Designing a Pre-Amplifier using LM358

In all the previous tutorial, a pre amplifier using MAX4468 was designed. On the basis of application, there are two types of audio amplifiers –

1) Pre-Amplifier

2) Power Amplifier

The pre-amplifiers are used to level up the audio signals from a microphone or audio source to standard voltage levels while the power amplifiers are generally used at the output stage of the audio systems to boost audio signals before they are reproduced by the speakers. As the name suggests, a Pre- amplifier prepares the signal which is coming from the microphone or line input for further processing and transmission of the signal. A preamp is the hidden part of any device. It is integrated in the USB microphones, sound cards and mixers.

The pre- amplifier can be classified into two types –

1) Passive Preamp – These pre-amplifiers use discrete transistor, OPAM or tube for amplification of input signal.

2) Active Preamp – These pre-amplifiers use transformer or other impedance matching components for signal amplification.

The Pre-amplifiers are generally used for providing the voltage gain to leveling up a signal in Milli Volts to few volts like increasing a signal amplitude in few milli volts to 1 V level. These are also used to remove any noise from the input signal coming from a microphone or audio source. Ideally, the microphone should be connected with shortest wire with the device to avoid any noise or distortion, but this is not practically possible. So, pre-amplifiers help removing the noise from the original audio signals.

Obviously, the pre-amplifiers have to necessarily level up the signal amplitudes to line levels. This is the most primitive and essential function of any pre-amplifier. The preamp sets the input signal to a line level. The line level is the standard strength of the audio signal to transmit the signal between audio sources to amplifiers, radios, TV and DVD players. Line level is expressed in dBu (decibel unloaded) or dbV(decibel voltage). According to telephone standards, if 1 Vrms is equal to 0 dBV, then the decibel unloaded reference voltage or 0 dBu is the AC voltage required to generate 1 mW of power across load having 600 ohms impedance. The most nominal line level is -10dBu i.e. 0.316 V and +4 dBV which is 1.228 V.

The pre-amp are meant to strengthen the input signal. They do not provide high output current. So after a pre-amp, a power amplifier must be used if audio signal has to be reproduced using a speaker, CD player or head phone at the output. So, a pre-amplifier can be assumed to be a small amplifier before the power amplifier which has output current in micro-ampere range.

The passive amplifiers are the preferred choice among the pre-amps as they do what actually a pre-amp must do. So, in this series, passive pre-amps are designed. In this tutorial, a passive pre-amplifier will be designed using LM358 IC. LM358 is a low power, dual operational amplifier. This IC operates over a supply voltage in range from 3 V to 32 V and provides high voltage gain up to 100 dB.

In the introductory article of this series, various design parameters of the audio amplifier circuits were discussed like Gain, Volume, Skew Rate, Linearity, Bandwidth, Clipping effect, Stability, Efficiency, SNR, Output power, THD and loop grounding. This amplifier circuit will be designed considering the following design parameters –

Gain (Voltage) – 20 dB

Bandwidth – 20 Hz to 20 KHz

Components Required –

Fig. 1: List of components required for LM358 IC based Pre-Amplifier

Block Diagram –

Block Diagram of LM358 Audio Pre Amplifier

Fig. 2: Block Diagram of LM358 Audio Pre Amplifier

Circuit Connections –

This Pre-amplifier circuit is built by assembling the following components together –

1) DC Source – There are two sources of power required in this circuit. A power supply of 5V is required for biasing of microphones while a power supply of 9V is required for the OPAM circuit. The 9V DC source is needed to provide the bias voltage to the amplifier. For making the 5V supply, two 9V batteries are connected in parallel.

2) Input Source – A Microphone is used to provide the input audio signal. An electret MIC has been taken as the input source. The electret MIC requires a biasing voltage between 1 to 5V to power the inbuilt FET buffer which is present in the MIC. Generally, this microphone is powered by 1 V to 5 V DC through a resistor having value from 1K ohms to 10K ohms. There must be sufficient voltage at the bias pin of the microphone so that it could sense the audio signal. The other pin of the electret microphone is connected to the common ground.

Fig. 3: Typical Image of Electret Microphone

The biasing circuit for this microphone has been shown below –

Circuit Diagram of Electret Microphone Biasing

Fig. 4: Circuit Diagram of Electret Microphone Biasing

3) LM358 OPAM IC – LM358 is a low power dual operational amplifier IC. It consists of two independent OPAM (operational amplifier). The maximum DC voltage gain of this OPAM is 100 db with 2 mA to 20 mA output current range. The IC has the following pin configuration –

Fig. 5: Table listing pin configuration of LM358 OPAM IC

The IC has the following pin diagram –

Fig. 6: Pin Diagram of LM358 OPAM IC

In this circuit, only one of its OPAM is used as an inverting amplifier. The inverting amplifier has negative feedback which makes them better than non- inverting amplifiers.

