simulate this circuit – Schematic created using CircuitLab


simulate this circuit

Hello,as you can see,I constructed a circuit which consists of an amplifier(first scheme) and a preamplifier(second scheme) connected together.The goal is to connect a low sound source(for example,a MP3) and amplify its sound many times to the point when it becomes very loud.It seems my preamplifier is useless,as the only thing I can hear at the speaker is a annoying parasitic noise.Perhaps the problem is the way it's built or maybe it's the way I connected it to the amplifier.What is causing the problem and what can I modify to make this work?

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    \$\begingroup\$ Did you try to simulate the circuit and analyze the results? Couple pointers: DC bias (R3) is poor; Large distortion will be introduced due to the omitted emitter resistor; The chosen circuit is really only useful for small signals where distortion is not an issue or for switching a load on/off. Add proper DC bias including an emitter resistor and your results will dramatically improve. \$\endgroup\$ – jippie Oct 16 '15 at 17:14
  • \$\begingroup\$ I don't know what R2 is for but the + side of the battery should be connected to top side of R1 if it isn't. \$\endgroup\$ – Rob Moore Oct 16 '15 at 17:15
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    \$\begingroup\$ Are you really connecting the single preamp output to BOTH LM386 inputs? All the sample amplifier circuits in the datasheet show pin 2 grounded, with the input signal connected only to pin 3. \$\endgroup\$ – Peter Bennett Oct 16 '15 at 18:39
  • \$\begingroup\$ @jippie Ok,but I'm not really sure for what parameters to bias it for.Can you give me a hint? \$\endgroup\$ – Daniel Tork Oct 17 '15 at 16:32

It's a crappy preamp but why should you need it. The gain of the LM386 can be made to be 200 and has a gain of 20 minimum. The output from an mp3 player is a few hundred milli volts and at a gain of 200 100mVp-p becomes 20Vp-p and clipping the end-stops of your LM386.

But if you really wanted to know why you are getting noise it's probably how you have connected your battery/power feed - if any current for the LM386 output stage goes thru circuits connected to the inputs then you'll get all manner of problems. It's called unwanted feedback.

Feed power directy to the power pins of the LM386 and then apply your signal by teeing off from the power 0V connection.

  • \$\begingroup\$ It's true that the mp3 player is a poor example.Actually I meant something like an electric guitar,which needs a preamp too,if it doesn't have one in its body.I tested the circuit by turning it into a radio.Since I connected the antenna as the input directly to the chip and the sound was lo even if the gain was set at max,I added the preamp,too,and once again the parasitic noise popped up. \$\endgroup\$ – Daniel Tork Oct 16 '15 at 17:53
  • \$\begingroup\$ A guitar needs a high input impedance interface. Your "pre-amp" is a low input impedance. \$\endgroup\$ – Andy aka Oct 16 '15 at 18:13
  • \$\begingroup\$ Perhaps a read of some of the articles on this site will shed some light on both guitar pre-amps and non-LM386 amplifiers. \$\endgroup\$ – CharlieHanson Oct 17 '15 at 2:21

Preamp circuit is wrong

preamp corrected

If you are using a microphone the 10k resistor can be kept.but the positive of battery should be connected to the top rail of preamp as shown in above image.

lm386 amp circuit is also partially wrong.

pin 2 must be connected to pin 4. and to negative of preamp and - of supply.

You are trying to use a circuit with gain control.100 mf is not needed 10mf is usually used. capacitor from pin 7 may also be omitted.

if u want a simple amp.use this 10mf=200 gain. no 10mf =20 gain.

also make sure to connect possitive of preamp,lm386 amp and battery possitive together.


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