Abstract:
A switching operation is performed in response to an externally applied DC control signal from an external load. An input connector receives an input audio signal from an external audio source, and an output connector that provides an audio output signal. A DC isolator couples the input audio signal from the input connector as the audio output signal on the output connector while isolating the externally applied DC control signal from the input connector. A switch is responsive to the externally applied DC control signal applied to the output connector so as to perform the switch operation.

Description:
TECHNICAL FIELD  
       [0001]    The method and/or apparatus described below relates to the use of phantom power from a console, such as an audio or other console, in order to switch various devices such as a direct box or an A/B box. 
       BACKGROUND  
       [0002]    A direct box is a device used to adapt an unbalanced AC audio signal to a balanced low impedance AC output for transmission to an audio console. A switch, commonly know as a “ground lift switch,” is provided in the direct box to connect or disconnect separate ground points between the input and the output of the direct box. The position of the switch is manually controlled by the user at the direct box. The ideal position of the switch is dependent on the types of equipment connected at both the input and output of the direct box and related grounds. 
         [0003]    An input selection box, commonly called an A/B box, is a device used to route one of two (or more) inputs to a single common output. A switch is provided in these units to select which input is routed to the output. The position of this switch is manually controlled by the user at the A/B box. 
         [0004]    Phantom power is a system for applying a DC voltage across the balanced AC inputs of audio consoles. Phantom power has primarily been used to power the electronics of amplifier circuits in active direct boxes or condenser microphones. Phantom power is usually configurable to be ON or OFF by a control switch on the audio console. 
         [0005]    The method and/or apparatus described below relates to the use of such phantom power to control a direct box, an A/B box, or other apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0006]      FIG. 1  illustrates a phantom power controlled direct box; 
           [0007]      FIG. 2  illustrates a phantom power controlled A/B box; 
           [0008]      FIG. 3  illustrates an exemplary variation of  FIG. 1 ; 
           [0009]      FIG. 4  illustrates another exemplary variation of  FIG. 1 ; 
           [0010]      FIGS. 5A ,  5 B,  5 C, and  5 D illustrate various solid state switches that may be used in place of the relay of  FIG. 1 ; 
           [0011]      FIG. 6  illustrates a solid state version of the phantom power controlled A/B box of  FIG. 2 ; 
           [0012]      FIGS. 7A and 7B  illustrate alternative switch configurations; 
           [0013]      FIG. 8  shows connection of a direct box between a source and an audio console; 
           [0014]      FIG. 9  shows connection of an A/B box between a source and an audio console; and, 
           [0015]      FIG. 10  illustrates another exemplary variation of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows an arrangement to facilitate opening or closing of the connection between input and output ground points in a direct box by applying or removing DC phantom power from the balanced output audio line. A phantom power controlled relay is used to connect or disconnect the separate input and output ground points. Other functions controlled by the switching mechanism are possible. The benefit of this arrangement is to allow the ground configuration to be remotely controlled from the audio console providing the DC phantom power rather than at the direct box itself. 
         [0017]    As shown in  FIG. 1 , a direct box  10  includes a three conductor connector  12 . The direct box  10  is referred to as a box because the components shown in  FIG. 1  are contained in a housing (not shown). The connector  12  functions as both the output of a balanced AC audio signal from the direct box  10  (to, e.g., an audio console) and the input (from, e.g., the audio console) for providing phantom power to the direct box  10 . A conductor  14  of the connector  12  carries the non-inverted portion of the balanced output audio signal, and a conductor  16  carries the inverted portion of the balanced output audio signal. Both of the conductors  14  and  16  function as positive DC poles for the phantom supply from an audio console. Phantom power is commonly  12 ,  24 , or  48  volts. A conductor  18  of the connector  12  provides a return path for the phantom power back to the audio console. This return path is designated as ground GND 2 . The conductors  14 ,  16 , and  18 , for example, may be pins. 
         [0018]    A two conductor connector  20  of the direct box  10  receives the unbalanced input signal. A two conductor connector  22  is connected in parallel with the connector  20  to facilitate a direct output of the unbalanced input signal provided to the connector  20 . A shared ground connection of the connectors  20  and  22  is designated as a ground GND 1 . A decoupling capacitor  24  blocks any DC voltage present on the unbalanced input signal. 
