Philips Motional Feedback Speakers


For a few years now I've been using a pair of fairly old Philips active speakers as part of my DJ setup at home. I didn't think they were anything special: indeed, the only reason I've got them is that someone gave them to my family because one of them didn't work properly and they knew we were prepared to take in electronic junk. One day I did a Google search for "philips motional feedback" just out of idle curiosity, and was surprised and delighted to find a wealth of information about these unusual speakers.

I won't attempt to repeat all the other information that's out there, but for the curious, here's a quick description of the speakers and why they're unusual. They were built by Philips in the late 1970s and early 1980s as active speakers, with power amplifiers built in. This sort of design wasn't common at the time, although these days every pair of cheap PC speakers is active. However, these particular ones have an extra ingredient: motional feedback. This means that the power amplifier for the bass unit actually senses the physical position of the cone and attempts to correct for any distortion in its motion. In theory this gives the speakers a flatter and deeper bass response than would normally be possible for the size of cabinet they're in.

For more information I recommend you read Thomas Baur's very comprehensive website at

Fans of these speakers will be delighted to know that this particular pair are in daily use doing just what they were designed to do.

My Speakers

The particular pair I've got are the model 545, which it turns out are the biggest of the range and quite rare: apparently only 1000 pairs were made. Here are a few photos of them, taken with a rather poor-quality webcam, for which I apologise.

First of all, the front view. Each speaker is about 650mm high, 440mm wide, and 360mm deep. The lower fabric grille covers the woofer and tweeter, and the upper one covers the midrange driver. The controls at the top left are for input switching and adjusting the frequency response of the speaker.

This is what one looks like with the grilles removed. Quite mean, with that 12" woofer staring you in the face.

Here's the tweeter in closeup. It's a hard-dome unit, rather dated by today's standards.

Here's the squawker (midrange driver). It's an unusual dome design, although I notice it's used on some of the other models in the range.

And that thumping great woofer, with its large rolled rubber surround. Notice especially the two sets of leadout wires in the cone: one for driving it, and one for the motional feedback.

Here's a closeup of the controls. The switches down the left hand side are marked "side to wall", "rear to wall", and "standing on floor", and adjust the bass response for different room positions. The other controls, from the top left, are:

That's quite a lot just for one speaker. This panel should be covered by a fold-down flap with the frequency response curves for the various filters printed on the inside in traditional Philips style, but the flaps are temporarily mislaid. They are held on by a feeble magnetic catch at the top and fragile plastic clips at the bottom, and have been stashed away in a box somewhere for safekeeping. I just don't know which box.

Here's a rear view. This is just the top quarter or so of the cabinet. Notice the mains inlet (an unusual 2-pin connector), power switch and fuses on the left hand side, the generous heatsinks in the middle, and the audio inputs on the right. There's a female XLR connector at the top for the balanced input and a pair of 5-pin DIN sockets for the daisy-chainable unbalanced input. I've seen another photo of a 545 which had phono sockets instead of DIN ones.

And here's an oddity. The pair of speakers isn't actually a pair. Their grille cloths are slightly different shades of 1970s beige, but more obviously, they have different labels in the top right hand corner. One says "MFB-STUDIO":

and the other says "ELECTRONIC":

Why did they change it? Who knows.

The Sound

This is the fun bit. How do they sound?

Well, in a word, they rock. They have a deep, pounding bass, and a clear, bright mid and top without ever getting shrill. From a purist audiophile point of view they have a lot of faults: I would never claim that they sounded particularly "natural". Classical music definitely sounds a bit odd through them, and they're unlikely to win any prizes for their stereo image.

The sound is best described as being rather like a really good club sound system: imagine a large stack of Turbosound or JBL cabinets, but in miniature. Which means that for my application, one either side of a DJ mixer, they're just perfect. And they'll play very, very loud: with 100 real, RMS watts available per channel, they can keep the neighbours awake all night if necessary. Of course I'd never dream of doing such a thing, but in case I should forget, they have a built in protection circuit which cuts off the power for a few seconds if you push them too hard. Apparently ;-)

One of them is actually faulty: when I got it it switched itself on and off constantly. I discovered almost by accident that disconnecting the motional feedback connection from the woofer cured this, and it's worked that way ever since. The bass response is noticeably poorer than the working one, and I have to turn the faulty one's bass control down about 6dB to hide its rather wooly sound. I never investigated any further due to the rather inscrutable discrete transistor circuit design which is impenetrable without a circuit diagram. With a copy of the circuit diagram I'll have another go one day.

Chris Jones, 18 January 2003