Abstract:
A signal conditioning system that receives inputs from at least one pair of conductors connected to its input. Each such input is processed by an input filter and presented to a buffer amplifier. Each such input filter and buffer amplifier refers to and is powered by independent power sources whose power return reference potentials are independently determined by the potential of the corresponding input signal potential reference conductor for the signal frequencies of interest. The outputs of all such buffer amplifiers, the power return reference potentials, and the power return reference potential of the conditioning circuit output are all appropriately added or subtracted in the next circuit stage. This circuit stage consists of an amplifier buffer having low output impedance which is powered by another independent power source whose power return reference potential is independently determined by the potential of the output signal reference conductor. The output of this circuit stage is connected to an output inductor circuit which in turn drives the output signal conductor. The output includes a filter, and is designed to decouple unstable loading conditions while rejecting external influences on the output signal. The invention also includes means that connect the reference potential of the destination of the output conductors to the system power ground potential. The present invention provides a relatively inexpensive and efficient way of reducing or eliminating interference caused by coax cabling in audio, power and video amplifiers, for example.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 of PCT/US95/05293 filed Apr. 28, 1995 and a continuation-in-part of application Ser. No. 08/234,343 filed Apr. 28, 1994, now U.S. Pat. No. 5,436,593, which is a continuation-in-part of application Ser. No. 07/879,941, filed May 8, 1992, now U.S. Pat. No. 5,386,148. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to circuits that use signal conditioning circuitry to eliminate interferences caused by magnetic fields, electric fields, and electro-magnetic or radio frequency fields on conductors that provide electrical connection between devices in a system. The present invention also relates to various electronic circuit that drive conductors, the electronic circuits including signal conditioning circuit to overcome the adverse effects of their loading on the signal source. More specifically, the present invention relates to audio amplifiers, power amplifiers, video amplifiers, etc. that use signal conditioning circuitry for achieving the above-noted benefit. 
     2. Description of Related Art 
     Conductors that provide electrical connection between devices in a system are often the source of many types of electrical interference. Magnetic fields, electric fields and electro-magnetic or radio frequency fields are known to interfere with the fidelity of signals conveyed over conductors which are subjected to those fields. Furthermore, the ground or reference conductor of a typical signal carrying pair of conductors are often connected to different local ground potentials between one end of the conductor as compared to the other, and currents are known to flow in such conductors which then produce voltage drops on that conductor which also interfere with the fidelity of the signals being conveyed. In addition, these conductors, especially when very long, present loads to the signal source that may adversely effect the fidelity of the signal. 
     The problems of conveying signals over conductor pairs in various types of electronic circuits including amplifier systems such as audio amplifier systems, power amplifier systems, video amplifier systems, etc. is well known. The conveyance of signals, especially between powered devices, is often plagued by electro-magnetic interference. 
     One method employed to reduce these interferences modulates the signal so that it can be easily separated from the interference, and then demodulates the signal at the destination. For example, an analog-to-digital converter can be utilized to convey digital impulses over the connecting conductors instead of analog voltage potentials. The destination device in such instances must then convert the signal back to an analog signal potential. Such approaches, while effective, can be very costly, and require extensive circuitry at both the sending and receiving ends of the conductors. Such methods are exemplified by U.S. Pat. No. 4,922,536 to Hogue. 
     Another common method to reduce these interferences is to convey such signals in a differential manner. A common approach utilizes a three conductor shielded cable where two of the conductors deliver the signal and its arithmetic inverse, and a third conductor, usually a shield, conveys the ground reference potential voltage. The conditioning circuit, usually placed at the destination end of the conductors, forms the difference between the potential of the first signal carrying conductor and the second signal carrying conductor. In theory, both conductors are subject to the same interferences, and the subtraction of the signals as conveyed will eliminate the common mode noises. This approach, while effective in eliminating most interference is nevertheless expensive and difficult to implement. To adapt this approach in the general case of processing signals between subsystems requires active circuitry at the sending end to form the inverse signal, and a separate active circuit at the receiving end to subtract the signals. Multiple conductors are also required to be contained within a single shield, which is more costly than conductors having only one conductor surrounded by a shield. Such methods do not, however, address any interference or other affects of the cables that connect the transmitter and receiver to source and destination respectively. Such methods are exemplified by U.S. Pat. No. 4,979,218 to Strahm, and is described at pages 69-71 of “GROUNDING AND SHIELDING TECHNIQUES IN INSTRUMENTATION”, by Ralph Morrison, 3rd Edition, 1986, Wiley-Interscience. 
