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steph_tsf 27th April 2010 12:57 PM

Servo-Sound made in Belgium
2 Attachment(s)

I just found two Servo-Sound loudspeakers. I just dismantled them. See the pictures. Can somebody provide the schematics ? It reads "type 15B" on the PCB.

This is a interesting device because the little black box on the PCB is sampling the voltage potential that's developping on a small resistor inserted in series with the loudspeaker, and from what I can see, this black box generates the feedback to the amplifier, in the bass range.

Obviously, the aim of such feedback is to extend the bass response, ironing out the typical closed box woofer resonance. Kind of motional voltage synthesis, so the whole amp gets motional-voltage feedbacked, instead of voltage feedbacked. In the bass range. That was the big sales argument claimed by Servo-Sound.

From my experience, if you try a bass extension like this, using equalization in a closed box, due to the limited power and limited excursion of the woofer it is necessary to reduce the bass extension at high volumes. That is maybe the role of the bulb, maybe used as variable high-pass filter at the input, with the cold wire resistance decreasing with the AC audio current it sees. I can't see any LDR, so I'm quite confident that this bulb sees the AC audio signal, and is the variable element needed. But, on the other hand, one may expect a lot of distorsion if the thermal reaction time is faster than the audio signal ...

Note the unusual design of the loudspeaker chassis. It is truly rectangular, covering the whole surface of the baffle. First first time I see something like this. Kind of integration of the driver and the front of the baffle. The idea sounds right, but do you realize that if you want to redesign or upgrade the loudspeaker you need again to order one with a custom-made chassis ? What a nightmare, indeed ...

Cabling looks high quality like military or aeronautics. But look how far the power transistors are from the PCB ! And see how small and how far the rectifying bridge is, far from the filtering cap, and far from the transformer. That's completely mad !

Doesn't surprise me too much they stopped trading.

Their concept of an active loudspeaker integrating some smart equalization and variable high-pass for meeting the safe operating area of the driver, is still valid.

Therefore I'm sad they stopped trading.

So, if somebody could provide more info like a schematic of this Servo-Sound 15B version, that will be very welcome.


steph_tsf 27th April 2010 04:58 PM


in past century (that's not so long ago) I did some simulations of motional feedback loudspeakers using spice, comparing the Philips MFB (using an acceleration sensor), the Servo-Sound (relying on an electronic synthesis of the motional voltage), and the Yamaha AST (Active Servo Technology which at the origin is an embryonic Servo-Sound arrangement with a limited correction range).

The problem with the Servo-Sound method and with the Yamaha AST method was to have the motional bridge accurately reflecting the electric equivalent schematic embedding the Thiele-Small parameters in the said-to-be-so closed box.

Later on Yamaha came with other sorts of AST designs, like the AST that are now used in their active subwoofers that are no closed box anymore. I should get a service manual and see what's inside. Can anybody point a link ?

Anyway, due to the fact that the motional impedance bridge was always inaccurate (factory tolerances and temperature effects in the loudspeaker), the feedback was never accurate. Coil resistance variation with temperature was one of the issue, disabling the possibility to drive the coil with a negative resistance exactly equal to to the coil resistance. This is actually needed for getting a purely motional-voltage driven system. In such an arrangement, the loudspeaker becomes a big and powerful microphone, telling you in realtime if the acoustic output is different from what it should be. And this mike becomes thus the feedback source of the whole amp. This is indeed the theoretical aim of a 100% pure Servo-Sound arrangement. Nice, isn't ?

I then wanted to measure the resistance of the coil, realtime, for driving a variable resistor in the motional bridge arrangement. Never came to something decent. Always a kind of rocket-scientist solution, unpractical.

I also wanted to be able to measure the accuracy of this simulated motional bridge, like the autofocus block of your your digicam is working. Back in the nineties, this was perceived like madness. Like autofocus was perceived like madness before World War II. What is the signal indicating that your synthesized electric schematic circuit is on focus ? You say it is the resulting signal itself ? Come on, are you mad ? Well, quite surprising, there might be something in the recovered audio signal itself (the motional voltage), indicating that the synthesized electric schematic is on focus. Kind of signature. Because there are magnetics involved. Nice field of research, isn't ?

