LM386 Audio Listening Circuit

386 High Gain Audio Amplifier, Listening Circuit - DAA

Audio Listening Circuit Board Artwork By Pete Zenner
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Resistors: 1/4 Watt
R1 = 4.7K
R2 = 100K
R3 = 2.2K
R4 = 10K
R5 = 100K                                       
R6 = 10K
R7 = 4.7K
R8 = 100K
R9 = 47K
R10 = 33K
R11 = 1Meg
R12 = 22K
R13 = 4.7K
R14 = 1Meg
R15 = 10K
R16 = 22K
R17 = 1.2K
R18 = 100K
R19 = 100K
R20 = 10K
R21 = 100 Ohm
Pots:
PT1 = 50K Pot
PT2 = 10K Pot
Capacitors:
Disc Or Electrolytic
( Electrolytic Shown with +/- )
All Low Voltage
C1 = .001
C2 = .05
C3 = .01
C4 = 2.2 Mfd
C5 = .01
C6 = .01
C7 = .001
C8 = .01
C9 = .05
C10 = .001
C11 = .01
C12 = .01
C13 = 4.7 Mfd
C14 = .001
C15 = 470 pf
C16 = .05
C17 = .001
C18 = .05
C19 = 100 Mfd
C20 = .05
C21 =  47 Mfd
C22 = .001
C23 = 100 Mfd
C24 = 470 Mfd
C25 = .01
Transistors:
Q1 = 2N3904
Q2 = MPF102 - JFET                             
Q3 = MPF102 -JFET
Q4 = 2N3904
Q5 = MPF102 - JFET
All Transistors
Digi-Key Electronics

Update: Some builders are having problems locating the MPF102. A Direct Replacement is the NTE451. A couple of Builders have reported to me that they are having better results by using a BF245A in place of the MPF102. The circuit was originally prototyped and built with the 102. They can be found here:  http://vakits.com/mpf102-n-channel-fet   The problems that have cropped up - and have been resolved where a few builders thus far have experienced with the circuit not working - is that they have mixed up the Gate, Source and Drain leads on the MPF102 during assembly. Once corrected they reported that everything worked.. Hope the Update Helps. John



The Audio Amplifier Circuit shown below was originally designed for state agents down in Georgia and used for intercepting conversations outdoors where bootleggers were operating illegal stills. They required a high-gain amplifier on par with a parabolic microphone, less the telltale dish.
The problem was They also required that the unit be able to hear a still cooking ( In Operation ). From my childhood I recall that the sound of a still cooking off was a low rumble so here was my problem. How do I engineer ( In The Lab ) the reception of a low rumble and also the normal voice frequencies + the fact that the leaves blowing on trees and also sounds being distorted and blocked by the woods, trees and swamp would come into play.
This amplifier circuit is truly unique - and one of a kind. I actually took a box of components into the same environment, under the same conditions along with a breadboard, two headsets, and a state agent in tow. The circuit took on its own - by using my knowledge, his experience and my breadboarding the circuit on-site and with both he and I listening to stage by stage,  until he was satisfied with what was coming over the headset. It required two days, a lot of experimentation, re-breadboarding, the addition of filters to tune certain frequencies or suppress others - and last an acoustic resonator ( See Details Below ). The end results was the combination of breadboarding and listening rather than hard core engineering. Once perfected, the breadboard went back to the lab, to schematic, to board - and then to assembly. It achieved every result they were looking for, and I left with my customers being very happy campers.


high gain audio amplifier circuit LM386
Audio Chip:
Standard LM386 Audio Chip. Place A Jumper
Wire Between Pins 2 and 4. The Pin
Connections are indicated in the Schematic.
J1:
This is a standard Mono Female jack that
allows a patch cord to be inserted at this
point to any standard Cassette Record Input
at the "Mic" input. - At the same token,
another mono female jack ( J2 ) is placed at
the - side of C23 that will allow a standard 8
ohm earphone to be plugged or unplugged
J2: Female mono jack, size to fit the male
jack off of your headset.
S1:
Any, On / Off Switch Will Do - connecting the
+ side of the 9 volt battery to the circuit.
BATTERY:
9 Volts D.C.
MC1:
This Unit uses any small electret element,
any electret will do. Digi-Key Electronics
sells quite a few of these sensitive elements
and virtually any type will work. The element
will connect to C2, the negitive side to
ground.

MC1:
This unit incorporates an electret element for it's
microphone. Digi-Key Electronics sells quite a few in
different sizes, all are sensitive and virtually all of them will
work ( Any electret element  will do ).

The microphone element is placed on the inside
of standard Kodak 35 MM plastic film can ( That Holds
Film - when you purchase a roll ). The two combined in
the manner below produces DB gains in excess of 21 DB
in the audio frequency range. The plastic film cup acts as
an "Acoustic Resonator" for certain audio frequencies -
with a hell of a lot of gain, before it even hits the
electronics ).

To make this unit, cut a small hole out of the back of
this plastic film can - and insert the electret element.  Put
the  element about 1/2 way into the hole you have made
and glue it. The positive side of the element will connect
to C2, the negitive side to ground.

Additional Information:
Current Drain @ 9.0 VDC = 6 MA - (.006A)
Internal Noise Level ( Mic. Shorted ) = Less than
.003Mv
BandPass: Adjustable