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Power supplies

Audio amplifier power supply design - Part 2: External supplies, inrush current & RF emissions

September 06, 2010 | Douglas Self | 222901413
Douglas Self considers different power supply technologies as well as the design considerations involved in choosing and evaluating a mains transformer.
Page 1 of 11
[Part 1 looks at advantages and disadvantages of different power supply technologies as well as the design considerations involved in choosing and evaluating a mains transformer.]

External Power Supplies
However much care is taken, it is very difficult to keep all traces of transformer-induced hum out of the signal circuitry. It is highly irritating to find that despite the cunning use of low-noise circuitry, the noise floor is defined by the deficiencies of a component for the ideal transformer would obviously have no external field rather than the laws of physics as articulated by Johnson.

The ultimate solution to the problem is to put the mains transformer in a separate box, which can be placed a meter or so away from the amplifier unit, and powering it through an umbilical lead.

Advantages
The transformer field hum problem is authoritatively solved.

Will appeal to some potential customers as a 'serious' approach to high-end audio.

Disadvantages
The cost of an extra enclosure plus an extra cable and connectors, indicator lights, etc. The connectors will have to be multi-pole and capable of handling considerable voltages and currents. The transformer box must have fuses or other means of protection in case of short-circuits in the cable.

A significant proportion of users will, exhortations to the contrary not withstanding, promptly place the amplifier box directly on top of the transformer box, immediately defeating the whole object. This is particularly likely if the two boxes have the same footprint, and so look as if they ought to be stacked together. However, all is not lost in this situation, as the transformer is still physically further away from the sensitive electronics (though if the transformer has a large field emerging from its ends things may actually be worse) and there are now two extra layers of steel interposed assuming the boxes are made of steel, that is.

The voltages involved will probably be above the limit set by the Low Voltage Directive, so it will be necessary to ensure that the connector contacts cannot be touched. If the cable has a connector at both ends then both must be checked for this. A cable that is captive at the power-supply end makes this issue simpler and will also save the cost of a mating pair of connectors, which may be considerable.

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