on carpeted floors. With wood floors in my home I use an overlapping hardwood X stand on the bottom of each speaker box. The X stand raises the enclosure 1.5 inches which places the ribbon tweeter at the ideal listening height for seated listeners.
In the near future my prototype towers will be painted so that raw MDF look will disappear. But the sound that they produce will still be rewarding and pleasing to the listener.
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