Here LM358 is experimented as an Audio Op Amp. Audio music signal from a mobile handset is sent through LM358 for amplification which at the output drives a common loudspeaker.
LM358 op-amp is a popular general purpose op-amp IC(Integrated Circuit) that can be used for variety of application such as transducer amplifiers, dc gain blocks and all the conventional op-amp circuits such as active filters, voltage controlled oscillator(VCO) etc. It a dual op-amp IC which means it has two op-amp inside and has parameters among others, a unity gain of 1MHz and DC voltage gain of 100dB.
Though LM358 is merely used to drive a speaker, here for sake of experimentation it will be used as an audio amplifier. It is better suited for pre-amplifier stage than audio power amplifier stage. Just to add to this, the LM386 IC is better suited for audio amplification application.
LM358 as Audio Amplifier in non-inverting configuration with Split resistor biasing
The following picture shows the schematic circuit diagram of LM358 as audio op amp in non-inverting configuration with split resistor biasing.
Following pictures shows the hardware connection of LM358, audio input, loudspeaker and other external components on a breadboard.
Here we have used +5V single power supply to power the LM358 op-amp IC. The input is connected to the +ve terminal pin 3 of the op-amp. We have used a feedback resistor of 100KOhm and a POT of 10KOhm to vary the gain. The audio input is connected to the op-amp via a 0.1uF electrolytic capacitor and we have biased the input to 2.5V using two 4.7KOhm resistors by forming a voltage divider. It was found that a bit of noise is reduced using the voltage divider at the input.
The output of the op-amp is from the inverting terminal, that is, -ve pin 2. The 8Ohm loudspeaker is directly connected to the inverting pin 2. Actually as suggested by many circuit diagram using op-amp as audio amplifier, an output coupling capacitor is used. Here it is found that no matter what size and type of capacitor used(experimented values 0.1uF, 1uF, 10uF or 220uF), the output was noisy and clipped. So we have interfaced the loudspeaker directly to the output of the op-amp.
The following video demonstrates LM358 op-amp as audio amplifier.
You can hear that the sound produced on the loudspeaker is not so sweet. If anybody wants to know which song was played, the song is "Stand By Me". The sound is so noisy and loud that it could damage your hearing. The problem with this LM358 audio amplifier circuit is that the audio signal is clipped and the noise is amplified too high.
In order to improve the above LM358 audio amplifier circuit with split resistor biasing, the feedback resistor which was 100KOhm is reduced to just 18Kohm. Now the sound is a bit better. Below is the new LM358 circuit diagram for improved audio.
Watch and listen to the audio in the following video which is much better than the previous LM358 audio op amp circuit.
Summary & Conclusion
Audio amplifier was designed using LM358 in non-inverting configuration and with split resistor biasing method. First a 100Kohm feedback resistor was used which created very noisy and loud output. This is because the signal is clipped from above due to op-amp limitation. The feedback resistor was then reduced to 18Kohm and with it sound was much better. Video demonstration and LM358 audio circuit was shown for both cases. Although LM358 may not be the best suited op-amp for audio amplification it can produce sound. It was observed that higher input coupling capacitor results in louder sound. Here we have used a 0.1uF electrolytic capacitor.
Next see improved version of the same circuit in next experiment LM358 Audio Amplifier in non-inverting & split resistor biasing Experiment 2
Can we use 3ohm speaker in output
normally, your speaker current(I) handling capacity multiplied with resistance R=3ohm gives voltage(or power) it can handle by V=IR. So you may need current rating of your speaker and find voltage handling capacity by the speaker. After that you need to measure output voltage from the LM358 op-amp in the circuit above which must be equal or rather below that of the speaker voltage handling capacity. Also you may want to add a capacitor before the speaker as shown in this blog post https://ee-diary.blogspot.com/2021/10/how-to-design-practical-lm358-op-amp.html