VJD Cab - Tube Amp Doctor
To design and build a ported bass cabinet
featuring the CF18VJD loudspeaker
APPLICATION NOTE
www.celestion.com
CF18VJD Bass Cabinet: Design/Build Notes
CF18VJD Specification
Celestion have designed a ported bass cabinet, specifically for use with the CF18VJD ferrite magnet,
cast aluminium LF driver. Cabinet specifications are:
General Specifications
Material
18mm birch plywood
Cabinet Volume
167 litres (5.9 cu. ft.) Total
135 litres (4.8 cu. ft.) Effective
Cabinet Resonance (Fb)
37Hz
Max SPL (1-watt/1m)
127dB
Recommended
high pass filter
Butterworth
24 dB/octave 33Hz cut-off
(-0.5dB at 40Hz, -3dB at 36.5Hz)
A ported or reflex cabinet is a simple box enclosure
with one or more holes (ports) added on the front
baffle. For most PA uses, this offers the best option
for building a clean sounding loudspeaker cabinet
that makes the most of the bass driver used. The
addition of the port can extend bass driver performance and will also marginally improve the speaker’s
low frequency power handling and efficiency.
The energy from the front of the driver is radiated to
the outside world. But the sound from the back of the
speaker also has an effect, as it vibrates the air in
the port(s). This has a resonant frequency, like an
organ pipe, adding to the sound output of the system. If the port(s) are designed correctly, the additional energy will extend bass performance without
adding distortion or sacrificing a smooth response.
As with all ported boxes, it would be sensible to use
with a high pass filter. This will ensure that no high
level input is presented to the box below its lower
limit. This is because, for very low frequency signals,
the driver becomes unloaded, (i.e. the box effectively
disappears.) Under these circumstances, driver excursion increases rapidly and the potential for driver
damage is significant.
For this CF18VJD cabinet, a suitable value for this
filter is 33 Hz. A sharp corner to the cut-off would be
desirable, so a 24 dB/octave Butterworth filter or
similar is recommended.
The cabinet should be built as a solid and nonresonant box with well sealed and secured joints. This
particular design uses 18mm plywood the advantages
of this are strength, durability and availability.
The panel joints shown are simple butt joints that are
screwed and glued. For those with advanced woodworking skills, more sophisticated joints can be used.
Whatever joint type you use, it is important they are
secure and airtight. In addition, to reduce panel resonances, bracing battens have been added.
Electrical connectors such as a high quality ¼” jack
4mm binding posts, speakons or XLRs can be used.
When fitting these it is recommended you solder on
flying leads, mount the socket then make airtight using a sealing compound.
Use mounting bolts and T-nuts to fix the driver to the
baffle. CF18VJD is compatible with M6 bolts.
There is a wide range of speaker cabinet accessories available, including wheels, hands, grilles and
corners, adding to convenience and durability.
These should be fitted carefully so as not to weaken
the box or cause leaks.
Nominal diameter:
460mm/18in
Power rating:
1600Wrms (AES Standard)
Nominal impedance: 8Ω
Sensitivity:
97dB
Frequency range:
25Hz-1500Hz
Voice coil diameter:
125mm/5in
Chassis type:
Cast aluminium
Magnet type:
Ferrite
Coil material:
Round copper
Former material:
Glass fibre
Cone material:
Carbon and Kevlar loaded paper
Surround material:
Cloth sealed
Suspension:
Double
Xmax:
9mm/0.35in
Gap depth:
12mm/0.47in
Voice coil winding width: 30mm/1.18in
Small Signal Parameters
D:
Fs:
Mmd:
Qms:
Qes:
Qts:
Re:
Vas:
BI:
Cms:
Rms:
Le (at 1kHz):
0.38m/14.96in
33.1Hz
235.488g/8.31oz
9.464
0.315
0.305
5.51Ω
164.01lt/5.79ft3
30.54
0.09mm/N
5.644kg/s
1.987mH
Mounting Information
Diameter:
462mm/18.19in
Overall depth:
233mm/9.17in
Cut-out diameter:
416mm/16.38in
Mounting slot dims:
11mm x 7mm/0.43in x 0.28in
Number mounting slots: 8
Mounting PCD range: 432-441mm/17.0-17.36in
Unit weight:
23kg/50.6lb
CF18VJD Bass Cabinet: System Response Curves
Unfiltered SPL Response (predicted)
Impedance Response (measured)
Cabinet Plan
External and Port Dimensions
Cabinet Plan
Internal Bracing Dimensions
Cut-Out Guide
2.4m x 1.2m Sheet (8’ x 4’); 18mm Birch Plywood
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