     One source of interference in the conveyance of these differential signals between electronic subsystems is referred to as the ground loop. Because it is common for there to be multiple electronic paths between the reference potentials of each subsystem, and since such paths commonly include sources of interference, these alternative paths are often responsible for the interference present in those systems. Such ground loops are generally overcome by eliminating any electrical connection by conductors between the subsystems. “GROUNDING AND SHIELDING TECHNIQUES IN INSTRUMENTATION” by Morrison describes the elimination of the effects of the electrical connections between subsystems that convey their signals by differential means through the use of tandem differential amplifiers powered by electrically isolated power supplies. 
     The first differential amplifier in the Morrison reference calculates the difference between the signals being conveyed, and the second differential amplifier adds the reference potential of the destination to the result of the first differential amplifier. The result is that the reference potentials of the source of the differential signal may differ from the reference potential of the destination without effecting the expression of the signal at the destination. However, such an approach is not easily adapted to electronic systems consisting of single ended two wire signal conductors. Consequently, this approach suffers from the same limitations as devices that convey signals by differential means. For example, there are no means suggested in Morrison for the elimination or suppression of the magnetic field interference that may be picked up between the two conductors enclosed in the shield, due to differences in the magnetic field voltages induced in those conductors. Moreover, Morrison does not address the pickup of electric field interference or any other cable affects due to the output cable. 
     The circuits shown in the Morrison reference are also particularly subject to the variation of op-amp characteristics. In particular the output impedance of the opamps used to determine A 1  will negatively impact the interference rejection of any common mode voltage differences between source and destination reference potentials as that impedance relates to the difference resistors of gain stage A 2 . As this circuit characteristic is extremely gain and temperature dependent, such inaccuracies are not easily controlled without increased expense in the design of the output stages of those circuits or without compromises inherent in the utilization of higher impedances than would be appropriate in achieving other performance objectives such as thermal noise and bandwidth which are adversely affected by higher resistor values in this case. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide various types of electronic circuits including amplifier systems, such as audio, power and video amplifier systems, with circuitry to suppress or eliminate the expression of all types of interference in the wiring conveying analog voltage potential signals from a source to a destination. This is accomplished in the present invention by the unique combination of novel interference rejection circuits that address the sources of interference in these systems in a less costly and more efficient manner than other approaches. 
     It is another objective of the present invention to remove any effect that the loading of such wiring or the effects of the loading of the destination in these devices may have upon the accuracy of the signal as conveyed by the source of the signal. 
     It is a further objective of the present invention to accomplish the preceding objectives with a minimum number of precision resistors producing greater effective rejection than the prior art for a given cost. 
     An additional objective of the present invention is the reduction in sensitivity of the circuit action to the characteristics of the gain circuits and/or operational amplifier circuits employed by the circuit to achieve the various aims herein described. 
     Another objective of the present invention is to provide an economical means of adjusting gain in these systems without affecting the resulting interference rejection in practical applications. 
     Yet another objective of the present invention is to afford greater rejection of electric field interference and any electric field affects, such as dielectric absorption, due to output cable physics. 
     A further objective of the present invention is to effect these objectives without altering the accuracy or fidelity of the signal(s) being conveyed between subsystems. 
     Another objective of the present invention is to provide a device which accomplishes every objective of the present invention as described forthwith by means of an independent circuit which can easily be inserted into the existing wiring between popular electronic devices such as the above-described amplifier systems, for example, and which can accomplish every objective of the present invention as described forthwith with a minimum amount of time required to install the device in these systems. 
     It is a further objective of the present invention to provide all of the functions associated with state of the art amplification systems while achieving the above-noted benefits with a minimum amount of additional circuitry and without adding any additional active circuitry in the signal path. 
     A further objective of the present invention is to provide for the conditioning of single ended or differential signals with the same circuit organization and interconnecting wire cable(s). 
     A further objective of the present invention is to provide for the conditioning of differential signals, or any number of signals, where each signal is produced with reference to independent potential references which make higher levels of interference rejection possible. 
