Review: iFi Audio USB iPurifier2 and DC iPurifier


At the very tail end of my review of Sonore’s microRendu, I talked about testing two of iFi Audio’s iPurifier products with my own DIY music server. In this review I’m going to go into a lot more detail about exactly what they do and how they do it, but before I get to all of that, I want to talk about the two major traditional digital audio connection formats: AES/EBU, and the Sony/Philips Digital Interface, more commonly referred to as “S/PDIF.” Professional studios often need very long cables, and for that type of environment, the 110 Ohm, balanced AES/EBU format was created to connect digital devices without the worry of electrical interference. Interconnect cables in a home environment on the other hand rarely need to be longer than a few meters, and that’s where the unbalanced, 75 Ohm S/PDIF interface comes in. A simple coaxial cable with a lone signal conductor and a shield that doubles as the return conductor is all that’s required, and that can be made far more cheaply than the balanced twisted-pair type necessary for AES/EBU.

When fitted with a Bayonet Neill–Concelman (BNC) connector, even a basic coaxial S/PDIF cable can maintain a precise 75 Ohm impedance, and is very good at its intended task. For the last several decades, if you wanted the best way to connect your CD player or other digital source to your DAC, this is all you needed. (I’ll leave the largely failed experiments with I2S cables for another day). Using far more common RCA connectors instead of BNC allows the cables to be made even more cheaply, though at the cost of reduced performance. Maintaining the precise 75 Ohm impedance from end to end is far more difficult with RCA jacks than with BNC, which leads to things like impedance mismatch, causing signal reflections inside the cable. Of course S/PDIF cables fitted with RCA jacks still work just fine, they just don’t sound quite as good as they would with BNC connectors – a compromise for the sake of convenience.

Back To Basics

Early commercial PC based music servers pretty much all used sound cards with S/PDIF or AES/EBU outputs for connection with a DAC. The problem with that is most sound cards aren’t that good, and the ones that are from pro audio companies like Lynx and RME tend to cost quite a bit of money. They also have very complex drivers and interfaces meant for multi-in/multi-out studio use, and really aren’t intended for the home market at all. With sound cards not up to the task, there were two options left for getting digital audio out of a PC and into a DAC: USB, and FireWire. A company called Weiss tried to push PC audio over FireWire for a number of years, but USB won that fight easily. Most PCs didn’t have built-in FireWire ports, and nobody wanted to go out and buy a FireWire add-on card. Again, compromise for the sake of convenience.

In the beginning, USB audio was horrible. Early USB DACs pretty much all used the same USB interface: the Texas Instruments PCM2704 chipset, which was limited to a max of 16-bit/48kHz, and sounded like a hot, steaming pile of garbage (You are too kind. -Alex). Little by little, USB improved. It gained the ability to do 24-bit/96kHz thanks to more advanced input chips made by companies like Tenor and Centrance, but the real breakthrough came with the asynchronous mode protocol, which put the DAC in charge of timing instead of the computer. Current asynchronous mode chipsets like the ubiquitous XMOS are capable of 32-bit PCM and sample rates of 384kHz and beyond, but despite all of that capability, there are still numerous issues that must be overcome.

The S/PDIF interface was designed specifically for its intended role. USB wasn’t. It’s roots lie with computer peripherals like keyboards and mice. Sure, isochronous mode allows USB audio to work, but that certainly doesn’t make it well suited to the job. One of the biggest problems that USB has as an audio connection is power. It’s very convenient to be able to plug in a bus powered USB DAC and have it just work with no AC adapter or power cable needed. That means however that the voltage lines and data lines in the USB cable are literally right on top of each other – not a great recipe for the best sound quality. Worse yet, not all USB interfaces even truly comply with the USB standard’s power specifications. That’s why companies like Schiit recently came out with the Wyrd; they too have seen discrepancies between what a port claims to deliver to what it actually delivers. The truth is depending on how compliant a particular port is and the number of devices you have, you can get surprisingly different results with respect to power.

Purify Your Mind

One of the first companies to recognize the problem of lousy DC power from computer USB ports hurting DAC performance was iFi Audio, the spunky, value focused offshoot of Britain’s Abbingdon Music Research. To try and solve it, they created the original iUSB, which supplied a fresh, clean source of 5V DC to the USB DAC. If dirty power was the only problem with USB audio we’d be able to stop there, but unfortunately things aren’t that simple. Asynchronous mode chipsets are far better at dealing with jittery incoming digital sources like PCs than their adaptive mode predecessors, but that doesn’t mean they are immune to jitter.

