TONE CONTROL/KARAOKE CIRCUIT INTRODUCTION The purpose of the lab is to come up with a working karaoke circuit. The input to the circuit is the audio out from any device. 0.5V is the voltage amplitude at full volume. The circuit to be designed shall have 5 distinct blocks. The first one is a mixer, the second is tone control, the third is volume control, fourth, a 4 LED volume display, and last one is an attenuator and output driver. The input to the first stage has two channels. The left and right audio channels. A 3.5mm standard audio jack is used. It has three configurations left channel, right channel and ground. DESIGN BLOCK 1: MIXER/KARAOKE OBJECTIVE: To design a mixer for both the left and right channel inputs from an audio device. In this block, an inverting summing amplifier is used. Depending on the position of the single pole double throw selector switch, there are various possible outputs. Output from the inverting summing amplifier is – (L + R) and that from the subtracting amplifier (L – R). The circuit that was designed with the resistors in the provided lab kit is shown below: The first function generator represents the right channel. The second generator represents the left channel. The right channel settings is as shown below, The settings for the left channel are as shown below, The SPDT selector switch is used to determine the output (whether inverted or non-inverted output). An oscilloscope is used to show the waveforms needed. SIMULATIONS For the “L/R Mixer mode” the circuit is The simulation results For the Karaoke mode, the frequency of both the left and right channels are set 50Hz. The simulation result is shown below, BLOCK 2: TONE CONTROL OBJECTIVE: To design a Baxandall circuit that gives a gain range of about approximately 1/10 to 10 for both bass and treble. The circuit modelled after a Baxandall circuit. It consists of 2 potentiometers, 2 capacitors, and an op amp. The circuit allows adjustment of base and treble control. The two potentiometers represent the controls. The base is controlled by the top potentiometer. Treble is controlled by the lower pot. The circuit is based on the fundamental characteristics of capacitors. At low frequencies, capacitors operate as open circuits, and at higher frequencies, they act as short circuits. The circuit was designed to maximize on the fundamental properties in order to construct a network with adjustable base and treble. Both capacitors operate as open circuits at low frequencies, and Vout is determined by R4 resistors and the top potentiometer. Since C1's capacitance is much greater than C2's at middle frequency ranges, C1 acts as a short circuit and C2 acts as an open circuit. This op amp simply inverts the signal. For high frequencies, the capacitors are short circuited. Just the bottom two resistors and the bottom capacitor determine the output voltage. The output of the mixer is input of the tone control circuit. CALCULATIONS The required calculations are for the values of R1 and R4. To calculate these values, equations given in the project handout for maximum gain and minimum gain were used. The maximum gain is (100,000 + R)/R and the minimum gain is R/(100,000+R). With these equations the calculated values of R1=R4= 11k Ohms. The values of the capacitors and other resistors were given, so no further calculations were made. SIMULATION RESULTS For the bass boost the first potentiometer is set to maximum. The second potentiometer is set to minimum. The graph is as shown below: For the treble boost, the second pot is set to maximum while the other is set to minimum. The graph obtained is shown below For a flat response, both the bass and the treble are at par. BLOCK 3: VOLUME CONTROL Volume control is majorly done just to increase the loudness of sound produced. The simplest circuit consists of a simple potentiometer. No calculations are needed. BLOCK 4: LED VOLUME DISPLAY OBJECTIVES: To design a 4 LED volume display. The circuit is for the basis of visual representation of the volume of the system. Each LED shall light up at various voltages 2V, 1V, 0.5V, and 0.25V. An op amp in this circuit is used as a comparator. Each level of the voltage is compared to the input voltage to determine whether the LED will light up. CALCULATIONS In order to obtain the desired output, voltage divider rule can be used. The following equation is used Vout = (Vin)*((R6)/(R6+R7)) Where R6 will be the resistors on the top, and R7 will be the resistors below. The other equations that are considered are : (R6/Req)(9) = 7; (R7/Req)(9)=1; (R8/Req)(9)= 0.5; (R9/Req)(9)=0.25; (R10/Req)(9)= 0.25 where Req = R6 + R7+ R8+ R9 + R10 and the 9 voltage from the batteries. For the limiting resistors, Rlim = (Va-Vb-Vd)/(desired Current). After substituting 9V for Va and 3.3V and desired current as 10mA, a resistor of 570ohms was suitable. Considering the ratios obtained from the above equations, the following circuit was created. BLOCK 5: ATTENUATOR AND OUTPUT DRIVER OBJECTIVE: To design a circuit to make sure that the maximum output voltage level stays in an appropriate range for driving headphone. This is the final block of the design. As it is the final stage, issues of gain are considered. For our case, the maximum desired output voltage was beteen 0.5V and 1V. An inverting amplifier was used to obthan the desired effect. Without sufficient voltage at the output, no sound shall be produced. CALCULATIONS For an inverting op amp, the gain was calculated as follows, Vout/ Vin = R1/R2. The location of the resistors can be seen in the diagram below. From this a gain of 1/10 was obtained. 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