US 3647969 A
A motional feedback amplifier, having an impedance bridge, one arm of which comprises a loudspeaker and which is balanced at the frequency of the loudspeaker with the diaphragm blocked, comprising a low-pass filter in the feedback loop between the reading terminals of the bridge and the input terminal for the feedback voltage to the amplifier and the characteristic of which is adjusted so as substantially to suppress the feedback voltage in the frequency zone in which the false readings appear and above that zone, a low-frequency corrector in the conventional negative feedback loop, the said filter being preceded by a transistor, two control electrodes of which, are fed with a signal from the reading terminals of the bridge and the output electrode of which delivers the motional feedback signal to the amplifier.
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Description (OCR text may contain errors)
United States Patent Korn 1 Mar. 7, 1972  MOTIONAL FEEDBACK AMPLIFIER  Inventor: Tadeusz Ken, 58 rue Mercelis, Brussels, ""9"" Exami'.wr Kathleen H'Claffy Belgium 1 Assistant Exammer-Horst F. Brauner Attorney-Waters, Roditi, Schwartz & Nissen  Filed: Aug. 28, 1969 211 Appl. No.: 853,665 [571 ABSTRACT A motional feedback amplifier, having an impedance bridge,  Foreign Application Priority new one arm of which comprises a loudspeaker and which is balanced at the frequency of the loudspeaker with the Aug. 29, 1968 Belgium ..720-1 15 diaphragm blocked, comprising a low-pass filt i h f d. back loop between the reading terminals of the bridge and the  U.S. Cl ..l79/1 F input terminal f the f db k lt to the lifi and  f 3/00 the characteristic of which is adjusted so as substantially to  Field of Search ..I79/ 1 F suppress the feedback voltage in the frequency zone in which the false readings appear and above that zone, a low-frequen-  Reerences cued cy corrector in the conventional negative feedback loop, the
UNITED STATES PATENTS said filter being preceded by a transistor, two control electrodes of which, are fed with a signal from the reading ter- 2,887,532 5/1959 wpmel' 1 79/1 i l fth b idge and the output electrode of which delivers g g the motional feedback signal to the amplifier. er ier..... 3,530,244 9/1970 Reiffin ..179/l F 2 Claims, 6 Drawing Figures 4F C01EGTOE I 1 1 A ---?MflL/F/Ee a F l A 120 F71. r52
A I J I 19 l 7 1 8 TEAMS/STOP Patented March 7; 1972 3,647,969
5 Sheets-Sheet 1 4/14/ 4 lF/EE Patentd March 1, 1972 3,647,969
3 Sheets-Sheet 2 FIGA Patented March 7, 1972 3 Sheets-Sheet 5 I7 r I I I I I l I I I I I I I I I I I 0 .l U Q a FIGLE FIG.6
.. I i l 1 I I 24 MOTIONAL FEEDBACK AMPLIFIER This invention relates to the construction of a motional feedback amplifier whereby the mechanical resonance of the loudspeaker to which it is connected can be suppressed, said motional feedback being generated by an impedance bridge, one arm of which consists of a loudspeaker and which is balanced when the loudspeakers diaphragm is blocked.
The invention relates to the construction of means adapted to make such amplifiers reliable and suitable for mass production.
Referring to the accompanying drawings:
FIGS. 1 and 2 show a prior art motionalfeedback amplifier;
FIG. 3 is a frequency-reactance graph of the amplifier of FIGS. 1 and 2;
FIGS. 4 to 6 illustrate a motional feedback amplifier in accordance with the present invention.
A motional feedback amplification method is already known of the type shown in FIG. 1 of the accompanying drawings, being based on the feedback principle comprising a bridge 2 with one arm formed by the loudspeaker 3, the said' bridge being prebalanced for the value of this arm equal to the electrical impedance of the coil of the loudspeaker with the diaphragm at rest. Thus the electromotive force induced in the coil by its vibrations in other words the appearance of the motional impedance of the loudspeaker-put the bridge off balanceand produces a reading voltage between the terminals 8 and 10. This voltage, which is assumed to be proportional to the velocity of the diaphragm, is applied as a negative feedback to the input of the amplifier 1 (terminal 8), in order to stabilize the velocity, i.e., suppress the mechanical resonance of the loudspeaker.
Unfortunately, the operation of the bridge 2 is not as simple as was assumed, and the circuit shown in FIG. 1 gives results which are hardly reproducible and frequently less satisfactory than without motional feedback at all.
Various known proposals have been published and even patented in order to improve this method, but without giving an industrially applicable result.
Thus despite its great theoretical advantages, it has never been possible to put this method into practice reproducibly.
The reason for these failures hitherto is that ithas not been possible to find the true cause of malfunctioning of the bridge 2. In order to explain the foregoing, reference will be made to FIG. 2, which illustrates the physical components forming the bridge 2.
It will be obvious that if theelectrical impedance of the loudspeaker with the diaphragm blocked 11 were composed solely of a pure resistance 12, the bridge 2 would readily operate in'accordance with the elementary principle set forth hereinbefore. However, the electrical impedance of the coil of the loudspeaker 11 includes an inductive component 13 which seriously obstructs the required operation of the bridge 2. The authors of known proposals had been aware of the existence of the element l3 but they have erroneously interpreted its obstructive action, i.e., they have considered it solely as a difficulty in correct prebalancing of the bridge 2 throughout the audio-frequency range (with the diaphragm blocked). As a remedy, they have proposed that one of the other arms of the bridge 2 should contain adequate'reactive elements, e.g., an inductance in series in the arm 4 or 5, or a capacitor in parallel with a resistor in the arm 6.
