How to find and fix hum in 3 easy steps

  Once a very long time ago I used to think this way. Then I became an engineer. When you spend other peoples’ money you often have to explain not only what you are doing but why and what other choices you have and why the one you selected is the best and most cost effective one. Building something 10 times, 100 times, 1,000 times what is needed must have strong justification or it will be unacceptable to those who must approve the expenditure. Rightsizing is a term invented to express that part of the engineering art that looks at current needs, possible or likely future needs, and uses sound judgment borne from experience to make intelligent choices. We don’t want solutions that are inadequate or marginal but we also don’t want to throw money away. Is a 1200 watt amplifier justified for powering a pair of Klipschorns? Not any more than a 3 watt tube amplifier is justified for powering a pair of Magnepan LRSs. One solution is modular design that is configured for later upgrades when changes are needed. This is done to avoid obsolescence of the original expenditure.

  What makes these oversized electronic units sound superior (assuming they do?) I’m going to take a guess. It’s the output impedance being low which is partly the result of a superior power supply. Circuit topology also matters. An preamplifier or DAC with a class AB output stage will have a far lower output impedance than an topology with an emitter follower or cathode follower output stage which is what is often traditionally used. ICs usually can’t handle much current either. Discrete transistors would be preferable IMO. This difference probably won’t matter when the input impedance of a tube amplifier is 100 Kohms or more but it can make a substantial audible difference when a solid state amplifier input is only 5 or 10 Kohms. A high capacitance or high inductance connecting cable only makes matters worse by rolling off the high end.

  Why stop at 1200 watts? Dick Burwin has a home sound system that is 20,000 watts.

  Will a loudspeaker/amplifier combination that can produce 126 db peak loudness without distortion in your home sound any more like a symphony orchestra than one that can only produce 106 db maximum loudness, 1/100 the power? I don’t think so. Less than 1% of all symphonic music produces sound in the audience louder than 106 db (I don’t know of any that reach even that level.) The real differences between live and recorded sound are qualitative, not quantitative.

  Oversizing equipment has major disadvantages and pitfalls. This is true for both electrical and mechanical systems but maybe not for structural systems. In one project for a large corporation a data center had been decommissioned, the equipment removed and the space was repurposed for document storage. The data center AC system was left as it was. Before long it was realized that not only was it very cold in that space but the documents were starting to grow mold. The oversized AC system achieved the desired temperature which controls the on/off cycle long before it removed much humidity from the air. An incompetent consulting firm came up with the brilliant idea to put terminal reheats in the HVAC air discharge system. A monster heating system to fight a monster AC system. Talk about inefficiency and sheer incompetence.

  I’ve encountered the following problem more than once and thankfully I was never responsible for it. When inadequate power was available from an electrical substation the engineer replaced the transformers with larger ones. These types of transformers have impedances rated in percent and are associated with their KVA (power) rating. The larger transformers could deliver much more current in a short circuit than the original transformers but the next downstream equipment, a circuit over-current interrupter was not rated to withstand that much short circuit current. The result of a short circuit on the load side of the next protective device could have resulted in an explosion rather than a circuit breaker trip. For a 2500 KVA transformer at 480 volts that’s 2.5 million watts in something the size of a shoe box. It will last for a few milliseconds after which it will heat to about 30,000 degrees and explode.

  What do you do to protect your expensive speakers from being overloaded and destroyed by an overpowered amplifier that can deliver far more power than the speakers can withstand?

  What if you replaced the engine in your car with one that could produce 800 horsepower so that you could go faster? Would you melt the tires? Would you burn out the transmission. This is one more reason to think about intelligent system design rather than just assembling components that you like.

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