The tone-adjustable microphone preamplifier can boost the signal of any electret (condenser) microphone but it can also operate with moving-coil dynamic type microphones. The circuit supports tone adjustment in low and high audio frequencies from two independent potensiometers. There is also a third potentiometer that adjusts the output level of the preamplifier. The preamplifier can operate with a single supply voltage from 5 to 30V.
The complete electronic circuit of the tone-adjustable microphone preamplifier is shown in Figure 1.
The preamplifier is based on the LM358 which is a high performance dual operational amplifier. LM358 is also factory optimized to operate with a single power supply.
The first stage of the prteamplifier uses U1A as a classic inverting amplifier. Resistors R5 and R6 are used as a voltage divider to bias the non-inverting input of U1A at a voltage level equal to the half of that of the supply voltage. The microphone signal is applied via capacitor C1 and R1 to the inverting input of the U1A. The gain of the first stage is determined by the ratio R11 to R1. With the values shown in Figure 1, this gain is equal to 10. At the output of the first stage, there is the P3 potensiometer which is used to adjust the volume. Then, follows a passive tone adjustment network. The P1 potensiometer in this network is used to adjust the low frequencies level (bass) while P2 is used to adjust the high frequencies (treble) level. The signal then is fed to a second amplification stage based on U1B. U1B provides additional gain but also ensures a proper low output impedance. In order for the operational amplifier U1B to operate on a single supply voltage, its non-inverting input is also biased by resistors R5 and R6.
The components values shown in Figure 1, refer to the case of a preamplifier which is designed to be used with an electret type microphone which normally requires some power supply. This power supply is provided via resistor R4. If you want to use the preamplifier with moving-coil type dynamic microphone you must remove R4 from the circuit and you must also modify the values of resistors R10 and R11 to increase the total gain. For a dynamic microphone, the R10 and R11 should be of 270K instead to the 100K shown in Figure 1.
You may assemble the circuit easily, on the circuit board provided below. All the components should be placed on the circuit board of Figure 2, according to the assembly guide of Figure 3. The potentiometers are also placed on the board as shown on the assembly guide and the photo of the prototype. The knobs axis of the potentiometers are passed through the board from three suitable holes.You must also place a small jumper wire on the circuit board as shown in the assembly guide.
All resistors we use in this project are of 1/4W through hole type and of 5% or better tolerance. We use some good quality capacitors with as low as possible tolerance. After assembling the circuit and soldering the components to the board, you will first be able to test it by connecting a microphone to the input and an amplifier or an oscilloscope to the output. Make the apropriate audio connections with a shielded audio cable. The volume and tone settings from P1, P2 and P3 allow the microphone to be adapted to your sound preferences.
You should use a good quality power supply unit with minimum ripple in order to avoid any noise from the power supply. Alternatively you can power with a 9V battery. The supply voltage can range from 5 to 30V, without any problems.