Audio Physic Cerubin - description by AP. - Roysen

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Audio Physic Cerubin - description by AP.

193.128.102.3

Here is a quoted text describing the new reference speaker called
Cerubin from Audio Physic:
"
Power Without Limits
The development of the Audio Physic Caldera made it clear to us that freedom from distortion across all frequency ranges
increases the transparency. As it is only the low level details, such as early reflections, that enable you to experience
the recording room, enable you to assign each instrument to its natural dimension, and recognise that it is a number of
individual violinists intoning the melody. But the dynamics of good music are high. Whereas "car radio" type pop music
only requires 6 dB dynamics for compressor reasons, with unmanipulated pieces of music peak values of 30 dB are no
exception. It was always our ultimate goal for the CERUBIN specification to maintain the low distortion level of the
Caldera right up to the ultimate limit.
To obtain an exact overview of what a no-compromise loudspeaker really has to achieve, a hundred pieces of classical music
and almost two hundred pop songs with various artists and recording techniques were scanned into the computer using a CD-ROM
drive and analysed using a self-written analysis program (Fig. 1). Additionally to the power spectrum displayed in Figure 1
a number of other properties of the music have been analysed. These include frequency dependent dynamics, amplitude density
and some parameters concerned with spatial perception. In this way the demands on the individual chassis could be matched
very precisely to the performance profile demanded by the music.
Eight 25 cm long stroke chassis with the patented SD-2 linearisation system per loudspeaker are required to be able to
generate 120 dB sound pressure at 30 Hz. With the medium-frequency speaker only one 18 cm chassis is needed to achieve this
value in the CERUBIN. By distributing the required bass area across 18 separate chassis an extreme control of the bass power
is achieved and no part of the diaphragm is further than 6 cm from the driving voice coil magnet system combination. In this
way partial oscillations are avoided effectively. By using several relatively small bass loudspeakers the coupling to the
room can be distributed across a greater area (Distributed-Energy-Coupling). Standing waves and cancellations by the initial
reflections of the playback room are suppressed effectively in this way. The standing waves and early reflections, that
provide us with an impression of the recording room are comprehensible to us for the first time without interference. The
result is an effortlessness in the reproduction, that is limited only by the reception capacity of the human ear, as in the
case of natural music reproduction.
With the tweeter the performance profile becomes critical, if we want to retain the important, generally small diaphragm
surface area for radiation behaviour and freedom from partial oscillations. More on this later!

The speed
Loudspeakers are popularly known as "fast" if they reproduce the impulsive sounds naturally. At Audio Physic we prefer to
use the term impulse accuracy. The Manger transducer installed in the Medea set new standards in this discipline. This has a
rise time of 15Âµs and an open period (the time after which the sound pressure becomes negative with a constant positive
input signal) of 800 Âµs, and therefore surpasses the capabilities of the human ear. We were successful for the first time
in the Audio Physic Caldera to also extend the open period to 800 Âµs by combining a very large (28 mm diameter) dome with
a first-degree crossover unit (6 dB/oct.) and a lower cut-off frequency of the mid-frequency range at 120 Hz (18 dB/oct.).
800 Âµs is the magic number here, because the maximum time difference between the two aural signals, which is of central
importance for spatial hearing is never more than 750 Âµs. In the CERUBIN for the first time the dome known from the Caldera
was modified with a metal diaphragm, that surpasses with its 33 kHz diaphragm resonance even most of the 19 mm metal domes
by far, and sets a new record for a 28 mm dome. The rise time is also greater than that of the human ear. In addition the
distortion values could be reduced once more in comparison with the Caldera tweeter. The level stability has been clearly
increased here and the lower resonance frequency could be lowered even further below the range of the original Caldera dome.

The fixed heat coupling between voice coil, metal voice coil bearer and metal diaphragm guarantees rapid dissipation of the
heat from the voice coil, thus ensuring a high degree of dynamic linearity. This is otherwise only possible by using
extremely large diaphragm tweeters (true ribbon-type speakers) or horns, together with their disadvantages such as partial
oscillation, directional effect or coloration. An additional diffuser also ensures that the CERUBIN radiates like a point
source even at very high frequencies. The result of this work is an impulse behaviour that remains precise across wide
auditory ranges and at all volumes, speed at all costs.
However all of the measures described above would not be sufficient to adapt the performance of the dome to the level
demanded by the bass and mid-range. In order to combine the good impulse behaviour of a 6 dB crossover with the good
distortion behaviour of an 18 dB crossover, the CERUBIN is fitted with an adaptive frequency crossover. An analogue computer
constantly evaluates the diaphragm displacement of the tweeter. If the linear tweeter range is exceeded for more than a few
milliseconds, then the crossover switches from impulse-precise 6 dB to low-distortion 18 dB. The exact change-over
characteristics has a psychoacoustical motivation. On the one hand the exact reproduction of short impulses is of great
importance for the localisation of sounds, on the other hand the brain needs a longer period of time to perceive the timbre
and therefore any distortions. Due to the adaptive crossover the maximum sound pressure that can be transmitted from the
tweeter is increased by more than 20 dB (cf. Figs. 2 and 3) compared to a purely 6 dB solution, and is therefore perfectly
adapted to the performance level of the other chassis.