         [0019]    An input attenuation pad  26  comprises resistors  28 ,  30 , and  32 . The attenuation pad  26  is switched into or out of the AC input line signal path by a toggle switch  34 . The attenuation pad  26  can be used to reduce the input signal to a level appropriate for a transformer  36 . In the position of the toggle switch  34  as shown in  FIG. 1 , the toggle switch  34  removes the attenuation pad  26  from the input signal. In the alternate position of the toggle switch  34 , the attenuation pad  26  is inserted into the signal path and creates a voltage divider to reduce the signal level induced across the transformer  36 . 
         [0020]    The conductor  14  of the connector  12  is connected to one end of the secondary of the transformer  36 , and the conductor  16  of the connector  12  is connected to the other end of the secondary of the transformer  36 . 
         [0021]    The unbalanced AC input signal provided to the connector  20  is converted into a balanced AC output signal by the transformer  36 . The transformer  36  also reduces the output impedance of the balanced signal for compatibility with the input of the audio console, allows the ground references GND 1  and GND 2  between the connectors  12  and  20  to be decoupled, and blocks or isolates the positive DC voltage of the phantom power on the conductors  14  and  16  of the connector  12  from the input of the direct box  10 . 
         [0022]    Resistors  38  and  40  combine the phantom power currents on the conductors  14  and  16  of the connector  12  while maintaining a load impedance on the balanced output. Diodes  42  and  44  may be included to isolate the load impedance from the resistors  38  and  40  across the balanced output when the phantom power is disengaged. 
         [0023]    The phantom power combined by the resistors  38  and  40  is fed to an electromagnetic coil  46  of a relay  48  such as a double pole, double throw, 2-Form-C relay. A first pole  50  of the relay  48  opens or closes the connection between the phantom power feed and an LED  52 . A resistor  54  limits the current through the LED  52 . A second pole  56  of the relay  48  opens or closes the connection between the ground GND 2  and a first pole  58  of a switch  60  such as a double pole, double throw slide switch. The switch  60  also has a second pole  62 . The switch  60  is used to select whether the switching of the ground configuration is performed remotely from the audio console by the relay  48  or locally by a manually operated switch  64 . 
         [0024]    A capacitor  66  increases the voltage transition time across the coil  46  of the relay  48 , delaying the operation point of the relay  48  relative to the phantom power control voltage (hysteresis). A diode  68  across the capacitor  66  prevents a large voltage spike from being created by the electromagnetic coil  46  of the relay  48  if the return path for the phantom power is broken. 
         [0025]    A resistor  70  and a capacitor  72  form a low impedance conduction path between GND 1  and GND 2  above audio frequencies to minimize RF interference. 
         [0026]    In the switch positions of  FIG. 1 , the first pole  58  of the switch  60  connects GND 1  to the switch  64 , giving control of the ground configuration to the switch  64 . The switch  64 , for example, may be a single pole, single throw toggle switch operated by the user at the direct box  10  to open or close the connection between GND 1  and GND 2 . The second pole  62  of the switch  60  breaks the phantom power return path GND 2 , preventing the relay  48  from needlessly operating when phantom power is applied at the audio console. 
         [0027]    In the alternate position of the switch  60 , the phantom power return path to GND 2  is completed through the second pole  62  of the switch  60 . Also, the connection between GND 1  and the switch  64  is broken by the first pole  58  the switch  60 . GND 1  is then routed through the first pole  58  of the switch  60  to the second pole  56  of the relay  48 , giving remote control of the ground configuration to the switch position of the relay  48 . Application of phantom power at the audio console is then able to operate the relay  48 , opening or closing the connection between GND 1  and GND 2 . 
         [0028]    Thus, with no phantom power applied to the direct box  10  from the audio console, the ground GND 1  is connected to the ground GND 2  through the first pole  58  of the switch  60  and the second pole  56  of the relay  48 . Also, the LED  52  is off. 
         [0029]    However, when phantom power is applied to the direct box  10  from the audio console, the first pole  50  of the relay  48  makes a connection between the phantom power feed and the LED  52 , turning on the LED  52 . Current through the LED  52  is limited by the resistor  54 . The LED  52  serves as a visual indicator to the user as to the state of the ground configuration in phantom power controlled mode. Also, the connection between the ground GND 1  and the ground GND 2  is broken by the second pole  56  of the relay  48 . 
