Patent Publication Number: US-3875334-A

Title: Multi-channel control circuit with D-C operated control devices

Description:
Apr. 1, 1975 1 1 MULTI-CHANNEL CONTROL CIRCUIT WITH D-C OPERATED CONTROL DEVICES Inventors: Francis H. Hilbert, Addison, 111.; Paul Dean Filliman, Fort Wayne, 1nd.  
 Assignee: Motorola, Inc., Franklin Park, 111.  
 Filed: June 19, 1973 Appl. No.: 371,437  
 11.8. CI 179/1 D, 179/1 VL Int. Cl. H04m 1/00 Field of Search 179/] D, 1 VL, 1 G;  
 [5 6] References Cited UNITED STATES PATENTS FOREIGN PATENTS OR APPLICATIONS 726,253 1/1966 Canada 179/1 VL TREBLE Primary Examiner-Ralph D. Blakeslee Attorney, Agent, or FirmVincent .1. Rauner; Donald J. Lisa [57] ABSTRACT A multi-channel control circuit arrangement is disclosed for effecting a plurality of predetermined signal control functions. The preferred embodiment comprises an audio control circuit in integrated circuit form with the required audio control functions being effected by simple, single-section. dc operated controls external to the IC circuitry, one for each function desired, regardless of the number of audio channels present. The d-c operated controls are in the form of variable resistance elements, which controls the bias of one portion of an associated differential amplifier acting as a current steering device for applied audio signal frequencies. The tone control arrangement. i.e.. treble and bass, is further characterized by a feed forward or feed back of applied signal frequencies to effect, respectively, the desired cut or boost action in the translated signal.  
 21 Claims, 6 Drawing Figures BASS  1 i. I r 5 l 1 LEFT LEFT LEFT mam mam FILTER SIGNAL 2: TREBLE 6ND TREBLE ag? SIGNAL INPUTS INPUT SIGNAL SIGNAL INPUT I6 15 14 13 I2 1 1 1O 9 j: &#39;5 0 1 2 3 4 5 s 1 6 LEFT LEFT meur mewmew BASS s mum. a gam a SIGNAL e455 FILTER sasmu. OUTPUT OUTPUT SIGNAL INPUTS L E P I F VOLUME l 29 BALANCE I l Q 1, r I 1 MULTI-CIIANNEL CONTROL CIRCUIT WITH D-C OPERATED CONTROL DEVICES BACKGROUND OF THE INVENTION The present invention relates in general to multichannel control circuits and, more particularly, to an improved multi-channel control circuit arrangement suitable for use in the audio section of an associated receiver wherein the required audio control functions, such as volume, balance, treble and bass, may be provided for any number of audio channels by a simple but inexpensive, single-section, d-c operated control element for each such function desired.  
  In the past, user-operated, manually adjustable control functions for a plurality of audio channels, such as encountered in stereophonic or quadrasonic music systems and the like, have required more than just one variable resistance control element. Usually, separate potentiometers are interconnected on a single control shaft with each such variable resistance element being interposed in an associated audio channel. Multicontrol elements previously were considered essential, since signal information was effectively controlled by passing through the resistance element itself, and any attempt to combine the function of one such element with that of another resulted in undesirable cross-talk.  
  As to tone controls specifically, i.e., treble and bass, the separate potentiometers in associated housings, and ganged together for operation from a common control shaft, created an unwieldly and rather cumbersome configuation. The further problem of insuring that each of the separate resistance elements track simultaneously was also imposed. Additionally, the associated circuitry utilized to effect the selective enhancement of the treble and bass tones in the translated audio signal information was customarily complex, and especially so in the multi-channel systems. They were also expensive, and frequently necessitated substantial space requirements.  
  Accordingly, it is an object of the present invention to provide an improved multi-channel control circuit arrangement having a plurality of manually adjustable controls for high and low frequency roll-offs as well as other control functions, which arrangement obviates the foregoing deficiencies.  
