Talk of powered speakers and the chances are you will think instantly of Bang and Olufsen or Meridian, or perhaps PMC and ATC. I was pleased to get hold of the Meridian’s M2 in the 1990’s, though had seen them in KJ LeisureCentre in Watford in the late 70’s and craved for them then. But the idea of powered speakers in the hifi industry goes further back than that.  Up until the mid-70’s record players with built in amplifiers and speakers were all the craze, though I never really got into Bush, Murphy, HMV, ITT, Ferguson and all the other mono record players that were up for sale in Tesco, Timothy Whites, Co-op, etc. The sad thing is that they seem to be making a comeback with HMV and others selling Crosley retro record players.

The idea of separate loudspeakers that were powered first came to light for me with the Philips Motional Feedback series back in the mid 70’s. This was not just a powered loudspeaker system, but one that could listen in and adjust the sound so that distortion was reduced.  Motional feedback in a loudspeaker means using a method of measuring the motion of loudspeaker diaphragm and using this observation to alter the speaker’s driving voltage and/or current so that the diaphragm now moves at its best.  This system works much the same as the computer system in your car, which maximises the potential of the engine and adjusts to your driving prowess.

The Philips idea was brilliant, though not the first system designed to get the best from your audio. The first attempts at controlling and optimising the movement of the loudspeaker cone came from a certain G.H. Brodie, back in 1958. His patent featured an assembly on the chassis, plus a contact on the cone, which formed a parallel plate capacitor, itself controlling the movement of the cone. A company called Servo Speaker continued this idea showing in their measurements that THD would drop below one fourth (12dB) of the THD of the same speaker driver without motional feedback. This would typically equate to a drop of around 50% (6dB) at 20 Hz.   A cylindrical capacitor was placed in the space inside the voice coil and above the pole piece of the magnet system. As the capacitance of this cylinder changes linearly according to the deflection of the cone, it sent messages to a control circuit to compare against what the amplifier was sending. The idea of controlling distortion at low frequencies has been adopted by a number of subwoofer manufacturers. Servo Speaker doesn’t exist now but there is a website showing you how to “modify” your loudspeaker to create your own capacitor monitoring system, should you feel inclined to mutilate your woofers. Paradigm Servo 15, for example, has an onboard comparator which controls the 400-watt RMS amplifier and compares the output of the subwoofer to the input signal and makes instantaneous corrections in order to get the best sound.

The Philips approach was to use a piezo-electric accelerometer to provide information about the movement, speed and direction of the bass cone to the amplifier, in order for it to be controlled and therefore reducing distortion. The ceramic sensing device on the cone ‘vibrates’ and as it does so an electrical signal is generated. This is then fed back and compared with the signal that made the woofer vibrate in the first place. If there is any difference then corrective action takes place in the amplifier. Close up view of the woofer shows two sets of lead-out wires connected to the cone, rather than the customary single pair. There were five models in the range; the smallest being the RH541 with 7” woofer and 1”dome tweeter, and increasing in size and cost through the RH544, AH567, RH532 and finally the RH545, the most talked about and biggest of the range. The RH544 and RH545 had complex switches and knobs on the front hidden under a plastic cover, where you could alter whether the speaker was against a side or rear wall, and whether on the floor, as well as knobs for cut off frequencies and sensitivity. Thus, highly complex alterations of sound architecture were possible. Their 3-way RH532 was advertised as a bookshelf speaker with true bass response, complete with a 20W and 40W amplifier for tweeter and woofer respectively. Using the system effectively extended the bass frequency, quite appreciably on the smaller RH541. This 22.9 x 29.4 x 17.3cm speaker could get down to 35Hz complete with a 30W amplifier. Speakers have a resonant frequency, particularly in infinite baffle motional feedback designs, caused by pressure forced against the cone as it moves. This, plus any distortion induced by the enclosure or the cone itself can be immediately corrected by the feedback system. To a small degree, the sensor-feedback system also compensated for non-optimal room acoustics, something stressed in their advertisements.  To do the “listening in” it is vital that the ceramic material is suspended in rubber blocks for freedom of movement. In addition, it must not sit in the flow of the voice coil of the speaker to avoid false readings. This was achieved simply by mounting the component in an airtight saucer.

Philips technology was brilliant and a shame that it wasn’t developed for more than a decade before being forgotten, like much of iconic Philips technology; the cassette, V2000, DCC, and currently their share in the design of the red-book CD. The Motional Feedback System unfortunately never really caught on in a big way. The Philips technology was expensive at the time, meaning price-wise it competed with high quality conventional loudspeaker and amplifier separates that actually sounded better. What the Philips technology couldn’t do was change overall tonal balance or change cabinet colouration.  I am pleased, however, that all those hours and much Dutch guilder were not used in vein, with the philosophy, if not the technology, being used by other companies for products such as subwoofers where it can be put to good use in monitoring the movement of a sub-bass drive unit to prevent damage at high excursions and volume levels. However, as loudspeaker drivers and cabinets improve reducing distortion and resonance, the need for a monitoring monitor becomes redundant.

Janine Elliot

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