IFI iPOWER audio power supply
iFi iPower audio power supply When a previous review yielded a quirk or two, Stephen Dawson decided to dig deeper into the causes, discovering the differences a better power supply can deliver.
Stephen Dawson digs deep into the differences a better power uspply can deliver.
Several issues back we carried a review of the Questyle QP2R high resolution portable audio player (you can read it at avhub. com.au/QP2R). Therein we mentioned the availability of the Questyle HB2 Hub. You can pop the QP2R player into this (as pictured from rear and front below) and charge it up. It also takes the analogue output from the player’s USB Type-C connection and makes it available via proper RCA stereo sockets (there’s also an optical output). I did run some quick measurements on the Hub, and things like frequency response were essentially identical to that of the player, the output was about a decibel higher, but the noise performance was significantly worse. Significantly, I say, not necessarily audibly. But if you can measure it, at least potentially you may be able to hear it. So what were these differences? Throughout I use 96kHz sampled, 24-bit test signals. (I also re-measured for this article, so there may be a tenth of a dB or so difference from the previous article here or there.) A-weighted, the noise level straight from the player’s headphone output was -107dB. From the Hub, -102.2dB. You will note that’s still way quieter than the 16-bit standard of around -96dB. But when I examined the output spectrum, things became interesting. Below about 500Hz, the noise floor was the same. Above 800Hz the noise floor via the hub was about 4dB worse. More importantly, there was a 40dB spike in noise at 50Hz. Around 38dB at 150Hz. And more major spikes for every odd harmonic of 50Hz, plus smaller spikes for some of the even harmonics (see Trace 1, green line). All that is clearly mains noise, and harmonic distortion products of mains noise, delivered by the USB power supply to the Hub. Of course, I’d like to think that the Hub wouldn’t let any of that through into the analogue output, but clearly it did.
The purist in me says: no, I don’t want that! But I didn’t publish, since the 5V USB-style power pack that was provided was not an official one. You don’t get one with the HB2 at all. Questyle envisages that it will be powered from a computer. (Though there are potential
noises that way too.) Which got me to thinking, surely one can do better with a better USB power supply? A bit of Googling soon revealed that Addicted to Audio carries a line of products from iFi (you can see one iFi product has won an EISA award elsewhere in this issue), and among the range is an “iPower audio power supply”. This is available in a range of outputs, but we asked for the 5 volt, 2.5 amp model.
iFi iPower
iFi says that the iFi iPower supply uses Active Noise Cancellation to reduce noise from its output, making it suitable for devices ‘that require a super-silent’ power supply. The unit comes in the form of a wall wart (pictured left), although not an excessively bulky one, with a two-metre cable attached. There are slide-on prongs for the four different kinds of power points: Europe, US, UK and Australia. At the end is a standard cylindrical power plug, positive for the centre connection. In the box are adaptors for four different sizes of power plugs, plus an adaptor for Micro-B USB, and one for standard USB Type A connections. There’s also an adaptor to reverse polarity for unusual equipment. I needed only the USB Type A adaptor.
I’d note that 2.5 amps is plenty of capacity for any device designed to operate from USB power supplies, including iPads.
iFi vs generic adaptor
Compared to the original adaptor, testing the QP2R in the Hub HB2 powered by the iFi iPower, the noise was significantly lower. Oddly, A-weighted, there was only 0.1dB in it (-102.3dB vs -102.2dB). But the results were far more impressive when looking at the spectrum. The noise floor was the same for both, but the 50Hz spike was only about half the height with the iPower, and all the odd-order harmonics of 50Hz were much, much lower (the third, for example, was only barely above the noise floor with the iPower, and around 38dB above it with the generic USB). Offsetting that a little were higher even order harmonics of 50Hz. But the worst of those was still less than the 3rd, 5th, 7th, 9th and even 11th-order harmonic produced by the other power supply. There was also a little bump in noise around 1300Hz for which I have no ready explanation. (The spikes up above 30kHz are, I think, produced by my ADC.)
At that point, a memory came back to me. A couple of years ago I reviewed the Google Chromecast Audio device, and noticed markedly better noise performance when the device was powered by the included power supply rather than the USB socket on an audio device. So I thought I might as well measure with that as well.
And, blow me down, the Google supply measured significantly better than either of the other power supplies: -103.5dB. The noise floor was the same with the Google power supply as with the iPower, but the 50Hz was almost entirely absent, and there were basically no harmonics breaking through (although a weird bit of extra noise around 440Hz). Note, the Google power supply is only rated at one amp output, so it may not be suitable for some devices.
How about a battery?
Then I thought: what about a power bank? This is inherently DC, not converted AC. No mains, so surely that would give great performance. So I grabbed a heavy 4000mAh Belkin power bank and plugged it in. I could see a low level of noise showing at the bottom of the recording meters, which is never good news. The final result was a very mediocre -88dB, and as the spectrum shows, it’s all due to a high noise floor, around 30dB higher for the bass and lower midrange, and gradually reducing the less than 10dB higher above about 6kHz. But, hey, at least there are no mains frequency spikes!
Conclusion
We are talking about tiny, tiny noise levels here, even with the battery. iFi says that the ‘average audio band noise floor’ of the iPower is around one microvolt, compared to a thousand microvolts for a ‘generic power supply’. I measured a 50Hz noise peak of 17dB lower than with one generic power supply. But then the Google Chromecast Audio power supply was a further 10dB quieter at that frequency, a dozen decibels quieter at 100Hz, and so on.
All of which means... very little certainty. I will recommend the iFi iPower, even though it costs more than a whole Google Chromecast Audio, including its power supply, because it has a higher output capacity and is a known quantity. Will a new Google Chromecast Audio power supply provide the same low noise today as my two-year-old one does? Will it even come from the same factory? Probably, but without specifications, who’s to know? How about Apple? I have a bunch of iPhone and iPad power supplies, but they are all several years old, so any results would be unreliable.
And what about the responsibility of the audio device? Should not they better isolate their analogue outputs from whatever noise the power supply is introducing? Or if they can’t, should not they be sold with a high quality power supply to avoid the problem? (Or, here’s a horrible thought, perhaps they are doing a great job of isolating the output, but the noise is so enormous that some of it’s getting through anyway.)
At this point, I’m thinking this is something of a messy result, and that our most sensible recommendation would be to suggest that if you’re using your portable device as a high quality source, you just charge up the battery to 100% then disconnect the power and listen for as long as the battery will last.