US2419615A - Hum reducing modulator - Google Patents

Description

APril'29 l94T .1.` o. wELDoN 'A HUM REDUCING MODULATOR Filed NOV. 25, 1942 y 1 high efficiency voperation signal frequencies.

Patented pr. 29, i947 HUM REDUCIN G MODULATOR James 0. Weldon, Del Rio, Tex., assgnor to Federal Telephone & Radio Corporation, New York, N. Y., a corporation of Delaware Application November 25, 1942, Serial No. 466.859

.'19 claims. (c1. 17e-171.5)

,This invention relates to Wave '.'transmission, and more particularly to the reduction of disturbing interference, hum and distortion tending to appear in thev output wave of an amplifying system. v

In a. carrier wave system utilizing a carrier Wave power amplifier at the output of the system and having an audio frequency signal amplifier for supplying amplified audio frequency modulating waves to the anode. circuit lof the power amplifier, the use of a lowfrequency alternating current for heating the cathode of theamplier, especially when the amplifier is of relatively high power, usually tends to cause modulation of the carrier wave at a plurality of hum frequencies corresponding to the fundamental and several harmonics of thecathode heating current. These hum frequencies extend over an appreciable frequency range, such as from 60 cycles to 300 cycles a second or higher, and thus may fall within the same frequency range as the desired signals, and

lead toserious complications and difficulties when l an attempt is made to reducethem to a negligibly small amplitude.

If an attempt is made to eliminate these hum components by theV usual negative feedback methods, many difculties arise which are not satisfactorily overcome by ordinary eXpedients.

- One of the difficulties is to maintain stable operation of thev audio frequency amplifying system without regeneration when .it is desired to provide a wide band high quality signal transmitting system, as for example in high fidelity'radio broadcasting. If the hum is opposed by the usual negative feedback circuit, the signal suffers a corresponding loss'of amplitude within the hum frequency range, and at the same timev a further Jserious difficulty arises due to the tendency of system employing amplification and subject tol interfering waves near the output of the system, and to reduce the amplitude of the undesired hum or other interfering waves appearing in f the output, without sacrificing the quality of the wave transmission.

Another object is to counteract .the above difficulties and to provide a wave transmitting system of relatively simple construction and of over a Wide range of Another object is to reduce hum or other interfering components produced in the output of a power amplifier by alternating current employed for heating the cathode of the amplifier.

Another object is to provide a substantially distortionless amplifier system having means for counteracting the effect of unwanted waves tending to be introduced near the output of the system.

Another object is to provide a negative feedback circuit for an audio frequencyramplifier, for transmitting to the input of the amplifier one or more vundesiredcomponents of hum frequency which have been introduced in the arnplifier, and to provide means for compensating for the resulting non-uniform frequency transmission of the amplier, so that the amplifier koperates substantially without distortion within the utilized frequency range including the hum frequencies, While effectively counteracting the near the output thereof, subject to interference,

is' provided with a first frequency selective negative feedback for suppressing interfering components in the output of the system, a second negative feedback being coupled between the f input of the amplifier and a part of the system in advance of that-subject to interference, the second feedback having a frequency selective characteristic,- theeffect of which is complementary to that of the first feedback, so that the distorting effect producedon the amplifier by the first feedback is counteracted without destroying the interference reducing effect of the first feedback.

In another aspect, the invention provides an interference or hum reducing negative-feedback circuit including a network having frequencyf amplitude and'frequency-phase characteristics adapted to improve the stability of operation of the amplier at vfrequencies outside the interference frequency band, as compared with the operation of an amplifier employing prior forms fnegative feedback networks.

'.From another viewpoint, the invention providesv an improved form of network particularly adapted for use in a negative feedback for an amplierfor permitting of stable operation of the'amplifier while providing a frequency selective characteristic therefor.

From anotherviewpoint, the invention provides an amplifier system in which two negative 3 ing eects with respect to desired signal waves and have non-complementary effects with respect to undesired interfering components, while having similar frequency-phase characteristics adapting the system for stable operation over a wide frequency range.

These and other objects and features of the invention will be understood more clearly in View of the following detailed description and claims and the accompanying drawings in which:

Fig. 1 is a schematic diagram of the circuit of a carrier wave transmitting system, such as a radio broadcast transmitter; and

Fig. 2 shows graphs of the transmission characteristics of networks A and B of Fig. 1.

