US2887532A

US2887532A - Audio frequency amplifier

Info

Publication number: US2887532A
Application number: US619603A
Authority: US
Inventors: Richard E Werner
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1956-10-31
Filing date: 1956-10-31
Publication date: 1959-05-19
Anticipated expiration: 1976-05-19

Description

May'19, 1959 R. E; WERNER 2,887,532

AUDIO FREQUENCY AMPLIFIER Filed Oct. 51, 1956 IIFVENTOR.

[Mam E Mme! ATZUBNEX' United States Patent OT AUDIO FREQUENCY AMPLIFIER Richard E. Werner, Medford Lakes, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application October 31, 1956, Serial No. 619,603

8 Claims. (Cl. 179-1) This invention relates to sound reproducing systems,

and more particularly to audio frequency amplifiers havmg negative output impedance characteristics for driving moving coil loudspeakers.

' Direct-radiator loudspeakers which are driven by an electrical signal applied to a moving voice coil of the loudspeaker have an effective electrical impedance that is determined by the electrical voice coil impedance and the equivalent electrical impedance of the mechanical impedance of the loudspeaker. The electrical impedance of the voice coil is generally called the blocked voice coil impedance, since it is measured with the voice coil blocked or prevented from moving within the magnetic field of the loudspeaker. The mechanical impedance, which is effectively in series with the blocked voice coil impedance, cannot be easily treated unless it is converted to an equivalent electrical impedance and when it is so converted it maybe called the reflected mechanical ims pedance. In typical loudspeakers, the blocked voice coil impedance is large compared to the reflected mechanical impedance.

speaker will result in a low resonant frequency, but will require considerable electrical power to drive the system. If, however, the blocked voice coil impedance could be cancelled or eliminated, the amplifier output voltage would then be applied directly across the reflected mechanical impedance rather than through the blocked voice coil impedance. The velocity of the voice coil driving the loudspeaker would then be the exact replica of the signal voltage applied to the loudspeaker. The system would be substantially free of resonances, since the reflected mechanical impedance could be effectively damped, and the system would be substantially devoid of distortion caused by non-linear compliance characterise tics of the loudspeaker. The cancellation of the blocked voice impedance may be effected by making the output impedance of the amplifier driving the loudspeaker the negative of the blocked voice coil impedance of the loudspeaker. i It is therefore an object of this invention to provide an improved audio frequency amplifier for driving a mov ing coil loudspeaker system which substantially reduces the distortion introduced by non-linearity characteristics of the loudspeaker. It is another object of the invention to provide an audio frequency amplifier circuit for driving a loudspeaker system having a negative output impedance to cancel or reduce the effects of the blocked voice-coil impedance of moving-coil loudspeakers.

It is another object of the present invention to provide an audio. frequency amplifier having a negative reactance output impedance.

The acoustic output of a direct-radiator moving voiceblocked voice coil impedance is in series with the reflected mechanical impedance introduces several problems. The reflected mechanical impedance is effectively a tuned circuit and thus has a resonant frequency, often within the audio frequency band. This resonance intro- In accordance with, the invention, an audio frequency signal amplifier is made to possess an output impedance that is, in effect, a negative'resistauce, and a negative inductance in series to compensate for the positive resistance and positive inductance of the blocked voice coil impedance of a loudspeaker which is driven by the amplifier. Such output impedance is provided by a negative impedance feedback circuit which includes positive current feedback and negative voltage feedback from the output circuit to the input circuit of the signal amplifier through abridge circuit and a common feedback path. When the bridge circuit is near balance, there will be a net negative feedback at audio frequencies, and a stable negative output impedance for the amplifier, including negative resistance and negative inductance, is provided. Positive feedback may be applied over a portion of the amplifier duces non-linearities in the acoustic output signal of the loudspeaker since the reflected mechanical impedance generally cannot be efiectively damped because of the blocked voice coil impedance in series with it. Even with the input terminals to the voice 'coil shorted, the mechanical resonance is usually still troublesome.

