Power supplies

By power supply we mean the part or parts of a component dedicated to converting the available electricity (alternating current from the wall socket, that is, 230V at 50Hz) into the voltage or voltages needed by the internal electronics, which are usually lower than those 220V (the only exception being tubes) and, it should be noted, always direct current. The power supply must therefore fulfill two tasks: reduce the incoming voltage (230 Volts) to the level the electronics need, and convert the alternating current into direct current so that the electronics can use it.

In both cases, the aim is for the process to be carried out as cleanly as possible, filtering out any noise or spikes coming from the mains and not adding any of its own. It must also provide a current that is as stable as possible under rapidly changing load conditions, something we can picture very clearly in the case of an amplifier when a quiet passage turns into a symphony orchestra in full flight.

Very often, the difference between a mid-range component and a high-end one is simply a higher-quality power supply (cleaner and more capable), and the paradigmatic example of this is the British electronics maker Naim, which directly offers, as an upgrade path for a component, the use of a separate power supply (or more than one) that improves on the component's original internal one.

When linear supplies with a transformer are used (see sidebar), the weight of the component can be a good indication of how generously dimensioned the power supply is and, therefore, of its quality a priori, since a higher-capacity transformer always contains more iron (and copper) and weighs more... and costs more, so the manufacturer is really giving us more for our money, all other things being equal.

INTERNAL OR EXTERNAL

Usually, the power supply is built into the component, and its only connection is an inlet for the power cord, usually a trapezoidal "IEC" socket popularized by computers. But we increasingly find, especially in mid- and low-range components, separate power supplies in the form of a small external box that plugs in directly or via another cable, and connects to the component it powers with a two-pin or multi-pin connector.

These are popularly known as external transformers, although this is not always the correct term: a transformer is a part with two windings (primary and secondary) which, through the magnetic influence of one on the other, converts alternating current between different voltages, for example, if with 220V at the input (primary) we obtain 12V at the output (secondary). Some external power adapters are just that, a transformer to adapt the voltage, but they normally also have components to convert that alternating output current into direct current, which is what the component's parts need in order to work.

When the external adapter is just a transformer, the component will indicate "AC input power" or something similar on its power inlet, and when the component needs to receive direct current, it will indicate something like "DC input power". If we want to experiment with a power supply other than the original (because of a fault, or out of curiosity in search of an improvement), it is important to pay attention to this detail and to the component's requirements since, in addition to whether it needs direct current (DC or a = symbol) or alternating current (AC or a ~ symbol), we must check the required voltage, which must always be respected, and the current required, which the adapter must be able to supply in equal or greater measure.

WHICH NEW EXTERNAL SUPPLY TO CHOOSE?

If we want -or need- to use a power supply other than the original, we must always respect the following rules when looking for, building or buying the new adapter:

- Identical type of output current: direct (DC) or alternating (AC).

- Identical output voltage (5V, 12V, etc.).

- Current capacity equal to or -better- greater than that of the original adapter or that indicated on the component: if it says 5V and 2A, choose one rated at more than 2 Amps. If it only indicates power, for example 5V 10W, remember that current is power divided by voltage, so in that example we would need 10W/5V = 2 Amps or more.

LINEAR OR SWITCH-MODE?

A linear or traditional power supply consists of a transformer that steps the incoming voltage (230V) down to the required level, plus a rectifier (diodes) that converts it from alternating to direct current. After this process, filters are applied to eliminate noise coming from the mains itself and/or from the rectification process. Diodes only conduct current in one direction, which is why they serve this purpose, but the moment of switch-off can generate voltage spikes and other noise (hence the interest in special diodes that minimize this effect). These filters consist, at the very least, of one or more high-capacitance capacitors, which in addition to filtering act as an energy reservoir for when the downstream circuit needs a large, momentary burst of current.

A switch-mode power supply works, in a way, the other way around: it first rectifies the 230V alternating current and then converts it to a lower voltage. To do so, it uses an oscillator circuit to generate an "alternating" current (switched, with a square waveform, hence its name) but at a much higher frequency than the original from the socket (in the order of several kHz, or thousands of Hz, compared with the usual 50Hz). Then, as in the traditional supply, a transformer steps down the voltage of this artificially generated alternating current which, being of such a high frequency, does not need such a large, heavy transformer for the same amount of current: a much more compact one is enough. Once at the required voltage, this high-frequency alternating current is rectified into direct current, although it needs better filtering than in a traditional supply to prevent part of the internal high frequency from reaching the component being powered, since it would impair quality across the audio spectrum. The essential difference lies in being able to do without a large, heavy and expensive transformer. This explains why switch-mode power supplies have become so popular and why it is rare today to find components that still use traditional power supplies (outside audio, in fact, they are almost impossible to find).