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I am trying to make an electret mic preamp with lm358. here are the pspice simulation results. I measured the voltage of the pins and the values were simulated exactly like the sample. I tried with 5 to 11 volts. Regardless of the slight noise, the circuit has good sensitivity to sound. But the speaker sound is weak. Low current consumption circuit (up to 10mA). mic voltage:60mv and speaker 560 mv (Vpp) Can you guide me? Thankful
The opamp is not designed as a power amplifier. You typically want at least 0.5W to get a decent sound output, even a 5532, which has pretty decent current drive, won't do this very well (although people have made power amps by using many of these ICs in parallel). You could either add a pair of discrete transistors as a push-pull output or use a dedicated audio power amp IC, such as the venerable TDA2030 (although you will need to choose one that works with whatever voltage rails you have).
EDIT : here is a somewhat better discussion of a push pull output. You need the diode biassing, for best results you can also take the feedback from the output of the transistors to the input of the opamp.
There are 3 main things to consider when designing each stage of a circuit of this nature.
Gain
Input impedance
Frequency response
For instance R2 is a 1k resistor. When combined with the 10k resistor (10k output resistance) used to bias the mic it means that there is immediately a divide by 11 effect before you even start doing any amplification. It is a good idea to make the first stage a non-inverting amplifier and make use of its very high input impedance.
You need a current amplifier of some description on the output to drive the speaker. An ordinary op amp hasn't got the "strength" to provide the current that the speaker needs. The simple way of doing this is to use a push-pull complementary follower, an npn and a pnp transistor with the required current drive capability. Feedback would be used from the output of the circuit back to the input of the last gain stage. Bias the bases with diodes and use small resistors in the emitters. The combination of the base biasing and feedback greatly reduces crossover distortion in the output stage and turns the push-pull driver from class B into class AB.
Loud speakers do not like dc current through them and this is the reason 2 complementary output transistors are required instead of just one transistor.
This design is a non-inverting amp followed by an inverting amp with a gain controlling pot in between them. Overall gain with the pot set to mid position is about 250. Maximum gain with pot wound right up is about 600. If you require more gain then increase the value of R3 or R6.
It's all about the maximum drive capability of your op-amp.
Since you said your load is 8R, and assuming 0.5W is the maximum output power that can be dissipated by it, then your amplifier should be able to sink 250mA of current.
However, since you have an LM358, it will have a typical sink capability of only 20mA with a 15V supply, i.e. it won't be able to drive it properly. Therefore, you'll see a drop in gain.
An important assumption done when assuming ideal op-amp conditions is that it'll be able to source/sink infinite current. Obviously this is impossible in real life.
For a quick fix, you can try using a buffer like the LT1010 within the loop of your 2nd op-amp stage and see how it much improves. There's probably better drivers out there, but off-hand I can recommend that one. I tried it myself once for driving a headphone amplifier with good results.
The output drive of an opamp, especially a generic one like the LM358 is rather weak. You will get optimum power out of your opamp if the load is about 1 kOhm.
If you want to drive an 8 Ohm speaker, the opamp will not manage a lot of power and will also distort as feedback cant be easily satisfied.
With your present equipment, the simplest solution is a to look for a small transformer with a 10:1 step down (e.g. a microphone transformer). That will make the 8 Ohm speaker appear like an 800 Ohm load, which is suitable for the opamp. It will substantially reduce distortion of the second opamp and increase speaker power about 10x. However this is only suggested, if you have it on hand. The transformer also has its own set of issues, e.g. it can saturate.
The proper way to drive an 8 Ohm load would be with a suitable power stage after the second opamp, such as Class A (NPN + 1 resistor) or Class AB (NPN + PNP + 2 diodes + 2 resistors).
Alternatively, the power stage functionality is also offered in some integrated parts, if you don't want to get your hands dirty :)
