Audio Amplifier with Volume Control
and Gain Switch
V.Sai Sankar
T.Sai Naveen
Dept of Electronics & Communication
Fifth semester , NIT Calicut
Calicut
vs9346@gmail.com
Dept of Electronics & Communication
Fifth semester , NIT Calicut
Calicut
Sainaveenthota333@gmail.com
AbstractThe Aim of this Project is to implement a low
voltage audio amplifier with volume control knob and gain
toggle switch using an LM386 Audio Op-amp IC by Texas
Instruments.
I.
Introduction
The Audio amplifier is designed using an LM386 audio
amplifier IC manufactured by texas instruments and test the
output on an 8ohm Speaker taking input from an audio
source requiring a 3.5mm jack (generic mp3 player , mobile
phone etc.). The LM386 is a power amplifier designed for
use in low voltage consumer applications. The gain is
internally set to 20 to keep external part count low, but the
addition of an external resistor and capacitor between pins 1
and 8 will increase the gain to any value from 20 to 200.
The IC is suitable for battery operation ( 6- 12 Volts) , has a
low quiescent current drain of 4mA , features low distortion
of about 0.2% for an 8ohm load and gain of 20. The inputs
are ground referenced and the heat dissipated during
operation at 6V is 0.2 Watts.
II.
Figure 1: Circuit diagram
CIRCUIT DIAGRAM
The circuit diagram of the amplifier is shown below.
The connections to pins 1 and 8 are left open as they are
shown in a separate image. These pins determine the gain of
the amplifier. The Circuit can be easily implemented on a
solder-less breadboard using connecting wires.
Figure 2: Circuit Diagram
The diagram above shows the external parts required for
gain toggle. In configuration A the gain is set 200 while in
configuration B the gain is set to 50. A toggle switch is used
for switching between the 2 values.
III.
Components Required
A. LM 386 Audio Amplifier IC
A brief introduction of the IC has already been given.
This section addresses the typical electrical and
Performance characteristics of the device. The following
data has been collected at T=25 Celsius [3]
Operating supply Voltage : 4-12 Volts
Quiescent Current at VS=6V , VIN =0 : 4-8 mA
Output Power ( Pout ) at VS=6V, RL=8 : 325mW
Voltage gain : 26 46 dB
Total Harmonic Distortion at VS=6V, RL=8 and
POUT =124mW : 0.2%
Power Supply Rejection Ratio (PSRR) : 50dB
Input Resistance (RIN ) : 50k
The Graph shows the variation of the quiescent current with
respect to the change in Supply Voltage. The Maximum
quiescent current required is 4.6mA at 12 Volt which is an
ideal value for battery operation
Pins 1 and 8 control gain. When not connected
(NC), the amplifier gain is 20. Adding a 10uF
capacitor between them gives a gain of 200.
Intermediate values are also possible, as described
in the datasheet.
Pin 2- is the negative input GND in our case.
Pin 3 is the positive input i.e. the actual signal to
be amplified. There is a 10K potentiometer before
it, which adjusts the input signal level, acting as a
volume control.
Pins 4 (GND) and 6 (Vs) provide the supply voltage
for the amplification. For this setup a 4x AA battery
pack is used, which provides ~6V.
Pin 5 is the output. It is biased to 1/2 of the supply
voltage Vs. In simple terms, this means that the
signal has two components: An AC component,
which is the amplified input signal, plus a DC
component of 1/2 Vs = 3V. This biased voltage
cannot be fed directly to a speaker. The 250uF
electrolytic capacitor filters out the DC component
and the remaining AC goes to the speaker.
The 0.05uF capacitor and 10 ohm resistor pair from
pin 5 to ground are used to prevent high frequency
oscillations.
A small electrolytic or tantalum cap of a 10 uF from
pin 7 to ground will isolate the high gain input stage
of the LM386 from power supply noise, hum,
transients, etc.
Another capacitor of 100 uF is connected between
the power supply and ground to deal power supply
noise (unstable power supply).This capacitor keeps
the input DC voltage stable for stable output.
B. Other Components
C1 Bypass Capacitor 10uF
C2 Gain Capacitor 10uF
C3 Output decoupling Capacitor 220uF
C5 Capacitor 47nF
R1 Resistance 10
R2 Gain Resistance 1.2 K
R3 LED resistance 1.2 K
R4 Potentiometer 10 K
R5 Speaker - 8
Green colour LED
Switches 2
DC voltage supply (battery bank) 6V
IV.
working of the circuit
This amplifier circuit uses an LM386-N Low voltage
audio Op-amp IC whose pin out diagram is given here:
V.
Dealing with Power supply
noise
Another common way of dealing with power
supply noise is adding a small decoupling capacitor
(100uF) next to the IC power pins C5
In my experience, the presence of these capacitors
turned out to be decisive, especially when working
on a breadboard.
Couple of other approaches may also be used to
reduce the noise likeA) Keep all components as close to the
IC as possible
B) Keep all signal lines as short as
possible.
Acknowledgment
We, Sai Sankar & Sai Naveen extend our sincere
gratitude to Professor Bhuvan of NIT ,Calicut for the
brilliant lectures and opportunity to realize Analog Circuit
Design practically, and thanks for being a constant pillar of
support and guidance.
References
[1]Datasheet of the LM-386 as provided by Texas Instruments.
The data sheet provides the typical performance
[2] audio amplifier, En.wikipedia.org.2016. [online]
[3]A.Sedra and k.Smith. Microelectronic circuit