In Fig. 1 the audio frequency source I transmits signal through transformer 2, through the push-pull amplifier stage 3, 4, of the electron tube type, through the additional amplifier stages 5,

through the output transformer 6 to the anode current supply conductor I of the carrier wave power amplifier 8 of the electronic type within which a carrier wave is modulated in well known manner in accordance with the audio frequency signal wave transmitted to conductor -`I, the re- .4

sulting modulated wave being transmitted to tuned circuit 9, coupled by the line I4 to the antenna or other load I0. The carrier wave source supplies waves to amplifier I2 which in turn excites the power amplifier 8. The low frequency y auxiliary source of power I3 is connected in well known manner with the cathodes, not shown, in amplifiers 8 and I2, for supplying heating current thereto.

The low frequency source I3 may be of the usual commercial frequency, such as for example as 60 cycles per second. As is well known, such a low frequency auxiliary current tends undesirably to modulate the carrier wave and produce hum components in the envelope of the modulated carrier wave transmitted from power amplifier 8 to circuits 9, and I8. The hum components have frequencies corresponding respectively with the fundamental of the low frequency source I3 and harmonics thereof as high as the fifth harmonic or higher. Since these hum components represent an interfering frequency within the same frequency range as that of the signal waves from source I, the usual methods of avoiding interference are not adapted to eliminate the hum, because such methods ordinarily tend to discriminate also against the desired signal frequencies within the hum frequency range, thereby distorting the desired signal wave.

A portion of the carrier wave energy, having an envelope modulated in accordance with the desired signal wave, is transmitted from the output line I4 over the line I5 and ground to the primary of transformer I6, the secondary of which has its terminals connected respectively with the anodes I1 of the rectifier I8. The cathode I9 of rectier I8 is connected through potentiometer resistances A2|) and 2| in series to a midtap connection 22 on the secondary coil of transformer I6. Rectifier I8 demodulates the carrier wave from the output of power amplifier 8, to reproduce the signal wave across the negative feedback line 23 which connects through'network B with the inputs of amplifier tubes 3 and 4, the network B being of such design as to cause the amplifier 3, 4, to discriminate against `waves within the hum frequency range, thus producing discrimination against signal wave components within the same hum frequency range and't'hereby distorting the signal'wave. In order to counteract the distorting effect of network B on the operation of the amplifying system, a negative feedback circuit 24 is connected from the output of amplifier stages 5 with the input of amplifier tubes 3, 4, the input of feedback 24 being connected to the system at a place in advance of the coupling of stages 5 with the power amplifier 8 where the hum or other interfering waives are introduced. The frequency-amplitude characteristie of network A connected in feedback 24, is complementary to that of network B and thus counteracts or compensates for the distorting leffect of network B on the desired signal wave.

Since `the frequency-phase characteristic of the amplifier 3, 4, 5 with its interstage couplings and input and output connections, varies with the frequency of the wave component being transmitted therethrough, difficulty arises in providing a feedback circuit 23 having such a frequencyphase characteristic as to maintain a proper value of negative .feedback to accomplish lthe desired results throughout the wide range of frequencies to be covered by the signal waves. Furthermore, any shift in the phase angle of network B must be kept Within suchlimits that the feedback circuit 23 can produce no appreciable positive feedback. In order to meet'these diculties, the frequency-phase characteristic of networks A and B are made similar to each other within the range of Afrequencies in which the amplitude of the feedback wave is sufficient to substantially affect the stabilityof the amplifier.

Further details of the circuit whereby the above results are obtained will now be described. An input circuitmay be traced from the control grid of tube 3 through the upper secondary coil of transformer 2, `through resistors 3U and 3| in series'to lthe cathode of the tube, the junction between resistors .30 and 3| vbeing grounded at V32. Similarly an input circuit may be traced from the control vgrid of tube 4 through the lower secondary coil of transformer 2 and resistors 33 and 34 in series to the Acathode of the tube, the vjunction between resistors 33 and 34 being also grounded at 32.

