fig1 illustrates the basic configuration of the present invention in a typical application . the audio distribution amplifier ( ada ) system 100 receives as inputs left and right line level signals from the home owner &# 39 ; s personal stereo 102 , substantially continuous audio signals from a telephone handset 105 , intermittently generated control signals from a paging device located within the telephone handset , and any of a plurality of intermittently generated doorbell signals from a multiplicity of different doorbell sources 106 , 107 or 108 at various locations . the ada system 100 generates , as outputs to speakers located in a plurality of different zones a - d , any of the following : a multi - tone doorbell signal indicating from which of a multiplicity of sources 106 , 107 , and 108 the doorbell signal originated ; a voice paging message originating from someone speaking into a telephone handset 105 after a page command is initiated ; or amplified left and right stereo signals corresponding to line level signals received from the home owner &# 39 ; s personal stereo 102 . the interface between the ada system 100 and the output of the stereo 102 is illustrated in fig2 . the interface includes a wall plate interface circuit 110 which receives and buffers left and right signals from a typical consumer music system , such as a stereo amplifier or radio receiver 102 . the range of stereo output signal levels and impedances is preferably from 0 . 1 v to 1 . 0 v ( peak to peak ) and 10 kohms to 100 kohms , respectively . a remote master gain control 111 in the wall plate interface circuit 110 ( mounted on printed circuit board 109 ) provides volume control . ideally , the wall plate interface circuit 110 ( and the gain control 111 ) are located near the associated stereo so that the stereo signal may be properly distributed without degradation . the interface illustrated in fig2 also includes a remote stereo defeat control switch 112 which disables the stereo signals presented to the ada system 100 when closed . when the defeat control switch is open , the left and right signals from the stereo 102 are delivered to the ada system 100 via two signal cables 113 . each signal cable preferably consists of two 22 a . w . g . conductors with a bell foil shield and drain wire . the cable lengths should be between 20 and 200 feet . fig2 illustrates the telephone handset 105 from which a paging command is initiated . in a preferred embodiment , a telephone handset with a page button , such as a square d elan paging button part no . 40603 - 120 - 01 or a gemini electronics two - line - button with hold feature , is used . upon pressing the page button 126 on the telephone handset 105 , the telephone handset generates a monaural paging signal . the monaural paging signal is supplied to the ada system 100 via a standard two - wire telephone tip and ring ( t and r ) interconnection 194 ( fig4 ). the monaural paging signal activates the station line interface circuit ( slic ) 192 ( e . g ., using an ams 2406 - type integrated circuit ). the slic 192 then disconnects the telephone handset and serves as a telephone handset power source during the page . power to the ada system 100 may be provided using a conventional ac domestic power source ( not shown ) and a transformer 103 providing + 15 v , - 15 v and - 24 v ac power levels to the ada system 100 . power to the wall plate interface circuit is provided by a power supply in the ada 100 ( wires not shown ). referring now to fig3 there is illustrated one of two identical wall plate interface circuits which may be used for converting and buffering a respective one of the left or right stereo signals from a single - ended to a balanced signal . the circuit includes a balanced termination resistor 150 for receiving the stereo signal , and conventionally arranged high and low feed amplification circuits based around tl074 - type operational amplifers 156 and 158 . the resistor and capacitor values in the circuit of fig3 may be implemented using 100 kohm for r1 , 100 ohms for r2 , pico - farads for c1 and 100 micro - farads for c2 . the potentiometer denoted 160 is preferably implemented using a 49 . 9 kohms component . fig4 illustrates , in block diagram form , the basic functional circuits internal to the ada system 100 of fig2 . from the stereo signal cables 113 provided by the wall plate interface circuit of fig3 the stereo ( or music ) signal is received by conventional pre - amplification and equalization circuit stages 170 and 172 and by threshold detection circuitry 174 , the latter of which monitors the left and right stereo signal feeds 176 for low level audio signals . in response to the stereo signals being absent or at a negligible level , the detection circuitry 174 initiates a control signal , over feed 178 , which effectively instructs a selection / control circuit 180 to block any noise at the output of the equalization stages 172 from reaching the speakers . a momentary closure of any of the plurality of doorbell switches ( e . g ., from 106 - 108 of fig1 - 2 ) will