Noise is the bane of audio design! Here are some of our tips to help solve hum problems in a PCB design.
Can you provide a connection from audio ground to mains earth? This can really help reduce hum and get rid of problems of moving your hand near a circuit and introducing mains born hum.
Check all components are soldered properly. Sounds obvious but on SMD prototypes that have been hand soldered it can be very easy for there to be a bad connection (elect capacitors in particular can be hard to manually solder).
You are looking for tiny voltages – 5mV at line level is plenty to give you hum, even though your signal of interest is say 1V p-p. Use a scope to identify hum areas by connecting scope ground to different GND points and then measuring other GND points. A good starter is to connect scope GND to the PCB’s GND star point and measure from there. Its amazing how simple thick tracks can still allow hum to appear. Also bear in mind that amplifier GND is very important to be compared with other IC GND’s – follow the signals path around your PCB.
Remove the audio source to remove noise that it may be creating. Solve any local noise problems before dealing with noise from other areas / devices. If a source is creating noise then there is nothing you can do about that at your end except lower its level into your circuitry. With the source removed short your input connections to their GND to avoid potential for these connections to be picking up noise now that they are un-driven (high impedance inputs can pick up noise if you leave inputs floating).
Try shorting different GND points togther to see if that removes the hum. If it does try soldering a wire link in and see if its still gone. If it is then try breaking the original track to see if its still good. If so you now have a new PCB track to add in place of the old.
There may be parasitic impedance / inductance on audio inputs which you can try removing by connecting a 220pF cap from each to their GND. If it works lower the value of the capacitor until it no longer works. This can often be a much simpler solution than re-designing the PCB to remove it.
Oscillations can cause large amounts of current to be flowing through parts of the circuitry and if speakers are connected these can cause very large currents which can easily damage the speakers or amplifier (disconnect them!). This current can obviously cause GND noise problems which make the problem worse.
Can the gain of the input be increased and then attenuated later on using a simple resister divider, so that the signal can be risen above the noise?
Use a basic signal generator to examine what happens to a signal as it goes through the circuit. This is a great way of diagnosing audio issues and seeing what will work to correct particular frequency problems.
This really is simply case of trial and error. You can find yourself in situations where linking to one point on a track will suddenly remove hum and to another point a couple of cm’s down it the hum remaining. Solving noise problems is often simply a bit of a black art and you just need to slowly and methodically go through each aspect of a design looking for the cause. Remember – you are looking for the source of the noise so that you can remove it. Somewhere the noise is being introduced and you need to find it, isolate it and then cure it.