     It is also an objective of the present invention to accomplish every objective of the invention as described forthwith while utilizing signal wiring between devices which consists of two conductors arranged concentrically. This type of cabling is known as “COAX” which is a shortening of the term “CO-AXIAL”, and which refers to a cable whose circular conductors share the same major axis. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawing: 
     FIG. 1 is  FIGS. 1A and 1B together constitute a circuit diagram of a preferred embodiment of the signal conditioning circuitry to be included in various types of electronic circuits according to various embodiments of the present invention; 
     FIG. 2 is  FIGS. 2A and 2B together constitute a circuit diagram of a typical audio amplifier; 
     FIG. 3 is  FIGS. 3A and 3B together constitute a circuit diagram of the typical audio amplifier including signal conditioning circuitry according to an embodiment of the present invention; 
     FIG. 4 is a block diagram of an amplifier system according to an embodiment of the present invention; and 
     FIG. 5 is a block diagram of another embodiment of the amplifier system according to an embodiment of the present invention; 
     FIG. 6 is a block diagram of a video circuit including signal conditioning circuitry according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, in which like reference numerals identify identical or similar elements, and in particular to FIG. 1FIGS.  1 A and  1 B, signal source  1  may be understood to be differentially related to signal source  26  in instances where signal  1  and signal  26  are both available and known to relate to each other in an arithmetically inverse manner. Such a relationship is not necessary to carry out the present invention. Other relationships may be appropriate; indeed, if the circuit were to be used as a mixer of signals, there may be no relationship at all. Also, signal  1  or signal  26  need not convey a signal at all, and either may be disconnected to apply the device to single ended signals utilizing only two conductors to convey a signal, without limiting the effectiveness of the device. 
     Signal  1  and Signal  26  are conveyed over coaxial cable(s)  4  and  29  respectively. Coaxial cables are preferred in connection with this invention because the coaxial nature of the cables offers the special benefit that both conductors respond nearly identically to magnetic field interference by virtue of the high degree of axial symmetry possible around the major axis of the cable run. By virtue of this symmetry, the outer conductor will respond to magnetic fields in substantially the same way as the inner conductor will in accordance with Lenz&#39;s law which relates an induced voltage to the rate of change of magnetic field strength. Hence magnetic field disturbances in the vicinity of said cables will induce substantially the same voltage in the outer conductor as such interference will induce in the inner conductor. 
     The present invention is particularly effective at rejecting the influence of any electric currents conducted by each shield that would tend to shift the reference potential of each signal at its source. This is often a problem because the ground potential impedance of the source is less than ideal in practice. Since each signal is generated with respect to its own reference potential more exactly than with respect to any shared reference potential, and since all such current will only substantially flow into that source reference impedance, affecting both the signal source ground potential and signal potential substantially equally, the present invention will be very effective in eliminating interference so induced. The extreme high impedances of the power supply circuit in the preferred embodiment provides for this principle in a novel and especially effective way at a much lower cost than other techniques. 
     This power supply circuit is developed in the preferred embodiment by very high impedance current source circuits composed of field effect transistors  47 ,  22 ,  50  and  24  in combination with current programming resistors  46 ,  21 ,  51  and  25  and opamps  48 ,  23 ,  52  and  26 . The opamps provide the biasing necessary for the transistors to conduct exactly that current required to produce that voltage across the resistors that match the biasing voltages produced by zener diodes  80  and  85  in combination with resistors  82  and  84 . Capacitors  81  and  86  are included to further reject any interference which may be present on the power supply as provided by contacts  91 ,  89  and  90 . In this way an exact current is precisely metered through each transistor to operate opamps  13 ,  16 ,  38 , and  41 . Because this current is metered so precisely, any changes in potential at the opamp supply pins will have no bearing on the current delivered. As a result, the effective impedance of the power supply will be extremely high. The quality of this high impedance current source is limited only by the gate-to-drain capacitance of the field effect transistors used, which can be very small depending on the device characteristics. Field effect transistors with capacitances on the order of 0.05 pf are known to exist in metal gate transistors designed for VHF mixers. Such transistors would easily provide 3 megs of equivalent isolation at 1 Mega-Hz provided opamps  13 ,  16 ,  38  and  41  could handle that frequency accurately. 
     Other transistors could be used as well. While field effect transistors are preferred, ordinary PNP and NPN transistors have proven to work satisfactorily, but available device characteristics are not as ideal as field effect transistors for this purpose. General purpose small signal transistors are limited in isolation by the offset of the base current which varies with collector voltage, and by larger base to collector capacitances, two effects which compromise the performance of this circuit. Careful selection of high B types or darlington configurations can go a long way towards improving such circuits. 