cause an intermittent doorbell signal to be received by a door bell generator circuit 182 ( fig6 b ). the door bell generator circuit 182 then creates two signals . the first is a unique tone sequence 184 which will identify which of the three doorbell switches initiated the doorbell signal . the second is a doorbell control signal 186 which the selection / control circuit 180 uses to mute the stereo signal inputs 176 and unmute the unique tone sequence 184 so it can be coupled to pre - amplifiers 188 and power amplifiers 190 , which amplify the signal selected from the selection / control circuit 180 using conventional circuitry . further , upon receiving the monaural paging signal over the t and r lines , the slic 192 generates a page control signal ( feed 129 ) which the selection / control circuit 180 uses to mute the stereo signal and unique tone sequence 176 and 184 , and unmute the page signal input 130 . the slic should then receive an audio message from the party initiating the page command , and transmit this page signal information to the ada system 100 . the page signal information is also passed through a feedback control circuit stage 132 which prevents significant signal degradation of the audio message , which may result if the telephone handset is located near a speaker . from the feedback control circuit stage 132 , the page signal information is passed through an equalization circuit stage 133 to enhance the voice integrity of the audio message . a conventional power supply / rectification circuit 210 may be used to convert the ac power levels to complementary dc power levels for the above described circuitry . fig4 further illustrates the signal selection / control circuit 180 as coupling the selected stereo 176 , unique tone sequence 184 , and page signal 130 to the output amplifiers 188 and 190 . when more than one control signal is presented to the selection / control circuit simultaneously , the input signal with the highest priority is coupled to the speakers . the signal priorities for the illustrated embodiment are from highest to lowest : page , doorbell , stereo and then mute . fig4 further illustrates the output amplification stages 188 and 190 of the ada . the arrangement uses conventional audio pre - amplification and power amplification circuitry 188 and 190 to distribute the selected information signal to speakers in a plurality of different zones a - d 134 . fig5 illustrates a preferred circuit for implementing the stereo pre - amplification and equalization stages 170 and 172 and the threshold detection circuit 174 of fig4 . the illustrated stereo pre - amplification circuit 170 is one of two identical circuits used to convert a respective one of the left or right stereo signals from a balanced signal to a single - ended signal . the circuit includes a balanced termination resistor 501 and a conventionally arranged amplification circuit based around a tl074 - type operational amplifier 502 . the resistor and capacitor values in the pre - amplification circuit 170 of fig5 may be calculated using 100 kohms for r1 , 100 micro - farads for c2 and 47 pico - farads for c3 . the equalization stage 172 is similarly one of two identical circuits used to compensate a respective left or right stereo signal for reduced volume levels and frequency response roll - off caused by the ada speakers ( fig9 ). the circuit includes treble and base equalization stages based around tl074 - type operational amplifiers 504 and 506 respectively . the resistor and capacitor values in the equalization circuit 172 of fig5 may be implemented using 10 kohms for r4 , 1 kohms for r5 , 22 . 6 kohms for r6 , 29 . 4 kohms for r7 , 18 . 2 kohms for r8 , 10 pico - farad for c1 , 0 . 039 micro - farad mylar for c4 , and 0 . 001 micro - farad mylar for c5 . the potentiometers denoted 508 and 510 are preferably implemented with 10 kohms as the maximum value . the threshold detection circuit 174 monitors the left and right stereo signal lines 176 for low level audio signals . as mentioned above , when the stereo signal levels are absent , or their levels become negligible , the dc control line 178 goes high , which instructs the selection / control circuit to mute the stereo channel ( 180 of fig4 ). the threshold detection circuit 174 includes two summation resistors 512 and 514 and a tl074 - type operational amplifier 516 in a buffered voltage - follower configuration . from the amplifier 516 , the buffered signal is ac coupled to a threshold - level adjustment potentiometer 520 and a second stage which provides amplification of the stereo signal using a tl074 - type operational amplifier 518 with a gain of 100 . a capacitor 522 connected between the amplifier &# 39 ; s ( 518 ) output and negative input provides noise reduction for the stereo signal . the final stages of the threshold detection circuit 174 include a pair of tl074 - type operational amplifiers 524 and 526 arranged for signal rectification and amplification . the first amplifier 524 provides rectification using a pair of diodes 528 and 530 , while the second operational