         [0030]      FIG. 2  shows an arrangement to facilitate the input selection of an A/B box  100  by applying or removing DC phantom power from the balanced output audio line of the A/B box  100 . The A/B box  100  is also referred to as a box because the components shown in  FIG. 2  are contained in a housing (not shown). The benefit of this arrangement is to allow the input source to be remotely selected from the audio console rather than at the A/B box  100  itself. The A/B box  100  selects one of two (or more) AC inputs which is then routed to the one AC balanced output of the A/B box  100 . The selection is controlled by application of external phantom power on the balanced output of the A/B box  100 . 
         [0031]    The A/B box  100  of  FIG. 2  includes a relay  102 , such as a double pole, double throw, 2-Form-C relay, that is used to connect an output connector  104  to either of two input connectors  106  or  108 , depending on whether phantom power is applied. Other functions controlled by the switching mechanism are possible but not shown. 
         [0032]    The connector  104  is a three conductor connector, functioning as both the output of the balanced AC audio signal and the input for the phantom power. A conductor  110  of the connector  104  carries the non-inverted portion of the balanced output audio signal, and a conductor  112  carries the inverted portion of the balanced output audio signal. Both of the conductors  110  and  112  of the connector  104  function as positive DC poles for the phantom supply. As before, phantom power is commonly  12 ,  24 , or  48  volts. A conductor  114  of the connector  104  provides a return path for the phantom power back to the audio console. The conductors  110 ,  112 , and  114 , for example, may be pins. 
         [0033]    Both of the connectors  106  and  108  are three conductor connectors functioning as inputs to the A/B box  100 . Conductors  116  and  118  of the connector  106  carry the AC input signal from a first audio source. Conductors  120  and  122  of the connector  108  carry the AC input signal from a second audio source. Conductors  124  and  126  of the connectors  106  and  108  serves as a common ground connection. 
         [0034]    The conductors  116  and  118  of the connector  106  and the conductors  120  and  122  of the connector  108  are connected to a transformer  128  through switch contacts of the relay  102 . The conductor  110  of the connector  104  is connected to one end of the secondary of the transformer  128 , and the conductor  112  of the connector  104  is connected to the other end of the secondary of the transformer  128 . 
         [0035]    A first pole  130  of the relay  102  connects either the conductor  116  of the connector  106  or the conductor  120  of the connector  108  to the one end of the primary winding of the transformer  128 . A second pole  132  of the relay  102  connects either the conductor  118  of the connector  106  or the conductor  122  of the connector  108  to the other end of the primary winding of the transformer  128 . The transformer  128  blocks the positive DC voltage on the phantom power on the conductors  110  and  112  of the connector  104  from the first and second sources connected to the connectors  106  and  108 , while allowing the AC components of the audio signal from the connectors  106  and  108  to pass through to the connector  104 . 
         [0036]    Resistors  134  and  136  combine the phantom power currents on the conductors  110  and  112  of the connector  104  while maintaining a load impedance on the balanced output of the A/B box  100 . The phantom power combined by the resistors  134  and  136  is fed to an electromagnetic coil  138  of the relay  102 . 
         [0037]    When phantom power is applied to the connector  104  and then through the combining resistors  134  and  136  to the relay  102 , the coil  138  is energized and the first and second poles  130  and  132  of the relay  102  switch to connect only the conductors  120  and  122  of the connector  108  across the primary of the transformer  128 . When the phantom power is off, the relay  102  is de-energized and only the conductors  116  and  118  of the connector  106  are connected across the primary winding of the transformer  128  through the first and second poles  130  and  132  of the relay  102 . Accordingly, application of phantom power from the audio console to the connector  104  operates the relay  102  through the combining resistors  134  and  136  so as to switch the primary connections of the transformer  128  between the connectors  106  and  108 . 
         [0038]    Variations on the use of phantom power can be made without departing from the scope of the invention as defined by the claims below. For example, one such variation is shown in  FIG. 3  by way of a direct box  200 . 
