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HASIB'S ARTICLES AND AUDIO PROJECT REVIEWS
2-WAY CENTER SPEAKER Introduction Home theater system needs a center speaker to drive center channel. Typically a center channel speaker need to produce dialogues as well special effects. This speaker is the most important speaker in HT setup because the “intelligence band” came along from it. On good HT system center speaker must be timber matched with main speakers (left and right) to keep good balancing on sound reproduction. It’s actually means the drivers used on center channel speaker must used same drivers on main speakers ie tweeter, midrange and optional woofer. This is important. Imagine a Ferrari car past from left to right speaker. When on left speaker, it sounds like Ferrari but when passing center speaker it sounds like Porsche and sounds like Ferrari again on right speaker. On 2-way speaker system usually there are two mid/bass drivers and a tweeter between them. The midbass drivers usually connected in parallel. This is called MTM design, which the tweeter sandwiched between the mid/bass drivers. Typical 2-way speaker system can be used instead (a mid/woofer and a tweeter driver). I also found an argument that MTM design is bad for center channel speaker but I can see it disadvantage. If somebody has a reason why, please contact this page webmaster. This is the project intended for my center speaker project for my current HT setup. On building this center channel speaker, I combined a number of goals and interests into one project and to build it all cheaply by using whatever materials I already have. Oh yeah, this speaker system use passive crossover instead of active. The Drivers I used 1” soft-dome tweeter and 5” mid/bass driver. The drivers are the same model for my main speaker tweeter/midrange driver except the impedance is 8-ohm. Tweeter has an impedance of 8-ohm and 8-ohm for each mid/bass driver. Using my dedicated measuring technique I found that tweeter resonant frequency is 900-Hz. As the drivers I used are equal to main speaker drivers I think timbre-match issue is not really a problem but more on this later. Drivers Math Here are the mid/bass driver specifications: Fs = 81Hz, Qms = 4.52, Qes = 0.48, Qts = 0.43 and Vas = 6.12 litres. I mount the two mid/bass driver in one enclosure (in parallel). After playing around with WinISD 0.44 version here is the plotted output:
The Box The box is made from 11-mm plywood. Yes from plywood, it’s really hard to find MDF (medium density fiberboard) in my area and I know that MDF is better than plywood in any aspect. I cut the plywood using handsaw gently and using paper sand to remove the rough cutting edges. The external dimensions are 46 cm by wide, 16 cm by tall and 23 cm by deep. Internal volume is 0.43 cubic feet or 12 liters. The two mid/woofer drivers share same chamber. The left and right panel are attached to the top and bottom panel using ¾ inch nails. The ½ inch front and back panel also attached to the main frame by same technique. I glued the attachments using high quality contact adhesive/hot glue to keep them air tighted. I cut the holes for drivers/vents and speaker terminal using saber saw. I roll the paper masking tape to whole area of the enclosure surfaces for letting them has smooth surface and I painted them with flat spray paint. For speaker terminal I use binding post speaker terminal (banana plug accepted). I secure the driver’s flange using rubber based foam for letting the air not leaking outside. I locate two vents, each one near the mid/woofer driver, which 4-cm in diameter and 5 cm long. Seeing as it is a two-way speaker system a lot of midrange energy will be coming out of the port. If you place the ports in the rear panel then this energy will arrive delayed compared to the midrange energy coming from the speakers. This will result in some blurring of the stereo image. Excuse me…this is sealed speaker project but there are two ports on front baffle. It is a convenience to me if I want to convert the closed box to vented box configuration. Right now the ports bunged with stuffing materials. I use flat black spray paint to paint the entire box – just very simple solution. Grill available for this speaker project to keep from prying fingers. The Crossover Network This is passive 2nd order crossover network that crossed over approximately at 2700-Hz. On designing this crossover I take out some measurements to determine the data(s) needed especially on L-pad attenuator, series compensation and Zobel network. Yeah I have own frequency sweeper but still don’t have frequency counter. You can read an article on designing passive crossover network from ESP page. You can follow the link below. Tweeter Crossover Network I combined series compensation network to combat tweeter resonance phenomenon (900-Hz mentioned above) and L-pad attenuator to tame the tweeter efficiency as close as possible to mid/bass drivers sensitivity. For L-pad the series resistance is about 4-ohm and parallel resistance is about 9.2-ohm. Tweeter efficiency about 5dB (!) more efficient than mid/bass drivers. For series compensation network the resistance is about 7-ohm, 760-uH of inductance and 40-uF of capacitance. All resistors have 2-watt power rating. The inductance is made from iron core and capacitors from bipolar caps. No exotic components because the cost will be high. For it’s 2nd order network the capacitance is 4.4-uF and 786-uH of inductance. The caps from ceramic caps (bipolar caps is no use here) and the inductance still made from iron-core. Tweeter driver polarity is reversed to keep it in phase with mid/woofer drives. That’s the “law” of 2nd order passive crossover network.
Here
is the tweeter crossover network diagram
Mid/woofer Crossover Network The two drivers connected in series actually to give total impedance about 8-ohm. I know that series connection is bad on speaker hook-up. Parallel connection is good but can’t do that here since the equivalent total load impedance is 2-ohm. The power amp drive this speaker not designed to handle this load and will fail. I include zobel network for mid/woofer crossover. Actually the zobel network circuit is to tame down high frequency that passed to mid/bass driver since passive crossover network is not too accurate on split the frequency spectrum. The series resistance is about 7-ohm and capacitance is about 55-uF. I use 5-watt resistor (you can use 10-watt rating if the power amp drive this speaker has high power rating) and bipolar caps. Do not use 1-watt or 2-watt resistor on woofer/midrange zobel network. For it’s 2nd order network the capacitance is 4.6-uF and 753-uH of inductance.
Here is mid/woofer driver crossover network The Result I like it sound where it doesn’t harsh or lacking in high. The midrange also close (but not too well) to timber matched with my main speakers. It’s naturally came no surprise to me since the main speaker is tri-amped. The only way to make them 100% matched I must use identical active crossover, power amps and perhaps cabinet layout. That’s way too far. There is a hint of chesty dialogue heard during very loud passage (that’s a problem with 2-way center speaker used as center speaker). Overall I satisfied with the sound. OK…..still not satisfied. Maybe I will think 3-way design. The Costs
Total costs: RM 164.00 Complete Pictures
Fig1 - MTM Center Speaker (Front View)
Fig2 - MTM Center Speaker (Back View) Link to ESP page Muhammad Hasib Md. Nor 1998 Any Comment And Question Email to : hasib_tangkak@yahoo.com
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