Less than a
thousandth of the thickness of this sheet of paper…
Have you
ever pausd to consider what a wonderful instrument our ear is? For example, at
a party we have no trouble in hearing people talking a yard away. Nothing very
special in that, you may say, … and yet it means that our ear is capable of
detecting sound waves with an amplitude of no more dan 1/10000 mm, or 1/1000 th
of the thickness of this sheet of paper. And what is more, we are able to
distinguish tiny details of these sound waves. The range in which the human ear
ezperiences vibrations as audible sounds runs roughly from 20 to 16,000 Hz. In
the frequency range around 1000 Hz, the limit of detectability of differences
in sound pressure is about 3 dB.
The Philips
545 MFB Studio loudspeaker has a transmission range extending from 20 to 20,000
Hz. Between 45 and 17,000 Hz, fluctuations in its sound pressure are kept
within the narrow tolerances of ±1,5 dB, as demonstrated by
"free-field" measurements.
A high
quality lousdspeaker must possess extreme purity of tone. The extent to which we experience tones which
are not present in the original sound (because they are first produced during
the transformation of electrical virations into acoustic ones) as impurity of
tone, or distortion, depends on the playback volume and the complexity of the
original sound. Even when only two notes are played together, combination tones
and diffecrence tones will be produced in addition to the harmonics of the
original notes. We will experience these as annoying if they are too loud and
if they don't harmonize with the original tones.
The discord
will be greater when chord in question is not played quite cleanly - and this
is almost inevitable in music-making, since no musical instrument is perfectly
pitched. The audibility limits of such discord are lowest in the sound-pressure
range from 50 to 70 dB. Only in this range,above about 300 Hz, and only under
extremely "favourable" conditions is non-linear quadratic distortion
(K₂) audible down to 1%, and non-linear cubic distortion (K₃) down to 0,5%.
For the real
music lover, there is nothing finer than following the individual instruments
in an ensamble. Our ability to do this depends on the ear's astonishing
capacity for hearing very quick changes in sounds. In the range from 50 to
10,000 Hz, the ear can distinguish the slightest differences in the build-up of
a no:e, even when the time involved is no more than 0,25 ms. The build-up time
of a note produced by a musical instrument is much longer than this.
For complete
characterization of the high-speed response of a loudspeaker (its "pulse
response"), we need to know how its sound pressure and acoustic phase vary
with time. The more linear the amplitude and phase response, the more exactly
will pulsed signals be reproduced. This is especially important in contemporary
music, where pulsed tones are relatively common.
So far, we
have been concentrating on the main characteristics of your own personal
acoustic "receiver" - your ear. We will now tell you something about
the corresponding quality characteristics of an acoustic "transmiter"
- the Philips 545 MFB Studio loudspeaker - which matches your ear very well. We
felt obliged to present you with both sets of data, because technical data
without reference to the reality of hearing satisfy us no more than they do
you.
The team
responsible for the development of the 545 MFB Studio was given the task of
building a loudspeaker whose tone reproduction was realistic enough to meet all
requirements made by professional studio workers - a real
"specialist" among specialists.
The
"specialist" had to have the following interesting
"qualifications", among others:
A small
housing, which still permits faultless reproduction of low notes right down to
20 Hz.
Playback
volume characteristics which meet all studio requirements, without audible
distortion.
Simple
adjustment of the linear acoustic response characteristic to meet the
requirements of the playback room ("positional matching")
Special
filters for matching the loudspeaker to the acoustics of the room in the high-
and low-frequency ranges.
These
stringent specifications could never have been met without the help of the
exhaustive work carried out in Philips Research Laboratories.
Some of the
concrete results of philips research in the 545 MFB Studio loudspeaker are as
follows:
A 70-litre
box into which are built three special loudspeakers together with trimming and
control circuitry, and three power amplifiers with outputs of 50, 35 and 15
watt RMS, sine-wave.
The 50 watt
amplifier drivers the woofer in accordance with the input signal it receives.
The woofer carries at the centre of its cone an acceleration transducer in the
form of a piezoelectric element (PXE for short). This produces a voltage
proportional to the acceleration of the cone, which is compared with the input
signal of the amplifier in a comparator stage. A correction signal produced by
the comparator as the result of the difference between the original signal and
the PXE-signal, is fed back by the comparator, with reversal of phase, to the
amplifier, thus making it possible for the motional errors of the woofer cone,
which would be inevitable in the absence of MFB (motional feedback), to be
eliminated at the source. The acoustic signal emitted by the woofer is hence a
faithful replica of the original driving signal.
The 35 watt
amplifier drivers the medium-frequency dome loudspeaker (squawker), while the
15-watt amplifier drives the tweeter (also a dome loudspeaker). The over-all
transmission range is divided into three frequency bands (at 500 and 3000 Hz)
by precision cross-over filters situated before the power amplifiers.
Since it is
important for Hi-Fi reproduction that the amplitude response should be correct,
even for the big dynamic peaks which occur relatively often in big orchestral
performances, the 545 MFB Studio can deliver a total sound-pressure level (over
its entire 7,5 -octave range) of 108 dB at a distance of 1 metre from the box.
Since the
sound radiation at low frequencies is highly dependent on the positioning of
the loudspeaker, three high-precision electronic filters (with gyrators) are
provided for accurate correction of the sound pressure (positional matching).
