Amp Repair Safety Tips from Douglas Self
This section considers the safety of the designer and the service technician. The recommendations here are advisory only. Regulations bearing on the safety of the user are backed by law; they are considered in the next section.
There are some specific points that should be considered:
- An amplifier may have supply-rails of relatively low voltage, but the reservoir capacitors will still store a significant amount of energy. If they are shorted out by a metal finger-ring, then a nasty burn is likely. If your bodily adornment is metallic, then it should be removed before diving into an amplifier.
- Any amplifier containing a mains power supply is potentially lethal. The risks involved in working for some time on the powered-up chassis must be considered. The metal chassis must be securely earthed to prevent it becoming live if a mains connection falls off, but this presents the snag that if one of your hands touches live, there is a good chance that the other is leaning on chassis ground, so your well-insulated rubber-soled shoes will not save you. All mains connections (Neutral as well as Live, in case of mis-wired mains) must therefore be properly insulated so they cannot be accidentally touched by finger or screwdriver. My own preference is for double insulation; for example, the mains inlet connector not only has its push-on terminals sleeved, but there is also an overall plastic boot fitted over the rear of the connector, and secured with a tiewrap. Note that this is a more severe requirement than BS415 which only requires that mains should be inaccessible until you remove the cover. This assumes a tool is required to remove the cover, rather than it being instantly removable. In this context a coin counts as a tool if it is used to undo giant screwheads. If you are working on equipment with exposed mains voltages, taking the time to improvise some temporary insulation with plastic sheet and tape might just save your life.
- Switch-mode supplies are even more dangerous, as they contain capacitors charged to 400 V DC by rectification of the incoming mains. DC supplies are proverbially dangerous as the contraction of the muscles may mean you cannot let go. Be VERY careful with these things. Set up the equipment so it is impossible to touch the 400 V section.
- Be very wary about leaning over equipment you are not sure of, and never do it when you are switching on for the first time. I was once involved with the use of an outside consultant to design a switch-mode supply, and I retain vivid memories of the first time he switched one of the prototypes on. There was a violent explosion directed upwards, followed by an almost perfect scale reproduction of the Bikini Atoll mushroom cloud. ‘Ah,’ said the consultant. ‘This is an opportunity to refresh the design!’ The moral of this story is not that you should never employ consultants, but that you should never lean over unproven equipment.
- A Class-A amplifier runs hot and the heatsinks may well rise above 70C. This is not likely to cause serious burns, but it is painful to touch. You might consider this point when arranging the mechanical design. Safety standards on permissible temperature rise of external parts will be the dominant factor.
- Readers of hi-fi magazines are frequently advised to leave amplifiers permanently powered for optimal performance. Unless your equipment is afflicted with truly doubtful control over its own internal workings, this is quite unnecessary. (And if it is so afflicted, personally I would turn it off Right Now!) While there should be no real safety risk in leaving a soundly constructed power amplifier powered permanently, I see no point and some potential risk in leaving unattended equipment powered; if you prefer big Class-A amplifiers, there may be a hefty impact on your electricity bill.
- The Dress Code for working on power amplifiers comes down to jeans and T-shirt, for practicality rather than as a style statement. Cotton is resistant to molten solder, while fabrics based on polyester and the like melt instantly and allow the hot metal straight through to your quivering flesh. Put on your steampunk goggles if you are likely to be spattering molten solder about, or there is a possibility that something will explode. Shoes with plastic soles provide some protection against electric shock, but all too often one hand or elbow will be resting on something grounded. Safety boots with reinforced toe-caps are a good plan if you are likely to be dropping 1 kVA toroids on your feet.
Excerpt from Audio Power Amplifier Design, 6th edition by Douglas Self © 2013 Taylor & Francis Group. All Rights Reserved.