Eminent LFT-VIII User manual

EMINENT
TECHNOLOGY
INCORPORATED
LFTVIII
HYBRID LINEAR FIELD TRANSDUCER LOUDSPEAKER
REFERENCE MANUAL
Revised: 7/22/03
Eminent Technology, Inc. 225 East Palmer Street Tallahassee, Florida 32301
Phone: [phone] FAX: [phone] Email: [e-mail]
Website: www.eminent-tech.com

2
Extremely strong magnetic fields are
present at and around this
loudspeaker. Devices that are
adversely affected by high levels of
magnetic flux, such as television sets
and pacemakers, should be kept at
least three feet away from each
speaker. Also, keep in mind when
any ferrous objects are brought close
to the speakers. Hold steel tools
securely when setting up and
adjusting the LFT-VIII, to prevent a
hex key or screwdriver from slipping
from your hand and damaging the
Mylar diaphragm.
ATTENTION:
STRONG
MAGNETIC
FIELDS
_____________

3
TABLE OF CONTENTS
Installation of the LFT-VIII........................................................5
Unpacking the Speakers.............................................................5
Speaker Assembly......................................................................6
Positioning the Speakers in the Listening Room.......................7
Imaging ......................................................................................7
The Tweeter Level Control........................................................8
Amplifier Requirements.............................................................8
Bi-Wiring and Bi-Amping.........................................................9
Bi-Amping .................................................................................9
Technical Description................................................................10
Electrostatic Loudspeakers ......................................................11
Planar Magnetic Loudspeakers................................................12
Ribbon Loudspeakers...............................................................13
Evaluating Earlier Approaches................................................14
Electrostatics............................................................................14
Planar Magnetics......................................................................14
Ribbons ....................................................................................14
The Linear Field Transducer....................................................16
Diaphragm Construction..........................................................16
The Magnet/Frame Structure...................................................16
Panel Frequencies ....................................................................18
General Specifications...............................................................19
LFT-VIII Impedance Curve.....................................................20
LFT-VIII Impedance Data........................................................21
Crossover Information..............................................................22
Square Wave Performance ......................................................23
Frequency Response Curve.......................................................24
LFT-VIII Panel Specifications..................................................25
Mid Range Panel Design...........................................................26
LFT-VIII Woofer Specifications..............................................27
Additional Woofer Specifications.............................................28

4
Woofer Design............................................................................29
Woofer Enclosure......................................................................30
Warranty....................................................................................31
Appendix A –Hex Cam Spacer Installation...........................33
Appendix B –Tweeter Diaphragm Replacement...................36

5
A complete technical description of the LFT-VIII is included in this
manual and begins on page 10. It is recommended that you become
familiar with this information because an understanding of the LFT
principals will assist you in the proper set up of these loudspeakers.
The LFT-VIII is shipped in 3 boxes. The larger square box contains the two
woofer cabinets. The 2 long rectangular boxes contain the panels and grills. To
remove the panels position the box on its side and open the end of the shipping
carton. Remove the padding from the bottom and slide the speaker from the box
as shown below.
Open
After removing the speaker from its carton it can be leaned against a wall
standing up or placed with the front face of the speaker flat on the carpet. Then
the bubble wrap should be removed. The wrap also holds the grill cloth (covered
by a large cardboard sheet) to the speaker. After the wrap is removed these will
separate from the speaker.
Do not attempt to remove or loosen hardware on the drivers themselves.
The magnets are held together under great force and personal injury could
result.
Installation of
the LFT-VIII
Unpacking the
Speakers

