The Plinth

 

The "plinth" is the base of the deck.  It is the structural attachment point of all other components of the deck.  One main function of the plinth is to rigidly position the platter and tonearm in relation to each other, among other important tasks.  Another of the plinth's main functions is to prevent internal and external vibrations from affecting the interface of the point of contact between the cantilevered stylus and the record groove.

The L-07D plinth achieves a super rigid positioning of the critical components by using a massive aerospace aluminum subframe.  On one side of this massive structure is the attachment point for the direct drive motor.  The motor bolts to the aluminum subframe of the plinth with six massive hex head bolts.  On the other side of the aluminum subframe is the massive 3 kg collet chuck VTA-adjustable tonearm base, which bolts down with four bolts inside a deep well.  The massive metal structure of the motor-subframe-tonearm base results in extreme rigidity of the structure which supports and contains all moving parts of the deck.

The aluminum subframe is cast into the main section of the plinth, which is composed of a concrete resin.  Kenwood calls this material ARCB, which stands for Anti-Resonance Compression Base.  These two components form two thirds of the plinth.  The remaining part is a one and three-sixteenths inch thick mahogany composite sub-plinth.  The mahogany sub-plinth is attached to the ARCB section with 34 screws.

Rigidity of the plinth is assured by mass and composite properties.  Any given material has its own natural resonant frequency, at which it will vibrate if excited by kinetic energy.  By bonding materials with different resonant frequencies together, the ability of the composite to vibrate is minimized to nil.  As far as shear mass goes, which helps absorb vibrational energy, this deck is heavy!  (It weights 35 kg fully assembled.)  The ARCB section alone weighs 10 kg/22.1 pounds.  The aluminum frame which is embedded in the ARCB section weighs 2 kg/4.4 pounds.  The mahogany sub-plinth weighs another 7 kg/15.4 pounds. 

Four massive feet at the corners of the plinth are height-adjustable.  Each footer weighs about 1.4 kg/3.0 pounds.  A spirit level is built into the top surface of the platter.  The adjustable footers and spirit level are used to level the deck.

The L-07D brochure states:

How Kenwood engineers designed the L-07D to absorb vibration over the entire audible frequency range

Kenwood engineers realized that only a highly rigid construction could prevent signal loss through undesirable vibrational movement.  They therefore ruled out the use of conventional elastic vibration-damping materials such as rubber or metal springs which merely damp vibrations through internal loss.  More importantly, such common materials can create partial vibrations of several orders that may cause even worse resonances than those intended for suppression.  On the other hand, since highly rigid materials have fairly precise resonant frequencies, bonding different materials together restricts resonant frequencies over a wide range.  This is the function of the massive 33 KG (72.6 lbs) triple layer base.  It uses a new ARCB (Kenwood Anti-Resonance Compression Base) material (about 10 Kg/22.1 lbs) made of a special resin-concrete which is bonded to a layer of mahogany complite (about 7 Kg/15.4 lbs).  This forms the entire cabinet.  But embedded in this highly rigid composite base is a third element: a hard aluminum frame (about 2 Kg/4..4 lbs) that provides the ultimate accurate localization of the motor and tonearm.  This is one essential link in the continuous loop comprising stylus, arm, base, motor, platter and record.  The result of this design is a freedom from resonance across the entire audible frequency range, and near perfect conversion of recorded to reproduced signal -  a unique achievement in the history of turntable design.


The cross sectional view shown above demonstrates the construction detail of the plinth.  A cross section of the wood sub-plinth with an attaching screw is seen on the bottom left.  The aluminum frame section on the bottom connects the motor to the tonearm base.  The aluminum frame is embedded in the ARCB concrete resin main plinth section.  The result is a rigid resonance canceling composite plinth which makes this deck impervious to vibrations which could affect playback sound.


The cross sectional view shown above demonstrates the construction detail of the plinth.  The aluminum  frame is seen, with rigid attachment of the motor and tonearm base to it.  The result is that all pivot points in the pickup loop are extremely accurate.


The image above demonstrates the effects of bonding materials of different resonances on the plinth's ability to resonate.  The ability of the L-07D composite plinth to resonate is nil.


The following photos show details of the various plinth sections:

The ARCB Plinth shown above.


ARCB plinth shown with motor support plate and mechanical brake in photo above.


Underside of the ARCB Plinth shown above.  Integral aluminum sub-frame and switching PCB are also seen.


Integral aluminum sub-frame molded into ARCB plinth is shown above.


The mahogany composite sub-plinth with handles shown above.


Reassembled plinth seen from the underside in the photo above.  Wood sub-plinth attaches to ARCB main plinth with 34 screws.


The massive L-07D corner footer is shown above.  Each footer is composed of two sections which are made from solid brass.  They are adjustable for the purpose of leveling the platter surface.


The two main sections of the adjustable height footer are shown above.  Note the key slots, which prevent the bottom section of the footer from rotating when footer height is adjusted.


The mating portions of the bottom and top sections of the footer are shown above to the left and center, respectively.  On the right is a top view of the top section.


The limits of height adjustability of the footers are shown above.


The plinth in relation to the rest of the deck is seen above.


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