8008-Center

8008-Center 93 dB/2.8V Surround Center Speaker
Copyright 2011 © Troels Gravesen

CROSSOVER CABINET MEASUREMENTS CONSTRUCTION PICS SPEAKER KIT

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With the number of TQWT/DTQWT kits sold, the quest for a center speaker is becoming more frequent as surround fans want a speaker to complement their (mostly) DTQWT systems.
This center speaker has been on the to-do list for a long time and started as a "classic" horizontal d'Appolito set-up. However, unless we cross over at a very low frequency between midbass drivers and tweeter, we may experience crossover lobing, that is uneven frequency response as we move from seat to seat in our home cinema. Usually we have an even horizontal dispersion from vertical aligned drivers if the chosen points of crossover have been done with respect to the diameters of the drivers and slopes of crossover. When wavelength hits a value equal to the diameter of the driver, we start having beaming (wellknown from fullrange drivers) and if the point of crossover is above this frequency we may have irregularities off-axis.
Most good center speakers are - at least - 3-way. Some manufacturers go even further and use a whole range of tweeters to overcome the problem - like McIntosh.
Having the small SEAS MCA12RC 4" unit covering the 800-3000 Hz range ensures wide dispersion and mated with the TW034 dome (this time without waveguide) we are certain of overcoming this problem and voiced around 92-93 dB, the MCA12RC can fill the gap between bass and tweeter despite modest sensitivity compared to the other drivers. In the end it even had to be attenuated 1 dB.

Cabinet drawing
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Main cabinet was made from 22 mm MDF. Front panel is laminated 22+12 mm MDF. Internal bracing is 12 mm plywood and mid enclosure top and rear panel is 22 MDF. View construction pics below.
Net volume: ~45 liter minus 3.7 liter brutto mid cab volume = 41.3 liter total. This provides a frequency response down to 50 Hz. Another 5 Hz can be achieved from increasing volume to 55 liter (make cab deeper).
Port tuning = 40 Hz, port = 68 mm (ID) x 70 mm (view components list).

Crossover
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Due to smooth frequency response of all drivers, an almost as simple as can be crossover.
I had hopes for 22 uF to middriver (cost) but 30 uF is what it takes.
Pay notice to L2021, DCR = 1.55 Ohm, no resistor needed here.

Go to crossover layout

Predicted individual and summed response of drivers driven from crossover.

Construction pics
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Cabs made from 22 mm MDF. Interior bracing panels from 12 mm Baltic birch.

Note serious chamfering of 8008 and MCA12RC holes.

Front panel made from 22 + 12 mm MDF.

Applying birch veneer to cabinet.

All interior panels damped with 10 mm felt material except for panels behind 8008 drivers. Here we need space for mounting crossovers; bass crossover
to one side, mid and treble crossover to the other side. On top of crossovers a piece of folded 30 mm acoustilux is placed, 22 x 35 cm. Pics to below.

Measurements
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Left: SPL/1 meter/2.8V. System sensitivity = ~92 dB. Right: System impedance: 5 ohms. Overall an easy load. Note port tuning @ ~40 Hz.

Left: Horizontal dispersion @ 0, 5, 10, 15 and 20 deg. displaying an even power response over a wide area.
Right: Vertical dispersion starting at middriver height (red) and 5, 10, 15 and 20 deg. (black) above.
All in all this speaker has an even sound distribution over a wide area, which was a prime target of the project.

Left: Individual and summed response of drivers driven from crossover. Right: Summed response with positive (orange) and negative (green) polarity of middriver.

Speaker Kit
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Complete kit available from Jantzen Audio. Mail: contact@jantzen-audio.com

Download sales presentation STANDARD Z-cap version

Download sales presentation SUPERIOR Z-cap version

Crossover Layout
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Crossover layout for Option 1 (Superior-Z version).
Same layout can be used for Option #2, all-STANDARD-Z-cap version, although components take up considerably less space.
If you want to spoil your drivers, use WAX coils for L2041 and L1031 (both must be 16 AWG).