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how to isloate audio ground from the amplifier
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Audioguru:
Beta is current gain and is used when a transistor or darlington is an amplifier with plenty of collector to emitter voltage, not when it is a saturated switch. The datasheet says that its maximum saturation voltage loss is 1.5V when its collector current is 100mA and its base current is 0.1mA but the saturation voltage loss is less when the base current is higher, use 1mA. If the saturation voltage loss is 1.5V then the LED string gets only 10.5V when the supply is 12V and it might not light. The graphs on a datasheet are for a "typical" device that you cannot buy, use minimum or maximum printed spec's.

The outputs of the LM3915 are collectors of transistors that go near 0V when activated. They have no voltage, your darlingtons provide the voltage. Their output current is adjustable but the calculations for the output current affects the reference voltage. Your supply is 12V and the darlington base-emitter voltage is about 0.8V and the output of the LM3915 is about 0.1V so for 1mA darlington base current the resistance should be (12V - 0.8V - 0.1V)/1mA= 10.2k, use 10k. The darlingtons will also need a resistor from the base to emitter to turn them off, use 10k.

The first page of the datasheet shows the reference pin7 set to about 10V with a 1.28k resistor and a 8.06k resistor. Use 1k and 6.2k instead to make a regulated reference of about 9.5V.
Adhith:

--- Quote from: Audioguru on March 27, 2018, 08:15:44 pm ---The outputs of the LM3915 are collectors of transistors that go near 0V when activated. They have no voltage, your darlingtons provide the voltage. Their output current is adjustable but the calculations for the output current affects the reference voltage. Your supply is 12V and the darlington base-emitter voltage is about 0.8V and the output of the LM3915 is about 0.1V so for 1mA darlington base current the resistance should be (12V - 0.8V - 0.1V)/1mA= 10.2k, use 10k. The darlingtons will also need a resistor from the base to emitter to turn them off, use 10k.

The first page of the datasheet shows the reference pin7 set to about 10V with a 1.28k resistor and a 8.06k resistor. Use 1k and 6.2k instead to make a regulated reference of about 9.5V.

--- End quote ---
Thank you again sir for your kind help. So things are clear with the base resistor.
1)In the circuit which I'm using the pin 7 is only grounded through a 1K resistor. it doesent have a second resistor and its connection to the pin8 instead, pin 8 is directly grounded. Seems like it doesn't create any voltage reference i guess ?? how bad is this to the performance of the VU meter??
2) you said about that the darlington also need a resistor from base to emitter to turn them off  right?? in the circuit which I have attached there is already two resistors from the transistor. one from the base and another from the emitter and its other ends are connected to the VU meter output pins. So the resistor from base to emitter for turn off is included in this right?? So replacing the two 1k with 10K will do right??
3)I have pointed out the corrections in the circuit which is attached below. could you please have a look and see its correct? the IN4148 diode and the 2.2mf capacitor at the audio input will be removed in actual circuit
Audioguru:
You have the regulated reference voltage not used then the input voltage that turns on each LED depends on the regulation of the power supply feeding the pot.
The current of each output can be as high as 10 x (1.25V/1k)= 12.5mA.
The transistor base current is 11.2mA and the darlington base current is 1.07mA.
Adhith:

--- Quote from: Audioguru on March 27, 2018, 10:27:59 pm ---You have the regulated reference voltage not used then the input voltage that turns on each LED depends on the regulation of the power supply feeding the pot.
The current of each output can be as high as 10 x (1.25V/1k)= 12.5mA.
The transistor base current is 11.2mA and the darlington base current is 1.07mA.

--- End quote ---
1)I'm using a 12v regulated supply from the buck/boost converted to feed the pots, anyhow I'll use the reference voltage of 9.5v with the resistors like you have suggested to be on the safe side.
2)also the 100K and 220 ohm resistors connected to the pin 5 of the LM3915N is not needed right?? since we are feeding the pin 5 from the peak detector
I'll make the circuit today itself and let you know about the results. Thank you once again
Adhith:
unfortunately the MPSA64 transistor was not locally available. So I had given an order for it and it would take around 5 days to reach me. So I let you know after i get it and finish buildinding my circuit.
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