Pink Floyds Studio Monitors ! *SOLD*
Type: RH 544
Number of speakers: 3
Low unit: AD 8067/MF4
Mid unit: AD 0210/Sq8
High unit: AD 0160/T8
Number of amplifiers: 2
Output low: 40 W
Output mid/high: 20 W
Cross-over frequency: 500 Hz (active) + 4 kHz (passive)
Dimensions (h x w x d): 387 x 287 x 216 mm.
Motional feedback refers to the incorporation of an idea that speaker engineers sometimes discuss in after-hours, knockabout, ‘what if’ brainstorms, but which is only rarely implemented in practice: low–frequency error correction. The problem that such error correction strives to solve is that the control an amplifier exerts over the movement of a driver diaphragm at low frequencies, say, below 200Hz, is relatively imprecise. The diaphragm has inertia, so is both hard to get moving and hard to stop. And the diaphragm suspension has both hysteresis, meaning it never quite returns to its original resting position, and non-linearity, meaning it doesn’t exactly follow the input signal. As a result, a bass driver diaphragm is hardly ever located exactly where the amplifier, and input signal, says it ought to be, or indeed moving at the intended velocity, and the result is distortion. Much of the characteristic increase in speaker distortion at low frequencies is a result of this lack of amplifier control.
Andy Jackson: “We used them quite a bit in mixing sessions, the way that maybe now you’d use NS10s: get some sounds up on the big monitors then go to the little ones for balance and stay there.
Philips’ “motional feedback” technique for error correction involved the attachment of a small and lightweight piezo-electric accelerometer to the underside of the bass driver dust-cap. The output of the accelerometer was returned to the amplifier and compared to the input, and the resulting error-correction signal used to modulate the amplifier output. As with any feedback error correction system, some latency is inevitable, but there’s no doubt the technique can significantly reduce low-frequency distortion, while at the same time enabling optimization of a speaker’s diaphragm–movement–limited power handing (as a by-product, it will also fix the effects of thermal compression as the voice coil temperature and electrical resistance rises). The RH series, by many contemporary accounts, worked as intended. They had really great bass, especially considering their compact dimensions.
But, I suspect you’re now asking, what has Philips’ curious motional feedback active hi-fi speaker range got to do with professional audio and music recording? Well, what if I told you that a number of classic Pink Floyd albums, including The Wall, were mixed on a pair of Philips RH544s? Andy Jackson, David Gilmour’s long-time engineering collaborator, takes over the story:
“I didn’t work on The Wall project but we were still using the 544s after that on The Final Cut, The Division Bell and on Roger [Waters]’s Pros & Cons Of Hitchhiking album.