please forget the "demanding upsampling procedures", the focus is "simply for sound quality"
Hi Matt, answer divided over 2 posts, was unaware there is a 10.000 character limit!
Thank you for raising the question, it is a very interesting topic which deserves an elaborate reply. For audio equipment power supplies we typically have 2 general mechanisms for which you have a choice between switching or linear solutions. The first is AC to DC conversion, the second is DC-DC conversion (regulation).
What is typically referred to as a SMPS (Switch Mode Power Supply) or LPS (Linear Power Supply) mainly addresses AC-DC conversion, now you could argue both LP and SM power supplies are switchers, a LPS switches at 50/60Hz or 100/120Hz (depending on the rectification circuit), with a SMPS you can choose/design the switching frequency, this is typically 50KHz or higher. A SMPS is always a more complex design, with a much higher component count, yet it can be cheaper as you can design a complete SMPS at a total cost of below that of just a mains transformer, however for audiophile applications you would want to use "audiophile quality" components which could make it much more expensive. It can also be more efficient, however you can also have a very efficient AC-DC LPS, as you can have very efficient rectifiers (active rectification for example) and toroidal transformers can have efficiencies up to 96%, combined that type of LPS rivals a SMPS for AC-DC conversion efficiency, leaving you with 2 factors in play, switching frequency and its associated noise spectrum on both the AC and DC side, and component quality which also affects the noise spectrum of the supply.
So far we've only been addressing AC-DC conversion, which turns an AC voltage into an unregulated DC voltage, this is typically not a constant voltage, it varies with AC grid voltage fluctuations and current draw. This unregulated DC voltage can be used to directly supply power to amplifier output stages, however we also need fixed voltages for which we need voltage regulation. Here we can again choose between linear and switching regulator solutions. Unlike AC-DC conversion, for DC-DC regulation efficiency there is no comparison between linear and switching regulator designs.
As an extreme example, let's look at powering a CPU. A CPU typically operates at 0.6-1.2 volts but can easily draw 100 watts or more, let's assume 1 volt for easier calculations. For 100 watts at 1V you are talking about 100 Amps(!) of current. As current drops voltage over resistance (Ohm's law), a CPU is typically supplied with a 12V voltage rail, so we need to regulate 12V down to 1V. If you would use linear regulation, and you'd have a 100 Amp current draw at 1V, you would also have a 100 Amp current draw at 12V (simplified), meaning 12V*100A=1200 watts. Then we need to feed this 12V by a 16-19V supply (again to account for voltage drop caused by current over resistance), let's assume 19V as that's a very common value to ensure broad compatibility, and we are talking 19V*100A=a shocking 1900 watts. The conversion efficiency here would be ~5%, the other 95% will just be converted to heat. A switch mode regulator is the inverse of this, it can regulate 19V down to 1V at a 95% efficiency wasting only 5% as heat. That is a 1900W versus 105W of power consumption.
As an hybrid solution, you could opt to perform the 12V to 1V conversion switched mode, and 19V to 12V linear. With 100A at 1V, and a regulation efficiency of 95%, your current draw at 12V would be about 8.8A (100*1*95%/12). You would then have about a 9A current draw (again simplified not accounting for resistive and other losses) at 19V, translating to a ~170W power consumption. Then you end up with a ~60% efficiency, burning about 70W in heat. Now you could stretch this a bit with a careful design, bring it down to say 16V, increasing efficiency to about 70%, and that is in fact a very typical efficiency for a carefully designed linear regulation circuit to serve a particular application. If you need multiple voltages, which you virtually always do, this would necessitate multiple unregulated DC power rails, meaning multiple transformers or a transformer with multiple secondaries, multiple rectification and DC filter stages. This is also where you need to start being really careful with your layout and circuit design as these rails can be supplying individual but interconnected circuits (creating loops), which is where for example your ground layout become a critical component affecting performance. RFI does not seek the shortest path to earth ground as is often assumed, it just travels everywhere it can (think antenna). In this scenario there is a potential benefit for an AC-DC SMPS as it's relatively easy to create multiple unregulated DC rails from a a single small footprint supply.
So where does this all of this leave us in our considerations to use switching or linear power supplies for audiophile purposes?
Approaching this question from a noise perspective, assuming a system's accumulated noise level directly affects sound quality, we propose the following aspects:
-higher currents increase noise
-vibration increases noise, higher currents increase vibration
-higher heat increases noise, where higher currents increase heat
-components produce noise, where we have their own operational noise plus additional noise caused by the 3 listed above
-circuit layout produces noise and is affected by it, by component parameters and circuit design including pcb traces, wiring, radiated noise and their interaction
Ideally we'd have the lowest possible currents flowing, low heat, high component quality and the best possible environment to allow these components to operate at their intended parameters or in their optimal range in order to lower distortion, and if possible a simple layout using a minimal component count where each of the components used can be of the highest possible quality with the lowest possible generated noise spectrum/levels.
Switching vs linear benefits:
-Higher efficiency, lower power waste therefor lower current flows and lower heat, DC-DC conversion efficiency also allows running higher voltage rails reducing currents even further for the same power requirements (as power (W) = I (current) * V (voltage) and therefor a lower vibration environment.
Linear vs switching benefits:
-Simplified circuit design, lower component count and a wide availability of audiophile grade (low noise and lower vibrations sensitive) components.
From a cost perspective:
For AC-DC applications: Linear design complexity is lower then switching, you typically need fewer parts, but we tend to spend more on individual parts quality in linear supplies and they are physically much larger. Design cost of linear circuits is less advanced, less complex and therefor (much) lower, the skill level requirements for designing switching mode supplies is magnitudes of orders higher then those for linear supplies. This nets out to a lower cost for Linear supplies for small quantity manufacturing, although the component and housing costs are higher, the development costs are much lower. For high volume manufacturing switch mode is cheaper with development costs divided by sales numbers.
For DC-DC applications: Switching costs are always higher when using high quality parts, the component count is higher and the design costs are higher. However this may not apply when attempting to regulate voltages in high current designs if you are going to waste dozens of watts in heat due to physical size and heatsinking requirements.
Reading back you may now think why are we using linear power supplies at all, but we have to refine that, as for AC-DC applications I currently do not see considerable advantages to using switch mode supplies for audiophile applications with the exception of lower costs for higher volume manufacturing and reduced space requirements. For DC-DC applications it depends on current needs, with some very obvious advantages for switching regulators for high current applications, in my book, high current is everything over about 1A, actually around 500mA is already borderline where you start running into issues with heat dissipation/power waste and associated noise.