Identifying sources of noise in audio equipment
Anytime you have an amplifier that is generating unwanted noise to the speakers you first need to determine what type of noise it is making. From there you can determine the likely source of the noise and rectify it. One thing to note is that there are a lot of variables that come in to play here based on the topology of the amplifier and the type of components used in certain areas, so the following will serve as a “general” guide.
These frequencies are more broadband and come down to the overall design of the circuit itself. For example components used throughout the entire signal path, how much gain, etc. All components whether opamps, transistors, or resistors all have an inherent noise floor, and there’s not much you can do here aside from use better quality components. Layout is rarely an issue. Keep in mind that if the hiss was never initially bad and it’s gotten worse then it may even be due to some faulty components in the signal path. I have certainly seen some instances where poorly soldered or oxidized solder joints, such as in the case with corrosion due to liquid exposure or excessive humidity variances over time, have caused excessive hiss. If that’s the case such components require replacing.
Low frequency Hum
This can come from transformers that aren’t shielded well enough. They generate large magnetic fields that can induce voltages into almost anything that is not shielded well. If this is the source it will be at 50 or 60hz hum coming out of your speakers depending on what frequency your AC voltage is at where you live. You may try moving the transformers or turning them a different direction if possible to see if this hum goes away. If you have any wires with an audio signal routed alongside a transformer or touching the outside of the transformer, even though it’s shielded, you may try routing those wires as far away as possible from the transformer to see if it makes any difference. Keep in mind it’s common for some transformers to make a very faint hum that is not present at the speakers but the transformer itself. This is rarely an issue as long as it is very quiet. If it’s noisy the transformer may need replaced with a better quality one or the AC source may need investigated for small DC on the line.
Higher frequency hum, wine, or buzz
The full wave bridge Rectified part of the power supply will be double the frequency of the AC which means 100-120hz depending where you live. A common reason for this type of noise on your speakers is due to poor filter capacitors on this stage. If electrolytics are used they may be worn out since they suffer degradation due to aging more so than other types.
Ground loops are another potential culprit. That is a poor ground return path that is not actually ground but has some potential on it. If you are using multiple pieces of equipment connected together and are using multiple AC outlets, try connecting all pieces of audio gear to a power strip that is connected to just 1 AC outlet from the wall. If this doesn’t solve it you may try a process of elimination starting with your source signal cables. If you disconnect them and the noise does not go away, go to the pre amplifier. If you can also disconnect it and there is still a noise then you know that nothing earlier in the chain than the amplifier is responsible.
*Ground loops can also be caused by a poor pcb layout but there’s no reason to suggest this is the cause unless it’s done so from day 1 and is common to that design specifically and not just that 1 device.
Other types of General “distortion” that did not exist before but now does.
If the amp is old: I would first check it for signs of oxidization or corrosion. For instance it’s very common for old audio gear to be stored in the not so best locations such as a garage and over time the exposure to varying degrees of humidity and temperatures can cause oxidization. If this is the case, all sources of corrosion on components would require replacing of said components and the board cleaned up with isopropyl alcohol. You may also look for the other obvious visual signs such as bulging electrolytic capacitors.
A very common reason for really bad distortion is bad input filtering capacitors especially if the amp used a liquid electrolyte type for this purpose since they could have dried up. This would make them horrible at blocking DC and having any of that pass to amplification stages can wreak havoc to your speakers and produce very loud noise. Dried up power supply and decoupling caps can definitely cause distortion as well by means of causing voltage drop especially during large inputs but it likely won’t result in the immediate and loud noise like the bad input filtering caps will cause when the amp is simply powered on.
Since we can’t possibly cover every possible scenario for every type of amplifier design, if none of this was helpful you may leave a comment below with more specifics regarding your noise and setup.