Typical Image of LM358 IC

Fig. 7: Typical Image of LM358 IC

The inverting amplifier changes the phase of output (signal amplitude) by 180 degrees with the input (signal amplitude). The inverting amplifier changes the phase of output (signal amplitude) by 180 degrees with the input (signal amplitude). However, this phase inversion does not affect the audio signal as human ears are response only to the intensity of the sound. The intensity is the energy flowing through an area in a given time, expressed in joule/s/m2. The energy of the wave is proportional to the square of its amplitude. So for a unitarea, the intensity is also proportional to the square of the amplitude.

I A2

So, changing the sign of wave has no effect on I.

The typical application circuit as given in the datasheet of LM358 is used for designing this pre-amplifier. The line output is drawn from the pin 1 of the IC.

The supply voltage to the IC is provided at pin 8 through filter capacitors. At pin 8, two capacitors (Shown as C2 and C3 in the circuit diagram) of 100 uF and 0.1 uF are connected. The high value capacitor C2 helps in filtering high frequency from the supply voltage and low value capacitor C3 helps in filtering low frequency from the supply voltage. The negative feedback is provided to the amplifier through a resistor (Shown as R3 in the circuit diagram) of 100 K ohms. At the output pin (pin 1) of the amplifier, a filtering capacitor (Shown as C6 in the circuit diagram) is connected to block any DC components from the amplifier to the line wire as DC components (due to clipping effect) can add noise and distortion in the audio output.

The following precautions must be taken care while assembling this circuit –

1. Always place the components as close as possible to reduce the noise in the circuit.

2. Follow star topology when grounding, this will keep the noise low.

3. Use the capacitor of high voltage rating than the input signal.

4. Always use the filtering capacitor at the input terminal of power supply to avoid the unwanted ripples.

5. Avoid clipping of the output signal as it may damage the speaker.

Fig. 8: Prototype of LM358 Audio Pre Amplifier

How the circuit Works –

The LM358 is a low power dual operational amplifier. As the pre amp is just to provide the gain and filtering so the IC is not capable of providing higher currents. The input signal from electret MIC ranges from 1 mV to 10 mV when the user speaks in normal voice. In the case of screaming, the input level may increase up to 50 mV. So the average value of input signal can be considered 20 mV. The maximum voltage that should be obtained at the output is 200 mV. So, the desired voltage gain can be calculated as follow –

Gain = Vout/ Vin

since, Vout = 200 mV and Vin = 20 mV

Gain = (0.2/0.02)

Desired Gain = 10

So, the desired gain is 10 or 20 dB. In the circuit, the voltage gain can be set through the resistor network R2 and R3 where gain can be calculated as follow –

Gain = -(R3/R2)

If resistor R2 is assumed to be 10K ohms and desired gain be 10 or 20 dB, then the value of resistor R3 is obtained as follow –

R3 = Gain * R2

R3 = 100K ohms

The negative sign signifies the inversion at the output. So, a resistor of 10K ohms is used as R2 and a feedback resistor R3 of 100K ohms is used in the circuit. The amplifier gives the output signal which is 10 times the input signal.

Testing the circuit –

For the testing of the amplifier circuit, the function generator is used as the input source. The function generator is used to generate a sine wave of constant amplitude and frequency. Any audio signal is also basically a sine wave so a function generator can be used instead of using a microphone or actual audio source. So, the function generator can be used as input source for testing the audio amplifier circuit. During testing, at the output, the line voltage need to be verified.

For testing of the amplifier circuit, first the input voltage is set between the applicable range between 10 mV and 50 mV. The frequency of the input signal is set to 1 KHz. Then, the output and input waveforms are observed on DSO. The following input waveform (red) and output waveform (yellow) are observed on DSO –

Fig. 9: Graph of Output Line Level from LM358 Preamp observed on Digital Storage Oscilloscope

So, it can be observed that the input audio signal from the microphone is successfully leveled up to the line voltage. So, the pre-amplifier is working at its best.

On testing this pre-amp circuit with a load of 100K ohms, the output power of the pre-amplifier can be calculated as follow –

Po = V2(p-p)/2R

Where

Input voltage, Vin = 20 mV

output voltage, Vp-p = 200 mV

load resistance, R = 100K

So,

Po = (0.2*0.2)/(2*100*1000)

Po = 0.2 uW

The output power of this preamp is very low as is the case for any pre-amplifier. This preamp is low cost and easy to design. It can be assembled with few components and its circuit is compact in size. This pre-amplifier circuit can be used in cell phones, Microphones, Hearing Aids and Portable Audio devices.