designing a power audio amplifier with OP AMP

[shaiqbashir]

Hi Guys!

well! i want to design a power audio amplifier with OPAMPS. Now i read somewhere that the one of the various Op Amps available for this is LM380.

Now here is the basic circuit:

now my dear friends! this circuit will work i guess. But i dont think that its a power amplifier. Now can you tell me what modifications should i have to make?

similarly can i use this circuit as a electret mic preamplifer :

and then the output of this preamplifier i want to use as the input of my power amplifier.

Is this possible, what i have to do?

plz help me as soon as possible.

I shall be thankful to you for this act of kindness.


remember that i want to operate speakers at the output (8ohms, 4 ohms or 2 ohms)

Good Bye!

 

[audioguru2]

i read somewhere that the one of the various Op Amps available for this is LM380.

The LM380 isn't an opamp. It is an audio power amp with built-in biasing and negative feedback.

i dont think that its a power amplifier. Now can you tell me what modifications should i have to make?

It is a power amplifier. With a 12V supply it provides 1W at clipping to an 8 ohm speaker.
I use the LM390 power amp (obsolete now) which is similar but has higher output power (2W) because it bootstraps its driver stage. It plays music at my workbench every day. It sounds great.
Your output capacitor has a value much too low. Mine uses a 1000uF output capacitor for good bass.
It needs 330uF for voices.

similarly can i use this circuit as a electret mic preamplifer :

Ron Elliott's mic preamp is fine.

Remember that i want to operate speakers at the output (8ohms, 4 ohms or 2 ohms)

The LM380 can't drive a 2 ohm speaker. With a 4 ohm speaker its supply voltage must be reduced to about 10V or the LM380 in an 8-pin package will melt. The 14 pin package must have a heatsink to drive a 4 ohm speaker and use a 12V supply.

 

[shaiqbashir]

Hi Audioguru!

well! a bundle of thanks to you for your tremendous help.


Now my dear friend! is Ron Elliot's preamp is the same whihc i have shown above:

and one more question

The power amplifer figure that is present on top most part, requires a mic at its input. Now can i use a mic preamplifier as its input. What will be the change in effects. Is it going to give some greeeeeeat results?

secondly if i want to play music in this amplifer, should i use a capacitor of around 1000uf?

Thankss in advance

Regards,

 

[audioguru2]

if i want to play music in this amplifer, should i use a capacitor of around 1000uf?

Yes, I've included it.

The LM380 has enough gain for a CD or MP3 player to be switched to the volume control and to disconnect the mic preamp.

I have put the two together for you with everything I can think about to make a nice little amplifier.View attachment 38159

 

[shaiqbashir]

Thank you very much Audio Guru!

you have really helped me much!

i hope that this will lead me towards  the correct design concept.

Thank you very much

Regards

 

[shaiqbashir]

Thanks once again audioguru!

i want to ask you one more question here!

if i want to arrange a power supply for this circuit, the voltage should be 12-15 volts but what should be the current requirement?

plz tell me this !

your early reply shall be highly appreciated!

thanks and Good Bye

Regards,

 

[audioguru2]

Hi Shaiq,
The current requirement for the LM380 amplifier is calculated with simple arithmatic using the power graph in the datasheet. The continuous max current is so low at 245mA that a better-quality 500mA transformer should be used. The voltage from a little 300mA transformer varies all over the place.
The amplifier won't be operating continuously at full power with music or voice, it would operate continuously if you like hearing a siren or something continuous. So it won't heat as much as calculated and a battery will last much longer than is calculated. ;D 

View attachment 38163

 

[shaiqbashir]

Hi Audioguru!

my dear friend!

i made the circuit which you have suggested above. Now what im getting is a booooooooooooooooooooom sound from my speaker continuously which increases or decreases on varyinf RV1 and volume control.

Im not hearing any sound when im speaking in mic. :'( what should i do now?

remmeber that im testing this circuit on breadboard right now. and the 1000uf capacitor which is connected with the supply, is of 16V instead of 25V.

secondly, i have made no ground in the circuit, just connected negative supply pin to the grounded connections. Im assuming negative supply as ground.

Please tell me if any mistake im making!

i shall be very thankful to you for this act of kindness.