6
Assemble the panel to each woofer cabinet as shown below.
Fasten the feet to the bottom of the woofer cabinet. Use the
drawing below to identify the correct hardware locations. Grill
cloths snap into place with Velcro fasteners at each corner on the
front and back of the speaker. Bolt the woofer box to the panel as
illustrated below. Laying the speaker on its side may be helpful
in assembly.
(5) #8X3/4 Sheet Metal
Screws In Front Of Panel
10-32 x 3/4 Socket Head
Screw (4)(4) #8X3/4 Sheet Metal
Screws In Bottom
Woofer Cabinet
Front View Showing
Position of The Five 8X3/4
Sheet Metal Screws
Location Of Terminal Board
Sheet metal Screw (2)
Connections to Mid-Range and Tweeter Panels
After attaching the woofer cabinets to the panels, you must attach
the three leads that come out of the mid-range/tweeter panel to the
screw barrier terminals on top of the woofer cabinets. The small
black wire (18 ga.) that comes from the tweeter panel is secured
to on of the terminals marked High, Med or Low on top of the
woofer cabinet. The larger diameter wires (12 ga.) will be
attached to their respective terminals atop the woofer cabinet,
Black to Negative, White to Positive.
Speaker
Assembly

7
Speaker placement is critical for correct imaging, frequency
balance, low frequency performance, and efficiency.
The LFT-VIII speakers are a mirror image pair and should be set
up with the tweeter panels to the inside.
Low frequency performance in particular can be determined by
the shape of the room and the speaker's distance from the wall
immediately behind them. Typically, the optimal distance
between the LFT’s and the rear wall is 1 to 5 feet in an average
room.
The overall frequency balance of the LFT-VIII is somewhat
affected by the degree to which the speakers are toed in toward
the central listening position. The on-axis frequency response of
the LFT-VIII is essentially flat, and it is often best to position the
speakers so that the main listening position is about on axis with
each speaker. Slight mid-range frequency balance changes can be
obtained by pointing the speakers slightly away from the listening
position. Adjusting the speakers’ degree of vertical tilt with the
pointed feet can also alter this balance.
Overall imaging depends primarily on the distance separating the
two speakers relative to their distance from the preferred listening
position; it is also affected by the degree of toe-in. We cannot
accurately predict what will work best in your listening room, and
can suggest only that you begin with the drawing on the previous
page as a starting point or general guideline. Keep in mind that
the parameters that affect frequency balance also tend to affect
imaging properties, and vice versa, so it is best to adjust speaker
placement in small increments and to note carefully all of the
changes effected by each shift in position before proceeding
further.
Positioning
the Speakers
in the
Listening
Room
Imaging

8
The high frequency performance of the LFT’s can be tailored with the
tweeter level control. The high frequency performance of the LFT-VIII
is adjusted with the tweeter level control. There are three tweeter level
positions: High, Mid and Low. These levels adjust the tweeter output
in approximately 3 dB increments. It is best to start with the tweeter
level set to Low, position the speakers for the best overall frequency
balance, and then decide if more high frequency energy is needed.
The LFT-VIII is wired for 8-ohm operation and is appropriate for use
with most moderately powered tube and solid-state amplifiers. The
efficiency is 84dB with a 2.83-volt drive (1 “8” ohm watt). The
efficiency rating is lower than average. However, the LFT-VIII
radiates a planar wave front, and as such, on axis its apparent efficiency
at the listening position is higher than the numerical rating implies. The
LFT-VIII has a minimum rating of 75 watts per side, tube or solid state.
It can handle “music power” levels (short term burst) of 300 watts or
more with out difficulty. The largest recommended amplifier size for
the LFT-VIII is 200 watts.
The LFT-VIII does not require a high current amplifier. A receiver may
be used if it has sufficient power. Tube amplifiers should be used with
the 8-ohm tap.
The Tweeter
Level Control
Amplifier
Requirements

9
The LFT-VIII is configured to allow bi-wiring or bi-amping with a
minimum of trouble.
Bi-wiring simply means connecting a single stereo amplifier (or two
mono amps) to a pair of speakers by using two pairs of speaker cables.
Connect the hot and ground conductors of a pair of cables to the same
output terminals on one channel of the amplifier; the other ends are
connected to the separate woofer and mid/tweeter inputs of the LFT-
VII (All speaker cables should be the same length). The effects of bi-
wiring tend to be subtle; the slight improvement may be worth the
relatively modest cost of an extra pair or speaker cables. Bi-wiring also
permits experimenting with different types of cables for the two inputs;
you may find that one type is best suited for bass performance, while
another works best on the mid/treble side.
Bi-amping requires and additional stereo amplifier or pair of mono
amps. You will also need some means of insuring that only the desired
portion of the frequency range reaches each amplifier. The simplest
way to accomplish this is with an external electronic crossover;
however, this can also be done by hard-wiring low-pass and high-pass
filters into the inputs of the bass/mid and treble amplifiers, respectively.
For the low/ frequency amp, a 180Hz low-pass filter (6 dB/octave) is
required; for the mid/treble amp, a 180Hz. high-pass filter (also 6
dB/octave) is required. If you wish to pursue this method, your dealer
or the manufacturer of your amplifiers should be able to help you
determine the specific parts necessary. Note that you will also need a
level control on either one of the stereo amps or on the crossover,
regardless of which approach you take to bi-amping. Contact Eminent
Technology or refer to the schematic in the back of this manual to
modify the crossover for proper speaker operation.
Bi-Wiring and Bi-
Amping
Bi-Amping