The output'of network B is connected with the ugrounded terminals of `resistors 3D and 33, so that the resistors 30 and 33 constitute couplings between the feedback 23 and the input circuits of tubes 3 and 4. The output terminals of network Aare .connected with the ungrounded terminals of lresistors 3| and 34, so that these resistors constitute'couplings between the feedback circuit 24 andthe input circuit of tubes 3 and 4. By providing separate coupling resistors for the Anegative feedback circuits 23 and'24, respectively,

interaction between these networks is reduced to a minimum land'couplings 30 and 33 are relatively independent of couplings 3| and 34.

Current is supplied to the anode of tube 3 through resistor 35 and to the anode of tube 4 through'resistor 36. The amplified output wave from tube 3 is transmitted from the anode thereof through the direct current blocking capacitor 31., and-through resistor 38 andcapacitcr 39 in parallel, to the input Vof the amplifier stages 5. Similarly the ampliiled'output wave from tube i4, is transmitted from 'the anode of tube 4 through-.the blocking capacitor 40, and through resistor 4I and capacitor 42 in parallel, to the input of stages 5.

jnparallellwiththe primary of output transformer Bis connected a 'circuit including the direct current blocking capacitor 43 the potentiometer 44, potentiometer 45 and the blocking caneutralized at all frequencies.

miams pacitor 4B all in series, the junction between potentiometers 44 and 45 being grounded at 41 and the reduced voltage tap-off points 48 and 49 be'- ing provided respectively on potentiometer 44 and 45, at which points are connected the conductors of the feedback .circuit 24.

Network A consists of a double, parallel T-network, that is,- of twopairs of T-networks, each pair including two Ts in parallel, the upper pair consisting of the resistors 50, 5| in series with each other in one line conductor of feedback circuit 24. The capacitor v52 being connected from ground to the junction of resistors 50, 5|. The other T of the same pair includes the capacitors 53 and 54 in series in the line conductor of the feed-back circuit; the resistor 55 being connected in'rshunt from ground to the junction between the capacitors 53, 54. In the lower half of network A, a similar pair of Ts are provided, one T including the series resistors 56, 51 and the shunt capacitor 58 connected between ground and the junction of resistors 56, 51, the other T including capacitors 25, 26 in series, the resistor 21 being connected in shunt from ground to the junction between capacitors 25, 26. The frequency-amplitude characteristic of network A and the frequency-phase characteristic thereof, are shown in Fig. 2 which also shows the complementary frequency-amplitude characteristic of network B, the frequency-phase characteristic thereof being represented by the same curve as that for network A.

Network B includes a lattice network having four impedance arms 60, 6|; 62, 63; 64, 65; and

66", 61; connected all in series with each other to form a closed circuit. The -two non-adjacent junctions 68 and 69, provide input terminals for tutes a Wheatstone bridge and the arms of the bridge or lattice are adjusted to balance according to well known practice in Wheatstone bridges, so that when a voltage is applied across input terminals 68, 69, no voltage appears across output terminals 10, 1I. In other words, the coupling which normally exists between the input and output terminals by virtue of the arms connected therebetween, is substantially completely Under these conditions, if the resistance of arms 60, 6| be called R1; that of arms 62, 63, R2; that of arms 64, 65,

R3; and that of arms 66, 61, RA; the relation of the resistances of the arms when balanced may be represented as follows:

The feedback circuit 23 would therefore produce no feedback eect at any frequency, if the balanced elements R1, R2, R and R4 were the only` elements coupling terminals 68, 69 with ter--A minals 16, 1I.

In order to provide a frequency selective path through network B, a frequency differs from that of the rst connection. For this purpose capacitors 10' and 1I' are connected'in series with each other from the junction of resistors 66, 61 in one arm to the junction of re-l sistors 62, 63 in a non-adjacent arin' of tlie lattice. `Similarly a circuit including resistor 12, capacitor 13, capacitor 14 and resistor 15 in series in the order named, is connected from the junction of resistors 60 and 6| in one arm to the junction of resistors 64,v 65 vin a non-adjacent arm. The junction between capacitors 10 and 1| vand the junction between capacitors 13 and 14 are grounded at 16'for the purpose of symmetry and balance of the network B with respect to ground, resistors 60, 61, 63 and 64 being preferably equal to each other, and resistors 6|,

