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Audio Listening Circuit - 386 Audio Amplifier Circuit

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Wall, Door, Window Audio Listening Circuit - A15
audio listening circuit
Resistors: 1/4 Watt
R1 = 1 MEG
R2 = 4.7K
R3 = 470 Ohm
R4 = 100K
R5 = 220 Ohm
R6 = 10K
R7 = 100 Ohm
R8 = 470K
R9 = 4.7K
R10 = 10K
R11 = 10K Pot
R12 = 100K
R13 = 10K
R14 = 100 Ohm
RT = 2.2K
Capacitors: - Disc Or Electrolytic ( Electrolytic
Shown with +/- )  All Low Voltage
C1 = 10 MFD
C2 = 4.7 MFD
C3 = .001
C4 = .001
C5 = .01
C6 = .1
C7 = .01
C8 = .1
C9 = .01
C9A = .01
C10 = .1
C11 = .1
C12 = 220 MFD
C13 = .01
C14 = 470 MFD
NOTE: There is a .1 Mfd Capacitor shown at R6 not listed here.
TD1:
Any sensitive Piezo Transducer will work. Digi-Key Electronics Supplies Them
ON/OFF Switch:
Any, On / Off Switch Will Do - connecting the + side of the 9 volt battery to the circuit
BATTERY:
9 Volts D.C. The Negative Side Of The Battery Connects To The On/Off Switch to ground
CP1:
A standard LM386 audio chip available from Digi-Key Electronics
S2:
A Standard On/Off Switch. This connects a filter into the circuit to filter out low frequency noises
JC1, JC2:
Standard Female Mono Jacks. JC1 conects to a standard 8 Ohm earphone jack. JC2 connects
into the "Mic" portion of any cassette recorder. Use A Patch Cord. The schematic shows the
solder points on the rear of these jacks
Operation:
Place TD1 up against any wall, door or window. Adjust the volume control  R11. Listen, and/or
Record
Transistors - and All Electronic Parts
Q1, Q2,  and Q3, = 2N3904 - NPN
                       Digi-Key Electronics
The A15 Circuit has to be the most reverse engineered unit, by other companies, I think I have ever produced. It was originally produced, manufactured and sold to law Enforcement Agencies back in 1979. I revised it in 1993, 4 transistors, 6 capacitors and 12 resistors removed. I added a 386 audio chip, a recording port and it has not been touched since. See This Circuit In Action In The Video Below. 'OVER 27,,000 HITS'

The Audio Listening Circuit A15 below was based on a ton of research that I did into STC ( Sound Transmission Class ) and STL ( Sound Transmission Loss ) thru different building materials, doors, windows, concrete, stud walls and how those materials affected frequency loss, suppression and the normal DB levels within a home, an office, a retail store and the audio being intercepted on the backside of the materials above.