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Designing PCB'sI love doing PCB layouts. I'm really crap when it comes to drawing, or most abstract creative stuff, but PCB's conform to rules whilst allowing plenty of opportunity for artistic impression. They appeal to the artistic element of my personality, but the design follows a process that I can relate to more readily. Thanks, I feel better for that ;) Having got to grips with Eagle, the first prototype artwork was duly fed from the laser printer. I used, for the first time, the Press'n'Peel system for the first boards, this is readily available from Maplin in the UK at reasonable cost, and comes highly recommended. I know people that have had varying degrees of success with Press'n'peel. Those that get good results swear by it, others just swear at it. I got excellent results first time, thanks to the help of one of my regular correspondents (thanks Jo!). The key for me (spot the engineer) was to get good process control by using a thermocouple to set the iron temperature to the correct range as specified by the manufacturer. The ironed-on artwork was duly passed to my silent partner-in-crime, who etched the board for me. My employer used to make lots of small-volume and prototype PCB's, but for some time now we go straight to professional manufacture, primarily because of the move to SMD (Surface Mount Devices). Doing a high-density double sided SMD board requires a lot of care and attention, and small batches of PCB's are readily available these days, at great prices. My employers loss is my gain, and the manufacturing facilities we once had at work now reside in a garage somewhere, available for use at almost any time. The PCB was designed as a drop-in replacement to a Naim SNAPS power supply, a picture of one of the early prototypes will be here soon. So where does one start with designing a PCB? Well, a schematic helps...and using most modern PCB packages creation of a schematic allows easy transfer to a PCB, with less potential for errors in design. The basic process is as follows: -
Eagle will give you a bunch of components all connected together by 'airwires'. These un-routed connections are the electrical connections one needs to make, in order for the PCB to be representative of the schematic you entered. The airwires will tend to follow the shortest point-point route though, and it is highly unlikely this will be the optimum solution. This is where the skilled part comes in. You MUST analyse the currents flowing in every track on your PCB - with high precision circuitry what you thought may be an inconsequential voltage drop along a PCB track (and we might be talking micro volts here!) could have significant effect upon the performance of the circuit. Techniques such as star-wiring, ground planes, physical layout all have a fundamental part to play. GOLDEN RULE NO.1THE PCB IS PART OF YOUR CIRCUIT - THE TRACES HAVE RESISTANCE, CAPACITANCE AND INDUCTANCE AMONGST OTHER EFFECTS.UNDERSTAND THE EFFECTS OF THESE ON THE VARIOUS PARTS OF THE CIRCUIT AND NEVER ASSUME THAT THE ERRORS ARE INSIGNIFICANT - CHECK, CALCULATE AND PERFORM ERROR-BUDGET CALCULATIONS FOR ALL BUT THE MOST BASIC, NON-CRITICAL CIRCUITRY - YOUR EARS WILL THANK YOU FOR IT.I cannot over-emphasize the above, so many people get hung up on component choice, circuit topology etc., when massive improvements are available just by optimising the circuit at hand. Ignoring subjectivity for a moment, audio electronics is all about minimising errors. These errors occur in many domains, and attention to understanding and minimising these errors WILL get you a better sounding circuit. The key is to look at the circuit as a whole, and understand what errors make the most difference - sometimes the 'cure' makes other things worse, and the final result sounds worse as a result. Never assume anything - the tips above cost nothing but your time, and hence are VERY good value for money. For the results of the above prototyping, take a look here and here. OK, cut the crap, and take me to the circuits!
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