Tannoy System 800A, User Manual

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attention paid to the sculptured front panel to provide smooth, rounded edges, which
minimise side diffraction.
Another problem involved in cabinet design is to ensure that the box will effectively
behave as neutrally as possible, ideally without interfering at all with the sound field
emitted by the drive unit and the port. There are two main causes for the cabinet to
vibrate. First there can be a mechanical transfer of energy between the drive unit and
the cabinet front panel. Preventing this requires the use of a rigid and stiff front baffle,
which is achieved on the System 800A by using a very thick, dense MDF panel. The
second way is transmission from acoustical to mechanical energy. Since high acoustic
pressures are present inside the cabinet this is quite likely to occur if no attention is paid
to minimise the effect. Here the use of rigid panels is also helpful but, since their
stiffness cannot be infinite and therefore their resonances only shifted towards higher
frequencies, enough damping has to be provided in the cabinet assembly, including
panels and joints. Due to its octagonal shape and its cabinet construction, the System
800A performs very well in that respect. Its shape tends to decrease the largest
dimensions of each side panel, which reduce low frequency resonances, while the
doubled number of side provides additional damping.
In addition to the cabinet construction the volume and port tuning have been carefully
calculated to give the best set of parameters for monitoring applications. There is a
fundamental relationship in loudspeakers between efficiency, cabinet volume and low
frequency performance given that minimal amplitude variations can be tolerated in
monitoring situations.
Active crossover and amplifier.
The integrated active crossover, which splits the input signal into LF and HF separate
amplification channels, has been designed using a dedicated computer simulation
program. The result is an unconventional topology giving optimum electronic transfer
functions, i.e. achieving the desired target response when combined with the acoustical
responses of the LF and HF units in the actual cabinet.
Thanks to the advantages of the Dual Concentric principle, filters with low phase
variations in the overlap frequency range can be used without detrimental effect on the
spatial dispersion, as with conventional multi-way speakers. As a result the group delay
can be maintained practically constant over the whole frequency range, which is
essential to a good transient response and an accurate stereo image.
Such a degree of optimisation and accuracy in matching the crossover to the drive unit
cannot be achieved passively, without inducing significant loss of sensitivity and
resulting in highly inconsistent performance due to the variations in the impedance of
the drive units.
The influence of the power amplifiers on the performance of a complete system does
not have to be demonstrated. However this does not reflect generally in any technical
figures, which most of the time - except for output power - seems close to perfection
(ultra low distortion, ultra linear response, etc...). Nobody would trust figures showing
that an amplifier with 0.002% distortion will sound worse than another with only 0.001%.
Not entering a technical discussion about why an amplifier can sound “warm”, or “harsh”,
or “dry”, this is another reason to choose a complete (e.g. a self-amplified speaker)
rather than a split system where you cannot predict the overall result until the chosen
amplifier is actually connected to the speaker.
Another obvious reason that favours an integrated solution is that, as in most
engineering work, designing an amplifier is a matter of making the right compromises
between different parameters, often in contradiction: voltage capability, current
capability, short term or long term power... Designing an amplifier for a given speaker
(electrically speaking, a given load) is a significant advantage which allows much better
optimisation.