10
The Eminent Technology Linear Field
Transducer is a full-range, push-pull, dynamic
planar loudspeaker. In a sense, it is the
magnetic equivalent of a push-pull electrostatic
loudspeaker, differing in that it requires no
step-up transformer or bias voltage, and that
the audio signal is applied directly to its
diaphragm.
The LFT-VIII
To fully understand the strengths of the LFT design, one must
first consider the design and operation of this speaker's three most
notable antecedents: the push-pull electrostatic loudspeaker
(ESL); the traditional, single-ended planar magnetic loudspeaker,
and the ribbon loudspeaker.
Technical
Description

11
The electrostatic starts with a very thin (half mil or less)
diaphragm made of Mylar or a similar material, to which a light
coating of mildly conductive substance such as graphite has been
applied. This diaphragm is suspended on a rigid frame and
sandwiched between two stationary conductive grids (usually
perforated metal plates) called stators.
FRAME
REAR STATOR
RESISTOR
TRANSFORMER +
--
TO
AMPLIFIER
TO BIAS
VOLTAGE
SUPPLY
[PERFORATED
METAL PLATE]
FRONT
STATOR
CONDUCTIVE
DIAPHRAM
A DC charge of high voltage (in the thousands of volts) but very
low current, known as the bias voltage, is applied to the
conductive diaphragm and kept constant. A step-up transformer
is introduced to increase the usable voltage of the amplifier's
output (while simultaneously decreasing the current), and the two
ends of the transformer's output coil are connected to the two
stators.
As the amplifier produces a continuously varying AC voltage,
(the amplified music signal), the charge on the two stators will
also continuously change in synchronization with the music; and
since the two stators are connected to two different ends of the
transformer's output, one stator will take on a predominantly
negative charge at the same time and to the same extent that the
other stator takes on a predominantly positive charge. The
constant-charge diaphragm will thus undergo a continuously
changing state of attraction to and repulsion from the two stators
as their polarization changes, and it is this motion that excites the
air to the front and rear of the speaker and produces sound.
Electrostatic
Loudspeakers

12
The traditional planar magnetic also starts with a thin Mylar
diaphragm, one side of which is coated with adhesive and fitted
with an aluminum wire voice grid, (analogous to the voice coil of
a conventional cone driver). The diaphragm is held taut in a
metal frame. On the front of this frame is a large sheet of
perforated metal, to which rows of vertically aligned strip
magnets have been fastened.
+
-
TO AMPLIFIER
FRAME
DIAPHRAM
VOICE GRID
[ACTUALLY A
CONTINUIOS LOOP]
PERMANENT
STRIP
MAGNETS
PERFORATED
METAL
SHEET
N
S
S
S
N
SINGLE-ENDED PLANAR MAGNETIC
[TOP VIEW CROSS-SECTION]
Spacing exaggerated to show detail
From there, the operation of a single-ended planar magnetic
loudspeaker is remarkably similar to that of a conventional cone
driver: The amplifier's output is sent directly through the voice
grid and, because it is suspended within a stationary magnetic
field, the grid moves back and forth within that field in
synchronization with the AC voltage that is the amplified music
signal. Since the voice grid is permanently fastened to a taut
diaphragm, the diaphragm also moves in synchronization with the
music signal, exciting the air and producing sound.
Planar Magnetic
Loudspeakers

