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Power Supplies


Power and Power Supplies

Most hifi and video products plug into the 240V power socket on the wall. But it may actually be a surprise to many that most of the gear actually does not need that much voltage, in fact most use a lot lower voltage, even down to 3.3V. However, don't get voltage confused with power or current; they are all interconnected. Just because a device has lower voltages inside doesn't mean it is also low power, it can still carry a lot of current hence a lot of power.

So we start at the fuse box where we have our 240V (for Australia anyway, this varies from country to country) and it is distributed around the house on various circuits. One may be for lights, another for the oven and so on. Power points are usually divided into zones, maybe one half of the house or similar. Some people with high-end systems will put in special circuits and high current-capable wiring to ensure the cleanest and best circuit possible for the hifi. Of course always use a licensed electrician for any electrical work!

The mains circuit in the house will carry the electricity to the power point where most people will use a power board to connect their hifi. The number of connections between the fuse box and the device can be numerous and each connections may add a little bit of noise and reduce the power slightly, that is why high-end system like their own circuits! Power boards are really not a good idea as most of them use flimsy internal connectors and wiring, best to get a good quality one or use power points directly on the wall. It lessens the mess of cables too and can make it safer. Equipment that uses a lot of power, especially amplifiers, should always be plugged into the wall socket but other units like CD and DVD players tend not to use very much so a power board is okay if there is no other option. Some power boards have special noise filtering and safety features built-in, these may protect your gear if lightning or surges occur.

There are many options when it comes to power filters and surge protectors. A surge protector will attempt to stop dangerously high levels getting to the connected equipment. These high voltages can come from lightning strikes, damaged street transformers and sub-stations, possums across wires and even industrial machinery turning on and off. Surge protectors often have one or more parts called a MOV (metal oxide varistor) or VDR (voltage dependant resistor) that will short circuit above a certain voltage. So any high voltages that shouldn't be there will be sent to ground and hopefully not affect your gear. Some brands of surge protectors offer insurance against any damage but read the fine print first!

Stopping noise getting into hifi gear is another issue. Some people argue that it is not necessary as good quality products already have enough filtering and extra filters can 'strangulate' the power. A power filter usually works by having various capacitors and filters in a separate box inline with the power cord. The circuitry uses these parts to remove any noises. Better ones will of course use bigger and more expensive components and sophisticated designs. The top-range versions are actually more power regenerators that just filters. They convert the 240V to a steady state, highly filtered DC (direct current) signal that is then converted back into a 240V signal. They are very complicated and certainly not cheap. As with most such devices, make sure you can test it out first. Their effectiveness will also depend on just how noisy your mains power is, you may be lucky and live in an area with very little noise.

Sometimes you can fix noise problems by yourself. If you have a noise issue, first try a different power cord or different wall socket, even one at the other end of the house in case that entire circuit has an issue. Check the house for possible sources of noise; for example an old washing machine or a faulty fridge may be the cause. Does it happen only at certain times, if so, what else happens at that time? The neighbour does their washing perhaps?! Check the power cords for any damage, a broken earth, apart from being dangerous, can also cause extra noise or hums. Dirty contacts can be a problem too, if your power points feel loose or are old; it may be a good idea to have them changed by an electrician. Never attempt to clean a power point yourself.

Now, back to the power supply. The power cord to the device in question will either be permanently fixed to the unit internally or be a plug in type, usually a three pin plug on the back of the unit. In this latter case it allows manufacturers to supply appropriate cords for each country plus allows the owner to upgrade to better power cords if required. There are a number of high-grade power cords available; they range in price quite a lot to many thousands. If your system is very revealing and you think that a new power cord will help, it is best to find a store that will allow you to return the cable if you do not notice a difference for the better.

Once the power enters the unit it usually travels through a fuse, some filters and the power switch. Believe it or not, you can actually purchase audiophile fuses! Some units do not have power switches that actually disconnect the power, the unit will remain on standby when off and this will use power. If your hifi does this, it is best to turn it off at the power point where possible to save power. The next part of the circuit in most gear is the power transformer. Many newer devices have a power supply call a switchmode power supply; we'll look at this briefly shortly. The transformer converts the 240V into lower voltages as required by the unit. It works simply by having many coils of wire around a metal core. The power supply is 50Hz, meaning it pulse as 50 time a second. This creates a magnetic field through the metal and another coil of wire (which isn't actually connected to the first one) picks up the magnetic field and converts it into electricity again at the lower voltages. The number of windings in the coil determines the exact voltage. Most transformers have multiple windings to give various output voltages.

The so-called switch-mode power supply generates the lower voltages literally by switching the 240V signal on and off very fast, 'grabbing' small parts of it to give the lower voltages. That is a very simple explanation, they usually have some sort of microcontroller and filtering too, this will remove most of the noise. They are becoming more popular as they are cheaper to make that using transformers (due to rising metal prices) but tend to generate more noise so are not yet so common in high-end products.

So now we have our lower voltages but they are still AC (alternating current) and for most hifi gear this is no good. They need steady state voltages called DC – direct current. A diode bridge is used to make the conversion. It works by flipping the bottom half of the AC wave to the top. It is still not smooth; the next step is to use large capacitors to filter out the 50Hz that is left (technically 100Hz half-wave now). The capacitors will charge up holding the power and release it upon demand. Usually several are used to give better performance. The capacitors also act as a power reservoir for the unit, a sudden increase in power demand such as from a crescendo in the music - the capacitors can deliver more and faster that what can come from the transformer itself. But they can only do it for a very short time. So you can see the quality and amount of the capacitor bank can make a difference to the performance of the unit. Many high-end brands promote the size of their power supplies and often rightly so! Transformers are heavy and the large ones can add many kilos to the weight of a unit, particularly amplifiers. Some units may even have two transformers, one for each channel; this may give better performance.

Now we have our DC voltages, as mentioned there is likely to be several different levels. A CD player for example may need 3.3V for the system control, 5V for the clocking and digital to analog circuitry, 12V for the motors that move things around and ±15 for the output stage. Yes, there is likely to be negative and positive voltages used; all referenced around zero volts. This is especially true in many amplifiers that need large powers supplies in the order of say ±60V. The transistors that operate of this voltage need that large range and create an audio signal that is a similar level voltage-wise and that relates to the output power. The great the output power, the larger the voltages needed in the power supply.

The smaller power supply voltages mentioned usually need to be kept stable; you don’t want the gear failing every time there is a voltage dip. So to protect against this, a part called a voltage regulator is used. A voltage regulator takes a voltage in and gives a voltage out. Not very exciting but the output remains fixed virtually whatever the input voltage is (provided it doesn't drop too low). So this makes designing of circuits very easy especially when you consider the different power supplies around the world. Each regulator usually has extra filtering both on the input and the output. This is because (especially at low voltages) noises can affect the operation and capacitors and miniature coils are used to filter out these noises. Better quality equipment will use better quality filtering parts and more of them.

Power distribution around the circuit is sometime done with wires, sometimes with circuit tracks and occasionally with bus bars. Bus bars are metal railings that can carry high current levels; this enables the voltages to remain stable and not loose much power as they travel. Some quality gear actually keeps the power supplies in a completely separate casing from the rest of the device. For example both Meridian and Shanling had CD players with separate power supplies. All the 240V and transformer parts were kept away from the more delicate digital and analog circuitry; this reduces the noise levels even further, hence improved performance.

Written by Leon Gross, originally published in Audio & Video Lifestyle magazine.