         [0039]    The direct box  200  includes a three conductor connector  202 . The connector  202  functions as both the output of a balanced AC audio signal from the direct box  200  and the input for phantom power to the direct box  200 . A conductor  204  of the connector  202  carries the non-inverted portion of the balanced output audio signal, and a conductor  206  carries the inverted portion of the balanced output audio signal. Both of the conductors  204  and  206  function as positive DC poles for the phantom supply from an audio console. A conductor  208  of the connector  202  provides a return path for the phantom power back to the audio console. This return path is designated as ground GND 2 . The conductors  204 ,  206 , and  208 , for example, may be pins. 
         [0040]    A two conductor connector  210  of the direct box  200  receives the unbalanced input signal. A ground connection of the connector  210  is the ground GND 1 . The unbalanced AC input signal provided to the connector  210  is converted into a balanced AC output signal by a transformer  212 . The transformer  212  also reduces the output impedance of the balanced signal for compatibility with the input of the audio console, allows the ground references between the connectors  202  and  210  to be decoupled, and blocks the positive DC voltage of the phantom power on the conductors  204  and  206  of the connector  202  from the input of the direct box  200 . The conductor  204  of the connector  202  is connected to one end of the secondary of the transformer  212 , and the conductor  206  of the connector  202  is connected to the other end of the secondary of the transformer  212 . 
         [0041]    Resistors  214  and  216  combine the phantom power currents on the conductors  204  and  206  of the connector  202  while maintaining a load impedance on the balanced output. The phantom power combined by the resistors  214  and  216  is fed to an electromagnetic coil  218  of a relay  220 . A pole  222  of the relay  220  opens or closes the connection between the ground GND 2  and the ground GND 1 . The ground GND 1  provides a return for both the connector  210  and the primary of the transformer  212 . 
         [0042]    When no phantom power is applied to the direct box  200  from the audio console, the pole  222  of the relay  220  connects the grounds GND 1  and GND 2 . When phantom power is applied to the direct box  200  from the audio console, the phantom power is combined by the resistors  214  and  216 , and the combined phantom power energizes the coil  218  to open the pole  222  and break the connection between the grounds GND 1  and GND 2 . 
         [0043]      FIG. 4  is another variation. As shown in  FIG. 4 , a direct box  300  includes a three conductor connector  302 . The connector  302  functions as both the output of a balanced AC audio signal from the direct box  300  and the input for phantom power to the direct box  300 . A conductor  304  of the connector  302  carries the non-inverted portion of the balanced output audio signal, and a conductor  306  carries the inverted portion of the balanced output audio signal. Both of the conductors  304  and  306  function as positive DC poles for the phantom supply from an audio console. A conductor  308  of the connector  302  provides a return path for the phantom power back to the audio console. This return path is the ground GND 2 . The conductors  304 ,  306 , and  308 , for example, may be pins. 
         [0044]    A two conductor connector  310  of the direct box  300  receives the unbalanced input signal and applies the unbalanced input signal across the primary of a center tap transformer  312 . The ground GND 1  serves as a ground connection for the connector  310  and the primary of the transformer  312 . The unbalanced AC input signal provided to the connector  310  is converted into a balanced AC output signal by the transformer  312 . The transformer  312  also reduces the output impedance of the balanced signal for compatibility with the input of the audio console, allows the ground references between the connectors  302  and  310  to be decoupled, and blocks the positive DC voltage of the phantom power on the conductors  304  and  306  of the connector  302  from the input of the direct box  300 . 
         [0045]    The conductor  304  of the connector  302  is connected to one end of the secondary of the transformer  312 , and the conductor  306  of the connector  302  is connected to the other end of the secondary of the transformer  312 . 
         [0046]    The phantom power from the audio console is fed by the center tap on the secondary of the transformer  312  to an electromagnetic coil  314  of a relay  316 . A pole  318  of the relay  316  opens or closes the connection between the ground GND 2  and a ground GND 1 . The ground GND 1  provides a return for both the connector  310  and the primary of the transformer  312 . 
