P1_____________22K Log. Potentiometer
R1_____________22K 1/4W Resistor
R2,R3_________100K 1/4W Resistors
R4_____________47K 1/4W Resistor
R5______________2K2 1/4W Resistor (See Notes)
R6______________6K8 1/4W Resistor
R7_____________22K 1/2W Carbon or Cermet Trimmer
R8______________2K7 1/4W Resistor
C1,C6_________100nF 63V Polyester or Ceramic Capacitors
C2,C3__________10µF 63V Electrolytic Capacitors
C4_____________22µF 25V Electrolytic Capacitor
C5_____________22nF 63V Polyester or Ceramic Capacitor
C7____________470µF 25V Electrolytic Capacitor
Q1____________BC547 45V 100mA NPN Transistor
IC1_________TDA7052 Audio power amplifier IC
SW1____________SPST miniature Switch
MIC____________Miniature electret microphone
SPKR___________8 Ohm Loudspeaker
Screened cable (See Text)
No complex changeover switching is required: the two units are connected together by means of a thin screened cable.
A "Private" switch allows microphone muting, if required.
The circuit uses the TDA7052 audio power amplifier IC, capable of delivering about 1 Watt of output power at a supply voltage comprised in the 6 - 12V range.
The same operation will occur when speaking into the microphone of the second unit: if R7 will be correctly set, almost no output will be heard from its loudspeaker but a loud and clear reproduction will be heard at the first unit output.
- The circuit is shown already doubled in the diagram. The two units can be built into two separate boxes and connected by a thin screened cable having the length desired.
- The cable screen is the negative ground path and the central wire is the signal path.
- The power supply can be a common wall-plug transformer-rectifier having a voltage output in the 6 - 12V dc range @ about 200mA.
- Enclosing the power supply in the box of one unit, the other unit can be easily fed by using a two-wire screened cable, its second wire becoming the positive dc path.
- To avoid a two-wire screened cable, each unit may have its own separate power supply.
- Please note that R5 is the only part of the circuit that must not be doubled.
- Closing SW1 prevents signal transmission only, not reception.
- To setup the circuit, rotate the volume control (P1) of the first unit near its maximum and speak into the microphone. Adjust Trimmer R7 until your voice becomes almost inaudible when reproduced by the loudspeaker of the same unit.
- Do the same as above with the second unit.