This eliminates the potential sources of sound distortion otherwise present
when the box is positioned against a rear wall, a side wall, or on the floor,
and making the sound-pressure characteristic independent of the positioning of
the loudspeaker. What that means to you in practical terms is that the low
tones are reproduced without the slightest distortion.
An active
low-pass filter with an adjustment range of ±10 dB and a passive high-pass
filter with a choice of cut-off frequencies (7 or 10 kHz) and an adjustable
damping slope (max.20 dB/octve) serve for matching the box to the acoustics of
the room.
For example,
the effect of anunwelcome rise in reverberation in the low-frequency range
can'be cut out with the aid of the low-pass filter.
A high-pass
filter with a cut-off frequency of 35 Hz protects against distortion from
infrafrequencies below 20 Hz.
Thanks to
the well thought out design, the boxes are easy to operate despite the
relatively large amount of circuitry thay house. For example, only 6 switches
are needed to adjust the box to the acoustic of the room - 3 for positional
matching and 3 for reverberation matching. The 3 remaining controls on the neat
little control panel are for facilitating the connection of the box. The first
of these is for selecting the nature of the input: symmetrical (studio norm) or
asymetrical (DIN home-studio norm 45 500). With the asymmetrical input, there
is no need to switch the box on and off manually. A signal-controlled relay
switches automatically from "ready" to "on", and back to
"ready" again if no signal is received for more than 2 minutes. The
second control adjust the input sensitivity to the desired level in 11 steps,
while the third and last adjusts the box for connection to the right-hand or
left-hand stereo channel.
The 545 MFB
Studio can thus be connected to several different types of signal source:
Amplifier
without power output stage, with symmetrical 1 V output;
Amplifier
without power output stage, with asymmetrical 1 V output;
HiFi
amplifier with power output stage, up to 100 W.
If the power
output available is inadequate, it is possible to connect several boxes to form
a group. For this purpose, each input connector has a corresponding output
connector in parallel with it.
This little
print card with the PXE acceleration transducer and field-effect transistor is
the heart of each MFB loudspeaker. It is mounted at the centre of the
loudspeaker cone. This element is subjected to high loads, as it vibrates with
the cone and undergoes the full changes in acceleration affecting the latter.
Careful design with over-dimensioned components and stringent life testing
ensure reliable operation for the loudspeaker as a whole.
The three
loudspeakers are protected against overload by means of a safety circuit which
interrupts the channels in question for the duration of the overload (e.g. an
overvoltage).
In the
interests of reliable operation, all components are overdimensioned, and all
transistors used are mounted in metal housings. The circuits are built up on
reinforced epoxy print cards, with fully metalized mounting holes.
Specifications
Transmission
Range: 20 - 20,000 Hz
Sound
Pressure level (over full 7 ½
-octave range): 108 dB at distance of 1
m
Volume: 70 litres (acoustic section 50 l, electronics
20 l)
Loudspeaker
Systems:
Tweeter
: 1" dome loudspeaker AD 0162/T8
Squawker
: 2" dome loudspeaker AD 0210/Sq 4
Woofer
: 12" low-tone loudspeaker AD 121
00/W4
Crossover
Frequencies: 500 Hz and 3 kHz (active
filter)
Amplifiers:
Low-frequency
Channel:
Output Power
: 50 W sine-wave (at 1000 Hz , 40 W;
k‹0,1%)
Bandwidth
: 5 Hz - 5 kHz
Medium-frequency
Channel:
Output Power
: 35 W sine-wave (at 1000 Hz , 25 W;
k‹0,1%)
Bandwidth
: 40 Hz - 30 kHz
High-frequency
Channel
Output Power
: 15 W sine-wave (at 1000 Hz , 10 W;
k‹0,1%)
Bandwidth
: 40 Hz - 50 kHz
Sound-pressure
Correction Filters
Positional
Matching:
200 Hz
: -5 dB
60 Hz : -5 dB
55-160
: -3 dB
Bass Tone
Control : Down from 350 Hz ±10 dB at 60
Hz in 11 steps
Treble Tone
Control : Continuously variable from 7
or 10 kHz, 0 - 20 dB/oct. (a pilot light shows when this control is switched
on)
Electronic
Automatic ON/OFF Switching (only with DIN asymmetrical input; can be switched
off):
Response
Time : below than 1 second for input
signals above about 1,5 mV
Decay Time :
more than 2 minutes (a pilot light shows when this function is switched on)
Input
Sensibility:
Continuously
Adjustable: 1 - 23 V
Symmetrical
: 1 V into 10 k Ω
Asymmetrical
: 1 V into 100 k Ω
Connections:
Cannon
: Studio Input connector symmetrical
DIN : Input connector, 5 pole asymmetrical
Main Input
terminal, DIN/IEC
Mains Output
Terminal, DIN/IEC
Semiconductors: 85 transistors; 39 diodes
Mains Power
Requirements: 220 V, 50/60 Hz (if
desired, can be adjusted by 110, 127 or 240 V)
Power
Consumption: 200 W max. consumption. A
pilot light shows when the box is connected to the mains
Housing: Wooden cabinet (black ash), textile front
panel (removable)
Dimensions
(W x H x D): 436 x 650 x 320 mm
Weight: 31 kg
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