how to isloate audio ground from the amplifier

[Adhith]

OK understood Sir. I'll let you know once I'm finished with the circuit

[Adhith]

First I apologize for the long delay for the reply. I was preparing for exams and stuffs and thus couldn't focus on my hobby project. I have now build the op amp  based filter and changed to the transistor to the darlington. But the circuit is not working at all. All wires were double checked still not working. I taped a wire from pin 5 and connected to the the audio then its working and i think the problem is with the filter circuit. I'm attaching the new circuit that i used. What could be the reason?? the diodes at the end is for a different sort of function to turn on the LEDs and there is is no problem with that function

Since the LM358 has two op amps and I need only one for my purpose, i could totally leave the pins for the other opamp right??  In the second pic of the opamp block diagram I leaved the pins 1, 2 and 3 and used the other pins of the second opamp

[Audioguru]

The circuit does not have a filter. Maybe you mean that the "peak detector circuit" is not working? Then a part is defective or its pins are connected backwards.

Since you have two opamps but one is not used then it should be disabled. With its pins disconnected then it has a voltage gain of hundreds of thousands and probably oscillates at a high frequency causing interference to the opamp you are using. To disable an LM358 opamp make its voltage gain= 1 by connecting its (-) input to its output and ground the (+) input.

[Adhith]

Yeah sorry ...I meant the peak detector .Its not working either. Used another LM358 by setting the unused op amp to gain 1 and still nothing. My soldering is proper and neat so I think the possibility of shorting the traces won't be a reason . My concern is towards a fault in the initial circuit layout that I have designed. I have attached a screenshot of my design with this. The layout is bit congested and may be annoying but it would be great if you could help with it. I'm also attaching the actual pic of my circuit also

1) the green trace between the components indicates the orientation in which the components are arranged and does not indicate a bottom  copper connection trace.
2) the long green trace connecting the two +12v pad indicates that the pads are connected externally with a wire.
3) The pins 1,2 and 3 of the LM358 are cutoff and thus it don't have a through hole on the PCB board. This is done to provide a way for the other traces to run through its bottom side.

The output pins side works fine since I have tested it separately, so I think the things at the left side could be taken as the area of analysis.

[Audioguru]

Your circuit board appears to be connected correctly but I do not see how the 47k pot is connected.

Use a digital multimeter set to measure DC volts:
1) With no input signal then pin 5 of the LM3915 should be close to 0V.
2) With an audio input signal of a few volts then pin 5 of the LM3915 should show the positive peak voltages of the signal.

EDIT: It looks like the 120k resistor is connected to pin 1 and pin 2 of the opamp that are shorted together, and both the 120k and 100k input resistors are connected to pin 3 of the opamp that is shorted to ground.

[Adhith]

The pad named"47K pot" is connected to the middle pin of the pot. The other pins of the pot is not shown in the diagram but is connected in the right way.
No no, like I have said in my previous message, the legs of the pins 1, 2, 3 are cut from the IC. there is no through hole made for it in the PCB board. I have attached an edited pic to explain it. Since the resistor is close to the IC it creates a confusion and I admit it.

I checked the circuit with the multi meter and found out a serious mistake
While taking a Voltage reference from pin 5 and ground I found that a voltage of around 12V (DC) is seen as soon as the the power supply to the VU meter is given without playing any audio. So its clear that the 12V power supply that i used for the pin 3 is some how reaching pin 5. But I still cant figure out that How this is happening. Could you help me out??

[Audioguru]

With a +12V supply, the maximum output of an LM358 opamp is about +10.8V. With a base voltage as high as +10.8V the maximum output voltage of the 2N3904 transistor is about +10.2V but yours is +12V.
Remove the 2N3904 transistor to see if the pin 5 voltage on the LM3915 drops to almost 0V. If its voltage does not drop then maybe the 330k resistor to ground at pin 5 is broken.

[Adhith]

With a +12V supply, the maximum output of an LM358 opamp is about +10.8V. With a base voltage as high as +10.8V the maximum output voltage of the 2N3904 transistor is about +10.2V but yours is +12V.
Remove the 2N3904 transistor to see if the pin 5 voltage on the LM3915 drops to almost 0V. If its voltage does not drop then maybe the 330k resistor to ground at pin 5 is broken.