I still believe that is the clear future of Rice-Kellogg moving coil loudspeaker patented in 1924, in the long range. Synthesizing the equivalent electronic schematic, isolating the motional impedance, measuring the instantaneous voltage on it, using this voltage as global feedback signal, plus a management block ensuring that the synthesized motional impedance is kept on focus whatever the actual conditions, factory dispersions and temperature effects.

You can't do it using bare DSP. Because of the processing delays. You only can do it with the DSP modifying the values of the analog components materializing your synthesized motional bridge.

Which means that you need electronic equivalents of a continuously variable resistor, capacitor and self, with the added difficulty that some cannot be single-ended (one end to the ground), but fully floating (no end to the ground).

Would be nice to deliver something special in 2024, for the 100th anniversary of the Rice-Kellogg moving-coil loudspeaker.

Today I just found this, from May 2009 in EDN :
by Louis Vlemincq
from Belgium

What do you think about it ?
Isn't this a nice first brick, for getting the DC resistance of the coil ?


bentoronto 27th April 2010 06:59 PM

I need more time to review Steph's many interesting ideas.

I also experimented with motional feedback in the last century and have used it for a lot years too. Following the clear analysis RCA did in the later 50's, a great idea. Lotsa ways to extend tone compass but miracle of motional feedback is pulse response - hard to believe how it helps in the test setting. In my case that was Bell Labs' anechoic chamber. Second benefit is harmonic distortion which is wonderfully reduced... and I won't rub it in for you bass reflex fans.

For sure, it is the way to go for all frequencies but doesn't have a hope of working except for low, piston-range frequencies.

I'm not sure I much understand Steph's detailed critique of modeling the speaker/box. But I believe it is misplaced. Essentially, what "current feedback" or "bridge" feedback does is ensure the coil moves in harmony the signal (and hence, we hope) the cone and the output (which is why tuned boxes are not possible but a horn like the Klipschorn might be).

To collect feedback, all you need is to collect feedback. Does that exactly mirror the output in light of the equivalent-circuit of the speaker/box? Not important since you've gone 90% of the way to controlling the cone motion.

Interesting to see what looks like a classic Philips driver with a whizzer cone.

Folks used to hook up incandescent bulbs in series to act as expanders. You know, it works nice!

Suggestion: make square waves or pulses and watch with a good mic.

bentoronto 27th April 2010 07:15 PM

Here's a reference that covers the issues pretty nicely. 1958.

Application of Negative Impedance Amplifiers to Loudspeaker Systems

Successful application of negative damping factors to loudspeakers is not so difficult a task as one would infer from the literature of recent years. Foremost in importance are the cancellation of voice-coil inductance as well as resistance, equalization for low-frequency radiation characteristics; and utilization of infinite baffle rather than a reflex or horn type. When satisfactory attention is granted these factors, substantial improvement in loudspeaker performance is almost certain. In a typical application of a negative impedance amplifier, resonant hangover is eliminated, the nonlinear distortion halved, and the low-frequency response leveled and extended about an octave.

Authors: Werner, Richard E.; Carrell, Ross M.
Affiliation: Radio Corporation of America, Camden, NJ
JAES Volume 6 Issue 4 pp. 240-243; October 1958

Click to purchase paper or login as an AES member. If your company or school subscribes to the E-Library then switch to the institutional version. If you are not an AES member and would like to subscribe to the E-Library then Join the AES!

This paper costs $20 for non-members, $5 for AES members and is free for E-Library subscribers.

steph_tsf 27th April 2010 07:40 PM


Originally Posted by bentoronto (
I'm not sure I much understand Steph's detailed critique of modeling the speaker/box. But I believe it is misplaced. Essentially, what "current feedback" or "bridge" feedback does is ensure the coil moves in harmony the signal (and hence, we hope) the cone and the output. To collect feedback, all you need is to collect feedback. Does that exactly mirror the output in light of the equivalent-circuit of the speaker/box? Not important since you've gone 90% of the way to controlling the cone motion.

