T+A CRITERION Serie Betriebsanleitung Seite 26

The cross-over unit
The crossover filters of passive loudspeakers have to
cope with high currents, and their internal resistance and
losses in the electrical components prevent them ever
working under optimum conditions. Signals are rounded
off, dynamic range is reduced, distortion is induced, and
the loudspeaker chassis are not coupled directly to the
output stages, with the result that the drivers cannot be
controlled effectively despite the use of external power
amplifiers with superb damping factors.
Not one of these problems is present with genuine active
loudspeakers, i. e. speakers featuring active crossovers
and separate, direct-coupled output stages for each drive
unit. Active crossover units have ideal characteristics:
there are no losses, while the frequency response and
phase characteristics are exactly as desired and cal-
culated by the design team. Active filters produce opti-
mum results with zero losses. Unlike passive filters, they
are not limited to simple first-order, second-order or (at
best) third-order network circuits; they can be designed
to incorporate much more sophisticated, refined circuitry,
capable of compensating for phase and frequency re-
sponse shortcomings in the drivers. Special filters are
employed in TCI Active speakers, designed to ensure
that all the drive units operate in phase at all times. In
conventional passive loudspeakers the mid-range drive
unit is operated out of phase, but this is not the case with
TCI speakers. Positive music signals are always cor-
rectly reproduced as positive signals, negative signals as
negative signals, regardless of frequency.
Correct phase is important, but equally crucial is correct
timing, and this is another area where TCI Active speakers
excel. In a normal three-way loudspeaker the high-frequency
and mid-range components do not reach the listener's ears
exactly simultaneously, whereas special measures are taken
in TCI Active speakers to correct these timing errors, with the
result that all parts of the frequency spectrum from 180 Hz to
40 kHz arrive at the listener exactly simultaneously, i.e. with
no timing errors. This technology ensures that every musical
event is reproduced with exactly correct timing, i.e. precisely
as it was recorded. An incidental advantage of perfectly
timed reproduction is a superb frequency response in correct
phase, retaining the entire dynamic range of the original
recording, and providing impressive spatial definition.
Bass regulation
It is impossible to regulate the drive units of a passive
loudspeaker, but it is certainly possible if digital concepts
are adopted. The regulation system involves monitoring
the travel of the speaker cones and correcting any error,
i. e. when the cone attempts to deviate from the travel
process prescribed by the music, the electronic regulatory
circuit intervenes and keeps the cone exactly on course.
The new, powerful digital output stages have opened up
new possibilities for regulating drive units: even the
TCI 2 A - a loudspeaker of quite moderate dimensions - is
capable of reproducing bass down to 30 Hz with only a –
3 dB roll-off. Even below this figure the sound pressure
does not decline abruptly as with passive loudspeakers,
but tapers off in a very flat curve; even at 20 Hertz there is
virtually a full-level output. The regulatory system also
optimises the drive units' travel characteristics and peak
handling capability. The result is extremely well contoured,
clean and well defined bass reproduction devoid of reso-
nance effects and rumble, and without post-oscillation
phenomena. However, even at low volume TCI speakers
provide a convincingly homogeneous sound image.
The output stages
As these are active loudspeakers, the output stage is
located in the speaker cabinet itself. Each TCI active
loudspeaker is fitted with three switched power amplifiers
of discrete construction, newly developed by , which
are matched perfectly to the requirements of each of the
drive units, and the net result is simply superb sound.
Our power amplifiers exploit the PWM process (PWM =
Pulse Width Modulation), combined with a modulator of
analogue construction. The analogue design of our
modulator stages permits a continuous, i. e. not quan-
tised, adjustment of the pulse width. This system can be
viewed as a multi-level modulator with an infinite number
of levels. Since there is practically no limit to the width of
the signal, the modulator is capable of setting the output
voltage very finely and precisely. This has enabled us to
achieve high linearity and extremely low total harmonic
distortion values of less than 0.001 %.
Power amplifiers are only as good as their mains power
supplies, and for this reason we have equipped the TCI
Active loudspeakers with stable, extremely high-capacity
torroidal transformers with exceedingly low induced field
characteristics. The mains sections of a pair of our little
TCI 2 A speakers feature power reserves of more than
900 Watts; those of the TCI 1 A more than 1300 Watts.
The new switched amplifiers offer superb reproduction
characteristics, high power and wide dynamic range,
making them the ideal partners for our demanding TCI
Active concept.
Since TCI loudspeakers feature separate power ampli-
fiers for each segment of the frequency range, i. e. bass,
mid-range and treble, they offer enormous power
reserves which guarantee an extremely wide dynamic
range. A normal amplifier driving a passive loudspeaker
has to reproduce all parts of the frequency range, and
there is little headroom for signals in the mid-range and
treble when a high-level bass signal is present. In con-
trast, TCI speakers still provide their full reserves in the
mid-range and treble even when the bass power ampli-
fier is delivering several hundred Watts of power. The
result is that the TCI's full, uninterrupted dynamic range
is main-tained even at high levels. A brief example will
clarify what would be necessary in order to achieve the
same dynamic range from a conventional passive
system:
If high peak signals occur simultaneously in the bass,
mid-range and treble, the TCI 2 A, for example, is
capable of delivering 70 Volts peak voltage in each of
these ranges. A power amplifier feeding a passive
speaker would need to supply around 200 Volts of output
voltage in order to achieve the same performance. This
would equate to a peak output of around 5100 Watts into
4 Ohms! Slightly more would be needed to provide the
same dynamic reserves as the larger TCI 1 A. This
example is very impressive proof of the superiority of a
fully active loudspeaker concept with separate output
stages for each segment of the frequency range.
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