  A more particular object of the present invention is to provide an improved audio control circuit arrangement for a multi-channel system wherein the desired audio control functions may be readily effected for any number of signal channels by a single-section, d-c operated, variable resistance element for each such desired function.  
  Another object of the present invention is to provide an audio control circuit arrangement of the foregoing type wherein the audio signal information does not pass through the individual adjustable control elements themselves.  
  Yet another object of the present invention is to provide an audio control circuit arrangement of the foregoing type that may be effected in a single monolithic integrated circuit with the required audio control functions, including treble, bass, volume and balance, being provided by simple and external potentiometer devices coupled to a source of unidirectional potential.  
  Still another object of the present invention is to provide an audio control circuit arrangement of the foregoing type in integrated circuit form wherein the d-c levels are effectively balanced and do not substantially change with differing settings of the associated d-c operated, audio control devices.  
 SUMMARY OF THE INVENTION In one aspect of the present invention, a technique is established for multi-channel audio systems so as to provide effective tone control as well as volume and balancing action by the use of single-section, variable resistance elements to which d-c voltage is applied at one terminal thereof. The audio control circuit arrangement preferably is provided in monolithic, integrated circuit form, with the various audio control functions, (treble, bass, balance and volume) included as external to the [C package. The control circuitry itself includes a plurality of current steering arrangements in the form of differential amplifiers. Tone control is accomplished by one such differential amplifier in conjunction with a pair of filter networks. The output of the differential amplifier is fed through the two filters, one a high-pass and the other a low-pass. and the respective signals, representing bass and treble signal information, is either fed back to the differential amplifier (comprising a cut action) or is fed forward (providing a boost&#34; action). The feed back or feed forward is effectively controlled by an associated variable resistance or potentiometer, one for treble frequencies and one for bass frequencies.  
  There is a related action provided for balance and volume control functions, except, of course, that the high and low pass filter networks are not required and feed back is not employed. Pairs of differential amplifiers, operating in parallel, are controlled by an associ ated potentiometer or control element coupled to an appropriate source of d-c potential. The referenced signal information is either shunted to ground, or effectively passed there thru without substantial change. For volume control, the right and left channel differential amplifiers are operated in a same direction, i.e., both fed forward or both shunted out, in unison. For balance control, the two referenced signals are controlled complementary to one another, one fed forward while the other is not, and visa-versa.  
 BRIEF DESCRIPTION OF THE DRAWINGS The novel features which are believed to be characteristic of the present invention are set forth with particularity in the appended claims. The invention itselt, however, will best be understood by reference to the following description when taken in conjunction with the drawings, in which:  
  FIG. 1 is a block and schematic representation of an audio control circuit arrangement in integrated circuit form and constructed in accordance with the present invention;  
  FIG. 2 is a block diagram of a portion of the audio control circuit of FIG. 1 and illustrating the operation of various of the audio controls;  
  FIG. 3 is a partial schematic diagram of the tone control arrangement included in the circuit arrangement of FIG. 1;  
  FIG. 4 is a partial schematic diagram ofthe balancing balance and volume control arrangement included in the circuit arrangement of FIG. 1;  
  FIG. 5a is a simplified schematic representation of the volume control arrangement; and  
  FIG. 5b is a simplified schematic representation of the balance control arrangement.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, an audio control circuit arrangement is shown in FIG. 1 in integrated circuit form, which control circuit has been constructed in accordance with the present invention. As indicated, the integrated circuit chip includes some 16 separate bonding pads serving as external connection points. Although not restricted to such application, circuit 20 is nevertheless especially suited for use in stereophonic systems which incorporate left and right audio channels. For quadrasonic audio systems, a pair of such integrated circuit chips 20 will suffice. It is to be expressly understood, however, that circuit 20 may find advantageous application in multi-channel systems other than stereophonic quadrasonic arrangements, nor is the invention limited to the control of audio frequencies.  
