Here is the circuit diagram of three input microphone preamplifier. This is a mic mixer preamplifier centered on the high gain operational amplifier IC LM 348. This IC LM 348 is a quad operational amplifier IC that is it has four operational amplifiers embedded in this chip. This IC has also got some advanced features such as very high gain, class AB output stage and very low input supply current of 0.6 mA for each op-amp.

The 3 channels of mic input require three amplifiers. The three op-amps (IC1b, IC1c and IC1d) out of four in the IC LM 348 are used here as input amplifiers for the corresponding channels. This three input amplifiers are made in this circuit work as non-inverting amplifiers. The operational amplifier IC1a is the output amplifier in this circuit. IC1a is wired as inverting amplifier here. The output from each input amplifiers are given to the inverting terminal of output op-amp. IC1a mixes the signals from each channel and produces the required output.

Circuit diagram

Microphone Pre-amplifier Circuit

Notes

• Use a good quality PCB to assemble this circuit.
• IC LM 348 operates from a dual power supply. A +12/-12V DC dual supply is used here for powering the circuit.
• The gain of individual channels can be adjusted by using potentiometers R5, R6 and R7.

The following circuit gives the circuit diagram of Telephone pickup Preamplifier. The central part of this circuit is the operational amplifier IC uA 741. This operational amplifier is used to pick up telephone signals. In this circuit, operational amplifier IC uA 741 acts as inverting amplifier. The gain of the amplifier is determined by the feedback resistor R4 (2M). This port resistor R4 can also be used for gain adjustments in this circuit. The gain to drive headphone Z1 is produced by the operational amplifier.

The tracking of telephone is done by high impendence magnetic core. If a high impedance magnetic core is placed near a telephone instrument or near one of the telephone wires, it picks up inductively both sides of the telephonic conversation without any electrical contact. The coil L1 used here can have 3000 turns of 0.4mm. The power supply for this circuit is 9 to 24 V DC supply.

Circuit Diagram

Telephone Pickup Pre-amplifier using 741 IC

Notes

• Use a good quality PCB or common board to assemble this particular circuit.
• The circuit can be used as a telephone bug too.
• Use 5000 turns for L1 if pick up with 3000 turns is not good.

Here is a simple guitar pre-amplifier circuit. The circuit is designed for high oimpedance type electric guitar pickups. An op-amp IC uA 741 is all that is required. uA 741 IC is IC1 which acts as the non-inverting amplifier. The volume can be controlled with the help of POT R1 and the tone can be controlled with the help of POT R6. The tone effects can be adjusted to “SOFT” and “BRILLIANT” modes with the help of switch S1.

An input voltage of 12 Volt DC power supply or battery is more than enough, though a maximum of 24 Volts can be used.

Circuit Schematic

Here is the circuit of a stereo preamplifier circuit with built in tone control facility.A very useful circuit using versatile tone control IC TDA 1524 from Phillips.

The circuit is designed as per the data sheet as an effective stereo-tone & volume control for car stereos , TV sets or  any sort of stereo amplifier circuit you want.The circuit  includes provisions like bass control, treble control , volume control with inbuilt contour or linear mode option (can be switched in betwwen) and balance. All these functions provided can be controlled by DC  voltages or by  linear potentiometers.

Circuit Diagram & Parts List .

Tips .

1. R1 ,R2 ,R3 andR4  can be used for controlling volume,balance ,treble and bass respectively.
2. If SPDT  switch S1 is in position 1 ,the circuit works in contour mode and  in position 2 the circuit works in linear mode.
3. The circuit can be powered from 7.5  to 12 V DC supply.  Do not give more than12V.

TDA1524 quick reference data .