Switch Mode Power Supplies, or SMPS, just sounds nasty. No, not the supply itself but the word "Switch Mode". Shades of choppy, digital, not linear, radiating, messy. Definitely something you want to stay away from.
A SMPS is a linear power supply only it doesn't have a big power transformer like what we think of when we mention a linear supply.
Audiophile wisdom suggests that linear supplies are better, especially in power amplifiers, than SMPS. If you try and nail down why this thought prevails the arguments quickly fall by the wayside because, well, we don't actually know why. It's just that a digital power supply seems so much nastier than one that isn't - in the same way that a digital audio product seems so much more messy than an analog music system - or an automobile electronic ignition system vs. a simple mechanical distributor.
I thought it might be helpful to spend some time examining what each of these two types of supplies actually are, how they work, why they are different and how they affect the products we listen to.
To start off let's ask the question: "why do we need a power supply at all"?
What comes out of our wall sockets at home is pretty useless to us for hi-fi. 120 or 230 volts moving between + and - 60 or 50 times a second isn't something we really want to try and make music with. Instead, we want a steady state + and - voltage, like that from a battery, at much lower levels: perhaps 5 to 30 volts for a solid state preamp or DAC and as high as 100 volts for a tube or power amplifier. None of that comes out of our wall sockets.
To further complicate the issue we have safety concerns to deal with as well. We don't want to have our music systems capable of connecting us directly up to the wall socket - like we can with an ordinary appliance like a hair dryer or toaster. We're a bit more cautious and concerned with the machines that make our music relative to the machines that dry our hair and make our toast - which makes sense because in the case of our stereo system the output is an electrical connection (interconnect or speaker cable) and in the toaster's case, a piece of non conducting bread and hot air for the dryer.
So the job of the power supply is to reduce the voltage from the wall, change it from AC to DC and protect us from possible electrocution. The main element that makes this possible, in both the linear and SMPS, is the transformer.
We've already spent a great deal of time on transformers, their importance to us and how they work, in these posts but I know it's probably helpful to review.
A transformer is nothing more than two coils of wire, one hooked up to your wall socket and the other connected to your equipment's power supply (there's also a whole bunch of steel to help the coils work better). When the AC power moves back and fourth between + and -, the coil of wire produces a magnetic field - meaning it becomes a magnet - and so the power in your wall socket is converted tomagnetism. The second coil of wire picks up this magnetic field and reverses the process to make electricity again.
The amount of wire in each coil determines the output voltage of the transformer - meaning if there's more wire in the wall socket side of the transformer (called the primary) than in the output side (the secondary) the transformer puts out less voltage than what goes in. The protection occurs because the two coils of wire don't actually touch each other and the only connection between them is magnetic, not physical.
So in each case, the linear and the SMPS, we have a power transformer that connects the wall socket AC to our equipment safely and at the proper voltage.
Tomorrow we'll start to understand what's different between the two.