Philips 545 MFB Studio

 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