amplifieraudioled
I have a PAM8403 3W dual channel audio amplifier board bought from Amazon and I would like to connect one of it's output signals to LM3915 dot/bar display driver.
The problem is that PAM8403 outputs differential signals and I can't use that as an input to LM3915.
simulate this circuit – Schematic created using CircuitLab
I tried to couple R+ signal to the led driver using a 100uF electrolytic capacitor but that didn't work for some reason. How do I source the signal to the LM3915?
I am not really an audio guy. But I took a look at Douglas Self's "Audio Power Amplifier Design Handbook." Those values, of 55 to 59 mV are ballpark. You might want to warm up the amp (by playing some music) and then check it again when it is warm.
As far as the mismatch goes, it isn't a huge mismatch. But were you measuring it with a load attached or no load? With no load, the two resistors are perfectly in series (or so I assume), so the currents have to be the same. In that case, any voltage mismatch is due to Re mismatch. You can tune Re by adding resistors in parallel with whichever one is higher in voltage until the two voltages match each other. Again, do this with no load attached to the amplifier.
If the two Re's are already matched, but you have different voltages across them when a load is attached, that is due to an output voltage offset. Unless it is large, I wouldn't worry about it.
Ultimately, bias current should be set based on what produces minimum distortion, not on anything else. But it is not very practical for most people to measure distortion while tuning bias current.
Good luck!
Connecting the potmeter at the output is a no-go. That will simply not work (assuming you want "decent" volume control). You have to regulate the volume at the input of the amplifier.
The 100 nF capacitor is there to prevent any DC coming out of the C.H.I.P. module to disturb the amplifier.
Also improve the supply arrangement by using separate wires for each module and 1 uF capacitors near the supply connections of the modules.
simulate this circuit – Schematic created using CircuitLab
Next time please educate yourself on "how things are done" regarding audio amplifiers and volume control. There are plenty of examples to be found if you just search for it. "Trying" things out in electronics seldom gives the result you want and only feeds frustration and can also damage components.
Best Answer
That PAM8403 is a class D amplifier so its output signal will be PWM (switched). So you need to filter the signal. Also, on average it will be half the supply voltage (as it switches between Vdd and ground) so you need to AC couple the signal as well as the LM3915 expects a ground referenced signal.
That the outputs af the amplifier are differential does not matter much, you can just use only one output. The other output carries the same signal but inverted (upside down). If you'd connect a speaker with a series capacitor between any of the outputs and ground, you'd hear the same signal (although inverted but your ear cannot detect that, only relative to another signal).
You could try this:
simulate this circuit – Schematic created using CircuitLab
C1 is the AC blocker cap
R1 and C2 are a low-pass filter with a cutoff frequency of around 10 kHz
This is just a suggestion and staring point. Don't blame me if it doesn't work as expected ;-)
You could also consider using the input signal of the PAM8403 as a signal for the LED driver. Optionally with a simple audio amplifier. A basic LM386 audio amplifier module will do the job and these are cheap.