100v audio source volume control / distribution

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.

Yes, it is allowed to have that variable a load on your amplifiers, given certain precautions. Most amplifiers can tolerate open circuit on their outputs, and, subject to limiting the output so they don't get too hot, somewhat lower than their 'rated' load.

A common method for supplying multiple speakers from a central location is the '100v line' method. Each speaker has a transformer before it to raise its impedance. This can get expensive in transformers, that have to be rated for the audio range.

You should be able to buy high impedance speakers, 32 ohm is not uncommon. Ten of those in parallel would present around 3ohms to your amplifier, which most audio amps should be able to drive at less than max output with little distress. How much fi and power do you want, I suspect these higher impedance speakers will be lo-fi and low power.

You could precede each speaker by its own local amplifier, which would raise the per room loading to a level where it ceases to be an issue. If powering the amplifiers via the CAT5, you would want a beefy electrolytic across the supply at the amplifier.