  For apparatus with multiple audio channels, the purpose of circuit 20 is of course to permit operation of certain basic audio control functions, i.e., volume, balance. treble or bass, for left and right audio channels, wherein each such control function is effected by a d-c operated control in the form of a simple, inexpensive, single-section, variable impedance device. As shown in FIG. I, potentiometer 22, having its fixed terminals connected between a source of unidirectional potential and ground, provides the desired treble control, while a similarly connected potentiometer 24 provides for bass control. Similarily, a potentiometer 26 effects the desired control of the volume while a potentiometer 28 serves as the customary balance control.  
  The operation of the audio control circuit 20 may be more readily appreciated by reference to the block diagram of the circuit arrangement as depicted in FIG. 2. As there shown. the audio signal derived in the associ ated reciver apparatus (not shown) is applied to an input terminal 15 from there to a differential amplifier circuit 30. One output of the differential amplifier 30, at a level of approximately one-half that of the input, is fed directly to a tone control output reference terminal 32. The other output of the differential amplifier 30, at approximately six times the input, is applied to a filter network 40. One output of filter network 40 passes frequencies above some predetermined reference, say 1,000 Hz, and thus functions as a high pass filter. while the other serves to pass frequencies below the referenced 1,000 Hz level for the required low pass control. The signal output comprising essentially high pass frequencies (treble) is applied to a first d-c control network 50, and the signal output of low frequencies, or bass, is applied to a similar, but separate, d-c control network 60. Each of the d-c control circuits 50 and 60 includes a pair of outputs. One output of each is fed back to the differential amplifier 30, while the other output of each is fed forward to the tone control output reference terminal 32.  
  When the movable contact arm of the treble control or potentiometer 22 is adjusted near the B-lterminal or end, the audio signal. comprised substantially of treble frequencies, is steered to the output which effects a feedback to the differential amplifier 30, thereby constituting the cut&#34; position or adjustment. However, with the movable arm of the potentiometer 22 near the ground terminal end, the reverse is true and the signal information is steered to the output providing a feed forward, or boost&#34; action. A similar action is effected for bass frequencies upon the adjustment of the bass control potentiometer 24, except that in this case the boost action is for frequencies below the 1,000 Hz reference.  
  It is to be noted that an important aspect of the present invention is the fact that three separate waveforms comprise the translated signal appearing at the tone control output terminal 32 and not just two as customarily employed in prior systems. In the disclosed arrangement at hand, one segment is obtained from the output of differential amplifier circuit 30, while the other two are obtained at the outputs of d-c control circuits 50 and 60. These latter two either boost or cut sig nal frequencies at the respective ends of the frequency range, but do not substantially change the mid-range frequencies. In this way, the undesirable cusping&#34; action heretofore encountered in prior systems, where only two waveform segments are employed, is effectively avoided. The referenced cusping action is the result of bass and treble frequency segments developing out of phase relationships as the composite audio signal is passed thru successive bandpass networks and then cancelling out as they interface. This cannot occur in the disclosed system of the present invention.  
  In any event, the referenced tone controls 22 and 24 are designed to provide a total boost, say, on the order of +21 db and a *cut&#34; of approximately -15 db from the 1,000 Hz reference level. That is, under the cut action, or feedback condition, the gain at the output of differential amplifier 30 is associated with the tone control output reference terminal 32 decreases approximately 6 db per octave, to the limit of 21 db (-15 db from the 1,000 Hz reference level). This represents a full cut position with a magnitude of attenuation of six. Conversely, in the boost action, or feed forward operating condition, the gain of the differential amplifier will increase at a rate of approximately +6 db per octave to the limit of +21 db, from the 1,000 Hz reference. The reference figures of course, are in no way critical and are intended for illustrative purposes only. They may be varied to fit the specific design requirements of the particularized application.  
  An operative schematic of the tone control portion of the integrated circuit arrangement 20 is shown at 20a in FIG. 3. The audio control circuitry is shown for only one channel, i.e., the left audio channel, in the interests of brevity. It is to be understood, however, that the circuitry for the right channel is essentially the same as that depicted for the left channel and operates in the same way.  