     Each signal from source  1  or  26  is then subjected to resistor  5  and  30  respectively. This resistance is provided primarily as a path for the input bias current necessary for the operation of operational amplifiers  13  and  38 . Such resistors are not, however, required to carry out the present invention, yet are preferred to enable the device to carry out the present invention in every possible context. For example, a common context for the device involves an input signal source that is blocked with a coupling capacitor which would not permit the necessary bias current to flow in the steady state without resistor  5  and  30 . These resistors should be designed with the highest resistance value practical considering the effect of the resistor on the source electronics and cable characteristics engendered in sources  1  and  26  and cable  4  and  29 . 
     Capacitors  6 ,  9 ,  33  and  34  along with resistors  8  and  32  and inductors  7  and  31  are all part of a Pi-filter arrangement designed to reject frequencies far higher than the frequencies of interest in the signal. Such a filter is preferred to prevent such high frequency interference from interacting with various non-linear elements in typical operational amplifiers, such as are indicated as  13  and  16  in the drawings, and so prevent the demodulating or converting of such high frequency interference to frequencies that would otherwise interfere with the frequencies of interest. 
     Opamps  13  and  38  along with resistors  10 ,  11 ,  35  and  36  form buffer amplifiers which may be designed to include gain according to the ratio of the values of resistors  11  and  36  with respect to resistors  10  and  35 . Because the gain of these amplifiers may be important in instances where differential signals are conditioned, these gains should be well matched. However, in the case of a single input, or multiple unrelated inputs, the precision of the gain of this stage will not be important because each input and each amplifier corresponding to each input refers to its own ground reference potential separately. In order to refer to each ground reference potential separately, such as local ground potentials as indicated as  3  and  28 , each such buffer opamp  13  and  38  must be powered by separate and independent sources of power whose ground return potentials can assume any value. Signal ground reference potentials  3  and  28  may present by way of the ground conductors of cable  4  and  29 . In this way the opamp circuit can not inject any currents into the signal carrying conductor, which would otherwise be possible through various stray capacitances and internal opamp circuits if the opamp&#39;s power supplies did not track the reference ground potential of each input. Such injected currents could produce severe feedback instabilities in addition to permitting the expression of any noises that may be included with those injected currents. 
     Each such opamp  13  and  38  provides, at its output, a signal proportional to the signal provided at its input, but it provides that signal potential with a much lower output impedance. Hence, components may then be connected to the output of the opamps, such as resistors  53  and  56 , which inject interference currents into the output of these amplifiers that are directly related to the interference potential between the respective source signal grounds  3  and  28  and the output signal ground  79 . The expression of these currents is suppressed, to a large degree, by the ratio of output impedance of the amplifier(s)  13  and  38  and the impedance of resistors  53  and  56  respectively. In the preferred embodiment configured for balanced signal sources as shown in FIG. 1FIGS.  1 A and  1 B, such interference due to said opamps is additionally cancelled by the differential action of the circuit comprising opamp  61 , and resistors  53 ,  56 ,  59  and  57 . In this way rejection of the common mode interference is limited primarily by the precision of the resistors  53  and  56  viz-a-viz resistors  57  and  59 . This result makes it possible to utilize smaller values for these resistors and hence makes wider bandwidth and lower noise levels possible. 
     Additional rejection of such opamp characteristics is offered by the incorporation of opamps  16  and  41 , which are especially beneficial when the signal source is single ended and comprises only signal  1 . In this case, the circuit comprised of opamp  16 , resistor  14  and resistor  15  may be altered in design by connecting resistor  39  to the positive input of opamp  38  instead of the connection shown, and by matching the ratio of resistors  36  and  35  with the ratio of resistors  40  and  39  so connected. Such an arrangement will produce the same common mode injected errors in opamp  16  as are produced by opamp  13 , and these errors will then be subtracted by the differential action of the circuit of opamp  61  in combination with resistors  53  and  55 . This improvement could also be applied to the balanced input source case by applying the aforementioned modifications to the input circuit for the complementary source. 
