Resonances in Analogue Playback

Resonances in Analogue Playback
Of Audio Signals and Resonances
Analogue playback begins with the vibration of the tracking of the stylus in the record
groove, which is a direct, mechanical contact. These vibrations are transformed into
electrical energy in the cartridge and transmitted through the tonearm wiring in the form of
an analogue signal.
This process distinguishes an analogue source from a digital source. Reading a CD with a
laser or playing a computer file occurs without this type of physical contact, and therefore
the playback is not initiated by strictly mechanical means. The music is transmitted as a
digital signal with embedded, coded information. The D/A converter finally transforms the
digital signal into an analogue signal, but in this case without the analogue mechanical
tracking as the origin of the signal.
The natural question is, what exactly happens when tracking a record? On the one hand you
have the original signal created by these vibrations that is eventually heard as music. On the
other hand, these same vibrations that create the signal also create unwanted resonances
that produce distortion.
The process is easier to visualize when comparing a turntable (including tonearm) with a
musical instrument. The basic concept of a guitar, for example, is a resonant body; the
resonances that result from plucking a string are an integral part of the music. A guitar
manufacturer uses special woods and shapes to generate and enhance specific resonances.
Basically, a turntable is also a potential resonant body; the stylus in the groove also
generates vibrations, which are accompanied by inevitable resonances. The difference
between a musical instrument and a turntable is that in the turntable these resonances are
unwanted, have a marked influence on the sound, and prevent neutral musical playback by
imposing unwanted distortions and colorations.
Consequently, every manufacturer of turntables, tonearms and cartridges is faced with the
challenge of allowing the wanted vibration of tracking the record’s groove to pass as
unhindered as possible while at the same time preventing the unwanted influences of the
concomitant resonances.
A very important aspect of analogue playback and preventing resonances is the choice of
construction materials in the turntable and tonearm.
Different materials have very different sonic qualities according to their density and
microstructure and they consequently react differently to resonances. Basically the harder
or denser the material is, the faster it transmits sound.
Wood, for example, is a soft material with a fibrous structure. It provides a magnificent
resonant body when hollow, but that of course is unwanted here. Due to its softness, solid
wood is a “slow” transmitter and absorbs resonances, but it also mellows the vibrations that
carry the musical signal. Different types of wood can have very diverse densities and cellular
structures, and therefore different woods can have very different effects on the sound. Built
into a turntable or tonearm, wood always imposes sonic colorations because the nature of
its irregular structure absorbs some resonances and supports others.
Plastic has musical qualities similar to wood. It’s also a soft material that absorbs resonances
and mellows the musical signal. The microstructure of plastic is irregular, so the absorption is
also irregular. It has an influence on the sound because the resonances are nonlinear,
meaning they are not evenly absorbed over the whole frequency range. Plastic “swallows”
vibrations and resonances and transforms them into heat.
Metals are a big group of very diverse materials. Typically, metals have a very organised and
regular microstructure and are ideally suited to transmit vibrations and resonances. Some
metals, like stainless steel for example, are very dense and therefore very fast in
transmitting sound. Metals rarely absorb resonances, but they can amplify resonances and
“sing along.” The high density and fast transmission can result in resonances bouncing back
and forth. Every metal has a different characteristic microstructure that results in marked
internal resonances and gives a characteristic sound. The most commonly used metal for
turntables is aluminium, and with good reason. Aluminium is less dense and heavy than
most other metals and has well-balanced acoustical qualities with no internal resonances
within the spectrum of human hearing and a certain capability to absorb resonances.
Connected Materials
Another important aspect is the connection between separate parts and different materials.
Naturally, a turntable or tonearm is built from a multitude of separate parts that are
connected one way or the other. One obvious idea is the combination of different materials
with different characteristics, for example the combination of a dense, fast transmitting
material like stainless steel or bronze with a soft, absorbing material like plastic. In this case,
the stainless steel would transmit the resonances into the plastic where it would be
absorbed. A combination like this can be ideal to supress resonances, but the resonances
would be transformed into heat by the plastic and would therefore not be available to
transform the vibration of the tracking into electrical energy. The result is a loss of dynamics.
The manner in which different parts are connected is also important. When two parts are
connected loosely, resonances also break and get absorbed. Micro vibrations get
transformed into heat. A tight connection between two parts enables the (wanted)
transmission of vibrations.
What Happens to these Vibrations and the Resonances?
A cartridge needs to be isolated from external disturbances to provide smooth tracking that
results in good sound and high resolution. Only the cantilever and stylus should move when
tracking the information in the groove and transmitting the vibrational energy; the rest of
the cartridge should be stable. The vibration is transformed into electrical energy in the coil
and the cartridge’s rubber damper absorbs resonances and prevents oscillation. This process
should take place without interference, but that’s difficult to achieve.