Variation in the value of the reflected mechanical irnpedance with frequency also introduces certain non-linearities. At frequenciesabove the resonant frequency of the reflected mechanical impedance,.the value of the reflected mechanical impedance is controlled largely by the mass of the moving system, whereas at frequencies below the resonant frequency it is controlled largely by the stiffness of the moving system. Since the stiffness is a non-linear function of the displacement of the cone of the loudspeaker, distortion will be introduced. The nonlinearities and distortion caused by the resonances and non-linear operation of the loudspeaker systemmay be reduced to some extent by the proper choice of cabinets or enclosures in which the loudspeaker is mounted. Also,

channel tov provide increased amplifier gain to more effectively cancel the loudspeaker impedance and to reduce distortion in the amplifier by increasing the net negative feedback of the negative impedance feedback circuit.

However, the invention will be further understood when the following description is read in connection with the accompanying drawing, in which:

Figures 1 and 2 are schematic circuit diagrams of audio at amplifier circuits constructed in accordance with the inan especially heavy cone and voice coil for the loudvention for driving moving-coil loudspeakers.

Referring now to the drawing and in particular to Fig;- ure 1, an audio frequency amplifier includes, basically, a self-balancing phase inverter utilizing first and second triode amplifier tubes 10 and 12 to supply push-pull signals to drive a pair of push-pull connected output tubes 14 and '16, which, in turn, supply driving power to a loud speaker 86. Audio frequency signals to be amplified and reproduced are applied to a pair of

signal input terminals

18 and 20 and appear across a grid return resistor 19. One terminal 20 is connected to ground or a point of referencepotential for the amplifier, and the other terminal 18 is connected through a series input resistor 22 to the control grid 24 of the first amplifier tube 10. The

anodes

26 and 28 of the first and second tubes 10 and 12 Patented May 19, 1959.,

assmsa are connected to a source'of operating potential, +13,

each through a separate load resistor 30 and 32, respectively, and the D.-C. circuit is completed by connecting the

respective cathodes

34 and 36 through individual

cathode bias resistors

38 and 40 to ground for the system.

The

anodes

26 and 28 of the phase invertertubes and 12 are connected to the

control grids

42 and 44 of the

output tubes

14 and 16 through

coupling capacitors

46 and 48. In order to supply an inversion signal to the second tube 12, a pair of resistors 50 and 52 of properly related values, are connected in series between the

control grids

42 and 44 of the

output tubes

14 and 16, and the junction of the resistors is connected to the control grid 54 of the second tube 12. The

anodes

56 and 58 of the

output tubes

14 and 16 are connected to the centertapped primary winding 60 of a push-pull output transformer 62, and operating potential is supplied to the

output tubes

14 and 16 by connecting the

screen electrodes

64 and 66 thereof and the center tap 68 on the primary 60 to the source of operating potential, +B. To complete the D.-C. circuit for the

output tubes

14 and 16 the cathodes 70 and 72 are connected through a common bias resistor 74 to ground for the system. The bias resistor 74 is by-passed by a capacitor 76 and the

control grids

42 and 44 of the

output tubes

14 and 16 are connected to ground through grid resistors 78 and 80, respectively.

Thus, audio frequency input signals applied to the

input terminals

18 and 20 are amplified and inverted in the phase inverter tubes 10 and 12 and applied as push-pull signals to the

control grids

42 and 44 of the

output tubes

14 and 16. Output signals developed by the

output tubes

14 and 16 are applied through the transformer secondary 82 and a resistance-capacitance network, which will be more fully described and explained hereinafter, to the moving voice coil 84 of the loudspeaker 86. .The operation of such an audio frequency push-pull power amplifier as so far described is well known and further description of its operation and characteristics are not necessary other than to state that by careful design the distortion characteristics of the amplifier may be reduced to a reasonably low value.

As previously noted, the equivalent impedance presented to an amplifier by a direct radiator moving-coil loudspeaker, such as the loudspeaker 86 in Figure 1, is rather complicated and includes components of reflected mechanical impedance and the blocked electrical impedance of the voice coil. Even if the effective output impedance of the amplifier were zero, the reflected mechanical impedance of the loudspeaker, which is in effect a rise to distortion and various undesired transient respouses. If the amplifier can be made to possess an output impedance that is the negative of the blocked voice coil impedance, the voltage across the reflected mechanical impedance of the loudspeaker can be made to be the exact replica of the signal voltage applied to the amplifier.

