Mains noise problem and reduced sensitivity

[arnab]

I had tried to build an audio amplifier using a preamp build out of a transistor and thereafter using lm386 for the audio amplifier. I had received a suitable response using an earphone. But when I use an 8 ohm 0.5 speaker the audio output gets distorted and also the sensitivity reduces. Also I am unable to eliminate the mains noise which distorts the output of the speaker. Along with it when I increase the volume control or the gain control too high i hear a sharp beep sound of 2.7 kHz. I would love to hear what improvments i could make in the circuit to make it sensitive as well as clear7487

[Stendall]

¿Are you powering preamp through a LED?

[arnab]

No. The led is just used to check the intensity of the signal entering through the mike

[JBeale]

Looks like you are using two electrolytic capacitors back-to-back to make a non-polarized 5 uF cap out of two polarized 10 uF caps. However, given your mic+ line is biased from +5V through the LED, and the transistor base won't go more than a diode drop above ground, I wonder if you need to do that. It seems like you should need only one cap, with the + lead towards the mic.

If you have AC powerline noise pickup, the first step is to make all wires short, and shielded, and make sure there isn't ripple on your +V input. Also, check for ground loops.

[Ben]

JBeale is right, the Mic coupling cap can be a polarized type. Also I don't understand the LED in the supply path to the preamp. Leave that out for troubleshooting. Next check the DC operating point of your LM386 input pin, its 0V. So half of the Signal will swing below the supply rail. I didn't look at the datasheet, but make sure the LM386 can handle that. Maybe you need to change that DC point to Vcc/2 ?
If you redo the input stage consider a small emitter resistor in the range of 10 to 100 ohms, that will help with distortion (but also lower the gain, since it's a feedback loop).

[Theremingenieur]

The LM386 can handle negative input voltages, it is designed for. And with the optional gain setting between pins 1 and 8, it can be made sensitive enough to be driven directly from the mic, no need for that preamplifier transistor stage. Next, the data sheet says that a 100uF electrolytic capacitor is needed close to the supply pins for stable operation. That IC is designed specifically as a driver for small 8 to 16Ohm loudspeakers with a minimum of external components, it can not be compared to a classic op-amp and one should respect the design rules given in the data sheet.

[arnab]

These are some of the modifications that I did in the circuit . But i am still getting acoustic howling. What can I do to prevent it dynamically. Also I want to increase the sensitivity to the extent that when I rub my fingers in front of the electret mic the speaker should be able to play it .749374947495749674977498749975007501

[Xenoamor]

Your gain of the LM386 is only 50. Try using just the 10uF to get a gain of 200

Also your pre-amp looks like it's operating in the lower end of the linear region which will give you a non-linear result. You should also try and fix Vbe with an emitter resistor to improve stability. Have a look over here (http://www.rason.org/Projects/bipolamp/bipolamp.htm)
I'm calculating:
R1 = 500Ω
R2 = 180Ω
R3 = 1.8kΩ
R4 = 3.3kΩ

[arnab]

Your gain of the LM386 is only 50. Try using just the 10uF to get a gain of 200

Also your pre-amp looks like it's operating in the lower end of the linear region which will give you a non-linear result. You should also try and fix Vbe with an emitter resistor to improve stability. Have a look over here (http://www.rason.org/Projects/bipolamp/bipolamp.htm)
I'm calculating:
R1 = 500Ω
R2 = 180Ω
R3 = 1.8kΩ
R4 = 3.3kΩ

Can i use a 220 ohm resistor in place of a 180 ohm resistor

[PaulStoffregen]

But i am still getting acoustic howling.

Perhaps you're getting acoustic feedback, from the speaker to the microphone? It's easy to focus so much on the electronics, but the air in the room is also part of this whole system!

Usually testing with headphones is the way to tell if the problem is or isn't related to acoustic feedback.

[Xenoamor]

Can i use a 220 ohm resistor in place of a 180 ohm resistor
Yep of course