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how to isloate audio ground from the amplifier
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Adhith:

--- Quote from: Audioguru on March 22, 2018, 11:33:01 pm ---Each speaker wire of the bridged amplifier has a DC voltage that is half the supply voltage. If your VU meter measures  DC then a coupling capacitor must be used in series to block the DC but pass the AC to the VU meter.

--- End quote ---
My Vu meter is based on LM3915N IC. The circuit which I'm using has already a diode at its positive audio input side to consider only the positive cycle of the audio. The circuit works fine since I have already used for an another project (with a another amplifier based on LA4440 IC ), but I don't know how to make it compatible to this class D amplifier IC board i.e the noise by connection the ground wire
Adhith:

--- Quote from: Buriedcode on March 22, 2018, 10:16:31 pm ---The modules ground is the bottom side of that DC barrel socket (from your picture).  I cannot see a specific pad for ground on that board so it looks like you'll have to solder a wire to the DC socket.

You mentioned an audio transformer which is one option, but I feel I am not qualified to recommend a particular part number.

Another option, possibly overkill would be a differential amplifier using an opamp: https://www.electronics-tutorials.ws/opamp/opamp_5.html 
You would still need to connect the amplifier boards ground (again that DC socket) to the ground of the VU meter.  But it would mean the VU meter shows the full voltage swing.

There is actually two differential amps on that PCB because the output from the bluetooth module is differential, which is then converted to single ended by the on board dual opamp (NE5532, one opamp per channel).  I'm unsure why they have done this since the input to the TDA7492 is differential, so that part of the circuit only adds noise.

--- End quote ---
I have removed the DC socket for the project and I have attached the pic of the board. You said that the ground will be below the DC socket and I have marked it on the board. Is this the amplifier ground??
Audioguru:
The LM3915 does not need a diode in series with or to ground at its positive input since it measures only positive voltages. It does nothing with negative input voltages. Adding a series diode causes its measurements to be 0.4V to 0.8V too low depending on the diode's current and its temperature. The datasheet shows peak detector circuits and active rectifier circuits to feed the input of the LM3915 that do not cause its measurements to be too low. The peak detector holds the peak voltage long enough for our slow vision to see it.

The LA4440 amplifier is an ordinary stereo amplifier with one wire of each speaker grounded (its outputs are not bridged) so your LM3915 is measuring only the live output signal. Its datasheet shows that both of its amplifiers can be connected to make a single bridged amplifier, that is like both amplifiers in your TDA7492 amplifier.

Your TDA7492 amplifier has two bridged amplifiers. You connect the input of your VU meter circuit through a series coupling capacitor to only one of its speaker wires and the ground wire of your VU meter to the ground of the amplifier. Shorting one of its speaker wires to ground will destroy the TDA7294 amplifier. Then your VU meter measure only half of the speaker voltage.

Use a multimeter set for OHMS to measure the ground on the pcb with the ground on the DC power jack.
Audioguru:
The instructions that came with the TDA7492 amplifier should show the positive and negative (ground) connections of its DC power jack. We haven't seen the instructions and we have not measured the jack so we don't know.
Adhith:

--- Quote from: Audioguru on March 23, 2018, 01:48:54 pm ---The LM3915 does not need a diode in series with or to ground at its positive input since it measures only positive voltages. It does nothing with negative input voltages. Adding a series diode causes its measurements to be 0.4V to 0.8V too low depending on the diode's current and its temperature. The datasheet shows peak detector circuits and active rectifier circuits to feed the input of the LM3915 that do not cause its measurements to be too low. The peak detector holds the peak voltage long enough for our slow vision to see it.

The LA4440 amplifier is an ordinary stereo amplifier with one wire of each speaker grounded (its outputs are not bridged) so your LM3915 is measuring only the live output signal. Its datasheet shows that both of its amplifiers can be connected to make a single bridged amplifier, that is like both amplifiers in your TDA7492 amplifier.

Your TDA7492 amplifier has two bridged amplifiers. You connect the input of your VU meter circuit through a series coupling capacitor to only one of its speaker wires and the ground wire of your VU meter to the ground of the amplifier. Shorting one of its speaker wires to ground will destroy the TDA7294 amplifier. Then your VU meter measure only half of the speaker voltage.

Use a multimeter set for OHMS to measure the ground on the pcb with the ground on the DC power jack.

--- End quote ---
Thank you very much sir for you help. I'm still having some doubts regarding it. I know its very annoying to ask these sort of simple things but for me its the only hope sir. I still dont get it on how to measure the ground on the pcb with the ground on the DC power jack. All the copper pads are coated with insulation right. I have an exposed power ground on the dc socket pin but dont see the ground of the amplifier. (by ground you mean the ground of the bridge amplifier right not the left and right ground right??)
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