  As indicated, the audio signal is applied to the input at bonding pad 15, and from there to the differential amplifier circuit 30. The base inputs of transistors 30a and 30b are interconnected by a resistance 33 and from there to ground thru a resistance 34. The emitters of transistors 30a and 30b are interconnected by a pair of resistances 35 and 36, and the junction thereof is in turn coupled to the collector of a third transistor 30c, acting as the current source for the former two. Resistance 37 serves as the emitter load for transistor 30c, while resistances 38 and 39 serve as collector loads for transistors 30a and 30b.  
  As will be appreciated, the collector of transistor 30b serves as one output of differential amplifier 30 and,  
 further, is associated with tone control output reference terminal 32. The other output of amplifier circuit 30 is taken at the collector of transistor 300, the latter being coupled to filter network 40 through a transistor 41 and resistance 42. The value of resistance 33 is chosen such that, in conjunction with the level of operating potential applied to differential amplifier circuit 30, an output is obtained at the collector of transistor 30b which is approximately one-half the input signal level present at bonding pad 15. At the same time, the output at the collector of transistor 30a is approximately 6 times the referenced input.  
  Filter network 40 includes a capacitor 43 coupled to bonding pad 13 and is of value to pass frequencies above the arbitrary l,000 Hz reference, thereby serving as the required high pass filter facility. Capacitors 44 and 45, and resistance 46, serially connected between bonding pad and ground external to the IC circuit 20, serves as the required low pass filter. The output, taken at the junction of capacitor 45 and resistance 46, is applied to bonding pad 1. Resistance 47 comprises a part of the emitter load for transistor 42, while capacitor 48 is for compensation and provides a roll-off at approximately 25 KHz.  
  The separated high and low signal frequencies at bonding pads l and 13, respectively, are applied to the d-c control circuits 50 and 60. Circuit 50 includes current steering devices in the form of pairs of differential amplifiers 52 and 54. Each is comprised of three transistors, 52a-52b-52c and 54a-54b-54c, suitably interconnected, as shown. Transistors 52c and 54(- serve as the current or signal sources, while the pairs of transis tors, 52a-52b and 54a-54b, serve as the associated current steering devices. The base inputs of transistors 52a and 54b are interconnected as are the base inputs of transistors 52b and 54a. Resistances 55 and 56 serve as emitters loads. Operating potential is applied to the collectors of transistors 52a and 540 thru a resistance 58 connected to 8+.  
  In operation, transistor 52c is biased to predetermined conduction level by a reference voltage applied to its bass thru the resistance 59 and, further, upon which the high or treble signal frequencies are superimposed. This serves as the current or signal source for differential amplifier 52 and will be steered through transistors 52a and 52b in accordance with the applied bias. A fixed reference voltage is applied to the junction of the bass inputs of transistors 52b and 540 as indicated, while a variable bias potential may be applied to the bass inputs of transistors 52a and 54b by the treble control or potentiometers 22 thru associated resistances 22a and 57.  
  Accordingly, when the bias voltage as applied to the bass of transistor 52a is at a higher level than that as applied to the bass of transistor 52b, the majority of signal current will be steered thru transistor 52a. The actual magnitude of course will depend upon the specific difference in applied bias voltages at the respective bases. In any event, a substantial portion of the signal frequencies at the bass of transistor 52c will be fed back through transistors 70 and 74, acting as level shifters, to the bass input of transistor 30b of differential amplifiers 30. This serves to reduce the signal information at the associated output and, in turn, the signal level at tone control reference terminal 32. Thus, treble control 22 in this position provides a cut action. However, with treble control 22 adjusted to a position whereby the bias at the bass of transistor 52a is lower than that as applied to the bass of the transistor 52b, the signal current through transistor 52b is increased while that of transistor 52a is decreased accordingly. The output of transistor 52b is then applied or fed forward to the tone control reference terminal 32, thereby reinforcing the signal information applied thereto from the output of transistor 30b. In this operating condition, a boost or feed forward obtains as previously described.  