         [0047]    When no phantom power is applied to the conductors  304  and  306  of the connector  302  of the direct box  300  from the audio console, the pole  318  of the relay  316  connects the grounds GND 1  and GND 2 . When phantom power is applied to the conductors  304  and  306  of the connector  302  of the direct box  300  from the audio console, the phantom power is fed by the center tap of the secondary of the transformer  312  to energize the coil  314  to open the pole  318  and break the connection between the grounds GND 1  and GND 2 . 
         [0048]    The center tap transformer  312  isolates the phantom power from the connector  310  and combines the two DC components of phantom power from pins  304  and  306  of the connector  302 . This manner of isolation can be used in the other embodiments described herein. 
         [0049]      FIGS. 5A ,  5 B,  5 C, and  5 D illustrate various solid state switches that may be used in place of the relays that are used in direct boxes. Thus, for example,  FIGS. 5A ,  5 B,  5 C, and  5 D illustrate various solid state switches that may be used in place of the relays  48  and  220 , and  316  of  FIGS. 1 ,  3 , and  4 . 
         [0050]    As shown in  FIG. 5A , a solid state switch  400  includes a resistor  402  having a first end connected to receive the phantom power and a second end connected to a first end of a resistor  408 . For example, the first end of the resistor  402  may be connected to the output of the combiner or to a center tap of the transformer. 
         [0051]    The solid state switch  400  also includes a pair of n-channel enhancement mode MOSFETs  404  and  406 . The drain of the MOSFET  404  is connected to the ground GND 1 , and the drain of the MOSFET  406  is connected to the ground GND 2 . The sources of the MOSFETs  404  and  406  are connected together and to the second end of the resistor  402 . The gates of the MOSFETs  404  and  406  are connected together and to the first end of the resistor  402 . The second end of the resistor  408  is connected to the ground GND 2 . 
         [0052]      FIG. 5B  shows a solid state switch  500  that is similar to the solid state switch  400  and, therefore, the same reference numerals are used to depict similar elements. In the case of the solid state switch  500 , a Zener diode  502  is used in place of the resistor  402 . Otherwise, the elements and connections are the same. The cathode of the Zener diode  502  is connected to receive the phantom power, and the anode of the Zener diode  502  is connected to a first end of the resistor  408 . 
         [0053]    As shown in  FIG. 5C , a solid state switch  600  includes a solid state relay integrated circuit  608 . The solid state relay integrated circuit  608  includes an LED  602 , two n-channel depletion mode MOSFETs  604  and  606 , and internal circuitry to drive the gates of MOSFETs  604  and  606 . The anode of the LED  602  is connected to receive the phantom power, and the cathode of the LED  602  is connected to the ground GND 2 . The drain of the MOSFET  604  is connected to the ground GND 1 , and the drain of the MOSFET  606  is connected to the ground GND 2 . 
         [0054]      FIG. 5D  shows a solid state switch  700  that is similar to the solid state switch  600  and, therefore, the same reference numerals are used to depict similar elements. In the case of the solid state switch  700 , a photovoltaic MOSFET driver  702  is used to drive the gates of MOSFETs  604  and  606 . The photovoltaic MOSFET driver  702  includes an LED  704  and one or more photo-sensitive diodes  706 . If more than one photo-sensitive diodes is used, they are connected in series. The anode of the LED  704  is connected to receive the phantom power, and the cathode of the LED  704  is connected to the ground GND 2 . A cathode of the photo-sensitive diodes  706  is connected to the gates of the MOSFETs  604  and  606 , and an anode of the photo-sensitive diodes  706  is connected to the sources of the MOSFETs  604  and  606 . 
         [0055]      FIG. 6  is an example of the use of solid state relays in connection with an A/B box  800 . The A/B box  800  includes phantom power controlled semiconductor relays  802  and  804  that are used to connect an output connector  806  to either of two input connectors  808  or  810 , depending on whether phantom power is applied. Other functions controlled by the switching mechanism are possible but not shown. 
         [0056]    The connector  806  is a three conductor connector, functioning as both the output of the balanced AC audio signal and the input for the phantom power. A conductor  812  of the connector  806  carries the non-inverted portion of the balanced output audio signal, and a conductor  814  carries the inverted portion of the balanced output audio signal. Both of the conductors  812  and  814  of the connector  806  function as positive DC poles for the phantom supply. A conductor  816  of the connector  806  provides a return path for the phantom power back to the audio console. The conductor  812  of the connector  806  is connected to one end of the secondary of the transformer  832 , and the conductor  814  of the connector  806  is connected to the other end of the secondary of the transformer  832 . The conductors  812 ,  814 , and  816 , for example, may be pins. 