In accordance with the invention, a feedback circuit to provide the negative output impedance is connected between the secondary 82 of the output transformer 62 and the control grid 24 of the first amplifier tube 10. The negative impedance feedback circuit is connected to provide both positive current feedback and negative voltage feedback over the amplifier. This is accomplished by connecting first and

second bridge resistors

90 and 88 in series across the secondary 82 and a third bridge resistor the pair of

bridge resistors

88 and 90 is connected to the control grid 24 of the first tube 10 through a bass compensation network comprising a capacitor 94 in series with a resistor 96, the action of which will be more fully explained hereinafter.

As can be seen from an inspection of the output circuit shown in Figure 1, the resistance-capacitance network and the voice coil of the loudspeaker 86 form a bridge circuit, the first arm of which is the bridge resistor 88, the second arm is the bridge resistor 90 together with the resistor 102 and the series capacitor 104 connected thereacross, the third arm is a third bridge resistor 92, and the fourth arm is the voice coil 84 of the loudspeaker 86. The output signal of the amplifier is applied to one diagonal of the bridge circuit by connecting the secondary winding 82 of the transformer 62 to two opposite corners of the bridge circuit, that is, the junction of the bridge resistor 88 and the voice coil 84 and the junction of the

bridge resistors

90 and 92. The negative impedance feedback circuit is connected across the other diagonal of the bridge by connecting the junction of the

bridge resistors

88 and 90 to the control grid 24 of the first tube 10, and by connecting the junction of the third bridge resistor 92 and the voice coil 84 to ground for the system.

The output impedance characteristic of the amplifier may be best understood by the use of mathematical equations. Let Z equal the impedance in the first bridge resistor 88; Z the impedance of the second bridge resistor 90; Z the overall impedance of the third bridge resistor 92 and the resistor 102 and series capacitor 104 connected thereacross; Z the blocked impedance of the voice coil 84 of the loudspeaker 86; A the open circuit volt age gain of the basic amplifier; the minus sign indicating a phase reversal, Z 'the' output impedance of the basic amplifier; A the open circuit voltage gain with the negative impedance feedback circuit connected; and Z the output impedance with the negative impedance feedback circuit connected. If Z +Z Z and the resistance value of the input resistor is sufificiently high to not effectively load the negative impedance feedback circuit, which will normally be the case, it then can be shown mathematically that 1 zl+zl The output impedance, then with the negative impedance feedback circuit connected can be shown to be Z Z A 1+ 2 ZgA 1 1+ z If the bridge circuit then is balanced against the blocked voice coil impedance (Z and A is very high I 25-23 (a) Utilizing equations (2) -and (3), it can then be shown that 92 between the secondary 82 and the voice coil 84 of the Thus, if the bridge circuit is balanced against the voice coil impedance and the voltage gain of the basic amplifier is very high, the output impedance of the amplifier is the complement or the negative of the voice coil impedance. The feedback voltage at the junction of the

bridge resistors

88 and 90 is a summation of a negative feedback voltage developed from the voltage on the ungrounded side of the voice coil 84 and a positive current controlled feedback voltage determined by the current flowing through the ,third bridge resistor 92. There will be no net feedback since the bridge circuit is balanced. However,

y it will be noted, that the reflected mechanical impedance of the loudspeaker was not considered in these equations. The reflected mechanical impedance when the voice coil is allowed'to move wilfunbalanceithe bridge and-"provide a net negative feedback without altering the output impedance of the amplifier. This. may be noted from Equation 2. where Z' the output impedance of the amplifier with the negative impedance feedback circuit connected, is seen to be in no way dependent upon theimpedance in the fourth arm of the bridge circuit in which the voice coil is connected.

