Review – A Tale of Three External Digital to Analog Converters (DACs) for your computer from High Resolution Technologies – the HRT iStreamer, Music Streamer II, and Head Streamer

HRT (short for High Resolution Technologies) is a U.S.-based audio engineering company whose minimalist design philosophy is consistent across their product line. For HRT, what matters most is the audio – not flash or bling but a “less is more” aesthetic. I came to view their gear as being very “cut to the chase;” their primary goal is obviously audio quality. I worked with three products – the HRT Music Streamer II, the iStreamer, and the HeadStreamer. The first two devices are designed to work with a desktop stereo system, while the HeadStreamer is a portable unit that fits in your pocket. Simple design, with an input that is either a USB for the Music Streamer II and HeadStreamer, while the iStreamer is designed to work with your iOS device’s dock. The Music Streamer II and HeadStreamer can convert just about any digital file and bit-rate you might use. In USB device mode, they support up to 24-bit @ 96kHz content using Audio Class 1.0 native drivers via an asynchronous data transfer protocol, while the iStreamer is limited to the lower data rates supported by iOS devices. In USB host mode it can support up to 16 bits @ 48 kHz @ 16 bits.

The design is pure minimalist Bauhaus; nothing superfluous. Construction consists of high-quality, gold-plated input and output RCA jacks, tough powder-coated rounded metal cases, and a set of small LEDs that indicate what sampling rate it is working with. That’s it. No buttons, adjustments, switches, options, EQ choices, nothing. Just plug and play. The same goes for their HeadStreamer, a small combination DAC and headphone amp in the same package but with a mini-jack for headphones or earphones. The HeadStreamer is small, light, and portable.

How did they sound? In a word, great. All the units were significant improvements on the audio output of either our Macs’ or PCs’ onboard audio. For the two-desktop system, the iStreamer and Streamer II, it is important to note that those units are but one component of a larger analog audio stage with more variables since they plug into a receiver or pre-amp and amplifier system, and then to loudspeakers. The gist is that because there are so many variables in such a scenario, I tested on different systems with similar source material to get a better sense of the DACs themselves.

I used the word “great” to describe their sound quality, a word that has virtually no measureable objectivity but which does describe the overall experience. While the difference in design, technology, and assumed impact on sound quality can certainly be measured, and the design and components can be precisely defined, that isn’t our goal or approach. Fortunately there are superb sites such as that are blessedly devoted to such metrics and look at the hardware from an engineering point of view. Thus, we feel it appropriate to leave the measuring and engineering conclusions up to them.

We concentrated on the far more subjective task of just listening and trying to gather impressions. We didn’t want to do this in a vacuum so we had fun inviting others to listen and gauge if they could tell a difference. In an age where we aren’t supposed to have developed a critical ear I’m happy to report that most listeners could instantly hear the change. While I wasn’t able to create true blind testing, I did take pains to hide the gear, software, or sampling rate in use.

The HRT line is based upon asynchronous USB, an elegant implementation of the USB protocol that allows for more accurate data transfer, and rare at this price point. I can’t tell if all of that results in better audio quality or not, but certainly HRT’s products all sounded uniformly good. Of course there are some wonderful sounding units from other manufacturers that don’t employ asynchronous USB, so go figure! The general rule is that jitter is bad, and asynchronous USB can reduce jitter. There is a lot more to the overall sound then a specific technology.

The minimalist, no-fuss, no-muss aesthetic in HRT’s design was consistent in the audio footprint. When using iTunes to play Apple-supplied and home-ripped files in MP3, AAC, or ALC formats, one could easily pick out the HRT components. But the real musicality was revealed when we switched to Decibel or Pure Music as our players, and switched to Apple Lossless, FLAC, and AIFF files with no compression. For source music we listened to pristine high-bit audio files from the B&W Society of Sound website, which features Peter Gabriel’s curated choices of music from around the world, most of which was available in three formats (24 bit FLAC, 16 bit FLAC, and 16 bit Apple Lossless).  We also listened to high-bit music from HD Tracks and Chesky Records.

The real rewards came from using contemporary high-bit carefully recorded and mastered tracks that revealed the limits of the technology we can use. As I have written previously, we had a tough time trying to pin any definitive improvement between 24-bit audio over 16-bit tracks from the same source. There were times I thought I heard a difference, but those impressions were far from reliable. In the case of some old low-fi recordings from bands like the Rolling Stones, the HD Tracks versions sounded great, but I’m not sure I could chalk up the difference between the high-bit releases and high-quality mastering from lovely SACD releases when the work was re-mastered and released years ago by Abkco, still the new versions seemed better. However, their recently high bitrate recorded jazz and classical tracks sounded exemplary. No matter the bitrate, the quality and generational differences in mastering are what seem to matter most. All the bits in the world and the fanciest DACs won’t make a difference if the source is third generation and the mastering is substandard.

