Repairing Audio Amplifiers – All that interest!
There must be thousands and thousands of pages on the web covering the repair of audio amplifiers, most often pointing in the direction of a specific make and model but yet audio amplifiers, as sophisticated as some of the circuitry leans towards is not normally out of the DIYer’s grasp for most repairs. Some of the most hideous sounds coming from an audio amplifier is often due to a small fault whilst of course the opposite is also true, slight distortion at lower levels which becomes ‘disguised’ at higher volumes can take a seasoned technician many hours to repair. I’ll point out some of these faults a little bit down the line but let’s look at the more common problems, unfortunately in my experience more often than not, man made – i.e. not reading the print before a test or installation.
Repairing Audio Amplifiers – Loading, Impedance and Mains Supply
If the amplifier is designed to be plugged into 8 ohm speakers please do just that, 4 ohms may be great way to get some extra power but unfortunately cheaper amplifiers do not have the extra safety margins built in, at worst you may burn out the mains transformer. Ditto playing into short circuits or with valve gear having no load at all. Oh yes, valve amplifiers don’t like playing into a zero impedance load and if you don’t want the most expensive component of all failing on you, the audio output transformer then heed this advice. Valve or thermionic tubes are voltage devices, bipolar transistors are current devices. Audio transformers and valves do break down with excess high voltage, especially if there is no damping taking place on the loudspeaker side.
Keep a clear head – if you don’t know anything about electricity or have not an inkling about audio amplifiers then do NOT open. Once you have prodded around for a few minutes with a unit powered up the chances are you will pay more for the repair than what you originally expected. Much more. Good technical staff charge for quotes – a lot of money. Free quotes is like a free lunch, remember this.
Before going on the warpath with your amplifier do yourself a favour, get the schematic and if it entails ensuring you have the right revision, do so – plotting your own schematic as you go along is time consuming. If you love your amplifier prepare to pay some money for the schematic and if possible get a schematic description – often you may think that you can read the diagram but the manufacturer has his or her own ideas in mind. Amplifier manufacturers use different tricks, especially in the front end (after the pre-amplifier) and in the current limiting stages. There is good news though – all amplifier designs follow a similar pattern. They are designed to amplify the mirror image of the input, perhaps out of phase but definitely an amplified version of the input signal, whether current or voltage.