Quite a hard attack. Need to respond. Motional feedback is not so simple if you want to overcome factory dispersions and if you want to attain a significant bass range extension, a significant distorsion decrease, and still maintain a safe operating range for the loudspeaker, and still maintain the stability of the feedback loop even with temperature changes of the coil (adapt negative resistance drive) and magnetics (adapt reactive components in the bridge). You need to drive using a resistance that is the negative of the coil copper resistance. If you don't do this, the feedback extraction won't be accurate. And if there is a crossover filter in the woofer, you need to take it into account and possibly persuade the main amplifier to deliver a usable signal for the tweeter. If it was so simple, all active loudspeakers would embed Motional Feedback. Have you tried Motional Feedback yet, yourself, practically ?


Originally Posted by bentoronto (
Here's a reference that covers the issues pretty nicely. AES 1958. Application of Negative Impedance Amplifiers to Loudspeaker Systems by Werner, Richard E.; Carrell, Ross M.

Thanks a lot. Nice suggestion. I like reading the AES before doing my homework. Seems there are a few more publications worth looking at, some quite recent :

The Use of Negative Source Impedance with Moving Coil Loudspeaker Drive Units: A Review and Analysis

The effect of negative source impedance on the frequency response and pole-zero pattern of a moving coil loudspeaker drive unit is explored from first principles, and closed form expressions for the transfer function and system poles are developed. Direct control of motor velocity via the substantial cancellation of voice coil impedance is discussed. Implementation using positive current feedback is analyzed, considering loop gain, damping and stability from a control theory perspective. Pole placement techniques are shown to be effective in controlling theoretical system behavior at high frequencies. Modeled and measured results are presented. A selection of previous papers and applications concerned with operation of loudspeakers from negative source impedances is briefly reviewed. Practical issues and some possible applications are discussed.

Authors: Turner, Michael J.; Wilson, David A.
Affiliations: University of Leeds; S.R. Drives Ltd.(See document for exact affiliation information.)
AES Convention:122 (May 2007) Paper Number:7072

Active Acoustic Absorption and Reflection

A single transducer that is part of a positive or negative feedback circuit can achieve active acoustic absorption and reflection. It is shown that the mobility of the diaphragm is dependent on the load of the voice coil. By using the four-pole mathematics, we show a direct relationship between the electrical and acoustical terminals of the transducer. The test setup as well as the results by the subtraction of impulse responses are discussed. Even the sound transmission through the diaphragm is a part of this investigation. Possible applications will be discussed.

Author: Swarte, Peter
Affiliation: P.A.S. Electro-acoustics
AES Convention:118 (May 2005) Paper Number:6520

Comparative Analysis of Moving-Coil Loudspeakers Driven by Voltage and Current Sources

The Thiele-Small method for speaker design considers the linear loudspeaker model driven by voltage sources and operating in a small signal environment. Subsequent studies have been made to introduce into the model some nonlinear characteristics due to the operation with large signals. This paper presents a comparative analysis of the sound pressure level and cone displacement of loudspeaker systems as an infinite baffle, a closed box, a vented box and band-pass enclosure driven by voltage and current sources, under small and large signals. The nonlinearities of the voice-coil, force factor and compliance of the loudspeaker are taken into account.

Authors: Bortoni, Rosalfonso; Noceti Filho, Sidnei; Seara, Rui
Affiliation: LINSE - Circuits and Signal Processing Laboratory, Department of Electrical Engineering, Federal University of Santa Catarina, Brazil
AES Convention:115 (October 2003) Paper Number:5910

Motional Voltage as a Distortion Mechanism in Loudspeakers

An alternate model of the loudspeaker/amplifier system suggests a reason to prefer amplifiers with damping factors near unity. This analysis revolves around the motional voltage or back EMF generated by moving-coil loudspeakers. This model and some examples are presented and examined. A circuit which uses motional voltage feedback to synthesize a damping factor of unity is presented. While listening tests tend to show a preference for systems with unity damping, little support for this observation appears in the usual battery of tests. Analysis of these and other tests point to a combination of factors that makes low damping factors desirable.

