Throughout the previous tutorial, a preamplifier was designed using MAX4468. Based on the application, there are two types of audio amplifiers –
1) Preamplifier
2) Power amplifier
Preamplifiers are used to level audio signals from a microphone or audio source to standard voltage levels, while power amplifiers are generally used in the output stage of audio systems to boost audio signals. audio before being played through the speakers. As the name suggests, a preamplifier prepares the signal coming from the microphone or line input for further signal processing and transmission. A preamplifier is the hidden part of any device. It is integrated into USB microphones, sound cards and mixers.
Preamplifier can be classified into two types –
1) Passive Preamplifier – These preamplifiers use OPAM discrete transistor or tube for amplification of the input signal.
2) Active Preamplifier – These preamplifiers use transformer or other impedance matching components for signal amplification.
Preamplifiers are generally used to provide the voltage gain to flatten a signal in milli Volts to a few volts, such as increasing the amplitude of a signal by a few milli volts to the 1 V level. to remove any noise from the input signal coming from a microphone or audio source. Ideally, the microphone should be connected with the shortest wire to the device to avoid any noise or distortion, but this is not practically possible. Thus, preamps help to remove noise from the original audio signals.
Obviously, preamplifiers must necessarily level signal amplitudes to line levels. This is the most primitive and essential function of any preamplifier. The preamplifier sets the input signal to a line level. Line level is the standard audio signal strength for transmitting the signal between audio sources to amplifiers, radios, TV and DVD players. Line level is expressed in dBu (discharged decibels) or dbV (voltage in decibels). According to telephone standards, if 1 Vrms is equal to 0 dBV then the reference discharged voltage in decibels or 0 dBu is the AC voltage required to generate 1 mW of power into a load with an impedance of 600 ohms. The most nominal line level is -10dBu which is 0.316 V and +4 dBV which is 1.228 V.
The preamplifier is intended to strengthen the input signal. They do not provide high output current. Therefore, after a preamplifier, a power amplifier must be used if the audio signal is to be reproduced using a speaker, CD player or headphones at the output. Therefore, it can be assumed that a preamplifier is a small amplifier before the power amplifier that has output current in the microamp range.
Passive amplifiers are the preferred choice among preamplifiers as they do what a preamplifier is actually supposed to do. Therefore, in this series, passive preamplifiers are designed. In this tutorial, a passive preamplifier will be designed using IC LM358. LM358 is a low power dual operational amplifier. This IC operates with a supply voltage in the range of 3V to 32V and provides high voltage gain of up to 100dB.
In the introductory article of this series, various design parameters of audio amplifier circuits were discussed such as gain, volume, slope rate, linearity, bandwidth, clipping effect, stability, efficiency, SNR, output power, THD and grounding. loop. This amplifier circuit will be designed considering the following design parameters –
Gain (voltage) – 20 dB
Bandwidth – 20 Hz to 20 KHz
Required components –
Fig. 1: List of components required for the LM358 IC-based preamplifier
Block diagram –
Fig. 2: LM358 audio preamplifier block diagram
Circuit Connections –
This preamplifier circuit is constructed by assembling the following components –
1) DC Source – There are two power sources required in this circuit. A 5V power supply is required for polarization of microphones, while a 9V power supply is required for the OPAM circuit. The 9V DC source is required to supply the bias voltage to the amplifier. To supply 5V, two 9V batteries are connected in parallel.
2) Input Source – A microphone is used to provide the input audio signal. An electret MIC was considered as the input source. The electret MIC requires a bias voltage between 1 to 5 V to power the built-in FET buffer that is present in the MIC. Generally, this microphone is powered by 1V to 5V DC through a resistor with a value of 1K ohms to 10K ohms. There must be enough voltage on the microphone's bias pin so that it can detect the audio signal. The other pin of the electret microphone is connected to common ground.