66, 62 and 65 being also equal to each other, and

resistors 12 and 15 being equal to each other, capacitors 13 and 14 being equal, and capacitors 10 and 1 being equal to each other. The values of capacitors 10', 1I', 13 and 14 are so chosen with reference to resistors 12 and 15 as to provide paths in the lattice network having the frequency-amplitude and frequency-phase characteristic represented in Fig. 2. It will be seen that network B may be viewed in a slightly different manner from a Wheatstone bridge, by considering the network to be composed of twopairs of T-networks connecting the input to the output rof network B. In one pair of T-networks, one T includes series line resistors 60, 6| having at their junction a shunt connection to ground including resistor 12 in series with capacitor 13. The other T of the pair of T-networks, includes the resistor 61 in series with the resistor 66, having at their junction the shunt connection to ground including the lcapacitor 10. The other pair,4 of T-networks includes one T consisting of resistors 64 and 65 in fseries, having the shunt connection to ground from the junction of said resistors and including resistor 15 and capacitor 14' in series, `the other T of the pair of T- networks including resistors 63 and 62 in series and having a shunt connection including capacitor 1| from there junction to ground.` The resistors 12 and 15 serve to decrease the phase angle of the shunt path 12, 13, 14, 15.

A high frequency inductive impedance `is provided in conductor I5 to reduce the amplitude of the carrier wave transmitted to transformer I6, the resistor 8| being connected from conductor I5 to ground. Conductor I5 is preferably arranged vas the inner conductor of the coaxial cable 83, the outer cylindrical conductor of which is grounded and connected with one terminal of the primary coil of transformer I6, theother terminal of the primary coil being connected with? conductor I5 through the D. C. blocking capacitor 84.

Sliding contacts 85, 86 are provided respectively on potentiometers 26, 2| for adjusting the amount of voltage transmitted to the input terminals 68, 69 of network B and for adjusting the balance of the feedback circuit with respect to ground. Capacitors 81, 88 are provided in each side of the feedback circuit 23 for blocking the passage of direct current, resistors 89, 90 in series with the line conductors of the feedback circuit 23, serving to isolate or decouple network B from the circuit of rectifier I8. Equal small capacitors 9| and 92 in series and having their junction grounded, provide a path for the high frequency carrier wave component of the wave being rectified in rectier I8, the grounded junction between the capacitors serving to balance the line conductors with respect to ground. Equalcapacitors 93 and 94 of relatively small values compared with capacitors 10', 1I",`13 and 14, provide a'shunt path for any high frequency carrier wave component reaching network B from rectier'l'o, their reactances being so high as to have a negligible effect on the network. Capacitors 93 and 9dy are in series in a circuit connected from the junction of resistors 00, l and the junction of resistors 80, 65, the junction kof capacitors 93, 90 being grounded.

While network B has been disclosed with a frequency selectiveimpedance path 10', 1I'; and a further frequency selective impedance path l2, 13, 'irland "l5, it will be understood that in some cases particularly where a relatively small frequency range is covered by the hum frequency components, only one frequency selective path, for example, 'i 'il' or 12, 13, 14, 15, may sufce to `provide a frequency selective transmission characteristic for the otherwise balanced, nontransmitting network B. Y

The anode current supply conductor 'l connects from the anode, not shown, of power amplifier 8, through the audio frequency choke coil 'l1 and the space current supply source 'l5 to ground, the secondary coil of transformiert being connected in shunt with coil 'li and having the blocking capacitor 19- in series therewith to prevent direct current from passing through transformer 5.

A resistor 28 is connected between network A and the cathode of tube it,Y and a resistor 20 is connected between network A and the cathode of tube 4 to isolate or Vdecouple the network from the other circuits of the tubes. A negative feedback circuitis connected from `the ungrounded terminal of resistor 30 through the capacitive reactancev95 in series with the resistor 96 to the anode or output circuit of tube 3, a similar negative feedback circuit beingconnected from the ungrounded terminal of resistor 33 through the capacitive reactance 9'! in seriesV with the resistor 98.130 the anode or output circuit of tube I4, for reducing the gain of theampliiier at the highest. signal frequencies and at` frequencies above the highest utilized signal frequencies to prevent instability and singing of the amplifier at very high frequencies. v

The parallel resistor 38 andcapacitor 39 and the parallel resistor M and capacitor 42 serve to reduce the amplitudeof very low frequencies at the lower end of the signal frequency range to prevent singing at such frequencies in the form of oscillations sometimes referred to in the art as motor boating.