  It may be appreciated that differential amplifier 54 is included to provide a d-c balancing action to avoid undesirable shifts in d-c levels. Differential amplifier 54 operates in parallel with differential amplifier 52, but is interconnected with the reference voltage source and that as applied from treble control 22 so as to affect the opposite conductive states. That is, transistor 54a is non-conductive when transistor 52a is rendered conductive, and transistor 54b will be non-conductive while transistor 52!) is conductive, and visa-versa.  
  The operation of do control circuit 60 is essentially the same as that set forth for d-c control circuit 50, such that further and more detailed description should not be necessary. The only difference is that low, or bass, frequencies are applied to the base of associated current source transistor 62c. Feedback, and reduced output from differential amplifier 30, will occur when most of the signal current is steered through transistor 62a, while a feed forward, or boost action, occurs when the signal current is effectively steered through associated transistor 62b.  
  Lastly, level shifter 80, coupled to the tone control output reference terminal 32, is comprised of transistors 81 and 82, resistances 84-86 and capacitor 87, all interconnected in the manner as indicated.  
  For the balancing and volume control functions, reference may be conveniently made to the schematic representation of FIG. 4. Both right and left audio channel segments are indicated so as to show the required interaction for the balancing control function. The left audio channel includes a pair of d-c control circuits and while the right channel includes similar circuits and 130. Each of the control circuits are comprised of current steering arrangements comprising pairs of differential amplifiers. DC control circuit 100 includes differential amplifiers 102 and 104 while circuit 110 includes differential amplifiers 112 and 114.  
  For left channel operation, the audio signal information is applied to the bass of transistor 1020, serving as the current source, which is then selectively steered through transistors 102a and 1021). Differential amplifier 104 serves to balance the d-c current levels as previously described in connection with differential amplifier 54 of d-c control circuit 50. The alternating current signal at the output of transistor 1112b is then steered through circuit 112 with circuit 114 providing a similar d-c balancing action. Accordingly, the signal output at the collector of transistor 112b is coupled to an emitter follower 116, and therethrough to bonding pad 2, to become the left signal output.  
  A similar action of course occurs in the right audio channel, and the signal information applied to source transistor 1226 is selectively steered through differential amplifiers 122 and 132 of d-c control circuits 120 and 130. The signal output at the collector of transistor 13212 is applied to emitter follower transistor 136, and  
 from there to bonding pad 6, to become the right audio signal output.  
  The balancing control 28 is effectively coupled to the junction of the bass inputs of transistors 102a and 104b, and also to the bass inputs of transistors 12215 and 124a. Additionally, a fixed reference voltage is applied from a source 150 to the bass inputs of transistor 122a and 124b, and also to the bass inputs of transistors 10212 and 104a.  
  Similarly, the volume control 26 is coupled to the junction of the bass inputs of transistors 112a and 114b, and also the bass inputs of transistors 132a and l34b. A fixed reference voltage is applied from a source 160 to the bass inputs of transistors ll2b and 114a, as well as to the bass inputs of transistors l32b and 1340.  
  The operation of the balance and volume control action may be best understood by reference to the simplified schematic representations as set forth in FIGS. a and Sb. The control differential amplifiers 102 and 122 are shown with the reference voltage source 150 applied to reference terminal 157, and in turn to the bass inputs of transistors l02b and 122a, in common. The balance control 28, external to the integrated circuitry, applied a selected d-c voltage through series resistances 152 and 154 to another reference terminal 155 common to the bass inputs of transistors 102a and l22b. A further resistance 156 is connected between reference terminals 155 and 157, and thus across the bass connection points of differential amplifiers 102 and 122, as indicated.  