amplifier 526 amplifies the control signal 178 . a capacitor 532 provides additional noise reduction . the resistor and capacitor values in the threshold detection circuit 174 of fig5 may be implemented using 100 kohms for r1 , 10 kohms for r4 , 1 kohm for r5 , 10 pico - farads for c1 , and 100 micro - farads for c2 . the potentiometer denoted 520 preferably limits at the upper end at 10 kohms . turning now to fig6 a , the circuit embodiments of the paging function &# 39 ; s slic 192 , pre - amplification , feedback control and equalization stages 131 - 133 of fig4 are illustrated . the slic 192 , which is preferably implemented using an ams 2406 - type integrated circuit configured as shown in fig6 processes the audio signal received through the tip and ring lines by passing the signal from its tx output to the pre - amplification circuit 131 . the illustrated pre - amplification and feedback control circuitry 131 - 132 includes a buffer - type circuit arrangement based around a tl074 - type operational amplifier 602 . the feedback control is accomplished using a potentiometer 604 as a current limiting device . if the audio signal begins oscillating , the current limiting provided by the potentiometer 604 prevents the voltage input to the equalization stage 133 from climbing excessively . the resistor and capacitor values in the pre - amplification and feedback control circuits 131 - 133 may be calculated using 100 kohms for r1 , 1 kohm for r5 , 100 micro - farads for c2 , 47 pico - farads for c3 , and 33 micro - farads for c9 . the illustrated page signal equalization circuit is identical in operation configuration and component values to the stereo signal equalization circuit 172 of fig5 ; a circuit description , therefore , is not necessary . fig6 a further illustrates the doorbell generator circuit 182 of fig4 . in response to the intermittent closure of any of three doorbell signal sources ( e . g ., from 106 , 107 , or 108 of fig1 - 2 ), the doorbell board 650 generates the doorbell control signal 186 for use by the selection / control circuit 180 ( fig4 ), and the unique tone sequence 184 for broadcast and identification of which of the three sources initiated the doorbell signal . the multi - tone sequence is then passed through an ac coupling capacitor 652 , and a unity gain buffer circuit based around a tl074 - type operational amplifier 654 ( operating in a manner similar to the pre - amplification and feedback stages 131 - 132 of fig6 a , and is ac coupled to a potentiometer 655 for level adjustment . fig6 b , which illustrates a preferred circuit embodiment of the doorbell generator board 650 , includes a timer 666 , three mc14017 - type decade counters 660 , 662 , and 664 and a mc14046 - type phase - locked - loop ic 668 . the timer 666 is implemented using a 555 - type monostable multivibrator ic which oscillates at a relatively low frequency , e . g ., about one hertz . the oscillating signal is provided to each of the three decade counters 660 , 662 and 664 as the clock input , and each doorbell switch ( when closed ) is used as an enable input . thus , a signal generated by the momentary closure of any of the three doorbell switches enables the respective decade counter chip 660 , 662 , or 664 to initiate the sequencing of the counter outputs , which are coupled through diodes 670 to a voltage divider ( resistor ) network 672 . as the enabled decade counter cycles , the voltage at the output 674 of the voltage divider 672 changes temporarily to a different dc level , which controls the input voltage of the phase - locked - loop ic 668 , which is configured to operate as a voltage - controlled oscillator ( vco ). for each dc voltage level input , the vco outputs a signal of a distinct frequency , and the signal is amplified through a 2n1101 npn - type transistor 678 , with the emitter of the transistor 678 connected to the audio line 676 ( fig6 a - 6b ). the number of tones that the doorbell generator circuit 182 ( fig6 ) produces in response to a doorbell signal from a particular doorbell source ( or switch ) depends on the configuration of the corresponding decade counter 660 , 662 , or 664 . as configured in fig6 a , an intermittent closure of doorbell switch 1 ( db1 ), doorbell switch 2 ( db2 ), or doorbell switch 3 ( db2 ) will result in a two -, four - or two - tone output respectively . when pin 6 of the enabled decade counter goes high , pin 13 , which is connected to pin 6 through a resistor , also goes high , and the counter is disabled . once disabled , the tone sequence is terminated . resistor and capacitor values in the doorbell circuit illustrated in fig6 a - 6b may be implemented using 100 kohms for r1 , 10 kohms for r4 , 1 kohms for r5 , 11 kohms for r11 , 680 ohms for r12 , 2 . 2 kohms for r13 , 10 pico - farads for c1 , 100 micro - farads for c2 , 2 . 2 micro - farads for c6 , 1 . 2 micro - farads for c7 , 0 . 