Resistors E0, 6l, 63, 50 serve to decouple the reactances of the frequency selective paths J', ll; and 12, 73, id, 15; from the other feedback paths and circuits of the amplifier tubes 3, and 4.

vIt hasfbeen ,found that the above described circuits are especially well adapted to provide vstable operation of the amplifier while substantially completely suppressing the-undesired components ofhum frequency introduced` in theloutput ofthe power amplifier as a result of the cathode heating current acting magnetically orv otherwisey to modulate the carrier wave current of the amplifier, at the same Ytimethe lnetwork A serving to rrestore the characteristic of the amplifier to a substantially fiat frequency amplitude characteristic throughout the range of the desired signal frequencies without destroying the hum reducing effect of network B. For example, in a particular case where a broadcast transmitter was adapted to transmit modulating signal 'frequency wavesv within arange extending fromapproximately 30 cycles vper second-to 10,000

cycles per second, without objectionable departure from a flat transmission characteristic, interfering frequencies representing hum components extending from 60 cycles per second to as high as 720 cycles per second at appreciable amplitudes were substantially completely suppressed by the effect of network B on the amplifier, while network A substantially completely compensated for the distortion introduced into theroperation of the amplifier by network B, without disturbing the hum reducing effect of network B.

In the particular case just referred to, in network A, eachof the resistors 50, 5|, 50 and 51 had a resistance of 1,500 ohms; each of the capacitors 53, 50,25 and 20 had a capacitance of 0.85 microfarad; and each of the resistors 55 and 2i had a resistance of 600 ohms. In the same case, in network'B, each of the resistors 00, 0l, 63, 60 had a resistance of 100,000 ohms; each of the resistors 5 l, 00, 02, 55 had a resistance of 50,000 ohms; each of the resistors 'l2 and 'l5 had a resistance of 2,000 ohms; each of the capacitors l0 'Il' has a capacitance of 0.05 microfarad; `and each of the capacitors 'i3 and 'M had a capacitance of 0.01 microfarad.v In this same case, each of the resistors 30 and 33 had a resistance of 25,000 ohms; each of the resistors 3l and 30 has a resistance of 2,000 ohms; each ofthe resistors 28, 20 had a resistance of 2,000 ohms; each of the resistors and 90 had a resistance of 50,000 ohms; each of the potentiometers 20 and 2! had a resistance from ground to its respective slider 05 and 85,*of approximately 10,000 ohms; each of the capacitors 9|,

B is ymade-negligible, so that the desired negative feedback relationmay be maintained over a wider frequency range of amplification than in the case where an amplifier is provided with the usual form Vof frequency selective circuit without any balanced Wheatstone bridge or lattice network that is balanced at all frequencies applied to the network. The network B is therefore particularly well adapted for use in negative feedback circuits in general, apart from the specific combination with which it is used in the vabove described carrier Wave system.

Instead of regarding the network A as a corrective network to compensate for the distortion produced in the amplifier 5, 4,5 as a result of network B, the network B may be regarded as a corrective element for the distorting effect of netin the hum frequency range in amplifier 8 to their normal amplitude relation to components outside the vhum range while `preserving the improved signal-to-noise ratio.

Various modifications of the abover'described was - amplifier and said power amplifier for applying modulating voltages to said power amplifier, a

rst negative feedback means coupling the output of said'power amplifier with the input of said audio frequency amplifier, said first feedback means including detecting means for producing an audio frequency current corresponding with the modulations of the high frequency carrierwave in the output of said power amplifier, said first feedback means including a network for selectively transmitting waves of said hum frequencies to the input of said audio frequency amplifier for reducing the effect ofsaid undesired hum modulation in said power amplifier, and second feedback means coupling the output of said audio frequency amplifier with the input thereof and including means for counteracting the distorting effect of said first feedback means on said audiofrequency amplifier without destroying the hum reduction produced by said first feedback means.

2. In a wave transmitting system according to claim 1, the combination in which the respective frequency-phase characteristics of said feedback means are substantially equal to each other.

3. In a wave transmitting system according to claim 1, the combination in which the respective frequency-attenuation characteristics of said feedback means are substantially complementary to each other.

4. In a wave transmitting system according to claim 1, the combination in which the respective,-

frequency-attenuation characteristics of said feedback means are substantially complementary to each other and in which the respective frequency-phase characteristics of said feedback means are substantially equal to each other.

