Paul McGowan writes: Of the two types of power supplies found on class D amps, linear or switch mode, the latter is taking over and the former is moving down in popularity. It’s a natural progression as combining a giant power transformer and capacitor bank supply with a small and lightweight power amplifier doesn’t make a lot of sense unless ……. and this is a big unless …… it sounds better. We’re here today to figure out if it does or not.
In the past the best sounding option was to go with a linear supply – making sure it was over built for the task. The problem with these supplies is that they are neither clean nor pretty – and the best results with any form of amplifier come when you have a pretty, clean and DC-like output on your power supply. Even your classic A or A/B amps benefit greatly from a better supply – and a few traditional amp manufacturers even go so far as to regulate their amp’s power supply – Roger Sanders (a fellow Coloradan) comes to mind. But the vast majority of amps do not regulate nor do they do anything about the ripple, jaggies and noise associated with this supply type.
The heart of any power supply is the power transformer which provides isolation from the power mains and a means of getting the desired voltage level to the amp. Power transformers must have AC to work their magic and that’s exactly what they get from our home’s wall outlet. 50 or 60 times a second, the mains voltage smoothly moves back and fourth from positive to negative. Place this moving voltage into a power transformer – which is really nothing more than two coils of wire surrounded by a lot of iron – and the input coil generates a big magnetic field that rises and falls 60 times a second. The second coil of wire picks up this magnetic field and converts it back to a voltage without any physical connection between the two coils; the only connection being magnetic which keeps us safe from electrocution.
The problem with power transformers is size, weight, efficiency and power restriction. If a power transformer is required to plug directly into your home’s mains, it needs to be really large to handle any power efficiently – and that power is restricted by the size of the transformer – bigger is always better when it comes to these devices.
There are two things that determine the size of a power transformer: the frequency of the mains AC and your power requirements. The mains AC is flipping back and fourth between positive and negative 50 or 60 times a second depending on where you live – and that requires a large size transformer to deliver any power – and the lower the frequency the larger the size. For example, our power transformers in our products have to be 25% larger for 50Hz countries than those that run on 60Hz. 25 percent! That’s a big difference for only a few Hertz.
But now go the other way – if you raise the frequency, the size of the transformer drops exponentially – and a 1500 watt power transformer can head down from a monster like in the latest video we produced about the Power Plants to something that fits in the palm of your hand.
So what engineers did was to figure out a way to take the incoming AC and raise it from 60Hz (60 times a second) to 100kHz (100,000 times a second) and that produced a tiny efficient supply that is essentially identical to the big power supplies – yet weighs in at 100th of what a monster supply does. How did they raise it up? Simple, they just placed the output stage of a class D amp right across the mains and connected the mains on and off again 100,000 times a second. The transformer doesn’t know any difference, so it’s happy, and the entire power supply is now 100th the size and weight of the big one – with performance that not only rivals the big one, but betters it.
