Problems with opamp circuit

[wellington]

Boy this is giving me too much headache ???
The above circuit was the one i've build as a headphone amp, I do have a working one, but the problem is that i can't build the second (also my friends).

The problem is that when i turn the power on, the sound start to crack(or something like that), i can hear only some cliks and pops very distorted.
Burning my brains out for a few days i have discovered that if i remove the 7806 regulator and bypass the voltage from the bridge the circuit works (with alot hum noise, but works fine).

Also, when i turn this on without the 7806 regulator, i get a huge pop in the headphone (almost got def because of that).
What i can't understand is that i have a working circuit with the same parts.

With the regulator i have a 5.5 DC output, so i change the 7806 to 7809 and T1 to 12V 500ma (+-15V output) that resulted in 8.9 DC output, but the still the same problem, remove the regulator and all works again >

I do have a kit from SONY, an video composite to RGB converter based in the old V7021 IC from SONY that has a LM386 for audio output and uses a 7805 voltage regulator to feed all circuit and works fine. ???

Searching around i have found this
Supplying power to opamps isn't as simple as you could think. It isn't enough to connect a 9V battery to the V+ and V- pins on the opamp, but an opamp needs to be connected to a bipolar power supply or it needs to be biased to half of the input voltage.

So this is what i miss??
Do I need do feed a -9V in to the pin 2???
Or "biasing" the voltage will do the same effect??
And Why the first circuit works and this second not??

Thanks in advance!

Wellington

View attachment 35548

 

[MP1]

Wellington, since the first circuit works, you can quit trying to redesign it in your head. Besides, the LM386 is not an opamp. It is a power Amp.
I would look for a mis-wired component or possibly you swapped some components. This is easy to do. If you have checked and you are sure that nothing is different between the two circuits, then the next question is whether the impedance is the same for both outputs. In other words, did you test the output with the same pair of headphones or were they different ones? The impedance might be different. The same question goes for the input. Is the input the same source on both units?

MP

 

[wellington]

Hello MP,

Well, i'm not trying to redesign the circuit but to build another since none of my friends could, what i think that was strange at first but doing again at my home i could not do it also. I'm sure all conections are fine and checked step by step.

I've checked the components, changed the IC's 5 times , they are from the same serie, same type (LM386-N1) and from same store, not only the IC but the components.

Rebuild the board also 5 times using the working one and schematic as base, checking every inch but the results are the same, it will only work if I remove the regulator, and if I do that, hum increases and oscillations occur, like a police siren if i plug the headphones with the circuit on.

Yes, the input sorce was the same on both units also the output headphone.

Thanks!

 

[audioguru2]

Hi Wellington,
Don't ground pin 8, just leave it disconnected from anything. ;D

 

[wellington]

audioguru I Love you ;D (you know what a mean, when i say love, not the LOVE you fell about other person, well...never mind)

It works!! ;D ;D ;D ;D
It works!! ;D ;D ;D ;D

All the five circuits, put the regulator back and disconnected the pin 8 from ground and is great, working!!
I've check the first circuit also, with or without the pin 8 grounded it will work the same way.

Did you build the circuit?

And one thing that i can't understand is why should i disconnect the pin 8 to make it work?

Thank you very much audioguru and MP!!!! ;D

 

[MP1]

Wellington, what audioguru did not explain is that your pin out of the LM386 is wrong on your schematic. Pin 8 is not a ground pin as your schematic shows. It is for gain. I would guess that you used the correct pin out on your first circuit and used this pin out on your second circuit. You should check for other inconsistencies as well.

Here I have attached the correct pin out from the data sheet.

MP

View attachment 35551

 

[audioguru2]

Wellington,
I'm glad to help you.
Pin 8 is an important part of the LM386's input stage. It is used only to increase the gain (and distortion and noise) with a capacitor to pin 1.
Your 1st circuit must have a "freak" IC with its pin 8 internally disconnected.

 

[wellington]

Hello audioguru and MP.
First of all, Thank you for providing me some light in this issue.

Strange, the Protel library was wrong about this device, checking the IC, now, it has pin8 GND >
It's a demo version by the way...

I've check the first circuit (again) and changed the IC's, it won't work if not disconnect the 8 pin. What i remember, I've mis-wired the first board and the IC start to heat very fast (I burned my finger after that...).

What i can think is that something inside this chips are burned out (my fault), that's why my friend and me could not build an second unit of this.
The sound with new IC's are different, because i've burned out the pin 8 ;D

Thank you audioguru and MP!