In the case of a properly aligned tonearm and cartridge, the most significant interferences
are the resonances that accompany the tracking of the groove. A tonearm is naturally less
solid than a turntable, so resonances built up and oscillate here much easier. Consequently,
tonearms work “backwards” and can influence the cartridge through the headshell. Often
the cartridge loses stability and the tracking process is disturbed with audible sonic
colorations when this occurs.
One potential way to prevent this is to absorb or “break up” the resonances. As described
above, this can be achieved by using soft and absorbing materials, for example a plastic
tonearm tube, or by having (relatively) loose connections between headshell and tonearm
tube (such as what can be found in removable headshells). The disadvantage of this method
is that the resonances can’t escape the headshell and the residual energy in the arm tube is
transformed into heat. No material can absorb completely, so there would always be “left
over” resonant energy that works backward and negatively affects the cartridge.
The other option is to channel and discharge the resonances. If this done fast and
unhindered, then this is the easiest way to prevent oscillating resonances in the tonearm
that work backwards and disturb the tracking process. The resonances are transmitted in the
direction of the tonearm bearing and with a suitable bearing construction, they can
discharge into the turntable plinth. Once the resonances have left the comparatively fragile
tonearm, it’s easier to discharge them without sonic consequence and the backward
influence on the cartridge is eliminated.
Tonearm bearings should be friction free and stabilize the tonearm to prevent oscillation of
the tonearm tube. Most common are play-free, preadjusted ball bearings or unipivot
bearings that are stabilized by the tonearm’s own weight. In any case, the resonances need
to be channelled through small points of contact. With a unipivot tonearm, there is only one
point of contact. The tonearm that rests on this single bearing contact needs to be heavy
enough to prevent the resonant peaks from “lifting” the arm off the bearing, which occur
when micro resonances build up in the bearing. Consequently, a unipivot tonearm usually
requires higher mass than a tonearm constructed with preadjusted ball bearings, which
affects the basic resonance of the tonearm/cartridge combination.
How are These Issues Solved in Brinkmann’s Turntables and Tonearms?
The first important question is the choice of construction material. Our turntables and
tonearms are mainly constructed of anodized aluminium. We have only sparsely combined
different materials for a very specific purpose. In our design concepts, it makes more sense
to channel and discharge interfering resonances than to break up or absorb them. Raw
aluminium is a comparatively soft metal that can absorb resonances and spread and
transmit them evenly through the organized microstructure. The anodized surface is very
dense, almost as hard as diamond, and is perfect to transmit sound very fast and at the same
time protect the surface. This combination provides a fast transmission of resonances on the
surface and a certain amount of absorption inside the aluminium, unique qualities found in
no other material.
Since the bulk of the resonances run through the tonearm, the headshell and tonearm tube
of our arms are tightly connected to prevent oscillation and discharge the resonances. Both
parts are made from anodized aluminium, and the tonearm tube has an especially hardanodized surface that is much harder than normal anodizing. Resonances can run over that
hard surface at high speed, while the inside of the tube is soft and absorbs some of the
resonances. With these measures, there is a certain amount of absorption but without the
resonances working backwards to influence the cartridge.
The rest of the resonances that result from tracking the record groove pass into the platter
and through the bearing into the plinth. It’s important that the platter material neither
dampens the wanted vibrations of signal tracking nor “sings along” with the unwanted
resonances. A dampening effect of the platter would be audible in the music as a lack of
dynamics. A platter that sings along with the resonances would influence the tracking and
add unwanted coloration to the music. All Brinkmann turntables have a crystal glass platter
mat. Glass has an amorphous but dense microstructure and spreads the resonances evenly
through the whole mat instead of absorbing them only at the area where the tracking takes
place, before they get transmitted into the platter. Our platters consist of a special
aluminium alloy that prevents resonances from swinging back and forth inside the platter,
channelling them instead through the bearing and into the plinth where they are discharged.
The plinth is the part of the turntable where both resonance paths – the one through the
tonearm and the one through the platter - meet. It’s important to discharge the resonances
of both paths separately to prevent them from affecting each other. The energy through the
tonearm is stronger, faster and more critical for the tracking process, so the major focus is to
discharge this energy. The resonances are discharged through a steel spike in the Balance
and an aluminium foot in the Bardo and Oasis tables. This provides a fast channel and easy
discharge. The resonances through the platter are already smoothed and slowed by the
heavy platter and plinth mass. They get discharged through two steel-copper spikes in the
Balance and two feet with plastic inserts in the Bardo and Oasis. The copper inserts in the
steel spikes allow a soft discharge of the already-smoothed energy. The feet of the Bardo
and Oasis have inserts of resonance-optimized plastic designed to absorb resonances evenly
over the full frequency range. In both cases these measures prevent the resonances that run
through the platter from interfering with the resonances that run through the tonearm. Both
resonance paths discharge their energy separately, resulting in clean, high-resolution
playback from Brinkmann’s turntable designs.
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