Fig. 3: Typical electret microphone image
The bias circuit for this microphone has been shown below –
Fig. 4: Electret Microphone Bias Circuit Diagram
3) LM358 OPAM IC – LM358 is a low power dual operational amplifier IC. It consists of two independent OPAM (operational amplifiers). The maximum DC voltage gain of this OPAM is 100 database with output current range of 2 mA to 20 mA. The IC has the following pin configuration –
Fig. 5: Table listing the pin configuration of the 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. Inverting amplifier has negative feedback which makes them better than non-inverting amplifiers.
Fig. 7: Typical image of the LM358 IC
The inverting amplifier changes the phase of the output (signal amplitude) by 180 degrees with the input (signal amplitude). The inverting amplifier changes the phase of the output (signal amplitude) by 180 degrees with the input (signal amplitude). However, this phase reversal does not affect the audio signal, as human ears only respond to sound intensity. Intensity is the energy that flows 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 unit area the intensity is also proportional to the square of the amplitude.
I A2
Therefore, changing the sign of the wave does not affect I.
The typical application circuit given in the LM358 datasheet is used to design this preamplifier. The line output is taken from pin 1 of the IC.
The supply voltage for the IC is supplied 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 high-frequency filtering of the supply voltage and the low-value capacitor C3 helps in low-frequency filtering of the supply voltage. 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 to the audio output.
The following precautions should be taken while assembling this circuit –
1. Always place components as close together as possible to reduce noise in the circuit.
2. Follow star topology when grounding, this will keep the noise low.
3. Use high voltage rating capacitor instead of input signal.
4. Always use the filtering capacitor on the power supply input terminal to avoid unwanted ripple.
5. Avoid clipping the output signal, as this may damage the speaker.
Fig. 8: LM358 audio preamplifier prototype
How the circuit works –
The LM358 is a low power dual operational amplifier. Since the preamplifier is only for providing gain and filtering, the IC is not capable of providing higher currents. The input signal of the electret MIC varies from 1 mV to 10 mV when the user speaks in a normal voice. In the case of screams, the input level can increase up to 50 mV. Therefore, the average value of the input signal can be considered 20 mV. The maximum voltage that must be obtained at the output is 200 mV. Therefore, the desired voltage gain can be calculated as follows –
Gain = Vout/Vin
since Vout = 200 mV and Vin = 20 mV
Gain = (0.2/0.02)
Desired Gain = 10
Therefore, the desired gain is 10 or 20 dB. In the circuit, the voltage gain can be defined through the network of resistors R2 and R3 where the gain can be calculated as follows –
Gain = -(R3/R2)
If resistor R2 is assumed to be 10K ohms and the desired gain is 10 or 20 dB, then the value of resistor R3 is obtained as follows –
R3 = Gain * R2
R3 = 100K ohms
The negative sign means the inversion in the output. Therefore, a 10K ohm resistor is used as R2 and a 100K ohm feedback resistor R3 is used in the circuit. The amplifier provides an output signal that is 10 times the input signal.
Testing the circuit –
For testing 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 a microphone or actual audio source. Thus, the function generator can be used as an input source to test the audio amplifier circuit. During the test, at the output, the line voltage needs to be checked.
To test the amplifier circuit, first the input voltage is set within 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 the DSO. The following input waveform (red) and output waveform (yellow) are observed in DSO –
Fig. 9: LM358 preamplifier output line level graph observed on the digital storage oscilloscope
Thus, it can be seen that the microphone input audio signal is successfully leveled to the line voltage. Therefore, the preamplifier is working at its best.
By testing this preamplifier circuit with a load of 100K ohms, the output power of the preamplifier can be calculated as follows –
Po = V2(pp)/2R
Where
Input voltage, Vin = 20 mV
output voltage, Vp-p = 200 mV
load resistance, R = 100K
Then,
Po = (0.2*0.2)/(2*100*1000)
Po = 0.2 u
The output power of this preamplifier is very low, as with any preamplifier. This preamplifier is low cost and easy to design. It can be assembled with few components and its circuit is compact. This preamplifier circuit can be used in cell phones, microphones, hearing aids and portable audio devices.