There is a likelihood that your power supply is contributing to modulation of the signal.
If you go to web sites obsessed with hand-crafting high fidelity high power audio amplifiers, you would realise our obsession with attaining extremely low noise/signal ratio.
There will be a point of improvement where the s/n ratio would be at a standstill due to signal modulation contributed by the power supply. So much so that a complex power supply design is inescapably an essential part of a hand-crafted ultra high fidelity audio amp.
But my preamble simply serves to illustrate that a power supply fluctuation does modulate the audio signal.
These are the reasons of power supply fluctuation
- Ground loops, the usual suspect
- Voltage dips due to interaction between variation in current drawn and internal impedance of power supply. Where voltage dips are directly proportional to the current drawn and internal impedance.
- The power supply being too small a hammer to hammer a big nail.
There must be more than sufficient materials on the internet discussing how to mitigate ground loops. A ground loop forms an antenna broadcasting and receiving signals whose wave-lengths are within the vicinities of the main harmonics of the effective diameter of the loop.
Therefore a ground loop also forms a resonant impedance to certain frequencies. Where modulation noise due to the power supply is more pronounced at those frequencies. With impedance, comes fluctuations.
An obvious but oft ignored cause of signal modulation due to power supply dipping is the power supply being too small a hammer for too big a nail. For example, if you are driving a 12V output, your input supply cannot be 12V, which is not your case.
Also, a 15 W peak supply can at most drive a 10W mean output. So, if your bluetooth is consuming 2 A, 15W is insufficient juice. I don't really know how much power bluetooth consumes. When I speak on my LG phone unplugged for 5 minutes, it stayed at 100%. When I have it on bluetooth (unplugged) for 15 mins listening to amazon music on my car amplifier, its charge dipped to 92%. I conclude that bluetooth must be pretty intense consumer of energy.
I am thinking these are the possible reasons for your hum
- 15W is not enough juice. Therefore your bluetooth preamp is noise-modulated by the 15W step-down.
- Your car has a lousy 12V supply. I am not well-informed in automotive electrical systems. Is it normal for the alternator to noise-modulate the battery's 12 V output? I can't even parallel park a car properly, so someone else needs to answer this question. Maybe you should try borrowing a strong big car battery and see what happens. As far as I have heard, the car battery should be a very stable supply of DC.
- Do your car lights dim or fluctuate when you turn up your audio? I am not an expert in this either. Does the hum frequency vary with the engine RPM?
- Maybe 15W is enough juice, but it is noise-modulated by your unclean battery supply. Such that even if you had a 25W step-down, the problem would remain unsolved.
You could experiment with a low pass filter (parallel-capacitor + series-inductance pair) at the 12V input of the step-down, using appropriately power-rated components. I suppose we could calculate the henries and farads using http://en.wikipedia.org/wiki/Capacitor-input_filter. Which I don't think is viable due to the resultant sizes of the components.
After all these troubles, you might as well spend $60-$80 on a well-meaning car audio that comes with bluetooth, which also would come with its own respectable switching supply, and you would use it as your preamp.