MANUFACTURER'S SPECIFICATIONS:
Frequency Response: 30 to 20,000 Hz.
System Type: Three way with one 8-in. woofer, one 5-in. midrange, and one 1 in. tweeter.
Crossover Networks: Active crossover at 500 Hz, passive crossover at 4 kHz.
Amplifiers: Two, self-contained; bass unit 40 watts, mid- and high-frequency unit 20 watts.
Power Consumption: 150 watts maximum at 117 V/60 Hz.
Size: 11 1/4 in. W x 15 in. H x 8 3/4 in. D.
Weight: 26 lbs.
Price: $365.00 each.
Big sound doesn't always require big boxes. Perhaps there is no better example of this than the new Motional Feedback System now marketed in this country and manufactured by Philips of Holland. With the unassuming title of RH532, this diminutive enclosure houses a three-way loudspeaker system, two power amplifiers with a total output of 60 watts continuous sine wave power, electronic and passive crossover networks, and a special electronic sensor to monitor and control the woofer excursion. The result is an innocent looking small box that can startle you with the volume of good sound it can produce. The first ads that appeared on this system refer to it as a David among Goliaths.
Jack the Giant Killer would perhaps have been as apt a title.
The enclosure itself measures 15 x 11 1/4 x 8 3/4 in. (378 x 283 x 212 millimeters if you choose to think metric). The top, bottom, and sides are finished in natural walnut, and a metal grille covers the entire front. When you pick it up to see what's on the back you realize that this is no ordinary enclosure, since it weighs a hefty 26 pounds. The rear of the cabinet does not have the familiar terminals for loudspeaker connection. Instead there are four phono connectors, two line-cord connectors, two push-button switches, one three-position slide switch, and three fuse connectors, all neatly labeled.
In addition to a very readable instruction manual, Philips supplies a 10-ft. a.c. cable, two two-channel 30-ft. audio signal cables with phono plugs at each end for connection to a pre-amplifier, and a 6-in. adaptor cable with phono sockets at one end and spade lugs at the other end for application to output terminals of a power amplifier if you wish to hook this unit onto an existing amplifier system.
The system is hooked up in the following manner. After placing the speakers where you want them to be for the best sound, the 30-ft., dual-wire, audio signal cable is plugged into the phono receptacles marked In L-R on the rear of one enclosure. The second such signal cable is then plugged into the receptacles marked Out L-R on this enclosure and run over to the In L-R of the second enclosure in the stereo setup. The first cable is then run over to your signal source, either a pre-amplifier or a power amplifier output. In this way, the signals from the audio source are routed through one system to the other.
A push-button selector switch adjacent to the signal input lines lets you determine whether the speaker is to respond to the left channel signal or the right channel signal.
Immediately below this is a three-position slide switch which allows you to control input sensitivity and source selection. The first position is labeled Pre-Amplifier 1 V, the second is labeled Med. Power Amplifier 7.5V, and the third is High Power Amplifier 19 V. If you are using a pre-amplifier source, then the first position is to be selected, with the other two positions reserved for those situations where you may be connecting to a power amplifier output.
Because this motional feedback system has self-contained amplifiers, the a.c. line cord is plugged into the receptacle provided. If you only have one utility outlet available near the speakers, Philips has provided an unswitched outlet that is in parallel with the input receptacle. The line cord from one speaker can then be plugged into the outlet on the second speaker, and the line cord from the second speaker run to the utility receptacle.
A red push-button on-off switch then lets you turn on the system. It here another example of Philips ingenuity crops up. When you push the power switch "on" with no audio signal applied, nothing happens. Instead of applying power to the self-contained amplifiers, a signal-sensing circuit is activated. After that any time you want to turn on the Philips loudspeaker system all that is required is to give it an audio signal. Snap, on goes the amplifier. What appears to be an innocuous piece of plastic on the grille now reveals itself as a pilot light which illuminates a Philips logo and lets you know that power is on. The power will stay on for two minutes or longer after the audio signal disappears. The signal level required to activate this turn on for the Pre-amp position was measured at 1.6 millivolts, which is high enough to be well above any hum level in the signal line and low enough to catch any signal that is audible.
In these days of concern for energy consumption, ecology-minded folks might feel uneasy about the use of this automatic turn-on feature as it is always drawing current. A clamp-on current probe revealed that the sustaining current of this signal-activation circuit is about 150 mA at 117 V, which amounts to 17.5 watts of continuous power consumption per speaker or slightly over five times that of a normal electric clock. Certainly the circuitry is to be admired as an example of Philips ingenuity, and if one is concerned about this amount of current draw, the system can be powered through a switched outlet on the preamplifier.
The line current drawn by this system is low enough to be handled by any switched outlet capable of handling 300 watts.
