LM3915 Issues

[macboy]

Now that the multiple gnds problem is solved. 

With the high signal voltage level on the speaker outputs (+/- 14V peaks), and the LM3915's very high input impedance, it should be possible to produce a simple peak/envelope detector out of a few passive components.

Here's using both speaker outputs so that both +ve and -ve peaks are detected, I don't know if the component values are very good - I've only simulated it with the 500Hz speaker signal.

This looks good in simulation, but won't work well due to the voltage drop of the diode. You will require considerably higher output level from your amplifier to light the first LED than normally necessary. This is why a precision rectifier is normally used for these high dynamic range type of signals.

[StillTrying]

This looks good in simulation, but won't work well due to the voltage drop of the diode.

I'm surprised that you think the voltage drop, resistance, and capacitance of a diode is not modeled.
I'll add it's output voltage at 5 Watt and at 80 Watt to my previous post in a bit.

[macboy]

This looks good in simulation, but won't work well due to the voltage drop of the diode.

I'm surprised that you think the voltage drop, resistance, and capacitance of a diode is not modeled.
I'll add it's output voltage at 5 Watt and at 80 Watt to my previous post in a bit.

I'm sure that at 5 W the output looks fine too. However, the LM3915 LEDs are separated by 3 dB steps. There are 9 steps or 27 dB betwen the lowest and highest LED. If you tune the meter to light the highest LED at 80 W, then the lowest one will light at -27 dB from 80 W, which is 0.16 W. What does the output look like at 0.16 Watt @ 6 ohm ?

[StillTrying]

I was thinking that the OP was only interested down to 5W, and 80W at the top end, but I think you could be right - better resolution needed at the low end. I'm only used to the linear 3914's and these don't have this problem.
Peak Watts in 6R vs. Volts to the lm3915
180mW = 0.065V
360mW = 0.170
720mW = 0.340
1.44W = 0.580
2.88W = 0.950
5.84W = 1.49
11.5W = 2.24
23.0  = 3.32
46.0  = 4.82
92.0W = 7.00

[Audioguru]

I am glad that you finally understand to use a coupling capacitor to block DC but pass the AC.

In the datasheet of the LM3915 there are good peak detectors shown. They charge the timing capacitor very quickly from a low source resistance so that short duration peaks can be seen. They discharge the capacitor slowly (about 30ms) so that our slow vision can see them.

Your circuit has the capacitor charging fairly slowly through high value resistors and has it discharging very quickly since the capacitor value is too low and the 270k resistor to ground on the input of the LM3915 should be 1M as shown in the datasheet.