         [0057]    Both of the connectors  808  and  810  are two conductor connectors functioning as inputs to the A/B box  800 . A conductor  818  of the connector  808  carries the AC input signal from a first audio source. A conductor  820  of the connector  810  carries the AC input signal from a second audio source. Conductors  822  and  824  of the connectors  808  and  810  serve as a common connection to the ground GND 1 . 
         [0058]    The semiconductor relay  802  includes a pair of n-channel depletion-mode MOSFETs  826  and  828 , an LED  830 , and internal circuitry to drive the gates of the MOSFETs  826  and  828 . The drain of the MOSFET  826  is connected to the conductor  818  of the connector  808 , and the drain of the MOSFET  828  is connected to one end of the primary of a transformer  832 . The other end of the primary of a transformer  832  is connected to the ground GND 1 . The sources of the MOSFETs  826  and  828  are connected together, and the gates of the MOSFETs  826  and  828  are connected together. 
         [0059]    A combiner  834  combines the phantom power. The combiner includes a resistor  836  having a first end connected to the conductor  812  of the connector  806  and a resistor  838  having a first end connected to the conductor  814  of the connector  806 . The second ends of the resistors  836  and  838  are connected together and to the anode of the LED  830 . Thus, the anode of the LED  830  receives the combined phantom power. 
         [0060]    The semiconductor relay  804  includes a pair of n-channel enhancement mode MOSFETs  840  and  842 , an LED  844 , and internal circuitry to drive the gates of MOSFETS  840  and  842 . The drain of the MOSFET  840  is connected to the conductor  820  of the connector  810 , and the drain of the MOSFET  842  is connected to the one end of the primary of the transformer  832 . The sources of the MOSFETs  840  and  842  are connected together, and the gates of the MOSFETs  840  and  842  are connected together. 
         [0061]    The cathode of the LED  830  is connected to the anode of the LED  844 , and the cathode of the LED  844  is connected to the ground GND 2 . 
         [0062]    Absence of phantom power from the audio console to the connector  806  operates the semiconductor relays  802  and  804  to connect the connector  808  to the primary of the transformer  832 , and application of phantom power from the audio console to the connector  806  operates the semiconductor relays  802  and  804  to connect the connector  810  to the primary of the transformer  832 . 
         [0063]      FIGS. 7A and 7B  illustrate alternative switch configurations. 
         [0064]    As shown in  FIG. 7A , a relay  900  and a switch  902  are in control of the ground configuration simultaneously. A change in the position of the switch  902  or a change of the phantom power results in a change of the ground connection. 
         [0065]    As shown in  FIG. 7B , closing of a switch  910  causes the ground configuration to be dependent upon on the state of a relay  912 . When the switch  910  is open, the ground connection is open and is not dependent upon the state of the relay  912 . When the switch  910  is closed, the ground connection is dependent upon the state of the relay  912 . An LED  914  indicates the presence of phantom power through a current limiting resistor  916 . 
         [0066]      FIG. 8  shows the connection between a direct box  920 , an audio or mixer console  922  that provides the phantom power to the output connector (such as  12  of  FIG. 1 ) of the direct box  920  and receives the balanced output of the direct box  920 , and a source  924  that provides the AC input to the input connector (such as  20  of  FIG. 1 ) of the direct box  920 . A switch  926  on the audio or mixer console  922  is operated by a user to control the supply of the phantom power to the direct box  920 . 
         [0067]      FIG. 9  shows the connection between an A/B box  930 , an audio console  932  that provides the phantom power to the output connector (such as  104  of  FIG. 2 ) of the A/B box  930  and receives the balanced output of the A/B box  930 , a first source  934  that provides a first AC input to one of the input connectors (such as  106  of  FIG. 2 ) of the A/B box  930 , and a second source  936  that provides a second AC input to another of the input connectors (such as  108  of  FIG. 2 ) of the A/B box  930 . A switch  938  on the audio or mixer console  932  is operated by a user to control the supply of the phantom power to the A/B box  930 . 
