This circuit is a simple amplifier of the audio signal
comming out directly from a magnetic head of a cassete
player (like the ones used for cars, before the CD era).
The design is focusing on simplicity and not on generating a
high fidelity output, which is also depending on the quality
of the cassette (tape). This amplifier differs
from most audio amplifiers based on common opamps because it
uses an LNA opamp (low noise amplifier), due to the
fact the audio source has microvolt level (uV).
Amplifying a uV voltage without adding internal noise is not
an easy task. Note that the output of a microphone has a mV
voltage level so it's easier to use common opamps for the
The circuit has two stages. The LNA preamplifier and a common
amplifier. Both are based on LM833N which is a low
noise operation amplifier. The LNA stage is connected is
such a way in order to generate the NAB equalization curve
for playback. The NAB equalization curve was used for
cassette tapes and was something like the famous
RIAA equalization curve. However it's not so strict like
RIAA (thus it's much easier to implement).
One LM833N opamp.
One 200KÙ resistor (1/4 W)
One 3.6KÙ resistor (1/4 W)
One 200Ù resistor (1/4 W)
One 100KÙ resistor (1/4 W)
One 5.6KÙ resistor (1/4 W)
One 47ìF electrolytic capacitor (16V)
One 15nF capacitor.
A 8 pin DIP socket (for LM833).
You will also need one cassette player deck mechanism (eg.
extracted from an old car tape player)
A small piece of a Veroboard
Wire for connections
Solder and soldering iron
Also you will need a stabilized symetrical +/- 12V external
You can use two 12V power supplies and connect them in such a
way to generate the symetrical supply (Note:
Connect them in series. The common of the first power supply and
the 12V line of the second, will be the common of the symetrical power
supply (GND). The common of the second power supply will be the
-12V line and the 12V of the first will be the +12V line). Note:
To reduce noise you can use a different 12V power supply for the
motor of the deck (but don't forget to connect the common wires
together). However if you have a good stabilized symmetrical
power supply you can use only that instead.
Here is the schematic diagram of the simple cassette
Attention: You can use two 100nF capacitors to the
power lines of the opamp. One to the +12V line and GND, as close
as possible to pin 8 of the LM833 chip. The second capacitor
connect it to the -12V line and GND again as close as possible
to pin 4 of LM833 chip.
Cassette player deck mechanism
In order for this circuit to work you will need a
cassette player deck mechanism, like the one in the following
can extract such a mechanism from an old car tape player, or you
can buy one (it is still possible to find these mechanism on
the internet). Notice that you don't need any
electronics on the deck. Only the magnetic head is required. If
you have a stereo magnetic head (wich is more possible) then you
can connect the channels together, or you can build two circuits
like the above one for each channel.
In this picture you can see the magnetic head. This is a stereo
magnetic head (it has three wires. Left,Right channel and
GND). You can connect the two channles together or build a
saparate curcuit for each channel.
following picture you see the circuit built on a piece of
following video you can see the circuit in action. For power amp
an old PC speaker set was used