Power supply design for the Innuos Statement

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Power supply design for the Innuos Statement

Sean Jacobs explains the work he did on the power supply for the remarkable Innuos Statement server.

The lower box contains the first stages of the PSU - transformer, rectifiers and capacitor arrays, along with some additional filtering components. There are eight separate transformer output windings, eight sets of rectifiers and eight banks of capacitors - all power rails are isolated from each other even at this stage (i'll come back to this later). These "raw" (unregulated) DC rails are passed through the umbilicals (one on each side to minimise circuit path length), and the final stage of the PSU - the regulators - are mounted inside the upper box, very close to the parts of the server that they are powering. This close positioning reduces noise in the supply significantly, in contrast to having the regulators in the lower box. The regulators are an evolution of the design used in the Zenith SE – they share some common ancestry but the Statement regulator is superior in many aspects, and to date it’s one of the best-performing designs I have produced.

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Innuos Statement power supply chassis with eight sets of rectifiers and capacitors.

The lower box acts as a shield too - when the boxes are stacked, the server is shielded from the AC currents and the transformer by the top of the lower box and also the base of the upper box. In addition, diode switching noise has been considered and minimised as far as possible without affecting PSU performance (Innuos have also added further EMI absorption to the various components in the upper box). The transformer is custom built to very high specs, with multiple shielding layers and an oversized core. And of course, the reservoir capacitors are our usual top-spec parts from Mundorf (as used in the Zenith SE and Zenith mk3). The wire used throughout the PSU sections is silver-plated OFC with a PTFE insulation, and the wiring is all kept neatly loomed to reduce ground loop areas.

Using short umbilicals (and having one on each side) minimises the overall length of each power path, which means that there is a lower circuit impedance. This gives a small but noticeable performance benefit in comparison to longer umbilical cables. It also helps to reduce absorbed EMI too. Using short umbilicals and stacking the boxes one atop the other does seem counter-intuitive if you consider EMI between the boxes and vibration isolation, but the lower impedance does make a bigger difference. I have always found supply impedance and overall layout to be an important aspect of how a PSU actually sounds, and it's rather under-rated compared to the noise figures that some seem to fixate upon.

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Innuos Statement server chassis with dedicated supplies on left and right.

As mentioned above, the lower box contains eight separate unregulated DC rails, each is carried into the upper box by the umbilicals. From here, there are eight separate regulator modules that get a dedicated DC feed, and each module has a specific task in the server such as supplying power to the CPU, or the SSD, or to one of the Innuos clock modules. Every part gets its own dedicated supply. The benefits of this approach are twofold - firstly, each regulator module has to supply less current (as opposed to, say, using one regulator to supply both of the clock modules at once). Less current means lower output noise, just as it does with any other regulator design. And secondly, there is virtually no noise due to crosstalk between power rails. For example, if we were powering the SSD and a clock module from the same regulator, then any supply noise created within the SSD (power usage is typically not constant, any variations mean a change in current draw and a very small change in voltage at the regulator output) would be free to travel along the supply wire and it would be fed straight into the clock module as well. When we use separate regulators each device is isolated from supply noise created by other devices. (A case in point, Naim preamps like the 282 offer a simple illustration of this, whereby they will sound better if you add extra supply rails with a HiCap or two HiCaps instead of using a single rail from a power amp).