Regards,

 

[audioguru2]

Hi Shaiq,
Happy New Year! ;D

It is too bad that your amplifier doesn't work yet.
You are correct, the negative wire of your 12V to 15V power supply is the circuit's ground.
Breadboards usually mess-up high gain circuits like this one. Maybe your noise is because you didn't use shielded cable from your microphone. Disconnect the microphone at the circuit to see if the noise stops.
Does your power supply voltage drop very much when the amp is making the noise?
Did you connect together all the ground pins on the LM380?

 

[shaiqbashir]

Hi Audio guru!

happy new year to you too!

Thanks a lot for your quick response. Now my speaker is still making noise "BOOOOOOOOOOOOOOOOMMM" even when the mic is disconnected.


im using a 14-pin LM380, so i think that you want me to connect all the ground pins with each other. is it?

and how i can see that my PSU is dropping much voltage while amplifying

thanks in advance!

 

[shaiqbashir]

what do u think making this circuit on a veroboard will improve its performance!

i mean to say NOISE REDUCTIOn to a great extent and somewhat THE EXPECTED PERFORMANCE of this circuit!

 

[audioguru2]

Yes, connect together all the ground pins on the LM380.
A breadboard can't pass the high current of an audio power amplifier. I've built many good ones on Veroboard.

Your power supply's voltage shouldn't change much, it is too weak and might be causing the noise.

 

[shaiqbashir]

hi again!

well! i have connected all the ground pins of LM380 with each other to the ground. But still im facing one strange thing:

this is happening almost regularly that whenever i press LM380 with my thumb on the breadboard, the noise stops and all become quiet. but when i release i hear the same bOOOOOOOOOOOOOOOm sound. But still the microphone is not working. I tell you that i have connected two jumper wires with the condenser mic. So, should i remove them ? what sort of wires should i connect with the mic?
Please help me because it looks like that there is a little problem and after that the amplifier will work fine.

Thanks in advance!

 

[audioguru2]

this is happening almost regularly that whenever i press LM380 with my thumb on the breadboard, the noise stops and all become quiet. but when i release i hear the same bOOOOOOOOOOOOOOOm sound.

Then the breadboard has intermittert contacts, doesn't it?

But still the microphone is not working. I tell you that i have connected two jumper wires with the condenser mic. So, should i remove them ? what sort of wires should i connect with the mic?

The circuit won't work with a condenser mic. A condenser mic needs an external 48V power supply. This circuit needs an electret mic that has the high voltage built-in inside its special electret material.
I don't know what you mean by saying you have jumper wires for the mic.
I solder my shielded cable to my mic so that the unshielded wire is only 3mm long. At my pcb or Veroboard I solder the shielded cable so that the unshielded wire is only 5mm long and it works fine.
I have many RCA plug to RCA plug shielded audio cables so I cut off the RCA plugs and use the small, flexible, shielded cable.

What is wrong with your power supply to cause its voltage to drop with a load?
Yesterday I tried a new AC-DC adapter with my new project. It is rated at 9V/400mA. Without a load its output is 10.6V. At full 400mA load its output is 9.2V with a small amount of doubled mains ripple.
I had another 9V adapter with an output of 19.5V without a load and it dropped to 8.6V with a high ripple when fully loaded. I threw it in the garbage bin.

 

[shaiqbashir]

Hey Audioguru!

sorry Boss!

it was my mistake!

im thankful to you for telling me the difference between the twos. According to your difference, im using an electret mic.


So my dear friend! today im going to solder my components on a veroboard. Then i will tell you if it will not work. Please pray for my project.

Thanks once again

Good Bye

Regards,

 

[shaiqbashir]

Hi Audioguru!


I Did it by the grace of God. The amplifier is working very fine on Veroboard.

Thanks a lot Audioguru.

I will be very happy if u will help me in understanding this circuit.!

thanks a lot again

 

[audioguru2]

Hi Shaiq,
Good for you and your god! You built it well and it works well. ;D
Can you turn up its gain high enough to get acoustical feedback howling?
Does it have enough power to be loud enough?
Does the LM380 get hot?
What don't you understand about its circuit?

View attachment 38201