Philips RH-532 (1975)
A 26-pound bookshelf box with two amplifiers, a motional-feedback servo, and more bass than its size suggests.
Overview
Twenty-six pounds. That's the number you notice when you lift one. The Philips RH-532 measures just 11¼ inches wide, 15 inches tall, and 8¾ inches deep, and it weighs as much as some small receivers. The reason is simple: this isn't a passive loudspeaker. It's a complete powered system, with two self-contained amplifiers (40 watts for the bass, 20 watts for the mid and high section, 60 watts total continuous sine wave) and the crossover networks built into the cabinet. You don't hook it to an amp. You plug it into the wall.
The technology inside is what Philips called MFB - motional feedback. A piezo-electric transducer sits at the center of the woofer diaphragm and generates a voltage proportional to cone movement. That voltage gets compared against the incoming signal in a comparator circuit, and the 40-watt bass amplifier corrects the woofer's excursion in real time, so the cone follows the input instead of approximating it. Audio (August 1975, p.44) traced the scheme back to the work of Klaasen and de Koning, developed roughly seven years earlier in Holland. So the RH-532 was the commercial arrival of an idea Philips had been refining in the lab.
The crossover setup is just as deliberate. The split between bass and midrange is an active electronic crossover at 500 Hz. The division between the 5-inch midrange and the 1-inch tweeter is a conventional passive network at 4 kHz. So the hard part, the low-frequency handoff where phase and power matter most, happens before amplification, and only the easy top end is divided passively. The system is a three-way: one 8-inch woofer, one 5-inch midrange, one 1-inch tweeter, rated from 30 to 20,000 Hz.
Power consumption tops out at 150 watts at 117 V, 60 Hz, and the on/off behavior is one of the more charming details of the design. A control circuit responds to any audio input above about 1 millivolt and switches the amplifiers on; the low-level stages stay energized so there's no turn-on thump. A light on each speaker glows red when it's live, and roughly three minutes after the signal disappears the system shuts itself down with a barely audible click. Owners who remember these describe walking away from the turntable and watching the red lights fade on their own — a small piece of automation that felt futuristic in 1975 and still feels thoughtful now.
Placement is fussier than the size suggests. The tweeter's acoustic position sits approximately 1.3 inches forward of the midrange driver, which gives an unusually smooth high-frequency transition in the left channel when the speaker is upright with the illuminated Philips logo on top. The magazine noted that slightly better overall sound results from mounting the right-channel speaker upside down. The cabinet looks identical inverted except for the logo, so this is practical in most rooms.
Frequency response is quoted at 30 to 20,000 Hz, which is a lot for a box this size and the whole point of the motional-feedback approach. The RH-532 can startle with the volume of good sound it produces, and Philips' own advertising billed it as a David among Goliaths. That's marketing copy, but the servo loop does more than extend bass. It reduces distortion by correcting the cone's actual motion rather than trusting the driver to behave.
Not everyone got a clean first impression. A Forecast FM column from the period described a CES demonstration where the program material was an amplified country-and-western group recorded with close-miked instrument speakers. That made it nearly impossible to judge what the speaker would do with a natural low source like an organ pipe or a well-hit bass drum. The same writer noted some breakup on loud highs that was probably the mic or preamp overloading rather than the speaker, and came away wanting a proper audition on his own music.
Construction drew praise as well. The instruction book was judged sufficiently detailed for most users, a schematic was supplied with sections and functions clearly identified, and the general build quality was called good. A considerable portion of the circuitry is accessible by simply removing the bottom cover. That's a big deal on a powered speaker with a servo loop and two amplifiers inside, which would be a miserable repair job behind glued panels. The RH-532 doesn't have them.
At $365 each when new, the RH-532 was never the cheap option. Buyers were paying for two amplifiers, an active crossover, and a feedback system in a package that fit on a shelf. Survivors are uncommon, and working examples get attention from collectors who care about the small-box-with-big-bass problem. Philips attacked it with electronics rather than cabinet volume.
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