Audio amplifier are electronics circuit used to amplify audio signals. Such amplifiers can be designed using transistors and operational amplifier. Here we look at audio amplifier design using 741 operational amplifiers(LM741,uA741 and similar 741 versions). Note that although general purpose operational amplifier like 741 can be used for audio amplifier it is advisable and better to use operational amplifiers that are specifically designed for audio amplifier application. High quality audio operational amplifiers requires Audio amplifiers with high slew rates, high gain-bandwidth product, high input impedance, low distortion, high voltage/power operation, and very low input noise. Some example of audio amplifiers include LM386, LM833, NE5532, NE5534, HA5112, OP-27, LT1115, OPA2604, AD842, AD847, AD845, OP249, LT1057 etc.
Audio amplifier with 741 can be build in inverting and non-inverting mode with dual and single supply. So we can have four design for 741 op-amp audio amplifier as follows,
1. Inverting Audio Amplifier with dual power supply
2. Inverting Audio Amplifier with single supply
3. Non-Inverting Audio Amplifier with dual power supply
4. Non-Inverting Audio Amplifier with single supply
1. Inverting Audio Amplifier with dual power supply
The following shows 741 audio amplifier circuit diagram for inverting and dual power supply.
The gain of the amplifier is set using the resistors R1 and R2 and is given by,
\[A = -\frac{R_{2}}{R_{1}}\]
The capacitor C1 is a coupling capacitor which couples the audio signal into the amplifier but blocks the DC signal or low frequency signal. The coupling capacitor helps in two ways, namely that the output signal is not saturated hence avoid distortion and that the low frequency noise are blocked. The value of this coupling capacitor can be determined using the cutoff frequency formula which is given by,
\[f_{c}= \frac{1}{2 \pi R_{1} C_{1}} \]
The following graph shows the audio input signal and the amplified audio signal.
2. Inverting Audio Amplifier with single supply
One can also create audio amplifier with LM741 op-amp with single supply. For this biasing at the input is required. The following is circuit drawing of audio amplifier with LM741 or uA741 operational amplifier with single supply of +5V.
As before in the above dual power supply amplifier, the capacitor C1 is the input coupling capacitor which is used to block DC and low frequency noise but allow the audio signal to pass through it. It is calculated using the cutoff frequency formula below,
\[f_{c}= \frac{1}{2 \pi R_{1} C_{1}} \]
The gain of inverting amplifier is,
\[A = -\frac{R_{2}}{R_{1}}\]
In the above circuit diagram, the resistors R3 and R4 are used to bias the input at 2.5V which is half of the power supply of +5V. The capacitor C2 is used to eliminate power supply noise so it acts as a noise suppression filter. This is calculated using the cutoff frequency below,
\[f_{c}= \frac{1}{2 \pi (R_{3} || R_{4}) C_{2}} \]
Also here an output coupling capacitor C3 is used to couple the output signal to the output load resistor RL. The value of this output coupling capacitor is calculated from the cutoff frequency formula which is as follows,
\[f_{c}= \frac{1}{2 \pi R_{L} C_{3}} \]
The following shows the amplified output audio signal from the LM741 or uA741 amplifier when an input audio signal of 10mV is applied to it.
3. Non-Inverting Audio Amplifier with dual power supplyor uA741
The circuit wiring of a non-inverting audio amplifier using LM741 or uA741 op-amp with dual power supply is shown.
One advantage of using non-inverting amplifier over inverting amplifier is that the input impedance is increased. In the above circuit, the capacitor C1 is a coupling capacitor whose value can be calculated from the cutoff frequency equation as follows,
\[f_{c}= \frac{1}{2 \pi R_{1} C_{1}} \]
The gain of the non-inverting audio amplifier is,
\[A = 1 + \frac{R_{2}}{R_{1}}\]
The capacitor C2 is a decoupling capacitor whose value can be calculated using the following cutoff frequency formula,
\[f_{c}= \frac{1}{2 \pi R_{3} C_{2}} \]
The following is graph of the amplified audio signal of the input audio signal of 10mV into the LM741 or uA741 audio amplifier.
4. Non-Inverting Audio Amplifier with single supply
The following circuit drawing shows how one can construct single supply non-inverting audio amplifier with LM741 or uA741 operational amplifier.
\[f_{c}= \frac{1}{2 \pi (R_{1}||R_{2}) C_{1}} \]
The gain of the non-inverting audio amplifier is,
\[A = 1 + \frac{R_{3}}{R_{4}}\]
The capacitor C2 is a decoupling capacitor whose value can be calculated using the following cutoff frequency formula,
\[f_{c}= \frac{1}{2 \pi R_{4} C_{2}} \]
The following is graph of the amplified audio signal of the input audio signal of 10mV into the LM741 or uA741 audio amplifier.
The above single supply amplifier circuit used split resistor biasing. This is explained more in details in the tutorial Biasing LM741 for Single Supply with Split Resistor Biasing and How to operate LM741 with single supply.
so which has the best audio quality?
i don't think there is much difference in quality whichever you use, the difference would be external component count.
I created this project and was able to create an amplifer for a crystal radio set. Would this also work if I use it to amplify the signal coming from the antenna before the tuner?
i am not sure, but i don't think it will work as as amplifier after the antenna because of the frequency of signal coming from the antenna. i think LM471/uA741 cannot handle high frequency of radio.