13
The third and final antecedent to consider is the ribbon: a
distinctly different sort of transducer, but one that is similar (in
principle, at least) to the single-ended planar magnetic. The
ribbon’s primary distinction is that its “diaphragm” and “voice
element” are one and the same.
A ribbon driver is based on a long, narrow strip of conductive
material; in practice, thus far, all true ribbons have used a strip of
very thin corrugated aluminum for this purpose. The two ends of
this strip are electrically connected to the amplifier’s output, and
are physically anchored such that the strip is suspended within a
stationary magnetic field--with said magnets positioned at the
edges of the strip.
PERMANENT MAGNET
PERMANENT MAGNET
ALUMINUM
RIBBON
ELEMENT
N
S
_
+
TO AMPLIFIER
[LEADS CONNECTED TO
TOP AND BOTTOM OF
RIBBONELEMENT]
RIBBON DRIVER
[TOP VIEW CROSS-SECTION]
The operating principle is straightforward from there: the
amplifier’s output passes directly through the aluminum strip--
which, because it is suspended within a permanent magnetic field,
moves back and forth in synchronization with the signal,
producing sound.
Ribbon
Loudspeakers

14
Not surprisingly, each of the approaches described above has its own
unique set of pros and cons. The electrostatic, because its diaphragm is
so thin and light, offers exceptionally good transient response and
reproduction of subtle, low-level musical detail. And, because it is a true
push-pull device (i.e., its diaphragm is, by design, driven from both the
front and the rear), the ESL operates in a linear fashion. Typically, gross
distortion results only when the driving amplifier clips into the speaker,
or when, in an attempt to play the speaker louder than its design allows,
its step-up transformer reaches a point of saturation.
On the negative side of the ledger, the ESL does require passing the
amplified musical signal through a transformer, which can introduce its
own colorations and non-linearities. Also, some ESLs are prone to a
condition known as arcing: Under the conditions of stress induced by
playing an ESL loudly, it is not uncommon for an electrical spark to jump
between one stator and the diaphragm (a phenomenon exactly analogous
to lightning), burning a minute hole in the diaphragm and, over time,
ruining it.
As for the planar magnetic, its strengths are similar to those of the ESL--
although the addition of several feet of wire and an adhesive coat make
for a somewhat more massive diaphragm, limiting this design’s transient
capabilities by comparison. But the planar magnetic requires no step-up
transformer or bias voltage supply, and it has the added benefit of being
an extremely manageable load for most amplifiers. However, the most
specific drawback of the traditional planar magnetic is that it is a single-
ended (as opposed to push-pull) device: As the diaphragm’s physical
excursion increases, the voice grid moves further away from its optimal
location within the permanent magnetic field (at least in one direction).
Thus, at the very instant when this speaker is called upon to reproduce
large-amplitude waveforms, it is least able to do so without distortion.
In many ways, a ribbon driver can be an excellent performer: the moving
element (the “ribbon” itself) is extremely light, allowing good “speed”
and transient performance as well as freedom from coloration. And there
is no significant physical structure on either side of the ribbon’s radiating
pattern. The ribbon’s main problem is not one of performance but of
application: it cannot be used to reproduce low frequencies. To create a
moving element large enough to generate frequencies lower than a few
hundred Hz would mean moving opposing magnetic poles so far apart
that they would no longer exert a sufficient magnetic field over the entire
area of the ribbon.
Evaluating
Earlier
Approaches
Electrostatics
Planar
Magnetics
Ribbons

15
Also, when a ribbon is operated at frequencies approaching the
element’s own resonant frequency (which is naturally quite low,
due to its high compliance), the ribbon element stretches and
“bows” to a point where it is no longer within the magnetic gap.
To get around either of these problems means to move the
permanent magnet structure from the edges of the element to one
entire side of the element, and/or to bond the element to a “host”
diaphragm, such as a sheet of Mylar, and to clamp that diaphragm
around its perimeter. In either case the driver is no longer a
ribbon; it is, in fact, a planar magnetic. To date, no one has
succeeded in creating a full range ribbon loudspeaker.