To obtain strict prebalancing of the bridge 2 throughout the whole audio-frequency range, some authors have proposed that the reactive elements used should be highly complex with a characteristic allowing for taking into consideration the resistive losses of the inductance 13 in the high-frequency zone.
To obviate these complications in prebalancing the bridge 2, some authors have proposed to attenuate the operation of the entire system in the high-frequency zone by means of a capacitor disposed, for example, between the reading terminals of the bridge 8 and 10. This solution was also supposed to attenuate the effect of the partial vibrations of the diaphragm appearing in the same high-frequency zone and which have also been considered as one of the main sources of difficulty.
Hereinafter it will be shown that a rudimentary solution of this type has not been able to eliminate the true cause of difficulty. It should also be noted that the provision of a capacitor directly at the reading terminals is contrary to the elementary theory of measuring bridges.
The present invention is based on a careful investigation of the circuit shown in FIG. 2.
This investigation shows that the true obstructive action is due to the combination of the positive reactance 13 with the reactive component of the motional impedance 14, which becomes negative above the resonant frequency of the loudspeaker.
FIG. 3 is a frequency-reactance graph showing this concept.
Referring to this graph, the line a denotes the reactance of 13; the curve b denotes the reactance of 14; the curve 0 denotes the resistance of 14.
The line f 0 relates to the frequency corresponding to the resonance of the loudspeaker where the motional feedback should be the most effective.
This Figure shows that the total reactance of the arm 3 is cancelled out at the frequencies f and f at which the moduli of said two reactances of opposite sign become equal (X 14= Thus the bridge 2 which was strictly prebalanced with the diaphragm blocked gives readings at frequencies f and j, which are in no case proportional to the velocity of the diaphragm and which seriously disturb the response curve of the complete system. These readings will hereinafter be referred to as false readings of the motional impedance by the bridge 2.
This behavior of the bridge 2 is somewhat similar to the operation of the known bass reflex loudspeaker but produces another which is still more disturbing since it is situated at a higher frequency. The amount of disturbance by the false readings of the bridge 2 at f, and f depends on the value of the series resistive component R 14 (curve c) of the motional impedance 14 which appears at these frequencies. This value is highly uncertain since it depends on the mechanical and acoustic loads of the loudspeaker, which are frequently uncontrollable. That explains why the known constructions gave unstable and nonreproducible results.
It is very important to note that the frequency zone at which the false readings of the bridge 2 appear is very close to the operating frequency of the system f,,. The problem is therefore physically and technically quite different from the known problems and more particularly prebalancing of the bridge or partial vibrations of the diaphragm which appear only in the much more remote frequency zone. Consequently the solutions forming the subject of this invention must be distinctly different from all those known in the technology.
The amplifier according to this invention comprises an inseparable group of means which provide an efficient motional feedback action (at least 10 d.) at the resonant frequency f of the loudspeaker but which sufficiently suppresses the reading action of the bridge 2 in the frequency zone at which the false readings appear and which is very close to the said frequency f,,. This group of means comprises the following elements (FIG. 4);
l. A transistor 18 for reading of the bridge 2, its input electrodes 19-20 being connected, according to the polarity of the required signal, respectively to the terminal 8 and the terminal 10 of the bridge 2, and the output electrode of which is shown at 21. The object of the reading" transistor is not only to bring the reading signal into reference with respect to the ground, but also amplify it and to form a high-impedance source for the circuits following one another in the motional feedback loop.
2. The reading transistor is followed by a filter I7 comprising two capacitors 25 and 26 (FIG. 5) connected between the signal line and ground. To prevent the transfer function of the filter from assuming excessive values at very low frequencies, which may result in a disturbing positive feedback, the filter also comprises a capacitor 27 in series in the signal line which attenuates the signal at the said very low frequencies. The said series capacitor also corrects the phase-shift of the filter in the effective frequency zone. However, the said filter with two capacitors to ground, in other words with two RC networks, does not yet have sufficient slope to select the effective frequency zone in relation to the disturbing zone. Unfortunately, any increase in the number of RC networks results in a disturbing phase-shift which interferes with the operation of the system. To obtain adequate discrimination of the effective zone in relation to the disturbing zone without disturbing phase-shift, the amplifier has a third element which forms part of the group of means proposed in the invention and which is:
3. A low-frequency corrector 24 (FIGS. 4 and 6) disposed in the conventional negative feedback loop with which the amplifier according to this invention must be provided and which corrector consists, for example, of the resistor 29 and capacitor 28 in parallel. The object of this corrector is to increase the amplifier gain at low frequencies, thus reinforcing discrimination between the effective zone and the disturbing zone of the motional feedback, because the said amplifier is situated in the loop of the latter.
What is claimed is:
1. A motional feedback amplifier comprising an impedance bridge for providing said feedback, one arm of said bridge including a loudspeaker, said bridge being balanced with a loudspeaker diaphragm blocked at the resonance frequency of the loudspeaker, a first feedback loop having a low-pass filter provided therein extending between reading terminals of the bridge and an amplifier input terminal for reading the feedback voltage to the amplifier, said amplifier being adapted to substantially suppress the feedback voltage in and above the frequency zone in which any false readings of the bridge appear, and a second feedback loop having a low-frequency corrector provided therein constituting a negative feedback loop of said amplifier, an active transistor element in said first feedback loop preceding said low-pass filter, said element including two control electrodes which form an emitter and a base of the element and which are supplied with a signal from the reading tenninals of the bridge, and an output electrode of the element adapted to deliver the feedback signal to the filter and to the amplifier.
2. An amplifier as claimed in claim 1, wherein said filter comprises at least two capacitors connected between a signal line and ground, and a capacitor connected in series in the signal line which attenuates the generated signal at very low frequencies and reduces the phase-shift in the operating zone of frequencies.