As you can see in the schematics, the circuit is composed of four stages:
- impedance adapt stage: this stage is composed of two OPAMPs ( TL082 ) in follower configuration. One of the operational amplifiers is for the right audio channel, and the other for the left audio channel. This gives to the mini amplifier a high input impedance to allow connecting different kind of audio sources without affecting to their output. The noise signal is also fed to the mixer through an operational amplifier in follower configuration to avoid affecting the noise generation circuit.
- mixer stage: this stage is placed after the impedance adapt stage and mixes the right, the left, and the noise audio signals into one single signal. This is achieved with an OPAMP ( TL082 ) in adder configuration. The “weight” on the output of each of the input signals is set with the input resistors values. The ratio between the value of these resistors and the output resistor sets the gain. The volume potentiometer works as the output resistor of this part of the circuit, and allows controlling the volume of the whole bizarre mini amplifier. This is the only OPAMP configuration in the circuit that inverts the resulting signal, but as all the input signals are affected in the same way this does not affect to the final sound.
- filter stage: this is a simple first order R-C low pass filter which allows removing part of the high frequencies present on the output signal. This part of the circuit is isolated from the rest of the circuit with another OPAMP ( TL082 ) in follower configuration. The resistor of the R-C network is the tone potentiometer and allows to the user to modify the filter cutoff frequency. So this is the tone potentiometer and changing it's value turns the amplifier sound brighter or more opaque.
- power stage: on the theory, ideal OPAMPs have 0 output impedance and can provide infinite current, but that is only on ideal world. Despite real OPAMPs have a very low output impedance and can provide a considerable amount of current for signal applications, they don't have enough power to move the 8 ohm speaker. So I had to use another specialized device in the power stage: a power amplifier integrated circuit ( LM386 ). The configuration of this device is the standard specified in the datasheet, and also added the bass boost improvement R-C network proposed in the same document. I had some noise problems with this IC which I solved placing a 20K resistor between the ‘-’ input and ground.