Paul Simon’s recent release, “So Beautiful or So What,” is available in high-bit 96K mode from HD Tracks, and using Decibel or Pure Music with the HRT units, the experience was warm, detailed, and had a well-defined bass. The HeadStreamer had a similar quality, albeit with a tiny bit of grain in the bottom end when pushed hard. This was a bit more apparent with some headphones but not others, and I charted it up to the limits of amplification with the low current load available in the USB specification. No matter, I found it gave the audio a bit of tooth. That the sound quality was so good is a testament to what you can accomplish with a USB powered device. The USB limits are difficult to design around; driving current hungry headphones becomes an engineering limit of available current. HRT obviously thought through their design and came up with a unit that can drive most any head or earphone you’d want to use.

Overall, the HRTs just stayed out of the way offering more dynamics, cleaner headroom, and an airy nature that produced a rich and complex soundstage. I did find that the Streamer II offered the best sound with a bit more airiness and detail in the bass, but it was so close to the other units that I wasn’t sure if it was conjecture or reality. The HeadStreamer was a bit more visible, as it included the amplifier in the chain, so I’m sure that identity was due in part to the amplifier circuit. While there were times I would have liked a bit more information detailing the unit’s operation, it really wasn’t necessary as long as I knew the sampling rate. That way I could confirm that the unit and software were working as expected. This was handy when I knew that the files were high-bit sources, but the LEDs indicated a lower bitrate. It turned out the problem was the software, and once I quit and re-launched, all was well and the indicators proved their worth.

Listening to the Black Keys’ “El Camino,” the grunge of the guitars, experimentation, and varying styles showed an improvement over running direct from iTunes. While Decibel and Pure Music improved the audio by themselves, it wasn’t until the HRTs were in the signal chain that all came together and the leap to truly believable musical experience began. So fear not, even with your existing collection of supposedly low-resolution normal CD audio, the combination of superior software and the outboard DACs made it clear that you wouldn’t be wasting your time or money.

When comparing the HeadStreamer to the new FiiO E-17 Alpen (Our review of it can be found here) that is a direct competitor, the HeadStreamer seemed a bit warmer in the bottom end, but a tad less airy in the mid and higher end. The sound was so close that the decision to buy one over the other might be most dependent on the feature set and the type of headphones you are driving. HRTs are made in the U.S., a plus for those who prefer to buy domestically produced products. Comparing the track “Big Bad Girl,” by Harry “Big Daddy” Hypolite from HD Tracks at 96hhz @ 24Bit, at the 3:00 minute mark when Big Daddy is straining his voice at volume, the FiiO seemed to handle it without grain, whereas the HeadStreamer picked up a bit of grit. However, when I lowered the volume a bit, the grit went away. Once again with the Audio Technica ATH-W1000X, it might be the limitations of the USB’s current limit. Not even the more efficient headphones that I tried revealed the grit. The Fiio with its own battery source-powered amplifier presumably had more current to work with.

The difference between the iStreamer and most other attempts to produce better sound from an iOS device is that HRT is tapping into the digital output pre-DAC, so that the conversion is handled by the iStreamer, not the phone, tablet, or iPod.  By comparison, most devices merely tap into the line-out analog output and pick up after the DAC has done its job. The analog output, even when coupled with good external amps, can improve the sound quality. It is still subject to increased noise and limited sampling rates, and there is only so much you can do after the fact, sort of like trying to fix a JPEG compared to a RAW file. Throw the bitstream into your own high-quality, audiophile grade device, and suddenly it’s a different ballgame!

If you use your iOS device as your music player, then we feel the HRT iStreamer is a solid choice, featuring far less noise, better musicality, and it’s a bargain at the price than Apple’s own hardware can deliver. Now, if they would only come out with a combination iStreamer & Head Streamer…

If you are using your computer as your music source, you’ll find that the HRT Streamer II allows the most flexibility and the best sound. You have your choice of music player software, from iTunes to more esoteric choices like Decibel, Pure Music, Amarra, or the dozens of solutions out available for Mac and PC. When I disconnected the Streamer II, and I went to back to using the straight output of the MacBook Pro, it became clear how muted the sound became without it.

Because I travel and often work at my desk with my laptop, the HRT HeadStreamer became a favorite and constant companion. Small, unobtrusive, tough, and easy to use, it comes highly recommended. If you have headphones that are difficult to drive, or at a high volume, then you might run into the limits of USB based amplification; if so, there are other approaches. My one wish for the Head Streamer (and other similar portable units) would be the use of a full-size ¼-inch jack, as many higher-end headphones such as the superb Audio Technical ATH-W1000X Limited Edition wood headphones don’t have the option of unscrewing the ¼-inch jack adapter and only have the full size jack, so I ended up being forced to use a little step-down adapter, and it wasn’t the most elegant solution. I doubt it colored the sound, but I suppose it would increase the size of the unit.

We enthusiastically recommend the HRT line of external DACs, from the iStreamer for  iOS devices, the Streamer II for streaming from your computer, and the HeadStreamer for a combination headphone amp/external DAC for great sound on the go. Highly recommended!

Harris Fogel, Posted 10/7/2012

For more information on the HRT Headstreamer visit:

For more information on the HRT Music Streamer II visit:

For more information on the HRT iStreamer visit:

For more information on B&W Society of Sound 

For more information on HD Tracks visit:

For more information on the Audio-Technica ATH-W1000X please visit: