Ground loop? Switch mode power supply noise

[jisoo]

I have a single 12V supply powering a 12V class D audio amplifier Tripath TA2020(TA2020-020 - TRTPATH - IC Chips - Kynix Semiconductor Hong Kong Limited.) based, model 'SMSL SA-36' (C) and a very cheap 12V to 5V 2A rated switch mode converter (A). The 5V is used to power a small computer that generates audio (B). The 12V and 5V grounds are tied together at the 12V input socket. The total 12V current draw is 0.5A-1.0A.

When I connect the audio from computer (B) to amp (C) (stereo mini-jack on the computer side, unbalanced RCA inputs on the amp side) I hear a tonne of switching noise (sounds like CPU/RAM/etc activity) on the speakers connected to the amp. If I connect a pair of headphones (D, only one channel shown) in parallel with the amp inputs, I hear switching noise on the headphones; if I disconnect the amp on one channel, then in the headphones (still connected to the computer) the switching noise goes away on that channel.

What's going on here? Do I have a ground loop? How do I get rid of it? Cut the ground wire to the RCA connectors? Use isolating transformers on the audio? Filter something? Throw out the 12V to 5V convertor and get a better one?

 

[MaSh]

When I connect the audio from computer (B) to amp (C) (stereo mini-jack on the computer side, unbalanced RCA inputs on the amp side) I hear a tonne of switching noise (sounds like CPU/RAM/etc activity) on the speakers connected to the amp. If I connect a pair of headphones (D, only one channel shown) in parallel with the amp inputs, I hear switching noise on the headphones; if I disconnect the amp on one channel, then in the headphones (still connected to the computer) the switching noise goes away on that channel.

What's going on here? Do I have a ground loop? How do I get rid of it? Cut the ground wire to the RCA connectors? Use isolating transformers on the audio? Filter something? Throw out the 12V to 5V convertor and get a better one?

Well I cannot provide a solution to this, do not have the knowledge. However, recently while trying out a self powered USB HUB with my two Raspberry Pis, I head exactly the same static noise you mention. Now this happened only when both the Pis were connected to the HUB. If only one is connected, there is no noise. Well I did not try to dissect it, just unplugged it.

Not sure if the two issues have the same cause.

MaSh

 

[omishra]

In place of switching 5V converter use simple 7805 regulators with proper 10uF bypass capacitors at in and out wrt ground.
Your 2020 based amp has own clock generator. Either use beads (LC filters) on all power lines and ground or on audio input side of amp, use 4700pF ceramic capacitors from main lead to chassis and ground lead to chassis. Or 1kpf ceramic between ground and main lead.