Authors: Peplinski, Chuck; Miller, Allen
Affiliations: Time Warner, Interactive, Milpitas, CA ; F. A. Miller Audio Inc., Kenwood, CA,(See document for exact affiliation information.)
AES Convention:99 (October 1995) Paper Number:4119

Audio Amplifiers with Tailored Output Impedances

Discussed is an approach to amplifier design which appears to be suitable for use in driving loudspeakers whose frequency response characteristics tend to vary with their electrical impedance. The approach deals with tailoring amplifier output impedances to control loudspeaker behavior. The paper discusses some general rules of design and gives an example along with experimental tests. It concludes with the significance that such amplifiers can have on loudspeaker design and the potential for increased flexibility afforded the loudspeaker designer.

Author: Evenson, Roderick J.
Affiliation: Consulting Engineer, Milwaukie, OR
AES Convention:85 (November 1988) Paper Number:2693

steph_tsf 27th April 2010 09:04 PM

2 Attachment(s)
My idea would be to get the focus (this concept is explained in a post above) basing on the power signature signal following Peplinsky-Miller AES publication in 1995. See attached pictures.

Once you have your "flat" power signature in the time domain as described by Peplinsky-Miller (instantaneous current times instantaneous voltage) on a couple of frequency bandwidths (need a few banked bandpass filters - easy with DSP), and if you keep track of the value of the positive impedance you are generating for effectively attaining the unity damping factor, then you know everything about your load.
The algorithm to be used is the autofocus algorithm used in digicams. It starts hunting, but the more signal you give him as food (contrast and details), the more accurate and faster he gets in focus.
When you are in focus, it means thus that you have acquired with great precision the equivalent schematic of your loudspeaker in the closed box. Or, possibly, any other loudspeaker arrangement like a 4th order bass-reflex, 4th order bandpass, or 8th order bandpass aka Bose.
This is the way to automate the obtention of Peplinsky-Miller unity damping factor with a 0.1% precision, say from 10 Hz to 250 Hz.
This is my idea #1. Is this a new idea ?

What to do with the precise knowledge of the load impedance ?
Answer : sample the current using a small shunt resistor. Feed this current into analog components emulating the equivalent schematic, with the values you just calculated. Extract the pure motional voltage. It may be precise at 0.3 % if you use 12 bits DACs for defining the values of those analog components, if you use 0.1% precision resistances in your gyrators and capacitance multipliers, and if the target value are precise at 0.1% from the autofocus process described above.
I know it's a kind of rocket scientist solution for getting a high-precision wide bandwith motional voltage.
This is my idea #2. Is this a new idea ?

What to do with the motional voltage ?
Answer : use it into an outer feedback loop, encompassing all what got discussed untill here, for finally getting an inner unity damping factor arrangement, but completely motional voltage driven from the outside.
That is my idea #3. Is this a new idea ?

I don't expect to have the magnetics non-linearities corrected by such feedback. Philips MFB using an accelerator sensor does.

With time passing, this looks less and less like a rocket scientist solution, if one dares to call this a solution, nowadays.
A tiny SoC containing everything, including ADCs, DACs, DSP, laser-trimmed FET opamps, laser-trimmed resistors used in gyrators for synthezising the reactive components, will be cheaper, one day, than a PXE (piezo) crystal needing two terminals, one precision FET amp as buffer, and two or three supplementary wires and associated connector going to the driver.

If my 3 ideas are all right, all quality active loudspeakers will be built like this : a rocket scientist approach. With possibly more tricks for compensating the magnetics nonlinearities.