The single most innovative feature about this system is the circuitry which gives rise to the name "Motional Feedback System." A piezo-electric element is incorporated in the center of the woofer cone. This miniature accelerometer is used to compare what the cone is actually doing relative to what the input signal wants it to do. The signal from this comparator is then applied to the self-contained bass amplifier to correct any differences. According to Philips, this not only results in lower distortion for the sound out of the woofer but an extremely smooth low frequency response which in a conventional system would not be possible from even much larger enclosures.
Fig. 1--One-meter anechoic sound pressure level.
While the concept of feedback from the speaker cone is well established in the audio art, Philips has developed a compact commercial version that appears to do everything claimed for it. The 8-in. woofer, mounted in a very small sealed volume, puts out a startling amount of bass for its size; the sealed volume plus the complete electronics are housed in the small enclosure! The internal electronics consists of a 500-Hz electronic crossover ahead of a 40-watt bass amplifier and a 20-watt treble amplifier. A more conventional 4-kHz passive crossover is used to separate the 1-in. tweeter and the 5-in. midrange.
Philips provides a warranty that the system is free of defects in material and workmanship for a period of one year from date of purchase. To utilize this warranty the complete unit should be returned postage prepaid and insured to North American Philips Corp. or authorized service station for examination. All parts which prove to be defective under normal conditions of use will be replaced under the warranty at manufacturer's option.
Technical Measurements
Because the Philips Motional Feedback system uses a self-contained amplifier, the input impedance is independent of frequency and is determined by the input mode selector switch on the rear of the speaker. The most sensitive input has an impedance of 3K ohms. The medium-power amplifier input, for connection to moderate power amplifiers up to 45 watts per channel, measures 27 ohms. The high-power amplifier input, for connection to sources of 45 watts or higher measures 25 ohms.
The one-meter anechoic amplitude frequency response is shown in Fig. 1. Three measurements are shown, corresponding to on-axis, a 30-degree left-channel-stereo location, and a 30-degree right-channel-stereo location. The on axis measurement has a slightly stronger high-frequency response than either of the off-axis measurements. The spectral balance for all three measurements is exceedingly good. The low bass remains good down to about 55 Hz, then rolls off at around 18 dB per octave below that frequency.
There is still a respectable output even at low C or 32 Hz, which completely belies the diminutive size of this speaker system. It has a lot of bass for such a small system. The general trend in response is slightly increasing at a rate of about 1 dB per octave from 60 Hz to 18 kHz. The only indication of resonance peaks are minor ones at 5, 10, and 15 kHz in the on-axis anechoic response.
Special attention should be paid to the very smooth high frequency response of the left channel measurement. In the RH532 the tweeter is to the left of the midrange, as seen by the listener. The acoustic position of the tweeter, by measurement, is approximately 1.3 in. in front of the midrange driver. When this speaker is used in a left-channel position, the extra time delay of the sound from the outboard tweeter is just enough to cancel its forward acoustic position-giving an extremely smooth transition in the frequency response. The measurement indicates therefore that there is a slight preference for left-channel smoothness when the system is mounted in the normal or erect position with the illuminated Philips logo on top. Slightly better overall sound will be obtained if it is possible to mount the right channel speaker upside down. Because this system looks the same inverted as it does erect, except for the illuminated logo, this may be feasible in many living room situations.
The on-axis anechoic phase response is shown in Fig. 2.
The woofer and midrange are in phase, that is, a positive going voltage applied to the input will produce an in-phase positive-going sound pressure for the direct speaker sound.
The tweeter has a 180-degree leading phase for the same input signal. The indicated acoustic crossovers are at slightly above 600 Hz and about 7 kHz. Two measurements are with the same result. I drew only a withering smile from the factory representative at such an unthinkable thought. And a quick and very deferential reprisal. If, of course, you had brought the machine to us in the first place ... but as it is ... Correct, entirely correct! And my private thought is that if they had made the original mistake I might even have achieved a free repair. They, too, have pride in their own workmanship. But now I will never know.
I will have to admit that the first motor, failing in the slow speed as did the second one, nevertheless took a whole year of gradual deterioration in its torque before its slow speed quit, leaving the fast speed OK. Whereas the second motor's slow speed apparently failed in seconds, leaving its high speed intact. And the grinding noise was new. The first motor simply got quietly tired and wouldn't start unless I first went to the high speed, then back to slow. No grinding. Mysteries.
Anyhow, now all is well and I've been working again with the machine, at last, and with both speeds. It's pleasant! About time.
There are moments like this when you feel that RESULTS, positive and definite, are worth any price at all. I think I'd pay six times as much instead of three, just to be able to get back to work this way. I feel GOOD.
P.S. Something has just been bothering me. Do I notice a very faint grinding sound in the new motor when it runs at the slow speed-which goes away at the fast [… more of this review, coming soon]
(Source: Audio magazine, Apr. 1975)
Also see: Philips Model AH-572 Preamplifier (Nov. 1977)
= = = =