Why not closed loop speaker amplification?

It has been done - some years ago Philips used to sell loudspeakers that worked on this principle. They weren't all that good, probably because the position of the diaphragm is measured at one point, and the diaphragm, not being infinitely rigid, distorts under load.

IIRR Philips used an accelerometer mounted on the drive coil as their "position" sensor.

------- Bill Sloman, Nijmegen

It has been done - some years ago Philips used to sell loudspeakers that worked on this principle. They weren't all that good, probably because the position of the diaphragm is measured at one point, and the diaphragm, not being infinitely rigid, distorts under load.

IIRR Philips used an accelerometer mounted on the drive coil as their "position" sensor.

------- Bill Sloman, Nijmegen

Or 'emotional feedback' ;-)

What low frequency corner can be achieved this way?

...Jim Thompson

I think this is done on shake tables, which can have fair bandwidth. Whether it would play Bach is another question:)

Bose uses linear actuators with feedback on all 4 wheels in a new suspension system they are developing. (Sorry I have no urls handy).

So it should work. Let us know when you make your first million:)

Mike Monett

Interesting. Thanks for the link.

Good day!

And it is currently being done on many 'servo' subwoofers.

Kal

I did a bunch of experiments back in the '70s. Had some basic problems.

Turns out you can go a long way by tailoring the output impedance of the amp to the loaded impedance of the speaker. That meant a negative amplifier output impedance.

Experiments with better coupling between the speaker and air resulted in purchasing a pair of Klipsch Corner Horns. At that point, getting more bass seemed a moot point ;-) mike

Bill,

You triggered an old memory of a feedback winding adjacent to the main coil. While looking for it, I came across the following:

---------------------------------------------------------------------- Voigt, Not Black

Editor:

I was surprised to find Martin Colloms---and John Atkinson in his footnote---perpetuating the notion that Harold Black "invented" feedback (January '98, p.87). As I recently pointed out elsewhere (Hi-Fi World, April 1997), that honor belongs to the British engineer Paul Voigt, a pioneer of high fidelity, who had incorporated it in his Motional Feedback Patent 231,972, dated January 29, 1924. Over 70 years later, motional feedback as applied to loudspeakers has made little further progress, and Voigt's use therein of selective negative feedback, which he regarded as obvious and not worthy of special attention, is the first reliably documented use of which I am aware---many years before the Black disclosure. I enclose a copy of Voigt's circuit in the patent, and a description of its operation in his own words that shows that he fully understood its application. The latter is taken from papers in my possession, typed and annotated by him in August 1970, and reads as follows:

(read the rest at

So it appears negative feedback occurred earlier than textbooks state, and involved our favorite topic - hifi. This should have generated some choice phrases for John's collection.

Mike Monett

AFAIK it is currently being done with subwoofers.

Trying this with actual speakers would be very difficult because of resonances and other transfer function effects in the speakers. If you were shooting for a doctorate you could consider an adaptive system that predistorts the drive going to the speaker to account for spectral and nonlinear distortion, perhaps with microphone feedback. I suspect that such a system would never be cheap enough for any but the highest-end audio, and may never be better than the Philips system mentioned above.

Interesting. What sort of bass drive amplifiers were they using? Compared to traditional systems, their 'open loop' gain must have been pretty high.

The idea of negative feedback can be traced back to the ancient Greeks. Hero of Alexandria used a negative feedback system for opening and closing temple doors.

Fred.

Yes, horns are the ultimate if one has the space.

As low as you want to, but eventually you will run out of diaphragm travel and power handling, so you can only go low at the cost of loudness.

I have an old article with a full design somewhere I'll see if I can find it, as far as remember the whole point was to move the low frequency corner down, achieve a "nice" Q and reduce distortion

-Lasse

Only two weeks ago, I came upon a tag sale with lots of old audio stuff including some 3panel Magnepans, Hafler amps, a Tandberg cassette deck, many(!) tapes and mikes and, among the myriad of speaker boxes, a set of Philips powered MFB speakers. Unfortunately, all the stuff looked dusty and grimy as if cats had been pissing on them for years. Too bad.

Kal

You want horns?

Richard Burwen did (maybe still does?) design Copley's switchmode power amps, used for servos and shakers and MRI gradients and stuff. He told me that for every new amp design he fills a bucket with blown fets. He did a chapter in Jim Williams' first book.

John

It occurred way before then. To wit, steam engines had a governor to regulate speed.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

Hmmm... Infinity ServoStatik (spelling? in the 70's) used electrostatic panels and a servo subwoofer.

generally referred to as 'motional feedback'.

I am sure there was an Elektor Circuit many many years ago that used a negative feedback system.