Electronic – DAC and/or output amplifier selection for a MP3 player

You likely have a ground loop, between the chips GBUF and the speakers GND, caused by the speakers power supply. Note the SFE MP3 player is like your referenced vs10XXan_output.pdf Section 2.1 Figure 4 or 5 (minus the ESD).

Note the GBUF (the return of the headset) is not GND. The VS1053's output is likely a Class D amplifier. Hence it's switching at high frequency, in reference to the return (or GBUF). Section 3.2 recommends that when the VS1053 is driving a amplifier, it requires the addition of C1,R1,C2,R2 as shown in figure 8.. The SFE MP3's schematic does not have these. leading to some complaints, such as your.

The SFE MP3 and UNO with a no-name amplified speaker set works for me. But I believe my speakers have AC isolation within, as it only has hot and neutrual and no ground, yours likely has some connection to ground. Hence you are hearing the switching across GBUF. Additionally there are warnings stating this can be harmful to the amplifier.

Until SFE changes their design, the only solution is either a isolated amplifier or hack a cable to include the addition AC coupling (aka R's and C's).

Regardless the VS1053 and its open source Library are a great companion for the Arduino. I have seen many successful projects using it.

It is always good to check the datasheet of a component when you are after information like this. The important thing to know is its input impedance, if that is high, then there is little or no problem connecting a couple of them in parallel. You'll have to develop a feeling for what is high and what is low. Sometimes you even find other useful information about the chip, eg. did you know about "power supply pumping" and how to address that? Interesting topic if you are driving these chips in parallel on the same supply.

Anyways, I admit input impedance is a bit hard to find in the datasheet, it is discussed in the chapter called "Gain setting via GAIN0 and GAIN1 inputs". You may notice some degradation at higher gain settings where the input impedance drops to about 9kΩ. When driving the amps from a headphone output, there should be no issue driving such low impedance. Notice that when driving multiple amplifier in parallel, the MP3 player will see an impedance 9kΩ/N where N is the number of parallel amps.

I'd start to frown when the parallel impedance drops below 1kΩ (9 in parallel) and I'd start to worry when it drops below 100Ω. Most MP3 players are designed to drive 32Ω headphones.