With the millions of forums available to musicians on the web the one thing that really stands out is the bad advice that is often given out to professionals seeking in assistance buying, fixing or modifying their rigs. I want to clarify from the outset that I am not a professional musician, neither, no longer, in the repair industry.
Audio power amplifiers, like television receivers, microwave ovens and car stereo systems have one thing in common and this is also the most prone to failure: the power supply. The power supply is there for two reasons 1) to optimise the correct voltages feeding the electronic circuity and 2) acts as an isolator between mains live and the user. Although you may be told to remove the ground cable from a plug to remove hum through the line or speaker outputs DON’T. It worked before. Hum is often caused by either filter capacitors (or decoupler) failure or poor shielding, as in broken cables. Ground loops are a regular occurrence in a multiple component installation and are 9/10 caused by earth return paths with a potential difference between two points. (simply put). Look at the many websites covering ground earth loops – they all cover the same thing and will usually point you in the right direction. Using balanced cable systems is usually the solution especially over long cable runs (more than 3m). Read up on DI boxes and transformer matching.
When a power supply filter cap goes faulty in a linear power supply (conventional using a 50 or 60Hz transformer) the user will get a low frequency 50 or 60Hz noise from the system. Shielding issues are more accompanied by a buzz than a hum.
Case point A: Audio amplifier used the chassis as ground. Unfortunately the power amplifier required two ground points so the manufacturer grounded the PCB to the chassis at three different points. At five years of age there was a miniscule amount of corrosion on the chassis. This caused the speaker protection relays to click in and out.
Case point B: Have you ever noticed the amount of technically minded people pulling jack plugs out of their sockets by the cable. If this is the case think of how many people pull out mains plugs by the cable. Never take things for granted – check the earth connections of your audio equipment.
Case Point C: Cleaning rotary and slider potentiometers with ‘special’ cleaners. This is a short term fix. Open them and clean them. Replace them if need be but adding oil into the mix only makes the matter worse.
Poor loudspeaker impedance matching is one of the worst evils known to man. Forget about damping factors, noise and distortion. If the amplifier is designed for a 4 Ohm load then never go below that for a solid state amplifier or above that for a tube (valve) amplifier. When bridging the solid state amplifier the configuration usually allows then for a minimum load of double the normal load impedance i.e. an amplifier designed for a 4 Ohm loudspeaker will now require an 8 Ohm load. Never ground either end of the speaker whether set up in bridge mode or not.
If in doubt always use a loudspeaker protector on the output of the amplifier – there are polyswitches, electric light bulbs and semiconductor /relay protectors which can be used. Most audio amplifiers do use protection circuits but not all. Make sure you know you have this – the clicking of a relay inside the amplifier does not mean it is protected through relays.
The fuse should be the weakest link and not the strongest link. Always replace with manufacturer spec. Aluminium foil and 20A fuses will cause more damage – possibly to your speakers and worse still, start a fire.