     Each signal presented at the output of opamps  13  and  38  is then appropriately summed along with inverted signals produced by opamps  16  and  41 . These signals and their individual complements hence produce twice the voltage level possible between their outputs than the single opamp  13  or  38  could provide. In addition, this complementary output is presented symmetrically with reference to the source ground potential as is related by the respective cable shield  4  or  29 , thus permitting the cancellation of the common mode interference included in that reference potential as compared to the destination reference potential. This arrangement makes it possible to design the following differential gain stage with opamp  61  and resistors  53 - 57  and  59  with ½ the gain that would otherwise be necessary, and this fact also reduces the sensitivity of the resultant output to that common mode interference. When all component sensitivities are taken into account, a worst case improvement of 4 db in said common mode rejection ratio can be expected without increasing the precision of the resistors required by said differential gain stage. 
     In any event, the signal from opamps  13 ,  38 ,  41  and  16  are summed by the following difference amplifier stage as follows: The signal from opamp  13  is applied to opamp  61  by way of resistor  53  to provide for the expression of that potential minus the inverted version of that potential expressed with reference to the input signal reference potential as presented by the shield conductor of cable  4  by way of resistor  55 . Likewise, resistor  54  provides for the expression of its respective signal potential minus the inverted version of that potential expressed about the input signal reference potential ground  28  as presented by the shield conductor of cable  29  by way of resistor  54 . Furthermore, resistor  57  provides for the addition of the output reference ground  79  as presented by the shield conductor of cable  75  to the signal output of opamp  61 . In this way the output potential of opamp  61  may present a potential at its output that not only calculates the differences between the input potentials and their ground reference returns, but also adds in the output reference potential so that the signal then tracks the reference potential used by the destination receiving the output of opamp  61 . These relationships expressed mathematically are as follows: 
     Where 
     Vout=Output of opamp  61   
     Vog=Output ground reference potential as presented by the shield of cable  75   
     Vin 1 +=Input from source  1  as presented by cable  4   
     Vin 1 g=Input ground reference potential from ground  3  as presented by cable  4   
     G 1 =gain of opamp  13 =(R 10 +R 11 )/R 10   
     VG 1 +=Output of opamp  13 =G 1 xVin 1 ++Vin 1 g 
     VG 1 −=Output of opamp  16 =−G 1 xVin 1 ++Vin 1 g 
     Vin 2 +=Input from source  26  as presented by cable  29   
     Vin 2 g=Input ground reference potential from ground  28  as presented by cable  29   
     G 2 =gain of opamp  38 =(R 35 +R 36 )/R 35   
     VG 2 +=Output of opamp  38 =G 2 xVin 2 ++Vin 2 g 
     VG 2 −=Output of opamp  41 =−G 2 xVin 2 ++Vin 2 g 
     G 3 =gain of differential amplifier stage with opamp  61 =R 60   59 /R 55 =R 60   59 /R 56 =R 57 /R 53 =R 57 /R 54   
     Vog=output ground potential as presented by the shield of cable  75   
     Vout=output of opamp  61 =VG 1 +−VG 1 —VG 2 ++VG 2 −+Vog. 
     Then the preferred embodiment of the present invention provides for the following relationship: 
     Signal Output=Vout−Vog=G 3 x( 2 xG 1 xVin 1 +)−G 3 x ( 2 xG 2 xVin 2 +)= 2 xG 3 x((G 1 xVin+)−(G 2 xVin 2 +)) 
     if G 1 =G 2 , which is normally the case: 
     Signal Output= 2 xG 3 xG 1 x(Vin 1 +−Vin 2 +) 
     In order to properly track the output reference potential, while also driving the signal with respect to that reference potential, it is also necessary to supply opamp  61  with power whose reference return potential will accurately assume the same value as the output reference ground  79  as presented by cable  75 . This is done in this embodiment by providing opamp  61  with its own separate and independent source of power whose reference ground potential can freely assume any value. Transistors  67  and  69  provide for such power by implementing very high impedance current sources in the same manner as described previously in connection with opamps  13  and  41 . 
     The output of opamp  61  is also filtered, according to the present invention to prevent the interaction of high frequencies picked up in the output cable from being detected by the non-linearities of the opamp at those high frequencies. Also, the inductor  72  and resistor  73  serve to provide a finite, but higher impedance to the opamp at higher frequencies than the capacitor  74  and the capacitance of the cable  75  would present at high frequencies, and which would otherwise render opamp  61  unstable in the servo action of its gain controlling feedback. Also, inductor  72  provides for very low impedance at lower frequencies so that the output cable  75  can be driven with extremely low impedance which can be very effective in shunting any currents that may be injected by electric fields along the cable, or by the electronic device to which the cable may be connected. In addition, the low output impedance afforded by inductor  72  makes it possible for opamp  61  to more accurately drive any cable capacitance that may be presented by cable  75 . This is possible because a higher output impedance, as would be necessary without inductor  72 , could result in a low pass RC filter (the R being resistor  73  and the C being the sum of capacitor  74  and the effective capacitance of cable  75 ) that would have a significant effect on the fidelity of the signal being conveyed. In addition, such low drive impedance also shunts the effects of the dielectric of the cable  75 . This dielectric may not be ideal as it may be subject to hysteresis-like effects such as dielectric absorption. Low output impedance will effectively reject such characteristics. 
