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DD8-MkII The DD8 Mk-II The evolution of the monitor Since the birth of the original DD8 nearly two years ago SEAS have updated their aluminium series of drivers. Besides that the older models will become more and more difficult to acquire, the new drivers should have improvements compared to their older counter parts. Also the new woofer allows for a larger floorstanding cabinet if prefered. The tweeter The Seas 27TBFC/G (H1212). 27mm High Definition aluminium/magnesium alloy dome tweeter with a wide, soft polymer surround. The dome and surround materials give high consistency and excellent stability against variations in air humidity. The diaphragm is protected by a highly perforated hexagrid carrying an acoustic lens, which tailors the high frequency roll off characteristic. The voice coil is wound on an aluminium voice coil former with adequate ventilating holes to eliminate noise from internal airflow. The voice coil is immersed in low viscosity magnetic fluid for high power handling capacity and simplified crossover design. A stiff and stable rear chamber with optimal acoustic damping allows 27TBFC/G to be used with moderately low crossover frequencies. The chassis is precision moulded from glass fibre reinforced plastic, and its front design offers optimum radiation conditions.
The mid-woofer The Seas L18RNX/P1-H1142 is an 18 cm (6,5'') cone driver, developed for use as a long throw high fidelity woofer or woofer/midrange unit. The stiff, yet light aluminium cone and the low loss rubber surround show no sign of the familiar 500-1500 Hz cone edge resonance and distortion associated with soft cones. On the other hand, the cone break-up modes at higher frequencies call for special attention in the crossover design work. The large magnet system gives good transient response, and the bumped back plate together with the very long, and lightweight copper clad aluminium voice coil allow for extreme coil excursion with low distortion. The extremely stiff and stable injection moulded metal basket, keeps the critical components in perfect alignment. Large windows in the basket both above and below the spider reduce sound reflection, air flow noise and cavity resonance to a minimum.
The cabinet It is a standard bookshelf type cabinet of 230x390x244mm with an internal volume of 12 litres (when heavily filled with damping material) tuned to 41,5Hz it gives a 3dB point around 56Hz. This size gives a relativly compact stand-mount speaker and is the minimum volume required for the L18RNX/P1. This driver is a very flexible driver and will also work in a floorstanding cabinet of 24 litres (when lightly filled with damping material) tuned to 35Hz it will give a 3dB point of about 37Hz. This version will give ample bass providing 4,5dB more output at 20Hz, +6dB at 30Hz, +4dB at 40Hz and +3dB at 50Hz. Up to you which one you want to build. All walls are made of 22mm MDF. Just the standard 6 planks plus one extra for an internal brace with a circular cutout of 140mm. The reflex port is placed in the rear panel and is a standard plastic trumpet shape tube from Monacor, the BR45-TR (Sd=13,8cm2).The inside of all the walls except the baffle (for both cabinets) are covered with 4mm thick bitumen and wedge-moulded foam to minimise cabinet vibrations and standing waves. The reflex port is kept clear. Bonded acetate fibre damping material is rolled up and densely fills the whole enclosure. Depending on the positioning of the speakers in the room and personal taste this fibre can be rolled up densely or loosely to give a tighter and dryer or a fuller and warmer sounding bass. The vertical edges of the front panels are routed with a 22x44mm angle for looks and to remove the sharp edges that would cause baffle diffraction problems.
The crossover network It uses a second-order series-filter with a crossover point spot on 2kHz. A notch filter across the woofer to cutout the woofers cone break-ups at higher frequencies. No impedance correction across the input terminals is needed because of the already flat nature of the system impedance. The tweeter is brought down to the level of the woofer with an L-Pad. Both units are connected with the same polarity.A single pair of gold plated binding posts on the rear, internal wiring is Van Den Hul CS-122 Hybrid for the woofer and silver coated OFC copper with Teflon insulation for the tweeter.
Crossover components: L1 = 0,47 mH air-core inductor 2,0 mm wire, Rdc = 0,11 ohms (tolerance max 5%) L2 = 0,82 mH air-core inductor 2,0 mm wire, Rdc = 0,15 ohms (tolerance max 5%) L3 = 0,22 mH air-core inductor 0,71 mm wire, Rdc = 0,39 ohms (tolerance max 5%) C1 = 15uF MKP polypropylene foil capacitor (tolerance max 5%) with 0,01uF MKP1837 bypass cap C2 = 10uF MKP polypropylene foil capacitor (tolerance max 5%) C3 = 4,7uF MKP polypropylene foil capacitor (tolerance max 5%) C4 = 22uF MKP polypropylene foil capacitor (tolerance max 5%) with 0,01uF MKP1837 bypass cap R1 = 1,5 ohms, 10 watts metal film resistor (tolerance 2%) R2 = 10 ohms, 10 watts metal film resistor (tolerance 2%) R3 = 6,8 ohms, 10 watts metal film resistor (tolerance 2%) R4 = 3,3 ohms, 10 watts metal film resistor (tolerance 2%) Note that the crossover can be large and therefore difficult to mount inside the cabinet.
Listening impressions and remarks First of all is it better than the Mk-I? Well, the main gain is the new tweeter. The original Seas 27TAFC/G (H883) tweeter is very good, but this new Seas 27TBFC/G (H1212) seems to have the leading edge. It gives a fraction more micro-detailling without getting over bright. The woofer seems to have the warmth of the L17RE combined with the power of the L17REX, exactly what I was looking for! Overall sound is very coherent with stong bass considering the size of the cabinet. The upper regions are detailled and calm. The soundstage has a realistic front-to-back size with a slightly forward projection. Effieciency is moderate at about 84dBs but even when driven hard things stay nicely in place, the long voice-coil of the L18 providing enough headroom. Measurements
1). Output level 200-20.000Hz horizontal division 5dB. Good linearity within +/- 3dB (note that I used very little smoothing) heavy smoothing used by some manufacturers would give +/- 1,5dB; the drop towards the lower end is due to the measurement being done in free-air (so no boundary re-enforcement) typical in room response is down to about 50Hz. The roll-off above 15kHz is the upper limit of my measuring equipment and not the speaker. 2). Step response time window 3,0ms. As can be seen from this plot, the tweeter has a fast rise of about 0,18ms and is connected in phase; the woofer follows at just under 0,5ms.
3). Waterfall plot 400-20.000Hz, time window 3,0ms corresponding with the step response. Smooth and even fall with no large ridges. 4). Impedance plot 20-20.000Hz, horizontal division 4 ohms, range 0-48 ohms. The impedance varies around 8 ohms. It should be a very easy load for most amplifiers with no extreme swings except for at the port tuning frequency. The graph is for the closed box simulation; the reflex will show two peaks in the bass with a minimum between at the tuning frequency of 45Hz. Due to the nearly flat impedance the electrical phase (dotted line) is also nearly perfect within +/-7 degrees except for the bass region.
NOTE: This design is strictly for the home DIY enthusiast and not to be used professionally without my permission! Tony Gee, The Netherlands April 2004 |
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