AS-EU08 + SP-USB Review: Budget HiFi Power & Data Filtering

In a world where high-end power filters, conditioners, and regenerators like PS Audio or AudioQuest can be costly, you might wonder if there is a more affordable way to improve the quality of the power supply going into your audio devices.

Many inexpensive devices claim to do this, but are they effective and a good idea to implement in your system? Let’s investigate.

I’ll present two solutions you might find helpful.

AS-EU08 Power Conditioner

The first is the AS-EU08 Power Conditioner, which comes at roughly $200. It’s not the cheapest option to simply get more power outlets out of a single one, but it does that much more, which in my opinion makes it at least worth considering.

The first thing we notice immediately after picking it up is the weight and the build quality. It’s outstanding. It’s made out of 3 solid pieces of metal and weighs about 3 kilograms. The solid metal enclosure effectively works like shielding, greatly reducing the noise getting in and out of it.

There are 8 pure copper European Schuko power sockets on top, each separated by about a centimeter of space to make plugging in larger barrel plugs easier - or in some cases, possible at all. Next to the outlets, there is a fancy power switch with a red LED that illuminates when the power is on and turns off when either the switch is turned off or the power strip is unplugged. The power switch feels very tactile and is of a high-quality standard.

On the side we’ve got even more audiophile goodies. Of course, there’s a breaker, but in addition to that, there’s a ground connector that can take a bare wire, a banana plug, or a spade connector. I didn’t end up using it, but I did check if the terminal works properly. I was pleasantly surprised to find out that it makes good contact with banana plugs especially, without causing any damage to them due to excessive clamping.

Speaking about contact - making good contact with a power cable going into the device is equally as important as making good contact with the cables going out of it, as this power filter lets you use your own power cables. I appreciate it a lot. I also appreciate the power connector used on it. It’s an audio-grade Furutech, model FI-06, in what seems to me like a rhodium-plated version. It securely holds my HiFi power cables without squeezing them too tightly, but ensuring a good connection.

For the internal wiring, rather than using some thin and unknown material options, they used 4mm² PVC-insulated pure copper cables and made them as short as possible to reduce resistance. Inside there’s an array of blue capacitors made for EMI filtering, accompanied by a metallized paper safety capacitor.

It may look good on paper, but you’re probably as curious as I am if it had any impact on the sound quality.

In the past, I’ve experienced instances where using low-quality power conditioners resulted in a worse sound - it was more restricted and less dynamic. However, with this product, it seems to be the opposite. After plugging in my integrated amplifier to this power conditioner, the sound seemed a bit more refined, with a darker background, and more dynamics.

Why would that be, you might ask? I assume that the blackness of the background comes from a lower noise floor or the amount of noise getting into the components. The same goes for the dynamics, as it allows for a higher difference between the lowest level signal (in this case, noise) and the highest level signal (the dynamic peaks). The slight improvement in refinement likely comes from the quality of the parts used, as even small things can have an impact on the sound. The sound path starts at the AC stageShort for soundstage; the perceived three-dimensional acoustic space of a stereo recording. Often used to describe headphone presentation specifically ("the Arya has a deep stage"). and then gets converted into other types of energy. So essentially the sound you’re getting goes through all of these internal parts.

If it’s not enough, this power conditioner additionally serves the function of protecting your expensive gear against power surges, lightning strikes, or any other similar accidents that could otherwise destroy your equipment.

SP-USB Data + Power Filter

One thing is AC noise that can be minimized by a power filter, while another is DC noise generated by low-quality switch-mode power supplies and imperfections in the USB data bus. These issues are more complex in their nature and require more specialized tools to reduce their impact on the audio quality. I’ve got a device which aims to do precisely that.

The SP-USB is a small yet weighty rectangular aluminum box that comes in black or silver. I have the silver one. On the front, there’s a nice front plate with a cutout to fit a blue LED digital display that acts as a voltage indicator.

All of the I/O is located on the rear of the unit, which is the perfect position to place it, in my opinion. It makes cable positioning, especially with thick audio-grade ones, much easier and cleaner. There’s a USB Type-B input, but not in a regular form. It’s a 3.0 port that can support higher speeds if needed, and despite its different shape, it’s backward compatible with the more popular USB 2.0 standard. There are 2 USB outputs - one carries both data and power, while the other is only for power and cuts out USB data.

There’s a unique switch that has a satisfying tactile feel when pressed. Its primary function is to control how the grounding process occurs. The switch can either connect the ground to the device’s chassis or disconnect it when it’s in the top position. It’s usually better to keep the switch in the bottom position - leave it on - but feel free to experiment with both options to determine which one works better for you.

There is a standard power socket that can fit even larger power cables, accompanied by a small power switch. There are four rubber feet on the bottom in the corners of the device, which keep it firmly in place, preventing it from moving even when a heavy cable is hanging from it.

Now let’s get to the internals. It uses a built-in low-noise linear stabilized power supply to replace the high-noise switch-mode power supply from the PC, so it essentially cuts out the power from your PC and replaces it with clean, stabilized 5V and up to 3A of power generated by a hermetic toroidal transformer. Moreover, it uses a high-speed op-amp and a MOS output stageThe final amplification block of an amp that directly drives the load (speakers or headphones), supplying the current and voltage the load demands. Unrelated to "stage" / "soundstage" - that's a listening term about perceived spatial width and depth; this is a circuit block. to reduce and regulate the voltage even further.

For the data line, the data coming in and out goes through Foxconn’s USB terminals. Then it goes through a balancedA signal transmission method using two opposite-polarity signal lines plus a ground; noise induced on both lines is cancelled at the differential input. noise elimination circuit, working together with HiFi filtering electrolytic capacitors and EMI filter inductors.

Regarding the sound quality improvements, quite a few things got better, which was surprising and proves digital isn’t always perfect.

First of all, a blacker background was noticeable when taking it in and out of the system. It’s definitely not a bad feature to have, as it makes for a much more enjoyable experience once you solve all your noise-related issues.

Then, the biggest improvement I heard was the lack of high-frequency glareAn unpleasant emphasis in the upper midrange (~2-4kHz) that makes voices and instruments sound strident or pushed forward. Often a sign of crossover error or an unhappy driver-amplifier interaction. that I didn’t even know I had to begin with. It made the upper range cleaner and more natural, although it didn’t sacrifice any details. Perhaps it made the sound ever so slightly more detailed.