5. In a wave transmitting system according to claim 1,'the combination in which an anode current supply conductor is provided for said power amplifier and said coupling with the output of said audio frequency signal amplifier iny cludes a coupling with said anode current supply conductor.

6. In a wave transmitting system according to claim 1, the combination in which said first feedback network includes a filter network of the Wheatstone bridge type having four arms in balanced relation with respect to the input and output terminals of the bridge at all frequencies to -be applied to the bridge, and a frequency selecl tive impedance connected from an intermediate c point vin one of said arms to a point on another of said arms having a different potential from said intermediate point in response to an alternating voltage impressed upon the input terminalsA ofthe bridge.

7. In a wave transmitting system according to claim 1, the combination in which said first feedf back network includes a filter having four resistor arms joined in series with each other to forma closed circuit, two non-adjacent junctions 10 of said arms forming-input terminals for said filter, the othern two non-adjacent junctions vof said arms forming output terminals for said filter, eachof two of Asaid non-adjacent arms including two resistors joined in series and providing an intermediate tap at their junction, and

a frequency selective unit connectedbetween said two taps. y

8. In a wave transmitting system according to claim 1, the combination in which said irstfeedback-networkincludes a filter having four resistor arms joined in "series with each other to form a closed circuit, two non-adjacent junctions of said arms formingr input terminals vfor said filter, the other two non-adjacent junctions of said arms forming output terminals for said filter, eachA said arm including two resistors joined in seriesand providing an intermediate tap at their junction, a first'unit having a predetermined frequency selective characteristic connected between the taps of two of said non-adjacent arms, v

and a second unit having a different predetermined frequency yselective characteristic from said 4first unit `connected between. the taps of the other two of said non-adjacent arms.

9.' In a wave transmitting system according to claim 1, the combination in which said first feedback network includes a, filterA having four resistor arms joined in series with each other to form a closed circuit, twonon-adjaoent junctions of said arms forming input terminals for said filter, the other two non-adjacent junctions of said arms forming output terminals for said filter, each of two of said non-adjacent arms including two resistors joined in series., and providing an intermediate tap at their junction, and a frequency selective .unit includingv two capacitors joined in series and having a ground connection at their junction, said unit being connected between said two taps.

, 10. In a wave transmitting system according to lclaim 1, .the combinationinwhich said first feedback network includes a filter having' four resistor arms joined in series with each other to form a Closed circuit, two nonadjacent junctions of said larms, forming'input terminals forsaid filter, the other two non-adjacent junctions of Ysaidl arms forming output `termiiials for said filter, each said arm including twoV resistors joined in series and providing an intermediate tap at their junction, a first unit'having a predetermined capacitance connected ibetween the taps of two of said non-adjacentarrns, and a, second unit including a predetermined resistance in series with a capacitance connected between thetaps of the othertwo of said non-adjacent arms.

11. In ay high frequency carrier wave transmitting system, a carrierwa've power amplifier of I the electron-discharge type having a low frequency auxiliary power supply circuit tending to produce undesired 'modulation Aof said carrier wave in said power amplifier at hum frequencies,

- an audio frequency signal amplifier, a coupling between the output of said audio frequency signal amplifier-and vsaidpower amplifier for applying modulating voltages to said power amplifier, a first negative feedback means coupling the output of said power amplifier with the input of said audio frequency amplier, said first feedback means including detecting means for producing an ,audio frequency current corresponding with the vmodulation of the high `frequency carrier wave in the outputof. said power amplifier,v said rst feedback .means including a network for selectively transmitting waves of said hum fre- 11 quencies to the input' of said audio' frequency amplifier for reducingthe effect offs-aid undesired hum modulation in saidA rpower amplifier, and second feedback means coupling theY output of said audioV frequencyv amplifier with the input thereof and including means for counteracting the distorting effect of said first feedback means on said audio frequency amplifier without destroying the'v `hum reduction Vproduced by said first feedbackjmeans, `and a'third negativev feedback means coupling, the output of one'of the stages of the signal amplifier with the input of one of the stages Afor reducing the gain of the higher frequencies l2. In av high frequencyA carrier wave transmitting. system, a carrier wave power amplifier of the electron-discharge type having a low frequency auxiliary power supply circuit tending to produce undesired Vmodulation of said carrier wave in, said power amplifier at hum frequencies, an audio frequency signall amplifier including at least a` vfirst stage', a coupling between the output of said audio frequency signal amplifier and said power amplifier for applying modulating voltages to said power amplifier, a first negative feedback f' means coupling the output' of said power amplifier with the input of said. audio frequency amplifier, said first Vfeedback means including detecting means .for producing' an audio frequency current corresponding with the modulation ofthe high frequency carrier Wave in the output of said power amplifier,- said first feedbackmeans including a network-for selectivelytransmitting waves of said hum frequencies to the input of said audio frequency amplifier for reducing the effect of saidundesired hum modulation in said power amplifier, and second feedback means coupling the output of said audio frequency amplifier with the input thereof and including means for counteracting the distorting effect. of said first' feedback means on said -audio frequency amplifier without destroying thehumv reduction produced by said first feedback means, and a third negative feedback means coupling the output of the first stage of thesignal amplifier with the input.