     In addition, capacitors  58  and  60  may also be added to minimize the tendency of some opamps to amplify higher frequencies in a manner that is not consistent with feed back resistor values, and which could compromise signal fidelity. Capacitor  60  shunts higher frequencies that may be delivered by resistors  55  and  56  while capacitor  58  increases the feedback applied to opamp  61  while it shunts higher frequencies that may be delivered by resistors  53  and  54  to the output. To minimize the effects of these capacitors in the frequency bands of interest, capacitors  58  and  60  should have values that are inversely proportional to resistors  57  and  59  respectively. 
     One problem with a signal conditioning apparatus intended to interface between two electronic subsystems is the range of different types of inputs to which the device will be connected. Destination devices can vary from having a ground connection that is ultimately connected to the system ground potential to fully isolated differential connections where there is equal, but finite, and sometimes large impedances between the destination signal ground connection and the system ground. Preferably, the present invention is utilized in a system in which the destination signal ground connection is connected to the system power return potential. Since this is not always the case with all destination devices, however, additional means to guarantee such a connection may be included in the present invention. 
     There is often the occasion to provide signal conditioning for more than one signal at a time. In such instances arrays of signal conditioning devices may be required, such that each signal may be processed by completely separate signal conditioning apparatus. In such instances the power supplies are independent in their ability to relate to the respective signal ground potentials. 
     Since the present invention utilizes only six matched resistors to condition a balanced source input, it requires only four matched resistors to condition a single ended source input. Further, because of the addition of an inverted circuit placed judiciously with the input circuit, an almost two fold increase in the effective interference rejection of the device may be accomplished for the same resistor matching. As a result, system cost for a given specification is reduced substantially. 
     As it is often desirable to adjust the gain of the subject signal conditioning embodiment, this gain may be adjusted without affecting the absolute level of unrejected interference. Hence, the present invention is unique in that common mode forms of interference are rejected with a rejection ratio that is actually proportional to the gain so that the interference residuals of each signal&#39;s common mode remains constant in spite of the increased gain. Only common mode interferences between differentially applied signals would be proportional to this gain, but these interferences are typically very small in relation to the normal common mode interferences. 
     FIG. 2 depicts  FIGS. 2A and 2B depict a front end of a typical audio amplifier. A first channel of the audio amplifier includes a first stage  100  consisting of opamp  101 , resistors  102 - 106  and capacitor  107  forming a non-inverting opamp circuit. A second stage includes a highpass active filter circuit  108  and a low pass active filter circuit  116 . Switch  124  is provided for selectively switching the output of highpass filter  108 , lowpass filter  116  or the output of first stage  100  directly, to the input of opamp circuit  125  consisting of opamp  126  and resistors  127 - 129 . 
     The second channel of the audio amplifier is similar to the first channel and includes a first stage  164  consisting of opamp  164 , resistors  137 - 141  and capacitors  142  and  143  also forming a non-inverting opamp circuit. The second stage includes highpass active filter circuit  165  and lowpass active filter circuit  166 . Switch  158  selectively switches the output of highpass filter  165 , lowpass filter  166  or the output of first stage  164  directly, to the input of inverting opamp circuit  167  consisting of opamp  162  and resistors  159 - 161 . Each of these circuits is well known in the art and each circuit, therefore, will not be described in detail below. These circuits will collectively be referred to hereinafter as the “front end” or preamp of the amplifying system. The output of opamp circuits  125  and  167  can be input to additional electronic circuitry  133  for power amplification, noise reduction, equalization or providing a noise gate function, etc., or any combination thereof. The outputs of processing circuitry  133  can then be input to speakers  134 . 
