LM3915 problem

[egrep]

Hi all.
Any help regarding lm3915 vu meter.
I have built 2 meters using attached scheme...For left and right channels.
The problem I have is separating left and right channels.
As I am powering both meters via a single 12v 500ma supply, they work but I have no channel separation.
Turning left or right up/down or off has no effect both meters keep responding.
Seems the common ground might be the problem as input signal connects to ground and carries the signal to both meters all the time.
Any ideas on this, if I use separate power supplies problem is gone, but I really would like to only have to use one supply.

 

[audioguru]

Maybe the speaker outputs of the stereo are bridged (very common on car radios) so the "ground" speaker wires actually carry signal. Connect the LM3915 circuits to ONE WIRE of a speaker and THE REAL GROUND.

 

[egrep]

Thanks for the response...AG.

I see what you mean.. did try this but same result..seems the neg of speakers is indeed common gnd for entire amp. The AC/DC power supply (12v) I am using to power both circuits carries a signal from the speakers the result is single output instead of separate left and right... Only solution I can think of is separate power supplies. Seems bit impractical to me. Simple pic attached of what I am doing...
Thanks again for your help.

 

[alec_t]

You could try using 1μF capacitors to AC-couple the speaker + terminals to the LM3915 sig inputs. Use of a peak detector circuit would probably give better results too.

The AC/DC power supply (12v) I am using to power both circuits carries a signal from the speakers

Why? How? A power supply would normally be just DC.

 

[audioguru]

The AC/DC power supply (12v) I am using to power both circuits carries a signal from the speakers the result is single output instead of separate left and right

That makes NO sense.
A power supply produces smooth DC with NO signal. The amplifier drives speakers with an AC signal. The power supply has NO SIGNAL. Maybe you do not have a DC power supply?

Your schematic shows a NEGATIVE 12V but the LM3915 does not want and must not have a negative supply. All the grounds on the datasheet schematic must be 0V, not a negative supply.

You need only a single +6VDC to 9VDC supply but it will work from +12VDC and get hot.
Each LM3915 has its own input and its own LEDs so there is no way that the channels are not separate except if the input is mono instead of stereo.
Deep bass sounds are usually in mono so that both channels produce it.