13e. In aV` high frequency carrierk wave transmitting system, a carrier wave power amplifier of the electron-discharge type having a low frequency auxiliary power supply circuit tending to produce undesired modulation of said carrier wave in said power amplier at hum. frequencies, an audio frequency signal amplifier, a coupling between the output of said audio frequency signal amplifier and said power amplifier for applying modulating voltages to said power amplifier, a

negative feedback means coupling the output of said power amplifier with the input of said audio frequency amplifier, said feedback means including detecting means for producingA an audio frequency current corresponding with the modulation of the high frequency carrier wave in the output of said power amplifier, and said feedback means including a filter network of the Wheatstone bridge type for selectively transmitting waves of said hum frequencies to the input of said audio frequency amplifier for reducing the effect of said undesired hum modulation in said power amplifier including four arms in balanced relation with respect to the input and output terminals of the bridge at all frequencies to be applied to the bridge and a frequency selective impedance connected from an intermediate point in oneof said arms to a point on another of said arms having a different potential from said intermediate point in response to an alternating voltage impressed upon the input terminals of the bridge. f

11i-1in a high frequency carrier wave transmitting system, a carrier waveV power amplifier of the electron-discharge type having Va low frequency auxiliary power supply circuit tending to produce undesired modulation of said carrier wave'in said power amplifier at hum frequencies, an audio frequency signa-l amplifier, a coupling between the output of said audio frequency signal amplifier' and said power amplifier for applying modulating voltages to said power amplifier, a negative feedback means cou-pling the output of said power amplier withthe input of said audio' frequency amplifier, said feedback means including detecting means forproducing an audio frequency current corresponding with the modulations of the high frequency carrier wave in the output of said power amplifier, said yfeedback means including a filter network for selectively transmitting waves of said hum frequenciesto the input of said audio frequency amplifier for reducing the effect of said undesired hum modulation in said power amplifier having four reistor arms joined in series with each other to form a' closed circuit,.two non-adjacent junctions oi" said arms forming input terminals for said filter the cthertwo non-adjacent junctions of said arms forming output terminals for 'said filter, each of two of said non-adjacent arms including two resistors joined in series Vand providing an vintermediate tap at their junction, and a frequency selective unit connected between said two taps.

l5. Ina high frequency carrier wave transmitting system, a carrier wave power amplifier' of the' electron-discharge type having a low frequency auxiliary power supply circuit tending to produce undesired modulation of saiolfcarrier wave in said power amplifier at' hum frequencies, an audio frequency signal amplifier, a coupling between the output of said audio frequency signal amplifier and said power amplifier for applying modulating voltages to said power amplifier, a negative feedbackmeans coupling the output of said power amplifier with the input of said audio frequency amplifier, said feedback meansincluding detecting means for producing 'an audio frequency current corresponding with the modulations of the high frequency carrier wave in the output of saidpower amplifier, said feedback means including a filter network for selectively transmitting wavesof said hum frequencies to the input of said audio frequency amplifier for reducing the effect of said undesired hum modulation in said power amplifier having four resistorarms joined in series with each otheito form a ,closed circuitjtwo non-adjacent junctions of said arms forming input terminals for said filter,l the other two non-adjacent junctions of said arms forming output terminals for said filter, each said arm including vtwo resistors joined in series and providing an yintermediate tap at their junction, a first unit having a predetermined frequency selective characteristic connected between the taps of two of said nonadjacent arms, anda second unit having a different predetermined frequency selective characteristic from said first unit connected between the taps of the other two of said non-adjacent arms.

