After the amplifier was constructed it was hooked to the computer and immediately I could hear everything on the CPU – hard disk, processor, Wifi dongle etc. They come in various hums, whines, static, clicks etc. Noise will start after the M3-ATX takes the amplifier out of standby through the remote/thump terminal. Through much reading in various forums (mp3car.com, diyaudio.com, avforums.com), it is concluded that I’m having ground loop issue.
Indeed, checking the ground of the amplifier and motherboard ground, there is 0.2V difference of potential. Further check results a 0.17V drop across the power wire to the M3-ATX. I’ve decided to take the power for the amplifier from the CPU motherboard itself and change the wire gauge to bigger one. … There is less voltage difference now between the ground and noise is reduced but still at an unacceptable level. Through more reading, it seems that this problem is common with cheaper off-the-shelf amplifier. Then, I almost decided going to build/buy a ground loop isolator which is more to a band-aid to the problem.
Since the amplifier is self-made and now grounds are common, I told myself that the circuit can be built better. The circuit was constructed with the typical application circuit from the datasheet. While researching I came to this page http://www.bcae1.com/ground.htm , which really hits the nail on the head. In the typical application circuit the signal and power ground are connected together which is applicable for simple setup for testing purposes. So a search for images on google yield an isolated signal ground to power ground schematic, http://www.hilo90mhz.com/electronics/audio_amp/audio_amp.html. I’d say give it a shot after work and it works! No more noise from the CPU. So guys make sure you separate the signal ground (S-GND,AC-GND) and power ground (P-GND) because one might be dirtier/cleaner from the other.
Quote from http://ludens.cl/Electron/audioamps/ta8215.html
If you compare this schematic to the one in Toshiba’s data sheet, you will notice that it’s almost an exact copy, except for R3! Toshiba ties the input ground directly to the output ground. In a PC, this leads to trouble! A computer is a noisy machine. There are quite strong noise currents circulating through ground, for example. If you tie all grounds together, it could well happen that a noisy power ground return from the hard disk takes a route through this power amplifier and the sound card! This makes some nasty noise show up in the speakers.
It’s important to understand that the TA8215, like many such chips, has the preamplifier internally separated from the power amplifier. This is very useful to get rid of the described ground loop problems! I left the input ground separated from the output ground, except for the 10 Ohm resistor. If you happen to use this amplifier with a signal source that has a floating ground, the resistor is low enough to apply proper power ground to the preamplifier stages, and it works well. But if you install the amplifier in a PC, the 10 Ohm resistor is high enough to break up the ground loop that would otherwise form! All sensitive input points, such as voltage divider grounds, feedback returns, and ripple filter capacitor ground, are directly connected to the soundcard ground, and separated by the resistor from the power ground. The result is a very good rejection of noise. In my system, I can’t hear power supply noise at all, but if I short out the resistor, the noise is all over the place!
Application circuit from TDA7560 datasheet
Schematic from hilo90mhz.com for TDA7386 which is pin-compatible with TDA7560