The inductive component of the blocked voice coil impedance of the loudspeaker 86 is not an ideal inductance, and may be termed an'impure inductance in that it has resistive components caused by hysteresis and eddy-current, losses associated with the magnetic structured the loudspeaker 86; Accurate and complete cancellation of such an impure inductance is quite complicated and'generally not economically feasible, since the inductance Varies slightly. If a large percentage of the resistance of the'blocked voice coil impedance in cancelled, the Q of the inductance of the-blockedvoice coil impedance is raised, which may result in resonance problemswith the mass of the moving system in the middle range of audio frequencies unless some reduction of the voice coil inductance also is provided. Since it is not a simplematter to exactly cancel the blocked voice-coil inductance it 'is more practical not to attempt to cancel the blocked voice .coil impedance completely. Excellent results may be obtained if a substantial portion of the blocked voice coil impedance is cancelled, preferably on the order of sixty to eighty percent, depending on the particular loudspeaker being used.

The feedback circuit of Figure 1 includes a resistor 102 and capacitor 104 connected in series across the second bridge resistor 90. The capacitor 164 provides that the feedback will make the amplifier output impedance appear to be a negative inductance, and the resistor 104 makes the negative inductance appear impure in approximately the same ratio that the inductance of the blocked voice coil impedance of the loudspeaker 86 is impure. Since the inductance of the blocked voice coil impedance is not can celled 'to the same extent as its resistance, the blocked voice coil impedance will not be as effectively cancelled at high audio frequencies as at loweraudio frequencies which willresult in reduced high frequency response. The capacitor 104 serves the additional function ofincreasing the amplifier response at high audio frequency signals to approximately compensate for this reduction in high frequency response by decreasing the net negative feedback at these high frequency signals.

At the low frequencies there is adecrease in the radiation efficiency of the loudspeaker, andthe input resistor 22 to the amplifier together with the resistor 96 andthe capacitor 94 in the negative impedance feedback circuit provide a. bass boost circuit to compensate for thisdecrease in radiation efiiciency. The circuit has a time con.- stant to provide. a frequency varying loadingof the input circuit to the tube It) by increasing the inputat lowfrequenci'es. as the'impedance of'the capacitor 94 rises. The values are also chosen so that there is negligible effect on the efiiciency of the feedbackcircuit providing the negative output impedance for the lowest frequency of interest.

Belowithe resonant frequency of the loudspeakerthe reflected mechanical impedance is :.essentially controlled bygthestitfness of the moving system and if. the blocked voice coiLimpedance is not completely cancelled. the stiff.-

shown in Figure 1 is provided only through the feedback 6 circuit providing the" negative output impedance. In order to 'more effectively-reduce the distortion in the amplifier proper, positive feedback may be applied around the phase inverter through a capacitor 98 and a resistor in series. This results in a high gain amplifier of relatively a few stages around which the feedback circuit providing the negative output impedance is connected. As 'will be seen from the equations hereinbefore mentioned, the high gain of the amplifier results in more effective cancellation of the loudspeaker blocked voice coil impedance, which gives rise to improved'frequency and transient response. The distortion of the amplifier itself is also reduced, since the gain of the amplifier is sufficiently high that the net negative feedback of 'the negative impedance feedback circuit is increased to a large value. The net negative feedback need not be sufficient to cancel the positive feedback around the phase inverter stage since the distortion of'the phase inverter stage can be made negligibly small by suitable design.

The amount of positive feedback around the phase in verter stage will be determined principally by the values of the resistors 96 and 160 and the capacitors 94 and in the negative impedance feedback circuit and the positive feedback circuit, since the value of the input resistor 22 is verylarge compared to the reactance of the capacitor 94 and the resistor 96 in the negative impedance feedbackcircuit. Since, the impedance of the bass boost circuit in the negative impedance feedback circuit, resistor 96 and capacitor 94, is a function of frequency, the value of the resistor 10% and the capacitor 98 in thepositive feedback circuit are made to have an impedance that is a similar function of frequency'so that the positive feed-' back is essentially uniform at all frequencies.