16. In a high frequency carrier wave transmitting system, a carrier wave power amplifier of the electron-discharge type having a low frequency auxiliary power supply circuit tending 113 to produce undesired modulation of said carrier Wave in said power amplifier at hum frequencies, an audio frequency signal amplifier, a coupling between the output of said audio frequency signal amplifier and said power amplifier for applying modulating voltages to said power amplifier, a negative feedback means coupling the output of said power amplifier with the input of said audio frequency amplifier, said feedback meansy including detecting means for producing an audio frequency current corresponding with the modulations of the high frequency carrier wave in the output of said power amplifier, said feedback means including a filter network for selectively transmitting waves of said hum frequencies to the input of said audio frequency amplifier for reducing the effect of said undesired hum modulation in said power amplifier having four resistor arms joined in series with each other to form a closed circuit, two non-adjacent junctions of said arms forming input terminals for said lter, the other two non-adjacent junctions of said arms forming output terminals for said filter, each of two of said non-adjacent arms including two resistors joined in series and providing an intermediate tap at their junction, and a frequency selective unit including two capacitors joined in series and having a ground connection at their junction, said unit being connected between said two taps.

17. In ya high frequency carrier wave transmitting system, a carrier wave power amplier of the electron-discharge type having a low frequency auxiliary power supply circuit tending to produce undesired modulation of said carrier wave in said power amplifier at hum frequencies, an Iaudio frequency signal amplifier, a coupling between the output of said audio frequency signal amplifier and said power amplifier for applying modulating voltages to said power amplifier, a negative feedback means coupling the output of said power amplifier with the input of said audio frequency amplifier, said feedback means including detecting means for producing an audio frequency current corresponding with the modulations of the high frequency carrier wave in the output of said power amplifier, said feedback means including a filter network for selectively transmitting waves of said hum frequencies to the input of said audio frequency amplifier for reducing the effect of said undesired hum modulation in said power amplifier having four resistor arms joined in series with each other to form a closed circuit, two non-adjacent junctions of said arms forming input terminals for said filter, the other two non-adjacent junctions of said arms forming output terminals for said filter, each said arm including two resistors joined in series and providing an intermediate tap at their junction, a first unit having a predetermined capacitance connected between the taps of two of said non-adjacent arms, and a second unit including a predetermined resistance in series with a capacitance connected between the taps of the other two of said non-adjacent arms.

18. In a wave transmitting system, an input circuit and an output circuit, wave amplifying means having means which introduces interference in a limited band of low frequencies, a iirst negative feedback means coupling said output circuit with said input circuit including a lter network for selectively reducing the gain of said amplifying means in the limited band of low frequencies and thereby producing distortion therein, and a second negative feedback means coupling said input circuit with a part of said amplifying system in advance of said output circuit including a network filter for reducing the effect of the distortion produced by said first feedback means in the limited band of frequencies, and a third negative feedback means coupling said input circuit with a part of said amplifying system in advance of said interference producing means for reducing the gain at the high frequencies.

19. In a wave transmitting system for transmitting carrier frequencies modulated by frequencies within a certain total band, an input circuit and a carrier output circuit, wave amplifying means, a rst negative feedback means selectively coupling said output circuit to said input circuit for reducing the gain of said system predominantly Within a restricted portion of said total band of frequencies and incidentally introducing both amplitude and phase shift distortion in said system throughout said total band of frequencies, and a second negative feedback means coupling said input circuit with a part of said transmitting system in advance of said output circuit and having attenuation and phase shift characteristics substantially completely compensating for said distortion produced in said system by said first negative feedback means.

JAMES Oi. WELDON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,093,751 Witt Sept. 21, 1937 2,173,427 Scott Sept. 19, 1939 2,133,410 Wirkler Oct. 18, 1938 2,163,670 Ditcham June 27, 1939 2,172,453 Rose Sept. 12, 1939 2,285,896 Brown June 9, 1942 1,853,929 Rettenmeyer Apr. 12, 1932 2,058,210 Bode Oct. 20, 1936

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