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

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.

The most important design issue is the distribution of the power-supply components between the two boxes. One approach is to put just the mains transformer in the power-supply box. This has the disadvantage that the current in the umbilical cable consists of short charging pulses of large magnitude at a frequency of 100 or 120 Hz, and these will not only experience a greater voltage drop in the cable resistance than a steady current, but also give rise to much greater I²R heating.

The latter is unlikely to cause problems in the cable itself, but can easily be fatal to the contacts of connectors. Speaking from bitter experience, I can warn that connectors that appear to have a more than adequate safety margin can fail under these conditions, and it is best to keep connectors out of charging pulse circuits.

The alternative is to put not just the mains transformer but also the rectifiers and reservoir capacitors in the power-supply box. The current in the umbilical cable is now rectified and smoothed DC, and it is much easier to specify connectors to cope with it. The snag is that the reservoir capacitors have two functions; as well as smoothing the rectified DC, they also hold a store of energy that can be drawn on during output peaks.

The resistance of the cable between the reservoir capacitors and the power amplifier will cause unwanted voltage drops when there are sudden demands for load current, which can significantly reduce peak power outputs during tone-burst testing. Another worry is that the extra resistance in the supply rails could imperil the stability of the amplifier, though the use of generous local decoupling capacitors should be enough to deal with this problem.

A solution to both problems is the provision of significant amounts of capacitance at both ends; the capacitors in the power-supply box deal with the smoothing, while those at the amplifier end provide a ready reserve of electricity. In this case the current through the cable will still show some charging peaks, the size of which will depend on the proportion of the total capacitance at each end and the cable resistance. This could be artificially increased by adding series resistors of small ohmic value but high wattage, making an RC filter that will reduce the ripple seen by the amplifier. This is a bit of a doubtful remedy as it will reduce the power output on sustained signals, and it is a very poor way to reduce amplifier noise derived from the supply rails, as will be described later.

There you have some of the pros and cons of external amplifier power supplies. It is not quite the expensive but foolproof solution it first appears to be, and the design issues require careful thought.