Fathom f112 Design and Construction
Introduction
In engineering, there is always more than one solution to a problem. Each approach has its pros and cons as various factors are balanced that can not all be simultaneously achieved. Cost, quality, performance, and size are just the surface of all the decisions that must be made in the course of engineering a device to satisfy design requirements. Throw in subjectiveness of opinion, aesthetics, and preferences for décor in people’s homes and designing a device that still does its job well gets harder.
Devices that are intended to output bass have many contradictory requirements. To meet the audio demands, the drivers must be large, able to move the substantial volume of air for long wavelength bass signals, and the cabinets must be large, to tune the system to reach as low as possible. This also implies expensive, requiring sturdy cones capable of large movements, large driver motors to move the air, powerful amplification, and stiff cabinets to contain the movement.
Now try to do all this while keeping in mind: cost limits what one can afford to place in a home theater, often one or more occupants of the house does not want something big living in the corner, and if you can see it, will it look good with the couch and drapes? Function follows form?
As different manufacturers balance these constraints and take aim at the market, powered subwoofers have taken many forms. At one extreme are small and inexpensive devices with flimsy cabinets and undersized drivers whose performance leaves it open to debate if they actually fit an audio definition of a sub, but hey, they can be easily hidden. At the other extreme are giant brutes that command attention when they rattle the house, without care for what anyone thinks.
After nearly twenty years in the mobile audio business earning patents and awards, high end car audio manufacturer, JL Audio, has decided to step into the home arena. With the introduction of the Fathom and Gotham subwoofer lines, they have made clear their stance on subwoofer design.
JL Audio’s solution favors the latter category.
Design and
Construction
The JL Audio Fathom f112
is a moderate sized subwoofer but at 115 lbs, it is a fairly heavy one. The unit features a solid cabinet with
radiused corners that is well braced and about as inert as any sub I can think
of. The f112 is available in two
finishes, matte black and high gloss black that defines the pricing difference;
the review sample sported the latter finish.
The black cloth covered grille is formed of heavy thermoplastic mesh
with recesses for steel lugs, mounted to the cabinet, for attachment. The subwoofer sits on four large,
hemispherical rubber feet but has no option for carpet spikes. In some ways this is a moot point as the sub
is heavy enough that will it likely compress any underlying carpet and padding
to the sub flooring so it’s pretty much going to stay put.
Of course, all the good stuff is under the grille and around back. The panel front sports most of the controls, set in a silver band above the driver with a musical instrument amplifier look: lever switches, knobs, and all. The 12 inch driver is hemispherical with an oversized foam surround that covers the mounting flange, which is recessed in a double layer front baffle. A JL logo badge adorns the bottom quadrant of the driver. All of this combines to give the f112 a rather serious look from the front.
Editorial Note on JL Audio Driver Surround Material
JL Audio uses a driver surround that is based on a multilayer polyester foam that considered environmentally resistant. Foam materials typically used for surrounds deteriorate from oxidation over time, requiring replacement to maintain the seal around the cone. After some discussion with design engineers, I learned that JL Audio has used this combination material and construction techniques successfully in mobile audio products dating back to the early nineties. The mobile version of the Fathom driver, the W7, has been in production since 2001 without problems where it withstands higher levels of user abuse and much harsher environments than are common in a home application. The engineers have also found that foam provides better support than rubbers with lower mass in high excursion applications.
I have included several links that give general environmental resistance information for various polymers:
The front panel controls provide access to most of the subwoofer
settings. From left to right we have:
the power toggle switch, mini XLR input jack with three room correction push
buttons: demo/defeat/calibrate, master/slave indicator light, level mode
toggle, master level knob, toggle for lights off/dim/on, low pass toggle
off/-12/-24 dB, low pass frequency knob, low frequency trim knob, the phase
knob, and the polarity toggle for 0/180 degrees.
One of the few minor complaints that I had with this sub
is the power switch located under the grille.
For those of us who are more environmentally conscious,
it forced me to
have to pull the grille off every time I wanted turn the sub on or off. Yes, standby mode is lower power consumption,
but it is still wasted power so I prefer to turn my gear off when not in use.
Around the back of the
unit is a black mounting plate cast with full height heat sinks to either side
that surround connection terminals located at the bottom, center. Connection options include balanced and
unbalanced stereo/mono inputs as well as a balanced mono output for connection
to additional Fathom subs. This
connection allows additional subs to be chained together in a master/slave
configuration to control all of the settings for the connected subs through one
master setting. The sub is supplied with
a heavy gauge detachable power cord that inputs below the balanced output. To the side of the outputs are two switches:
one to set the sub master/slave option and the other to set the sub as
grounded/isolated when using the unbalanced inputs. The purpose of the grounded/isolated switch is
to provide a fix for any possible ground loop problems between equipment from
sundry manufacturers. When the switch is
set to isolated, the RCA ground is disconnected and optical isolation is
engaged. If the sub hums, set the switch
to the other position; as stated in the manual, the correct position is the one
without hum.
Exposed banks of large heat sinks are a hallmark of hardcore audiophile amplifiers, many of which use them to dissipate the substantial heat generated from class A and A/B operation. These sinks may be overkill for the much more efficient class D amplifier that the sub uses, but if thermally coupled to the voice coil/motor assembly, they might insure extra dissipation for the large excursion driver. When I spoke with JL Audio’s engineers, they did indeed acknowledge the heat sink’s thermal dissipation capacity was considerably in excess of typical designs, by intent. The sinks are meant to keep the amplifier completely cool and unstressed under full load in any installation, including built in types.
