Volume control with DS1807 digital potentiometer

Yes, you can adjust the top of the PWM signal to effectively get volume adjustment. However, there are two problems with this:

1. If the rising and falling edges are not symmetric then the signal will have some distortion. A CMOS digital output, like the PWM signal out of the micro, will be pretty symmetric, and any assymetry will be limited to a very narrow time window, which again ensures that the overall assymetry is small relative to the signal amplitude. With circuitry that has to allow for a variable top level, you probably won't have the same level of symmetry, and probably slower edges too.

2. The volume setting will introduce a variable DC level on the AC signal. This will generally be slow enough so that most of it can be filtered out downstream, but it can cause problems if not thought about carefully.

I think a better scheme is to incorporate the volume control into the micro and produce the PWM already scaled to the desired volume. You can hook up a pot straight to the A/D input of the micro to give it volume input, and the rest is simple firmware. This has another advantage in that you can implement log volume if you want to. Log pots for the analog solution are getting harder to find nowadays, or more expensive when you do find them. Again, a digital processor can do this easily.

I was having the same problem. I tried a few grounding strategies to no avail. I then looked at the application notes for the Texas Instruments TPA3116 amp and noticed that they recommend decoupling capacitors of at least 100uF near the power pins on both ends of the chip. These caps are absent on this board.

Looking around eBay, I noticed one that doesn't have the big caps on the TPA3116 either, but it has a 10uF cap on the power pin of the op amp. I had one lying around so I soldered it between the resistor it sits next to and the ground plane and now I consider the amp usable.

I used a 10uF 16V cap because that is what I had. The board on ebay uses a 50V cap. Because the op amp runs at 5V, that seems like overkill.

I will probably add the recommended caps on the TPA3116 to see if I can reduce the little remaining noise and maybe help the low end a bit.

For clarity, here is a pic of the cap I added:

The wires are my failed attempt to solve the problem by directly connecting the op amp and the bluetooth module grounds to the TPA3116 ground.

Others have noted the same issue, even when powered by batteries.