az00025 Posted November 17, 2011 Report Share Posted November 17, 2011 Does any Know how to design simple Stereo Audio Amplifier(Pre-Amp+Tone Control+Output Stage) by using LM358 op-amp and (8 ohm Speaker) ,the circuit should be working with 4.5V (e.g. 3 AA batteries)..?I designed several circuits but I got 2 much noise >>>>one of the circuit that I built is show below(Op-Amp Audio Amplifier)...However the main two problems that I found are:1) Too much voltage gain (x300), and a noisy signal reference due to thermal noise from the 1M resistors. Assuming a 20kHz bandwidth I calculate 5.4mV of noise at the output, which is likely to be audible.2)Another possible issue is instability. RF oscillation sometimes shows itself as excessive audio noise.I'm experimenting this circuit but unfortunately,i'm still facing these problems.... any suggestion with some details to how reduce excessive audio noise....any help, I will appreciate it..<<<Does anyone have a solution for these problems??PLZ>>>.......... Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 18, 2011 Report Share Posted November 18, 2011 A lousy old LM358 is low power so its output operates in class-B to save power instead of in class-AB like all other opamps. Therefore it has horrible crossover distortion.It also has trouble above only 2kHz. It is not a low noise opamp so of course it produces a lot of noise when the gain is very high.With a 4.5V battery that drops to 3V during its life, the output power from your amp is about 0.06W into 8 ohms which is almost nothing. The very high value of the emitter resistors also reduces the max output power.Why is the gain so high?Why is there an opamp for the "ground"? Quote Link to comment Share on other sites More sharing options...
az00025 Posted November 18, 2011 Author Report Share Posted November 18, 2011 I got stuck on this circuit and I need to find a solution...and even I reduce the gain by use smaller values for resistors still facing some troubles above 2 KHz . Quote Link to comment Share on other sites More sharing options...
Hero999 Posted November 18, 2011 Report Share Posted November 18, 2011 The answer is to use an audio amplifier IC such as the LM386, TDA2822, TDA7052, TDA7053 etc. The schematic is on the datasheet which can be found using a search engine. Quote Link to comment Share on other sites More sharing options...
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