Cassette Players work by amplifying a small voltage generated from a cassette tape playback head from about 0.1mV up to enough to drive a speaker. Some key considerations in these circuits are equalization, minimization of noise, and clean amplification. In this project, we build a circuit that will output a clean audio signal from a cassette tape head using basic electronics components.
Test cassette motor power requirements.
Open the case of the cassette player you will be using, and measure the voltage across the two pins labeled MO+ & MO- while it is operating. For the ByronStatics cassette player, this should be about 1.6V. This is the voltage you will use to drive the motor.
Harvest the necessary physical components.
Taking off the back of the cassette player, desolder the connectors for the tape head, the motor, and the switch. You also might have to desolder some of the other connections to remove the PCB from the case. Our goal is to remove the cassette mechanism, which should include the tape head, tape motor, and a switch connected to the play button.
Wire up the motor.
With alligator clips, connect the negative end of the +1.6V power supply to the motor, and connect the positive end to one of the wires on the cassette chassis switch. Connect the other end of the switch to the positive end of the motor. Now, when the play button is pushed down, the motor should get power and begin to turn. I chose to use a separate external switch for the motor power, but the choice is up to you.
Testing the preamplification common emitter amplifier circuit (recommended but not required).
The first circuit we will build for our project will pre-amplify the signal from the tape head. To ensure that our signal doesn’t get cut off, we will first verify that our transistor is at a linear operating point. To begin, construct the circuit depicted in the diagram for this step using a 2SC1815 NPN audio transistor and the relevant resistors. First, measure the voltage between R1 and R2. It should be approximately 0.57V. To verify that our transistor is at a linear operating point, we will output a 1V peak-to-peak sine wave from the function generator at 1kHz. Connect this function generator output to the circuit, with the positive connection between R_1 & R_2, and the negative connection to ground. Using a splitter, connect the same signal from the function generator output to CH1 on the oscilloscope. Connect the CH2 input to the oscilloscope between the transistor & R_C using an oscilloscope probe. From here, navigate to the acquire menu on the oscilloscope, and choose XY timebase mode. You should see an output similar to the transistor_characterization.png in the supporting files. If your curve doesn’t show the same horizontal range, you can adjust the amplitude of the sine wave, but it should not exceed 4V. Using the cursors, verify that the voltage you measured between R_1 & R_2 is in the sloped range of the plot. If this is the case, the circuit should amplify audio signals properly!
Construct the remainder of the common emitter amplifier circuit.
Building on the circuit from the previous step, add the capacitors & connect the input to the tape head to create the full common-emitter amplifier circuit. At this point, you can test a cassette tape, measuring the output with an oscilloscope probe. The signal may look noisy, but as long as it’s in a detectable range on the order of about 10 mV peak-to-peak, the circuit should be working properly.
Build an inverting amplifier.
Here, we construct the next stage of our circuit, the inverting amplifier. Construct the circuit below with the relevant resistors & capacitors and an LM4562 op-amp. Ensure that pins 8 and 4 (V+ & V- rails) of the op-amp are at +15V and -15V respectively. I used pin 1 for the output and pins 2 & 3 for the inverting & non-inverting inputs respectively. These pins come from the op-amp datasheet, with the pinout attached in supporting files. If using a different op-amp, ensure that you consult the datasheet for the right pins. After constructing this circuit, you can connect the output from the first common-emitter amplifier stage to the input, and test the output with an amplified speaker. It’s a good idea to start with the speaker at a minimum volume, then progressively increase until you hear sound.
Create and Characterize an RC equalization circuit.
Using the 160kΩ resistor and the 1 nF capacitor, build the following high-pass filter circuit. The audio output will be connected to the resistor, but to test the filter, you can connect a function generator to the input, and an oscilloscope across the capacitor (this part is also optional). With a 5V peak-to-peak sine wave, start at a 20 Hz signal and gradually increase the frequency. The 20 Hz signal should be close to the full 5V peak-to-peak, which should drop to about 1V by 500 Hz. Up to 20000 Hz, the output should decrease further to about 0.1V. Attached in the supporting files is a frequency response curve for this circuit, which you could replicate by plotting 1-(Vout/Vin) against the frequency. We characterize the corresponding low-pass filter instead of the high-pass filter to avoid creating a ground loop by measuring the voltage across the resistor. Keeping the capacitor between the resistor and ground also helps to reduce noise from the ground connection.
Connect the different functional blocks.
At this point, we’re almost done! Following the functional diagram below, connect the tape head to the common emitter amplifier, connect the common emitter amplifier output to the inverting amplifier, connect the common emitter amplifier to the RC high pass filter, and connect your amplified speaker across the final 160 kΩ resistor! One final step we will take to reduce circuit noise is to connect 47nF capacitors between the voltage rails (+15V, -15V, and +3.3V) and ground. The +1.6V rail only powers the motor, so noise from this rail does not affect sound quality.
Test the Output!
With our circuit fully constructed, pick your favorite cassette, push the play button on the cassette chassis to bring the playback head into contact with the tape, turn on the power supplies, and switch on the speaker and the cassette motor.