         [0068]      FIG. 10  shows an arrangement in which two capacitors are used to replace the transformers in  FIGS. 1 ,  2 ,  3 , and  6  to perform the function of blocking DC voltage and passing AC voltage. An A/B box  1000  selects one of two (or more) AC inputs  1002  or  1004  which is then routed to the one AC balanced output of the A/B box  1000 . The selection is controlled by application of external phantom power on the balanced output of the A/B box  1000 . 
         [0069]    The A/B box  1000  includes a relay  1006 , such as a double pole, double throw, 2-Form-C relay, that is used to connect an output connector  1008  to either of the two input connectors  1002  or  1004 , depending on whether phantom power is applied. The connector  1008  is a three conductor connector, functioning as both the output of the balanced AC audio signal and the input for the phantom power. A conductor  1010  of the connector  1008  carries the non-inverted portion of the balanced output audio signal, and a conductor  1012  carries the inverted portion of the balanced output audio signal. Both of the conductors  1010  and  1012  of the connector  1008  function as positive DC poles for the phantom supply. As before, phantom power is commonly 12, 24, or 48 volts. A conductor  1014  of the connector  1008  provides a return path for the phantom power back to the audio console. The conductors  1010 ,  1012 , and  1014 , for example, may be pins. 
         [0070]    Both of the connectors  1002  and  1004  are three conductor connectors functioning as inputs to the A/B box  1000 . Conductors  1016  and  1018  of the connector  1002  carry the AC input signal from a first audio source. Conductors  1022  and  1024  of the connector  1004  carry the AC input signal from a second audio source. Conductors  1020  and  1026  of the connectors  1002  and  1004  serves as a common ground connection. 
         [0071]    The conductors  1016  and  1018  of the connector  1002  and the conductors  1022  and  1024  of the connector  1004  are connected to capacitors  1034  and  1036  through switch contacts of the relay  1006 . The conductor  1010  of the connector  1008  is connected to the capacitor  1034 , and the conductor  1012  of the connector  1008  is connected to the capacitor  1036 . 
         [0072]    A first pole  1028  of the relay  1006  connects either the conductor  1016  of the connector  1002  or the conductor  1022  of the connector  1004  to the capacitor  1034 . A second pole  1030  of the relay  1006  connects either the conductor  1018  of the connector  1002  or the conductor  1024  of the connector  1004  to the capacitor  1036 . The capacitors  1034  and  1036  block the positive DC voltage on the phantom power on the conductors  1010  and  1012  of the connector  1008  from the first and second sources connected to the connectors  1002  and  1004 , while allowing the AC components of the audio signal from the connectors  1002  and  1004  to pass through to the connector  1008 . 
         [0073]    Resistors  1038  and  1040  combine the phantom power currents on the conductors  1010  and  1012  of the connector  1008  while maintaining a load impedance on the balanced output of the A/B box  1000 . The phantom power combined by the resistors  1038  and  1040  is fed to an electromagnetic coil  1032  of the relay  1006 . 
         [0074]    When phantom power is applied to the connector  1008  and then through the combining resistors  1038  and  1040  to the relay  1006 , the coil  1032  is energized and the first and second poles  1028  and  1030  of the relay  1006  switch to connect only the conductors  1022  and  1024  of the connector  1004  to the capacitors  1034  and  1036 . When the phantom power is off, the relay  1006  is de-energized and only the conductors  1016  and  1018  of the connector  1002  are connected to the capacitors  1034  and  1036  through the first and second poles  1028  and  1030  of the relay  1006 . Accordingly, application of phantom power from the audio console to the connector  1008  operates the relay  1006  through the combining resistors  1038  and  1040  so as to switch the capacitors  1034  and  1036  between the connectors  1002  and  1004 . 
         [0075]    Thus, the capacitors  1034  and  1036  block the positive DC voltage phantom power on the conductors  1010  and  1012  of the connector  1008  from the first and second sources connected to the connectors  1002  and  1004 , while allowing the AC components of the audio signal from the connectors  1002  and  1004  to pass through to the connector  1008 . 
         [0076]    Certain modifications of the present invention have been described above. Other modifications will occur to those practicing in the art of the present invention. Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.