See attached pictures.


latala 27th April 2010 09:41 PM

I can remember the launch of the short lived servo sound loudspeaker in the seventies
I came across them in an exibition
I thought that they were really good for their size.
They in fact inspired me to produce some motional feed back spk of my own !
I fed the spk voice coil with an rf signal and had a pickup coil located in front! This amplitued modulated the signal so that I could determine the relative posistion of the cone and apply corrective feed back to put the cone where ir should be !
I was promised financial support by a then famous loudspeaker manufacturer to advance my ideas but whem push came to shove! they let me down and the project had to be dropped
regards Trev

steph_tsf 27th April 2010 09:47 PM

seen here :

May 26, 2009 11:44 AM
by Bill Whitlock

There's a much, much simpler way to monitor winding temperature. Simply include a winding that has an equal number of clockwise and counter-clockwise turns (hence, no AC output). If it is made of suitably small wire, it's easy to measure temperature using the well-known temp-co of copper. We do this in some of our transformers designed to be driven by amplifiers with negative output impedance.
Bill Whitlock, president & chief engineer

Jensen Transformers, Inc.
Chatsworth, CA, USA

My question : what if you replace the word "transformer" by the word "loudspeaker" ?

steph_tsf 27th April 2010 10:02 PM


Originally Posted by latala (
Some motional feed back spk of my own : I fed the spk voice coil with an rf signal and had a pickup coil located in front. This amplitude modulated the signal so that I could determine the relative posistion of the cone and apply corrective feed back to put the cone where it should be.

The aborted Grundig motional feedback system ? On some leaflets and pictures, some Grundig speakers had a "thing" facing the membrane, but I never managed to know what this was, actually. What you've done is wonderful ! You must be an RF specialist. Everybody else failed trying such an approach. Congratulation ! Can you tell us more about the practical results ? Signal / noise ratio was useable ? Do you still own the prototype ? Do you see obvious improvements to be made now, with the newest silicon ? Can we now replace the amplitude demodulation by triangulation using geometric wave processing ? Say a wavelenght of 5 cm for a max excursion to be 50% of the wavelenght. What is the complexity of a triangulation demodulator ? Would a passive metallized reflector suffice, glued on the center of the cone ? What signal / noise to expect using this approach ?

bentoronto 28th April 2010 01:12 AM

Thanks for references. Things are coming into clearer focus for me.

Yes, this stuff is unstable and I doubt it can be made to work commercially or to work except in the experimenter's environment. Likewise, unfeasible except when you are working with a piston-like woofer from say, 200 Hz down, not trying PA sound levels, using good drivers, etc. But WOW, great woofing if you do.

Yes, some of the earlier demos (including my guide, Werner at RCA) acted as if the goal was getting big sound from tiny boxes. That's because MF can drive the speaker right down through resonance, whether the driver is willing or not. A worthy enough goal and with obvious commercial value if anybody can do it, but hardly the issue for us.

Yes, producing what is essentially a mirror-image (positive output impedance, if I am not confused too badly) of the speaker is the goal with bridge (or resistor or possibly a dedicated winding on the voice coil, as Whitlock and other in history have suggested) MF. The other systems of sensing speaker output (capacitive, accelerometer, mic) are fairly unfeasible feedback concepts and because they use the term "feedback" rather loosely.

Hate to seem anti-intellectual, but a basic issue is that you are messing about trying to make a simulation work. For example, fussing over coil temperature is not a real-world issue. As I said, perhaps too cryptically, if you can grab some feedback voltage, run with it.

Footnote: I recently became involved with Kenwood Basic M1 Amp with "Sigma Drive" - really remote sensing like with lab power supplies. Did you know this is really a motional feedback circuit, providing you have a long enough (resistant enough) line to the speaker? No kidding. Check it out. You can create the same circuit WITH ANY AMP by just taking your feedback from the speaker terminals the same way (instead of internal to the amp). The speaker leads are the sensor resistor or add .5 ohms.

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