  Accordingly, with the reference voltage applied to terminal point 155 from balance control 28, at a higher level than that as applied to terminal point 157 from source 150, the audio signal information present at source transistors 102C and [22(- will be selectively routed or steered through transistors 102a and 122b, respectively. This results in a decrease in the left channel since the signal output is taken at the collector of transistor 1021). However, an increase in the signal level will occur for the right channel, since the output is taken at the collector of transistor 1221). The reverse, of course, occurs when the reference voltage applied to terminal point 155 is below the level effected at terminal point 157 by the source 150. A smooth transition from one operating condition to the other is established by the action of control 28.  
  The operation of the volume control 26, as shown in FIG. 5b. is substantially the same as above described in conjunction with the balancing control 28, except that the respective outputs in this case track in the same direction. That is, the outputs of the right and left audio channels, as taken at the collectors of transistors 11217 and 132b, both increase or both decrease with an appropriate adjustment in the reference voltage being applied from control 26. Also, as in the case of the balancing control arrangement, potentiometer 22, resistance 162 and capacitor 168 are supplied external to the integrated circuit package itself.  
  While only one specific embodiment of the present invention has been shown and described herein, it will, of course, be understood that certain variations and alternative constructions may be effected without departing from the true scope and spirit of the invention itself. The appended claims are intended to cover all such modifications and alternative constructions that may fall within such true scope and spirit.  
 What is claimed is:  
  l. A control circuit arrangement for effecting a plurality of predetermined control functions in a multichannel system, comprising in combination:  
 means for providing at least one signal path including a signal current steering device having first and second outputs for supplying varying relative amounts of an input signal to such outputs in response to a variable control signal;  
 an output terminal;  
 means for receiving and amplifying a desired signal and selectively applying the same to said signal path and to said output terminal;  
 means for supplying a variable control signal to said current steering device;  
 means coupling the first output of said steering device with said output terminal for feeding forward signals from said steering device for a boosting action; and  
 means coupling the second output of said steering device with said receiving and amplifying means in a feedback circuit for attenuating signals applied from said receiving and amplifying means to said output terminal.  
  2. A control circuit arrangement in accordance with claim 1, wherein the same is disposed in a single integrated circuit.  
  3. A control circuit arrangement in accordance with claim 1 wherein said signal current steering device is in the form of a differential amplifier having a signal input to which the signal from said receiving and amplifying means is applied and having first and second respective current signal paths therethrough coupled with said first and second outputs thereof, respectively, the selection of which is determined by said variable control signal.  
  4. A control circuit arrangement in accordance with claim 3 wherein said differential amplifier includes first, second and third transistor devices, each with base, emitter and collector electrodes, said emitter electrodes of said first and second transistors being connected together and to the collector electrode of said third transistor, said base of said third transistor serving as the input for said applied signal, and the collector electrodes of said first and second transistors serving as said first and second outputs, said variable control signal being coupled to one of said base electrodes of said first and second transistors for varying the relative conductivities thereof.  
  5. A control circuit arrangement in accordance with claim 3 wherein said signal receiving and amplifying means includes a differential amplifier having one output coupled with said output terminal; another output coupled with said signal current steering device to supply input signals thereto, a first input coupled to receive said desired signal and a second input coupled with the second output of said steering device.  
  6. A control circuit arrangement in accordance with claim 4 further including a d-c balancing complementary differential amplifier interconnected in parallel with said first named differential amplifier.  
  7. An audio control circuit arrangement for effecting a plurality of audio control functions in a multi-channel system comprising in combination:  
 means providing a plurality of signal paths, each of which includes at least one signal current steering device having first and second outputs for supplying varying relative amounts of an input signal to such outputs in response to a variable control signal;  
 an output terminal;  
 means for receiving and amplifying a desired signal and selectively applying the same to each of said signal paths and to said output terminal;  
 filter means interposed between said receiving and amplifying means and the input to said signal current steering device in at least one of said signal paths to restrict the frequency range in said signal path;  
 means for supplying variable control signals to said current steering devices;  
 means coupling the first output of each of said steering devices with said output terminal for feeding forward signals from said steering devices for a boosting action: and  
 means coupling the second output of each of said steering devices with said receiving and amplifying means in a feedback circuit for attenuating signals applied from said receiving and amplifying means to said output terminal.  
