Poor man's guide to remove power supply noise from audio
Well... You just connected your notebook to a car power adapter and to the amplifiers... But... WTF! You hear a very loud buzz from the speakers, and you can barely hear the music playing on your PC! Please, help me Mastro Gippo!
Example of a noisy environment:
Car -> DC-DC Inverter, 12V to 19V -> Laptop -> 3.5mm Jack to RCA cable -> Amplifiers
Here is the solution!
What you need:
-2 old modems (56K or lower, with the little transformer)
(get them from your nearest dumpster)
-the 3.5mm Jack to RCA cable (or whatever connector you need)
-soldering iron and standard stuff
-optional but recommended: a proto-board
Before you start
Standard disclaimer: I'm not responsible if you break your stuff or die. This method works for me and for that kind of noise. It won't eliminate any other kind of noise. Please do this only if you know what you are doing, and ask help to someone that knows electronic better if you are in doubt.
Carefully remove the tranformers from the modem PCB using the soldering iron (heat one pin and gently pull the transformer, then heat another pin, then another, then the last, repeat until the transformer falls into your hand - warning, hot pins!). Beware, these transformers are very fragile and you can break a pin if you pull out too heavy!
Now you should have two transformers, just test them to check if they work using an ohmeter.
The windings should have an impedance (resistance...) of 150ohm.
Cut the adapter cable in half, or at the point that suits you best, strip a bit of wire out of the ends and pre-solder them.
Solder the transformers on the proto-board, then solder the cables following this schematic:
I'm not so good at paint, so I added 2 pictures of the finished product so you can get an idea of the circuit. The white wire is only for holding the other two wires,so I don't break them if I pull too hard.
Connect the cale between the PC and the amplifiers... And it's done! Total cost: about 10$.... (even less if you already have the two modems, or if you find them for nothing).
And the quality??
This circit will not add any interference/distortion to your signal. You may have some impedance issues (the signal should increase a bit in amplitude) but the output will be 99.999999% the same as the input.
I hope you apprecate this topic and maybe make it sticky... I'm very busy at wok, but I will try to give support to anyone who needs help.