Electronic – Reducing noise for a TDA8947J amplifier

Several things:

1. A long cable carrying the most sensitive node in the whole system is a bad idea. Carefully shield and then tightly couple the amplifier to the pickup. Then you can send the higher level lower impedance signal over the long cable.

2. A 741 is a joke in this application. Look for a low noise opamp. There are amplifiers specifically for audio applications where noise matters a lot. Even a general purpose device like a TL07x will be lots better than a 741, both in gain and bandwidth.

3. Applications that need good lineariry and frequecy response usually use a photodiode in reverse bias configuration. Think of it as a diode that leaks proprtional to light.

4. Don't try to get all the gain in a single stage, particularly the first one. The first stage should take the little input signal and make is stronger and lower impedance such that it is much less susceptible to noise. The first stage amp can do this better if not run close to its gain-bandwidth product. You don't have to care about offset voltage since audio can be AC coupled between stages. The second stage can then make a nice strong signal that can be sent over 2m wire.

Audio circuit design is mostly about thinking carefully about noise every step of the way. The bandwidth may be low, but the signal to noise ratio needs to be very high.

The solution was to physically separate the power connections for the audio components. While @Dave mentioned in the comments something about a star ground topology, the problem was that while the ground connections were all centralized, the Vcc connections were also, and shouldn't have been.

In the original design, power was supplied to the entire assembly via a small PCB with header pins for each separate component. Two rows of male header pins, one for 12V DC and one for ground. In this arrangement, all of the noise was being dumped onto the audio components each time the servo motor was actuated. Adding capacitors helped slightly, but certainly not enough.

The main two wires supplying the small power distribution PCB are about 10 cm long. I added a second pair, and created a second power distribution PCB. The original one was used for the servos and receiver, while the new one was used for audio and amplifier. The "Y" split in the power cable ultimately had the effect of keeping the power clean on the audio side.

I also removed the ground connection between the RC receiver and Arduino, though this was unnecessary. Its presence did not add noise, nor did its absence cause triggering problems.

An electrical engineer coworker advised me that it was important to have separate power supplies for motors versus digital components. While this project still only uses one power supply, the solution was to split the power as early as practical, and dedicate each to its respective purpose.