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Resistors: 1/4 Watt
R1 = 220K
R2 = 1.8K
R3 = 33K
R4 = 4.7K
R5 = 10K
R6 = 4.7K
R7 = 10K Adjustable Pot
R8 = 1K
R9 = 100 Ohm
Capacitors: - Disc Or Electrolytic (
Electrolytic Shown with +/- ) All Low
C1 = 4.7 MFD
C2 = 1 MFD
C3 = .1
C4 = 1 MFD
C5 = .1
C6 = 470 MFD
C7 = .1
C8 = 470 MFD
C9 = 100 MFD
Q1, Q2 = 2N3904 Digi-Key Electronics
9 V.D.C. - Use a standard SPST switch for the on/off switch. The
Positive Side Of The on/off switch connects to the circuit, the
negative side of the battery to ground as shown on the schematic.
LM386-1 - Standard Audio Chip. Place a jumper wire between pins 2
Any 8 Ohm Speaker
Any 8 Ohm earphone or headset
J1: Female Mono Jack to match the size of your male headset jack.
Operation Of This Unit:
Turn the on/off switch on. Adjust R7 ( the 10K pot ) to a comfortable
Low Impedance Audio Amplifier Circuit - Using A Speaker
As A Microphone - Circuit AA5.
I Could not count the number of requests I have had over the years for an audio amplifier circuit
or a circuit diagram where a regular speaker was used as its microphone rather than an electret
or high impedance pickup.
The below circuit produces surprising results, Q1 and Q2 amplifying the circuit and then the
LM386 chip driving its output to full bloom in a really high and adjustable gain.
The Combination of C4,C5, R5 and R6 keeps the frequency flat in the voice frequencies.
I usually don't like to get into the technical aspects of any of my circuits since experience has
taught me that those who build them could care less, and only that it works when they turn the
switch on - and on the other hand those who are highly skilled on the technical and math end of
electronic circuits - usually can figure it out in a heartbeat. For those who own Multisim Software
Shown below - Directly Below This Text - is a schematic setup for Multisim
so you can bode plot it, or run any form of analysis you wish, and cater the signal with a
bandpass circuit to your own desires. Parallel a 50k resistor with R7 to load the output to
simulate the input impedance of the 386 chip and cut R7 to 1/2 the volume control or 5K