01 micro - farads for c8 , and 33 micro - farads for c9 . preferred component values for the potentiometers r9 and r14 are 10 kohms and 50 kohms , respectively . for further information concerning the operation of the components depicted herein , reference may be made to 101 sound , light and power ic projects by charles shoemaker , tab books inc . of blue ridge summit pa . referring now to fig7 the signal selection / control circuit 180 ( fig4 ) is illustrated as including two 4053 - type multiplexers 702 and 704 , mute control circuitry centered around 706 , 708 and 710 and the pre - amplifiers 188 ( fig4 ). if no stereo signals are present , the threshold detection circuitry 174 of fig4 - 5 will supply a threshold control signal 178 . this control signal is processed by a peak detector 710 and bufferred by a tl074 - type operational amplifier 706 . from the amplifier 706 , the control signal is compared to a voltage threshold set at the positive input of a comparator 708 to determine if the stereo signal is above this predetermined mute cut - off level . a delay circuit 712 provides approximately one minute of delay to allow changes in music selection without the channel being suddenly muted . resistors r15 and r16 and capacitors c10 , c11 and c12 may be implemented using 220 kohm , 3 . 3 megaohm , 0 . 1 micro - farad ( mylar ), 0 . 1 micro - farad ( monolithic ), and 22 micro - farad ( tantalum ) values , respectively . a mute control signal 716 , which is received by the stereo defeat control switch 112 of fig2 is or - tied ( through inverting buffers 715 ) with the output of the delay circuit 712 so that either the output of the delay circuit 712 or the stereo defeat control switch 112 can mute the music . each identical left and right multiplexer 702 and 704 is controlled by the mute function , the doorbell control signal 186 and the page control signal 129 by the respective x , y and z control inputs . as feed inputs , each multiplexer 702 and 704 receives left or right stereo signal feeds 716 ( fig4 ) at the x1 input , ground at the x0 input , the page signal 130 at the z0 input and the doorbell tone sequence 184 at the y1 input . the y0 input receives the x output , and the z1 input receives the y output , with the z output being provided to the pre - amplification circuit 188 , for priority control . any audio received at the output ( z ) 720 of the multiplexers is converted from a single - ended signal to a balanced signal using the conventional pre - amplification circuitry 188 shown in fig7 . in accordance with this configuration , if the one - minute delay circuit 712 lapses , or if the mute switch 112 is closed , the multiplexers 702 and 704 will choose input x0 and effectively mute the stereo inputs . otherwise , the multiplexers will pass the stereo signals through to the output ( x ) 718 , to the output ( y ) 720 and then to the output ( z ) 721 of the multiplexers . if a doorbell control signal 186 is present , the multiplexers will pass the doorbell tone sequence 184 to the output ( y ) 720 and then to the output ( z ) 721 of the multiplexer ( provided the page control signal 186 is not present ). if a doorbell control signal 186 is not present , the multiplexers will pass the output ( x ) 718 to the output ( z ) 720 . if a page control signal 129 is present , the multiplexers will pass the page signal 130 to the output ( z ) 721 of the multiplexer ( regardless of the presence of any other signals ). if a page control signal 129 is not present , the multiplexers will pass the output ( y ) 720 to the output ( z ) 720 . fig8 illustrates the power amplification circuit 190 of fig4 as including four identical lm675 - type power amplifiers 810 , 812 , 814 and 816 . the gain of each amplifier is 20 . the resistor and capacitor values in the power amplification circuit 190 of fig4 are preferably as follows : 10 kohms for r4 , 1 kohms for r5 , 22 kohms for r17 , 1 ohm ( 1 watt ) for r18 , 0 . 1 micro - farad for c14 , and 500 pico - farads for c13 . fig9 demonstrates four exemplary alternative hook - up configurations 902 , 904 , 906 , 908 for lines running from the amplifiers 190 to the speakers located in each of the zones a - d . because the output amplifiers 109 are in a balanced bridgeable output configuration , the bridgeable left and right output amplifiers 109 can accommodate left , right , and left plus right speaker hook - up configurations . preferably , each speaker has its own volume control located within the room where that speaker is placed . preferably , an audio transformer configuration 1002 ( fig1 ) is used to provide volume control . the amplifiers 109 connect to the inputs 1004 of the audio transformers 1002 . the 10 position selector switch 1006 provides adjustability from zero to full volume . each audio transformer &# 39 ; s output 1008 then feeds a corresponding speaker . due to the use of four ohm speakers , the volume control audio transformer has a built - in six decibels of attenuation so that the impedance presented to the ada system from each transformer with a four ohm speaker is not less than sixteen ohms . unless otherwise indicated , component values represented by the same notation from the different figures ( e . g ., r1 , c50 ) are the same .