     According to this embodiment of the present invention, signal source  131  and  136  can consist of any desired source of audio signals, such as a tuner, tape player, compact disc player, etc. When operating in stereo, signal source  131  can represent a left channel and signal source  136  can represent a right channel of an audio signal, for example. When operating in mono, signal sources  131  and  136  can represent the same signal source. By providing a non-inverted signal on line  132  via opamp circuit  125  and its complement on line  163  via inverting opamp circuit  167 , it is possible to achieve twice the output voltage by bridging the outputs of circuitry  133 . 
     First stage opamp circuits  100  and  164  form buffer amplifiers providing at their outputs signals proportional to the signals provided at their inputs, but with a much lower output impedance. 
     Highpass filter circuits.  108  and  165  block the low frequency signals and DC while passing the high frequency signals. These high frequency signals can be used for driving a high frequency speaker such as a “tweeter”, for example. The output of highpass filter circuit  108  is provided to terminal A of switch  124  and the output of highpass filter circuit  165  is provided to terminal A of switch  158 . 
     On the other hand, lowpass filter circuits  116  and  166  block the high frequency signals while passing the low frequency signals. These low frequency signals can be used for driving a low frequency speaker such as a “woofer”, for example. The output of lowpass filter circuit  116  is provided to terminal C of switch  124  and the output of lowpass filter circuit  166  is provided to terminal C of switch  158 . 
     Switches  124  and  158  are manual switches used for directing signals from one of terminals A-C to the respective inputs of opamp circuits  125  and  167 . Opamp circuit  125  provides a non-inverted signal on output line  132 . Inverting opamp circuit  167  provides an inverted signal on output line  163 . The signal on output lines  132  and  163  can then be input to additional electronic circuitry  133  for power amplification, noise reduction, etc. The signals output from circuitry  133  can then input to right and left speakers  134 , for example, when operating in stereo. When operating in mono, the outputs of circuitry  133  can be bridged and input to a single speaker  134 . 
     The “front end” provides most of the voltage amplification in such an audio amplifier system and electronic circuitry  133  (e.g., a power amplifier) provides most of the current amplification. 
     Some of the problems typically encountered by the audio amplifier circuitry as described above is that the conductors (e.g., coax cables) tend to introduce interference into the audio signal thus degrading audio quality. As shown in FIG. 3FIGS.  3 A and  3 B, the present invention provides signal conditioning circuitry, similar to that described above with respect to FIG. 1FIGS.  1 A and  1 B, that eliminates noise introduced by the coax cables, without compromising signal quality. Such benefits are possible using a very minimum amount of additional circuitry. 
     FIG. 3 shows  FIGS. 3A and 3B show the typical audio amplifier depicted in FIG. 2FIGS. 2A and 2Bincluding signal conditioning circuitry similar to that described above with respect to FIG. 1FIGS.  1 A and  1 B. Circuit  200 , consisting of resistors  230 - 232 , transistors  233  and  236 , zener diode  234  and capacitor  235  and circuit  201  consisting of resistors  240 - 242 , transistors  243  and  244 , zener diode  245  and capacitor  246  provide high impedance constant current sources for delivering the exact current necessary to operate the operational amplifiers in the circuitry. Constant current source circuits  200  and  201  are illustrative of circuits that can perform this function. Other well known circuitry may also be used for delivering constant current. As shown, the ground lines for each of the elements for each channel are tied together and not to a common ground. 
     Input amplifier circuits  100  and  164 , highpass filters  108  and  165  and lowpass filters  116  and  166  perform the same functions as described above with respect to FIG. 2FIGS.  2 A and  2 B. The present invention includes inverting opamp circuit  202  consisting of resistors  203  and  204  and opamp  205  and inverting opamp circuit  250  consisting of resistors  251  and  252  and opamp  253 . The gain of inverting opamp circuits  202  and  250  can be varied by varying the resistance ratios of resistors  203  and  204  resistors  251  and  252 , respectively. 
     A basic difference in the circuitry between FIG. 2FIGS.  2 A/ 2 B and FIG. 3FIGS.  3 A/ 3 B is that the output signals from switches  124  and  158  and the output of opamp circuits  202  and  250  are summed by differential amplifiers  125  and  167 , respectively, as follows: The signal from switch  124  is applied to opamp  126  by way of the voltage divider consisting of resistors  127  and  128  to provide for the expression of that potential minus the inverted version of that potential expressed with reference to the input signal reference potential as presented by the shield conductor of cable  130  by way of opamp  205  and resistor  206 . The inverted version of the signal from switch  158  is expressed with reference to the input signal reference potential as presented by the shield conductor of cable  135  by way of opamp  253  and resistor  159  to provide for the expression of that potential minus the signal from switch  158  applied to opamp  162  by way of resistor  160 . As described above with respect to FIG. 2FIGS.  2 A and  2 B, the output of opamps  126  and  162  may then be further processed by additional electronic circuitry  134  (e.g., a power amplifier) and output by one or more speakers  134 . 