16
Eminent Technology’s Linear Field Transducer, introduced
as the LFT, represents a new approach to the design and
construction of a high-quality loudspeaker*. It builds on the
strengths of the above designs while eliminating many of their
drawbacks.
The construction of the LFT -8 begins by laminating a very thin
sheet of aluminum foil to a half-mil-thick sheet of Mylar. A voice
grid pattern, created by means of CAD (Computer-Aided Design)
technology, is silk-screened onto the foil side; the remainder of
the aluminum--the part not covered by ink from the silk-screening
is chemically etched away, in a manner similar to the etching of
traces on a printed-circuit board. The ink is then washed away,
leaving a voice grid of near-perfect uniformity. This technique
results in a diaphragm/voice coil grid that is still less than one mil
in total thickness, and also permits relatively narrow spaces
between the individual traces, so the diaphragm can be evenly
driven over its entire surface.
The magnet/frame structure developed for the LFT-VIII is also
unique. Eminent Technology builds its strip magnets into
individual steel channels, the size and shape of which have been
carefully designed to help “focus” the magnetic flux lines and
concentrate the strength of the magnetic field on the appropriate
area of the diaphragm/voice grid. These channels are then welded
to steel crossbars, which in turn are bolted to the frame that holds
the diaphragm in place.
Interestingly, one of the biggest challenges faced in creating a true
push-pull dynamic speaker was not a design consideration but
rather a matter of construction difficulty: to assemble a perfectly
rigid structure with very powerful permanent magnets at the front
and the rear, both sides opposing each other with tremendous
force. It was not until Eminent Technology developed a special
method for this assembly procedure that the Linear Field
Transducer became a reality.
* The design and construction of the LFT is patented.
The Linear
Field
Transducer
Diaphragm
Construction
The
Magnet/Frame
Structure

17
By applying such new techniques to planar loudspeaker
construction, Eminent Technology has been able to eliminate
many of the flaws inherent in earlier designs. The use of a welded
channel-and-crossbar frame dispenses with the need for
perforated sheet metal (an “off-the-shelf” material presumably
used for reasons of economy and ease of manufacture.) thus
greatly improving dispersion, especially at high frequencies.
FRAME
DIAPHRAM
PERMANENT
STRIP
MAGNETS
STEEL CROSS
BAR
STEEL
CHANNELS
VOICE GRID
CONTINUOUS
LOOPS
N
S
N
N
N
S
S
S
TO AMPLIFIER
+
_
LINEAR FIELD TRANSDUCER
[TOP VIEW CROSS-SECTION]
Spacing exaggerated to show detail
Since it is now possible to have a powerful, precisely aligned
magnet structure on both sides of the diaphragm, true push-pull
operation has been achieved: Regardless of the degree of
excursion the diaphragm undergoes, the voice element is always
optimally positioned within the magnetic field. The result is
extremely linear performance throughout the audible range, with a
profound increase in dynamic range and an absolute minimum of
distortion.

18
Each LFT-VIII has two individual driver panels and a cone type
woofer. The placement of the individual drivers is shown in the
following diagram.
10 KHz and above
180 to 10 KHz
To 180 Hz
Panel
Frequencies

19
General Specifications:
Power Requirements 75 Watts minimum
Sensitivity 84 dB (pink noise, 20 -20kH) at 1
watt/1 meter (2.83 V)
Frequency Response 25 Hz-20 kHz ±4 dB (typical
room)
Phase Accuracy ±20 100 Hz -31 kHz
High Frequency Level Flat, -6dB, -12dB at 20kHz
smooth roll off
Impedance 8 Ohm rating
Maximum SPL 105 dB at 1 meter
Dimensions 13” wide by 60” high by 1” thick
Shipping Weight 90.5 lbs. Each
Warranty 3 years parts, 1 year labor
Available Finishes Oak, Walnut, Black Painted Oak
Technical
information

20
LFT-VIII Impedance Curves
The LFT-VIII impedance is shown below. The impedance
generally averages much higher than 8 ohms. This means that the
speaker does not require a high current amplifier. Because of the
lower than average efficiency you will still need an amplifier with
a fairly high power rating (75 watts per channel or more). If an
amplifier clips into the speaker it will probably be due to a voltage
limitation.
Technical
information