In the last couple of years several companies have created a number of products to try and clean up the jitter coming from the PC before it even gets to the DAC, including iFi with their second generation Micro and Nano iUSB 3.0 models, but most of these cost at least a couple hundred dollars, and most require a source of AC power. But what if you don’t want to spend that much? What if you want to be able to clean your computer’s USB output on the go, without needing a wall outlet? And more interestingly, what if you already have an AC powered USB scrubber, like my UpTone Regen, and you want to experiment with scrubbing the signal again before it finally makes it way to the DAC’s USB input? iFi has an answer for you in the form of the USB iPurifier, now in its second generation.

The sound quality of my DIY music server hugely depends on what’s plugged into it. If I power it with an off the shelf, laptop style switch-mode power supply, and connect it directly to my Lindemann USB DAC via one of its onboard USB ports, it pretty much sounds the same as my desktop computer: OK, definitely not great. If I replace the $20 power brick with a $370 TeddyPardo 12V linear power supply, now we’re getting somewhere. The bass firms up and becomes more prominent, the soundstage expands, midrange details are more fleshed out, and the highs are more extended. If I then add the Regen as a middleman between the server and DAC, things get a lot better. Now my humble little DIY server is competitive with multi thousand dollar commercial servers. I’ve always wondered just how much room might be left for further improvements, which is why I was itching to try the USB iPurifier2, and its DC power cousin which I’ll get to later.

The USB iPurifier2, like the other iPurifiers, is encased in an attractive aircraft-grade CNC-aluminium shell that gives it a nice, weighty feeling in the hand. Once inserted into the signal chain, it uses an active noise cancellation system much like noise cancelling headphones. A new signal is generated at the opposite phase of the incoming noise, and the two cancel each other out. The iPurifier2 then goes to work on the data stream, reclocking and regenerating the data with as low jitter as possible. Finally, iFi’s “Rebalance” technology removes DC offset and corrects the impedance to help eliminate signal reflections.

That brings us to the DC iPurifier. I’ve experienced first hand how much better my DIY server sounds when powered by the Pardo LPS as opposed to a switch-mode power supply, and I’ve contemplated getting a second one for the Regen, but I’ve never quite been able to justify a $380 power supply for the $175 Regen. The DC iPurifier is nearly $300 less expensive than a TeddyPardo supply, and seems perfectly suited for the role. Obviously with DC power there’s no data to reclock or regenerate, so it simply uses the same active noise cancelling system. A 5.5×2.1mm barrel is fitted as standard, and adapters for 5.5×2.5mm and 3.5×1.35mm jacks are included along with a pigtail cable in case there isn’t room on your device to plug the DC iPurifier in directly. The suitable incoming voltage range is 5-24V at up to 3.5A/84W, so just about all DC powered audio devices should work without issue.

I started my testing with just the USB iPurifier2 and “Broken Mirror” from Trees of Eternity’s Hour of the Nightingale. iFi recommends placing the USB iPurifier2 at the very end of the signal chain, and indeed, after some experimentation, that’s where I found it to sound best. This method also allowed me to cut out the USB pigtail cable that I otherwise have to use to connect the Regen to my DAC. With the iPurifier2 in place, the rumbling bass on this track was really brought to the fore, and the vocals from the now sadly departed Aleah Stanbridge seemed to have a much larger presence in the room.

For something a little more light hearted, I moved on to “1985” from Haken’s Affinity, this time with both the USB iPurifier2 and the DC iPurifier in the chain. As I’ve said in earlier reviews, it’s a good sign when you keep forgetting to listen critically because you’re just having so much damn fun listening to music, and that’s exactly what happened with the UpTone Regen, USB iPurifier2, and DC iPurifier all working together. Everything that the Pardo/Regen combo add to the sound of the server, the two iPurifiers added again and more. Moving on to a track I’ve had on near continual repeat lately, “Faithless By Default,” from Dark Tranquillity’s Atoma, the story was much the same as “1985.” As soon as I took the iPurifiers out, I immediately wanted them back in.

Regretfully, I never had the chance to test the USB iPurifier2 with the Sonore microRendu as I only had the two together very briefly, but I am definitely curious how that combo would fare. I can say that my DIY server with the Regen/iPurifier combo was not quite as good as the microRendu, but it was at least fairly close, and the Sonore is among the best sounding digital sources that I’ve ever heard at any price.

That’s A Wrap

To my own question of “how much room might be left for further improvements,” the answer with the USB and DC iPurifier combo is quite a bit. If you’re using a computer or server/streamer with a USB output, you should definitely give the USB iPurifier2 a try. If you already have a product like the Regen, Wyrd, or Recovery, or one of iFi’s own USB power supplies, you should also definitely give the USB iPurifier2 a try. The same goes for the DC iPurifier if you have a suitable device, it’s a great low cost alternative to spending hundreds of dollars on a Hynes or Pardo if those are out of your budget, and it can also be moved to devices with different voltage requirements, which linear supplies cannot. It’s cliche in the professional audio reviewer world to say “I liked it so much I bought the review sample,” but in this case, it’s absolutely true. These aren’t going anywhere.