     The additional electronic circuitry  133  can be provided as a separate and distinct unit from preamp circuitry  600  depicted in FIG. 3FIGS.  3 A and  3 B. In the alternative, the additional electronic circuitry  133  and the preamp or “front end” circuitry  600  can be provided as a unit in a single case, for example. Using the circuitry as described in FIG. 3FIGS.  3 A and  3 B, the present invention is capable of eliminating noise introduced caused by the conductors and providing a clean output signal. 
     In addition to conditioning audio signals in an audio amplifier as described above, the signal conditioning circuitry of the present invention can also be provided in other arrangements. For the description of the following embodiments of the present invention, the signal conditioning circuitry depicted in FIG. 1FIGS. 1A and 1B(or the “front end” depicted in broken line box  600  in FIG. 3FIGS.  3 A and  3 B) is shown in block diagram form as block  403 . 
     As shown in FIG. 4, the fight and left outputs of signal source  400  can be fed through coax cables  401  and  402 , respectively, to the input of preamp  403 . As noted above, preamp  403  can consist of the signal conditioning circuitry depicted in FIG. 1FIGS.  1 A and  1 B, or can consist of the “front end” circuitry  400  as depicted in FIG. 3FIGS.  3 A and  3 B. Preamp  403  provides voltage amplification and eliminates noise in the signals being processed, without compromising signal quality. The output of preamp  403  can then be provided to the input of power amplifier  410 . Power amplifier  410  provides current amplification of the signals on lines  411  and  412  for driving right and left speakers  420  via lines  413  and  414  when operating in stereo, for example. When operating in mono, lines  413  and  414  can be bridged to drive a single speaker. 
     According to this embodiment of the present invention, preamp  403  can be provided as a separate unit from power amplifier  410 , or can be provided as an integral unit with power amp  410 . If preamp  403  is provided as a separate unit, instead of providing individual power supplies for power amp  403  and power amp  410 , power can be supplied to preamp  403  from power amp  410  via cable  430 . 
     FIG. 5 is similar to FIG. 4, but includes an additional electronic component  500 . Component  500  can consist of an equalizer, crossover, noise reduction or effects processor circuitry, etc. or any combination thereof. The present invention can be used in any state of the art amplification system to eliminate noise without altering the effect of any other type of processing and without compromising sound quality in any way. 
     It is preferable that the majority of the voltage amplification of the amplifier system of the present invention take place in preamp  403  and that any circuitry that reduces gain be provided after preamp  403 . The majority of the current amplification of the amplifier system of the present invention occurs in power amp  410 . However, it may also be preferable that speakers  420  include their own current amplification circuits, particularly if speakers  420  consist of bass speakers. In addition, the present invention makes it possible to provide such current amplification without the necessity of isolated power supplies which are costly and lower in performance. In order to achieve the best results, it is preferable that if intermediate processing units are provided between the voltage amplification circuitry and the current amplification circuitry, that the power supplies for driving each of these units be isolated power supplies. 
     The signal conditioning circuitry of the present invention can also be used for conditioning other types of signals besides audio signals. For example, as shown in FIG. 6, the video signals from camera  600  can be fed through coaxial cable  610  to the input of signal conditioning circuit  620 . Signal conditioning circuit  620 , consisting of the signal conditioning circuitry depicted in FIG. 1FIGS.  1 A and  1 B, suppresses or eliminates noise or interference introduced by the coaxial cables and provides a relatively clean output signal. The output signal can then be fed via coax  630  to video display  635  for immediate display and/or to remote video recorder  640  for recording. Signal conditioning circuit  620  can be provided as a unit separate from camera  600 , display  635  and recorder  640 , or can be incorporated into an input or output stage of display  635  and/or recorder  640 . 
     The foregoing has set forth exemplary and preferred embodiments of the present invention. It will be understood, however, that various alternatives will occur to those of ordinary skill in the art without departure from the spirit and scope of the present invention.