I didnt removed the transistor but instead removed from the LM358 IC (removing the IC would do right?? or should I remove transistor??) from the IC stand and then powered the circuit. It is found that still there is around 12v (actually 11.3- 11.5v), So its clear that the IC is not giving any output to the transistor to create the 12v.
On further analysis its found that the pin 3 (where the 12v is applied) and the pin 5(audio input) has a resistance of around 330k and i think its the problem right. These pins should not be connected by any resistance right??
to be precise the emitter and collector legs is having around 330K resistance

[Audioguru]

With the LM358 removed then the base of the 2N3904 transistor will be 0V by R11 and its emitter will be 0V by R13. with its emitter at 0V then pin 5 input of the LM3915 will also be 0V. But if the base of the 2N3904 is shorted to +12.0V then its emitter will be about +11.4V.
Find the short circuit which is a shorted 2N3904 or shorted wiring.
Is the transistor an NPN 2N3904?? If it is a PNP 2N3906 then its opposite junctions will cause your problem.

[Adhith]

With the LM358 removed then the base of the 2N3904 transistor will be 0V by R11 and its emitter will be 0V by R13. with its emitter at 0V then pin 5 input of the LM3915 will also be 0V. But if the base of the 2N3904 is shorted to +12.0V then its emitter will be about +11.4V.
Find the short circuit which is a shorted 2N3904 or shorted wiring.
Is the transistor an NPN 2N3904?? If it is a PNP 2N3906 then its opposite junctions will cause your problem.

Ok understood sir. Thank you very much for your help. Like I said before the resistance between the emitter and collector is showing around 335K ohm. actually the resistance should be zero right since there is no connection between the emitter and collector. There is no visible shortening of the legs or traces in the PCB though. I'll double check everything again and let you know
Yes its a NPN 2N3904 transistor.

[Audioguru]

When you measure the resistance of the transistor in the circuit then you are measuring the resistance of the circuit, not the transistor. Since the emitter of the transistor has a resistor of 330k to ground then the transistor would measure 3.3k or less for it to make it emitter near +12V.

Disconnect one end of the 100 ohm resistor to see if the pin 5 of the LM3915 or the transistor is shorted to +12V.

[Adhith]

Disconnect one end of the 100 ohm resistor to see if the pin 5 of the LM3915 or the transistor is shorted to +12V.

Done it but there is no shorting. and the traces are perfect

Seems like I founded out the problem
Since there is no way of shorting I concluded that it may be the transistor and removed it. To my surprise I founded out that the transistor is not manufactured in the actual standard. During the diode test it is found that the base and collector are switched between each other.
I never had encountered this type of problem and totally feel stupid. So this is the problem right?? I'm very sorry to waste your time upon this ridiculous thing.

[Audioguru]

That is what happens when you buy a cheap fake Chinese transistor from ebay, Amazon or Ali.
Oriental little transistors like a 2SC945 have the collector pin in the middle. A 2N3904 transistor is American.

[Adhith]

That is what happens when you buy a cheap fake Chinese transistor from ebay, Amazon or Ali.
Oriental little transistors like a 2SC945 have the collector pin in the middle. A 2N3904 transistor is American.

Yup I understood Sir. The transistor was bought from a local supply. I have bought a lot of 2N3904 from the same shop but never had any problem with it. previously you have also warned me about the fake LI ion batteries from china  in this same discussion itself. I have taken it seriously from that moment onwards . But I never thought that the fake ones will change the standard pin configuration itself, that's totally unfair. The thing is that I didn't knew it was fake. I'll be more careful next time

[Adhith]

The transistor has been replaced and the circuit works great. It has good response and the LM3915 barely heats up. I sincerely thank each and every person who helped me with this discussion. Special Thanks to Audioguru for supporting me till the end.

[Audioguru]

I am glad you got it working well.
Mine has a microphone and it shows sounds in my home. I leave it in the BAR mode all the time and it is fairly bright.