Referring now'to Figure 2, an audio frequency amplifier embody-ing the invention'includes many of the same components and configurations as previously described with respect to the circuit of Figure 1. However, instead of using a self-balancing phase inverter, as shown in Figure l, a voltage amplifier and phase splitter circuit are shown. The input signal applied to the terminal 18 is applied through the input resistor 22 to the control grid 1% of a voltage amplifier tube 108. Operating voltage is supplied to the anode by connecting it through a load resistor 112 to the source of positive operating voltage indicated at +B, and the cathode 114is connected to ground for the system and B through a'cath ode resistor 116.

The amplified signal appearing at the anode of the voltage amplifier 108 is'applied directly to the control grid 113 of a phase splitter tube 129. In order to provide substantially balanced output signals from the'cathode 122 and the anode 124, an anode load or output resistor 126 is connected between the anode 124 and the source of positive operating potential, +3, and a cathode load or output resistor 128 is connected in thecathode circuit. It will be noted that at the cathode load resistor 128 is connected back to the cathode 114 of the voltage antplifier tubeltl and thence to ground through the-cathode resistor 116. This connection supplies positive feedback from the phase splitter to the voltage amplifieracrossthe cathode resistor-.116 to provide'an increase in overall gain for the amplifier for the reasons mentioned inthe previous descriptionof Figure 1.

The signals appearing at the anode andrcathode- 124 and 122 of the phase splitter tube are applied

through'the coupling capacitors

46 and 48 .to the-

control grids

42 and 44 of the. output tubes. The remainder of the amplifier circuit including the negative impedance feedback circuit connections are similar to those previ* ously described with reference to Figure 1.

Negative voltage feedbackand positive current feedback 'is supplied throughthe negative impedance feedback circuit in'the. same manner aspreviously described with reference to Figure 1. The negative output impedauce for the amplifier thus provided by the positive and negative feedback appears inductive because of the first capacitor 104 connected across the first bridge resistor 90. A second capacitor 105 connected across the second bridge resistor 88 provides that the negative inductance is impure in the same sense as the positive inductance of the blocked voice coil impedance of the loudspeaker 86 is impure. This second capacitor 105 performs the same function as the resistor 102 in series with the first capacitor 104 as shown in Figure 1. The resistor and

capacitor

96 and 94 in the negative impedance feedback circuit provides bass compensation in the same manner as hereinbefore described, and the positive feedback provides the desirable high gain for the amplifier.

In some instances it may be desirable or necessary to change the character of the output impedance of the amplifier at extreme low frequency signals. For instance, if an inexpensive amplifier is used, low frequency transients may cause the amplifier to tend to drive the loudspeaker at a subsonic or a near subsonic signal and it is necessary to reduce the power applied to the loudspeaker at these extreme low frequency signals because the available power from an inexpensive amplifier is relatively low. This action may be accomplished by connecting a third feedback circuit including a resistor 130 between the ungrounded side of the voice coil 84 and the junction between the bass compensation resistor 96 and capacitor 94 in the negative impedance feedback circuit. At frequencies of interest, that is, those desired to be reproduced, the resistance of the resistor 130 is made sufiiciently high as to have no effect on the balance of the bridge circuit for the negative impedance feedback circuit. However, at the extreme low frequencies which may cause the conditions mentioned above, a high degree of unbalance will result which produces relatively high negative feedback. The large amount of negative feedback changes the output impedance of the amplifier in a positive direction at very low frequencies and also reduces the amplifier gain at these subsonic or near subsonic signal frequencies. It should be noted that the use of the resistor 130 between the voice coil 84 and the input circuit may also be used, with exactly the same connections, in the amplifier circuit shown in Figure 1, with the same results.

An audio frequency amplifier constructed in accordance with the present invention to drive a direct-radiator, moving-coil loudspeaker is characterized by its virtually resonance and distortion free reproduction of audio frequency signals with extended low frequency response. Thus, it is adapted for Wide application in sound reproducing fields requiring an accurate transducer of electrical signals representing sounds.