  8. A audio control circuit arrangement in accordance with claim 7 wherein the same is disposed in a single integrated circuit.  
  9. An audio control circuit arrangement in accordance with claim 7 wherein said signal current steering devices are each in the form of a differential amplifier having a signal input to which the signal from said receiving and amplifying means is applied and having first and second respective current signal paths therethrough coupled with said first and second outputs thereof, respectively, the selection of which is determined by said variable control signal.  
  10. An audio control circuit arrangement in accordance with claim 9 wherein each of said differential amplifiers includes first, second and third transistor devices, each with base, emitter and collector electrodes, said emitter electrodes of said first and second transistors being connected together and to the collector electrode of said third transistor, said base of said third transistor serving as the input for said applied signal, and the collector electrodes of said first and second transistors serving as said first and second outputs, respectively, of each of said steering devices, said variable control signal being coupled to one of said base electrodes of said first and second transistors for varying the relative conductivities thereof.  
  H. An audio control circuit arrangement in accordance with claim 9 wherein said signal receiving and amplifying means includes a differential amplifier having one output coupled with said output terminal; another output coupled with said signal current steering devices to supply input signals thereto, a first input coupled to receive said desired signal and a second input coupled with the second outputs of said steering devices.  
  12. An audio control circuit arrangement in accordance with claim 10 further including a d-c balancing differential amplifier interconnected in parallel with each of said first named differential amplifiers.  
  13. An audio control circuit arrangement in accordance with claim 7 wherein respective signal paths are provided within which bass, treble, balance and volume control functions may be effected.  
 14. An audido control circuit arrangement in accordance with claim 7 wherein said filter means includes a high pass filter in one of said signal paths for restricting the same to treble frequencies and a low pass filter in another of said signal paths for restricting the same to bass frequencies.  
  15. An audio tone control arrangement for effecting treble and bass tone control functions in an electronic apparatus, comprising in combination:  
 a first signal path including at least one signal current steering device and a high pass filter;  
 a second signal path including at least one signal current steering device and a low pass filter;  
 a reference output terminal;  
 means for receiving and amplifying a desired signal and applying the same to said reference output terminal and to each of said first and second signal paths;  
 a source of potential; and  
 variable resistance means interconnected between said potential source and a respective one of said signal current steering devices whereby the applied signal to each of said signal paths may be selectively fed forward to said reference output terminal for a boost action or fed back to said receiving and amplifying means for a relative attenuation.  
  16. An audio tone control circuit arrangement in accordance with claim 15 wherein the same is disposed in a single integrated circuit with said variable resistance means being provided external thereto for manual adjustment accessibility.  
  17. An audio tone control circuit arrangement in accordance with claim 15 wherein said signal current steering devices are in the form of differential amplifier arrangement having respective current signal paths therethru, the selection of which is determined by setting of said variable resistance means.  
  18. An audio tone control circuit arrangement in accordance with claim 17 wherein each differential amplifier includes first, second and third transistor devices with base, emitter and collector electrodes, said emitter electrodes of said first and second transistors being connected together and to the collector electrode of said third transistor, said base of said third transistor serving as the input for said applied signal, and the collector electrodes of said first and second transistors serving as respective outputs, said variable resistance elements being coupled to one of said base electrodes of said first and second transistors.  
  19. An audio tone control circuit arrangement in accordance with claim 18 wherein additional means in the form of complementary current steering devices are included to maintain balance in the d-c levels at the outputs of said differential amplifier.  
  20. An audio tone control circuit arrangement in accordance with claim 19 wherein said d-c balancing complementary current steering devices include an additional differential amplifier arrangement interconnected in parallel with said first named differential amplifier.  
  21. An audio tone control arrangement in accordance with claim 15 wherein said signal receiving means includes a differential amplifier with one output connected to said reference output terminal and a second output connected to said filter means in said signal paths.