Servo Controlled Speakers

I'm sure most of us have heard of servo controlled subwoofers. These are basically subwoofers where the driver has a motion detection device attached, or a second voice coil which generates a current proportional to it's motion in the motors magnetic field. That signal is then fed back to the amp and any differences between the input signal and the motion of the woofer are cancelled out.

My question is this - why don't we see servo controlled mid-bass or midrange cone drivers? It seems perfectly possible in theory since the devices which feedback the drivers motion are capable of these frequencies, and the feedback itself happens almost instantaneously. So why not?

 

Philips, 70s, and again in the 90s.

http://www.stumpie.com/tech/mfb/

Sadly most information will be in dutch.


AFAIK keeping the closed-loop system stable was always a problem. I remember other manufacturers' attempts at MFB often ending in total destruction of the driver.







And in Germany Backes&Muller always had motional feedback from sense coils.


http://www.backesmueller.de/#/technik_text?sprache=en

 

Google Backes & Müller history.

 

I had a pair of the smaller Phillips monitors in the 90's - rescued from a theatre sound box where I worked for a while. They were quite good but not startlingly so - just a nice pair of active monitors.

DPA used speaker feedback (sensed via specially designed speaker cables - XLRs used) in their 50S and 100S power amplifiers to excellent effect.

 

Hi Neil,

Trio (kenwood) did a similar thing back around 1980 with their upmarket Sigma Drive series of amplifies. Effectively used sense wires to place the speaker cables inside the feedback loop.

Nice amps. I bought the top model with large external PSU a few years ago to try and while it sounded fine it went faulty soon after. Very complex thing and i just couldn't face removing hundreds of (different sized) screws and removing the plethora of pcbs to investigate.

 

Ah, Rob, if the PSU was for the power amp then it would have been the "all but impossible to find" 500S - there were only prototypes made if I recall and the design was prone to becoming unstable (so Tom Whelan -I think- informed me) but the 100S and 50S designs were very good.

 

Neil, sorry I meant the Trio had the external PSU.

 

Ooops, my misunderstanding -I'd have liked to have heard the Trio though!

 

This was the beast:

 

I recall tripping over that Rob. It would be good to hear it one day.

I can confirm the Deltecs get better with psu mods - but the kelvin-sensing feedback design doesn't really make a 'servo'd ' system.