Adaptability is the order of the day
The problem is not new: a large passive loudspeaker is overkill with its bass foundation when it is placed in a small room,
whereas it sounds like a mini-bass in a large hall. The reason for this is the bass amplification that affects small rooms
because the sound waves can no longer completely spread out and pressurises rooms small rooms. The CERUBIN bass controller
amplifier with its five tuning options for the room size and seven tuning options for the wall stiffness makes it possible to
adapt to any type of listening room. The individual settings are selected in such a way that on the one hand differences
between various settings are easily heard, but on the other hand an exact comparison of the room properties is possible. The
CERUBIN bass loudspeakers are designed in such a way that inevitable transient effects occur below the auditory range
(fg = 5 Hz, Qts = 0.5), and the filters required for the room adaptation are aperiodic first order filters and therefore
practically transient effect free. This technique, requiring only a minimal number of additional components in the signal
path, is possible due to a microprocessor control governing the necessary filter mistunings.
The power stages with their total output of 2 kW are adapted to the load capacity of the bass loudspeakers operate according
to the current-drive technology. Instead of generating an amplified voltage output following the input voltage, they feed the
loudspeakers with a current in proportion to the input voltage. As the driving force of a loudspeaker is proportional to the
flow of current ( F=B*I ), this drive method leads to a distortion level that is reduced by 10-20 dB. The bandwidth of the
power stage, that only needs two amplifying stages (one of these as voltage amplifier in "foldback cascode" technique and
the other a current-amplifying lateral Mosfet output stage), is overdimensioned with its 1 MHz upper corner frequency well
above the range of this application.
The power output circuit is supplied by a choke power supply ( 2 * 32 mH / 50 A power chokes and a total of 210000 ÂµF / 100 V
smoothing capacitors). The connection between the mains unit and the power stages is by the shortest route via 2 cm * 2 cm
copper blocks. The choke power supply has the decisive advantage that it takes the power from the mains approximately in a
sinusoidal manner, and the spike-like power uptake of conventional high performance power supplies that interferes with other
components is avoided. The energy stored in the mains unit is 850 watt seconds, meaning that theoretically over 40 kW are
available during a mains cycle.
With the exception of the choke power supply serving the output stages all other modules are supplied separately from each
other so that no less than 23 power supplies are active in the bass controller amplifier. The processor required for control
(room adaptation) and monitoring (incorrect mains voltage, DC offset, overvoltage, faulty fuses,...) communicates here via
an optically insulated IÂ²C bus with the individual circuits.
Details, details, and more details
A concept with the best available chassis is not of much use if the boundary conditions, such as the housing and crossover,
have not been optimised. To be able to offer vibration-free operation the housing should be as small as possible, the
individual walls should be as light and rigid as possible so that they do not store any energy. The actual housing should
however be as heavy as possible so that it is not moved by the diaphragm acceleration. A sandwich of MDF-PDP-MDF is used as
the optimum material in the CERUBIN. PDP is a paper-polymer-paper layered material, that effectively insulates both the
inner and the outer housing (up to 20 dB better than untreated MDF of the same thickness) and that reduces the time of the
vibration processes to less than a third (cf. Figs. 4 and 5). Reinforcement shells, like those used in the wings of
aeroplanes to combine strength with low weight, additionally reduce the wall movements. In the mid-range housing these are
arranged on the isobaric lines in order to suppress a mass-and-spring coupling of individual air volumes to the diaphragm.

The defined position of the CERUBIN in the room, except for the known push-push principle used in other Audio Physic
loudspeakers, is ensured also by marble slabs above and below the bass housing. Anyone who has already tested this measure
with a bass-intensive loudspeaker will be convinced of its effectiveness.
The tweeter housing is in monobloc form, which is a technique already tried-and-tested in the Caldera.
Mid-range and treble speakers have been optimised to exploit their performance ranges to the maximum, and the number of
components required in the crossover has been reduced by more than half compared to the Caldera.
Further improvements could be achieved with the damping material used. For the scientific investigation of the influence of
early reflections on the spatial perception we were looking for a damping material that enabled the fading out of single
reflections in as targeted a way as possible, but without influencing the general characteristic of the auditory space. The
material that functioned here best of all, that damps reflections by up to 20 dB more than sheep's wool for example at the
same volume and same thickness and that is also considerably less frequency selective, is now also being used in the API..
It is clear that many minor details are necessary to achieve something that is truly extraordinary, on the way to the goal
of "No Loss of Fine Detail".

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Topic - Audio Physic Cerubin - description by AP. - Roysen 08:25:50 11/15/00 (0)

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