Simple cassette player amplifier

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 amplification.

Details

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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)

Components

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• One LM833N opamp.
• One 200KΩ resistor (1/4 W)
• One 3.6KΩ resistor (1/4 W)
• One 200Ω resistor (1/4 W)
• One 100KΩ resistor (1/4 W)
• One 5.6KΩ resistor (1/4 W)
• One 47μF 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 power supply. 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.

Schematic Diagram

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Here is the schematic diagram of the simple cassette player amplifier.

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

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In order for this circuit to work you will need a cassette player deck mechanism, like the one in the following picture

You 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.

 

Pictures

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In the following picture you see the circuit built on a piece of Veroboard.

Videos

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In the following video you can see the circuit in action. For power amp an old PC speaker set was used