Audio Amplifier Plz check this out

[shaiqbashir]

Hi Guys!

well! i have designed this audio amplifier. It is intended to operate an 8ohm speaker.


Tell me whether it is correct or not?

what are the technical deficiencies and how to remove them.

remember that the input AC source actually representing a mic

Thanks and Good bye!

[audioguru]

Hi Shaiq,
I had to adjust the contrast on your schematic so I could see it. I also transferred it to a much smaller PNG file.

Your mic preamp looks nice. The value of its R3 seems to be too high.

The old LM312H was an opamp, not a power amp, that hasn't been made for 10 years.
You don't have a bias voltage for its input pin 3 so it won't do anything.
If its input was biased properly then your circuit doesn't have negative feedback for it so its voltage gain of a few hundred thousand might make square-waves in the speaker, if its input offset voltage is extremely low. If it has any input offset voltage then its output would just be saturated to a supply rail.

The LM312H is a low-power opamp that limits its output current to only 5mA. It is designed for not less than a 2.7k ohm load. It is spec'd with a 10k load.
If it had negative feedback to operate properly then it would produce a whopping 100uW max in the 8 ohm speaker. You might hear a faint sound if you held the speaker directly against your ear.

The low value of your output capacitor cuts low frequencies below about 4kHz, 909Hz will be at half power.

[shaiqbashir]

Thanks audioguru!

but actually this is the circuit made on Multisim. Actually the power amp section of this circuit uses LM380 instead of teh one used in the diagram above.

Sorry for not mentioning that,

Now tell me will it work or not?

Thanks and Good Bye!

[audioguru]

Hi Shaiq,
No, your circuit won't work with an LM380 as its power amp. :'(
You have its pin 2 connected to the feedback for the TL071, so its voltage is not at ground like it is supposed to be. It is also probably messing-up the TL071.
R7 and C5 don't do anything and should be removed.
The value of its output capacitor is far too low.

Don't you have its datasheet?

[shaiqbashir]

Hi Audioguru!

im thankful to you for your immediate responses.

Actually i have taken this idea from a website:

http://www.mines.edu/Academic/courses/physics/phgn217/lab10/lab10handout.htm

here you can see that LM380 is used in almost the same configuration as i have used. Now my dear! according to your advice, if i just remove the pin2 of LM380 and connect it to the ground. will this circuit work then?

PLease help me !

actually i should have made these tests myself but my simulation software doesnot have this LM380 in it.

I have the datasheets as well!

Thanks in advance

Regards,

[audioguru]

Hi Shaiq,
Even the tutorial has the circuit for the LM380 messed-up:
1) It spec's a non-polar cap for the input when the mic is positive and the input of the LM380 is internally DC grounded.
2) The value of the input cap is huge. Its low frequency response begins dropping at 16Hz. Half-power is at 3.2 Hz.
3) The value of the output cap is tiny. With an 8 ohm speaker its low frequency response begins dropping at 4kHz. Half-power is at 909Hz.
4) It doesn't show the polarity of the output cap.
5) It doesn't have a supply bypass cap.

Pin 2 of the LM380 is internally DC grounded. It can pickup interference so connect a 0.1uF cap from it to ground. If you used a 5k or 10k volume control at pin 3 then pin 2 should be grounded.

I just remembered that National Semi has an excellent applications note about the LM380. I think I learned all about it there. ;D