Editorial Note on Class D Amplifiers
Class D have traditionally been the amplifiers of choice for subwoofers where high efficiency provides high output wattages and limited frequency response sees demand roll off before any complications arise from these amplifiers when used for full range audio. Amplifiers of this sort are based on a concept called pulse width modulation (PWM) that allows the output transistors to operate as true switches in only an on/off state, greatly increasing efficiency. The down side is that these amplifiers can generate EMI/RF that affects audible sound quality and they have to operate at a frequency significantly above the audio signal. PS Audio, with whom I have some familiarity, has engineered a number of successful, well regarded full range PWM designs such as the HCA-2, which operated at a frequency of 500 kHz to make sure trouble was away form audibility.
For more reading on class D amplifiers and PWM use the following links:
Switching Amplifiers: The Technology and the Issues
en.wikipedia.org Pulse-width_modulation
JL Audio requested that the f112 not be disassembled for internal pictures. According to the manufacturer, special care has been exercised during assembly to make sure that the integrity of the seals is adequate for the internal pressures developed by the f112. Internal pictures were provided by JL Audio that consists of promotional shots of the internals for the larger f113 including a very nice exploded view as well as shots of the f112 during assembly. Other than a slight difference in size and electrical ratings, such as the larger driver and transformer, the two models of subwoofers are identical.
Inside the cabinet, the construction is of ¾” MDF
with substantial internal bracing and support rings for the driver. The preamplifier/signal processing and
amplifier circuitry is housed at in a separate chamber from the driver at the
top of the unit. The substantial driver
frame is thick cast aluminum and supports a motor structure built around a 7½”
diameter by 2½” thick magnet. A large
tordial transformer is mounted to the center of the back plate with an Allan
head screw. The amplifier is mounted
above the transformer where one can see a bank of five large output capacitors
to supply reserve power.
The Fathom subwoofer series is equipped with a host
of signal processing features. In
addition to the standard 0/180 degree polarity switch, phase can be set over a
continuous range from 0 to 280 degrees.
The low pass filter can be set to bypass, -12dB/octave Butterworth, or
–24dB/octave Linkwitz-Riley filter slopes with a variable crossover point from
30-130 Hz to provide flexibility when blending speakers. Gain can be set at a fixed reference or
variable from mute to +15dB. The
controls also include an extreme low frequency trim (ELF) with a –12dB to +3dB
range at 25 Hz to allow additional control over any troublesome room boundary
gain that might prevent flat frequency response. Lastly, there is an automatic single band
parametric equalizer function. Called
Automatic Room Correction (ARO), this feature can be used to eliminate the
largest room mode peak at the listening position. The ARO uses a high
quality analogue
equalizer circuit to avoid cumulative error from redigitization and uses a DSP
controller that curve fits to the room response from an extensive library of
filter functions.
JL Audio lays claim to extensive R&D, testing, and high
tech manufacturing efforts for their driver designs. Extensive analytical simulation is performed
using a combination of software developed in house and the commercially
available Cosmos finite element software package. Rigorous prototyping and physical testing is
also performed to select the correct materials to meet design goals. Evaluating material alternates, advanced
construction techniques with stringent tolerances, and even testing product
durability under simulated aging are all part of the development process.
Amplifier designs for each product are highly optimized for the impedance curve of the system. To allow these amplifiers to develop the substantial output power quoted within the limits of typical home wiring, JL Audio looked at the demand trends from various source material. Home theater use for movie LFE finds extreme peaks at intervals that occur over relatively short time periods while music tends to a more sustained demand but rarely with the intensity of LFE. Using these observations, JL Audio tuned the current draw of the amplifier section with the real world dynamic demands from typical source material.
Research into high excursion driver behavior has lead to
designs said to be dynamically stable and capable of linear displacement in
excess of 3 inches. The
design
methodology applied to the Fathom subs has been to engineer a system, driver,
cabinet, and amplifier, etcetera that can develop the full excursion
envelope. This research has earned quite
a few U.S.
patents, with many of these technologies applied to the Fathom sub designs.
Innovations in electrical design include:
The Fathom amplifier section
consists of a pair of class D switching amplifiers that use a patented feedback
method (#6,441,685). The amplifiers used in the f112 are said to
be capable of unclipped output voltages equivalent to 1500 watts at 3 ohm
nominal impedance when compared to an equivalent loudspeaker.
Dynamic Motor Analysis (DMA):
proprietary optimization system used to improve the dynamic behavior of JL
Audio’s motor designs. The methodology
is used to design drivers that provide linear motor force at high excursions
and to stabilize the gap magnetic field over a wide power range. 
Mechanical
design innovations include:
W-Cone (#6,496,590):
a unit body cone assembly said to produce drivers with a high stiffness to mass
ratio and improved torsional rigidity for high excursion voice coil alignment.
The Floating Cone Attach
Method (#6,501,844):
a manufacturing technique that is said to maintain proper surround geometry in
assembling the driver for improved dynamic alignment.
Plateau-Reinforced Spider Attachment (#6,118,884): derived JL Audio’s VRC technology that provides stress relief to the spider under high driver excursion.
Over-Roll Surround (#5,687,247 and #5,949,898): a method for attaching the surround to the outside of the mounting flange to allow a large surround roll required for high excursion movements without sacrificing radiating cone surface.
Clamp ring (#6,568,503):
a fitting used to connect the over-roll surround to the
frame.
Thermal design improvements include:Radially Cross-Drilled Pole Piece (#6,243,479): directs airflow across the voice coil for improved power handling.
The drivers also employ what JL Audio terms Elevated Cooling Technology (#6,219,431 and #6,229,902): slots placed directly above the voice coil top plate allow additional airflow to minimize compression and enhance power handling.
