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how to isloate audio ground from the amplifier
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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:

--- Quote from: Audioguru on May 19, 2018, 09:54:17 pm ---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.

--- End quote ---
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:

--- Quote from: Audioguru on May 20, 2018, 03:18:34 pm ---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.

--- End quote ---
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.
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