What is claimed is:

1. In a sound reproducing system for driving a direct radiator loudspeaker having a moving voice coil, the combination of an audio frequency signal amplifier having a signal input circuit, a signal output circuit for said amplifier including a transformer having a secondary winding, a first and a second resistor connected in series across said secondary Winding, a third resistor, means connecting the voice coil of the loudspeaker and said third resistor in series across said secondary winding, means connecting the junction of said voice coil and said third resistor to a point of reference potential for said amplifier, circuit means connecting the junction of said first and second resistors to said input circuit for applying positive and negative feedback over said amplifier and providing a negative output impedance therefor, and means including a fourth resistor and a first feedback control capacitor connected across said first resistor to make said negative output impedance substantially the complement of the electrical impedance of said voice coil.

2. In a sound reproducing system, the combination in accordance with claim 1, wherein positive feedback is applied over at least a portion of said amplifier to increase the overall gain thereof to a relatively high value.

3. In a sound reproducing system for driving a direct radiator loudspeaker having a moving voice coil, the combination of an audio frequency signal amplifier having a signal input circuit, a signal output circuit for said amplifier including a transformer having a secondary winding, positive feedback means connected over at least a portion of said amplifier to increase the gain thereof to a relatively high value, a first and a second resistor connected in series across said secondary winding, a third resistor, circuit means connecting the voice coil of the loudspeaker and said third resistor in series across said secondary winding, said third resistor being connected between the voice coil and the junction of the first resistor and secondary winding, circuit means connecting the junction of said voice coil and said third resistor to a point of reference potential for said amplifier, feedback means connected between the junction of said first and second resistors and the input circuit of said amplifier providing a negative output impedance for said amplifier, means including a fourth resistor and a first capacitor in series across said first resistor to make said negative output impendance substantially the complement of the electrical impedance of said voice coil, and means including a fifth resistor and a second capacitor serially connected in said feedback means to reduce the loading of said signal input circuit at bass frequency signals and provide bass frequency boost.

4. In a sound reproducing system for driving a direct radiator loudspeaker having a moving voice coil, the combination of an audio frequency signal amplifier having a signal input circuit, a signal output circuit for said amplifier including a transformer having a secondary winding, a first and a second feedback control resistor connected in series across said secondary winding, a third feedback control resistor, circuit means connecting said voice coil and said third resistor in series across said secondary winding, said third resistor being connected between the voice coil and the junction of the first resistor and secondary winding, means connecting the junction of said voice coil and said third resistor to a point of reference potential for said amplifier, feedback circuit means connecting the junction of said first and second resistors to said input circuit for applying positive and negative feedback over said amplifier providing a negative output impedance therefor, and means including a high frequency feedback control capacitor connected across said first resistor to make said negative output impedance substantially the complement of the electrical impedance of said voice coil.

5. In a sound reproducing system, the combination in accordance with claim 4, wherein positive feedback is applied over at least a portion of said amplifier to increase the overall gain thereof to a relatively high value.

6. In a sound reproducing system, the combination in accordance with claim 4, wherein a fourth resistor and a third capacitor are connected in series in said feedback circuit means to reduce the loading of said signal input circuit at low audio frequency signals and provide bass frequency boost.

7. In a sound reproducing system for driving a direct radiator loudspeaker having a moving voice coil, the combination of an audio frequency signal amplifier having a signal input circuit, a signal output circuit for said amplifier including a transformer having a secondary winding, positive feedback means connected over at least a portion of said amplifier to increase the gain thereof to a relatively high value, a first and a second resistor connected in series across said secondary winding, a third resistor, circuit means connecting the voice coil of the loudspeaker and said third resistor in series across said secondary winding, said third resistor being connected between the voice coil and the junction of the first resistor and secondary winding, circuit means connecting the junction of said voice coil and said third resistor to a point of reference potential for said amplifier, feedback circuit means connected between the junction of said first and second resistors and the input circuit of said amplifier providing a negative output impedance for said amplifier, means including a first capacitor connected in circuit across said first resistor and a second capacitor connected across said second resistor to control the frequency response of the amplifier and make said negative output impedance substantially the complement of the electrical impedance of the voice coil, and means including a fourth resistor and a third capacitor serially connected in said feedback means to reduce the loading of said signal input circuit at bass frequency signals providing bass frequency boost.

References Cited in the file of this patent UNITED STATES PATENTS 2,358,630 Fay Sept. 19, 1944