An integrated circuit audio amplifier
Like the simple amplifier in the previous section, this project starts
with the solderless breadboard version of our Three Penny Radio, and
adds an amplifier. But this amplifier has several important improvements
over the simple one transistor amplifier.
This project uses an integrated circuit with 10 transistors to amplify
much better with much less power drain on the batteries than our simple
amplifier.
Click on photo for a larger picture
The amplifier needs these parts:
- An LM386 integrated circuit amplifier chip
This is the main working part of the amplifier.
- A small speaker
- Some jumper wires
- A 9 volt battery clip
- A 9 volt battery
We will later improve the amplifier, using these parts:
- A 10 microfarad capacitor
- A 220 microfarad capacitor
- A 0.033 microfarad capacitor
- A 0.047 microfarad capacitor
- A 10 ohm resistor
The colors will be brown, black, black, and gold.
- A 10,000 ohm resistor
The colors will be brown, black, orange, and gold.
- A 1,200 ohm resistor
The colors will be brown, red, red, and gold.
We carry all the parts for the amplifier (except the battery) in our
catalog.
The Three Penny Radio normally has a piezoelectric earphone attached at
points J-20 and E-22. We replace the earphone with our amplifier.
Below is a closeup of the amplifier section:
Click on photo for a larger picture
Here you can see that we have connected the Three Penny Radio output at
J-20 to the ground rail below the blue line. This rail has all of its
holes connected together. We connect the black (negative) wire from the
battery to the ground rail. We connect the red (positive) wire from the
battery to the power rail, just above the red line at the top of the
photo. Having the power and ground connected to these rails makes it
easier to connect the other parts, and makes it easier to see where all
the connections are.
The other output from the Three Penny Radio us plugged into E-22.
Using the labeled grid as before, the parts are connected this way:
- LM386 amplifier chip at E-24, E-25, E-26, E-27
and F-24, F-25, F-26, and F-27.
- Jumper wire: F-20 to ground rail.
- Jumper wire: C-22 to C-26.
- Jumper wire: A-25 to ground rail.
- Jumper wire: A-27 to ground rail.
- Jumper wire: J-26 to power rail.
- Speaker: red wire to H-27 and black wire to ground rail.
- Negative 9 volt battery wire (black): ground rail.
- Positive 9 volt battery wire (red): power rail.
When all the wires are connected properly, you should be able to hear
radio stations coming from the speaker. They will not be particularly
loud, but we can increase the volume 10-fold with a simple adjustment.
Click on photo for a larger picture
We put a 10 microfarad capacitor connecting pins 1 and 8 of the integrated
circuit (put the negative capacitor lead into hole D-24 and the positive
capacitor lead into hole G-24).
This bypasses a resistor inside the integrated circuit, boosting the gain
from 20 to 200.
One problem with the circuit so far is that the speaker will get warm
and the battery will not last long. This is because a certain amount
of DC current is going through the speaker. Direct current (DC) does
not make sounds, and so this current is a complete waste of battery
power, and simply warms up the speaker coil.
Click on photo for a larger picture
We fix this problem by putting a 220 microfarad capacitor between the
integrated circuit output pin (pin 5, in hole F-27) and the red speaker
wire. The positive capacitor lead is put into hole J-27, and the
negative capacitor lead is put into hole J-29. The red speaker wire
is moved to hole G-29.
To prevent the capacitor from changing the sound, we add two more
components to create a "filter" that lets only audio frequencies
get to the speaker.
Click on photo for a larger picture
Here we have moved the negative lead of the 220 microfarad capacitor
to hole J-28, and moved the red wire of the speaker to H-28. We put
a 10 ohm resistor into holes G-27 and G-30. A 0.047 microfarad capacitor
goes into hole F-30 and the ground rail.
Our amplifier is working pretty well now. But our little speaker has
a high tinny voice, because of its size. It would be nice if it had
a little more power on the lower frequencies, what we call "bass response".
We can arrange to amplify the lower frequencies more than the high ones.
We make another filter from a 0.033 microfarad capacitor and a 10,000
ohm resistor, and connect that between the output pin and pin 1 of the
integrated circuit. This will feed some of the low frequencies back into
the amplifier to be amplified again.
We put the 10,000 ohm resistor in holes C-24 and F-29. We put the 0.033
microfarad capacitor in holes I-27 and I-29.
Now our amplifier has a pleasing, more realistic sound.
A more permanent version
As before, we can copy the circuit onto a printed circuit board
and solder all of the parts firmly in place.
This amplifier can be used for many projects, from amplifying crystal radios
to amplifying the signals from electric fish.
Some schematics for those who aren't using the solderless breadboard:
Next
A Digital Thermometer.
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Simon Quellen Field
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