Listening procedure of the HD25 included connections both through the standard S/PDIF, coaxial input (using the Teac Esoteric P70 as a source) and through the USB port. The device was used primarily as a DAC, with its output (adjusted for zero attenuation) connected to the Melos Plus Series Line preamplifier and for some time as a basic preamplifier driving directly the single ended inputs of the Parasound HCA3500 power amp. The loudspeaker used in all cases was the ATC SCM-50PSL.
As is customary in any case where the device offers some user-defined options, the first part of the listening tests were used to explore the audible differences between the two available digital filters. These differences seemed to be really small and the listener should give his whole attention to identify some specific sonic characteristics relevant to each of them. Personally, I came to the conclusion that the second filter (F2, with less pre-ringing) sounded a little better, being more friendly to the listener's ear, more accurate in focusing individual sound objects and slightly better in describing the feeling of space. Most of the listening was, therefore, realized with this option.
First impressions of HD25 included a feeling of neutrality, good dynamic contrast and some very good potential on description of sound scenes and spaces. One of the first things noticed was the almost "black" background, overlaid by a detailed disclosure of the mix, an impression that could be attributed to the device's low noise. Soundstage was clearly defined in the horizontal axis with good perception of depth. The listener faces a very accurate description of the air between the music instruments either as individual objects (in a small orchestra recording) or as groups (in the case of a larger orchestra). Details associated with the acoustic spaces described in the recording were clearly present and, thus, augmented the feeling of a live music event. These features seem to become even more dominant in high-resolution recordings. In such a case, HD25 has proved able to generate a highly realistic result while at the same time -in some way- provided a relaxing and uncompressed overall sound signature.
Listening to HD25, creates the impression of a device that is never close to its limits, always staying on the neutral and transparent side of things with no significant program embellishment traces.

Low frequency extension was quite impressive, offering good control and detail. HD25 behaved in a way that was quite near the DAC regularly used as a reference (Teac Esoteric D70), being just a little more subdued, a feature which I'm not sure that should be classified as a weakness. Although the DAC perfectly conveys the imposing character of some specific musical projects, it remains, probably, at the austere side, compensating with detail and a tactile sensation in the region. Given its price range, this behavior should be regarded as more than satisfactory. Overall, the reference system with HD25 in place kept its good timing attributes and much of its description potential in the low frequency part of the audio spectrum. The result was impressive, given the price tag, the rhythm section being fast, full and imposing without any traces of exaggeration.
Midrange behavior is probably primarily responsible, for the relaxed nature levied during the listening sessions. The listener has a sense of closeness, but everything remains clear, with detailed description of any nuance and a correct rendering of the size of each sound object. Even at high level listening, HD25 will not exaggerate when describing the total scene and would never be tiresome, as it is that another part of the audio chain (the loudspeakers or the room acoustics, most probably) will surface as the weak link under such conditions. Projects with significant content in human voice were reproduced particularly realistic with very good description of the position and articulation of soloists and, also, a very good description of the dimensions, position and movements of the choir.
The high frequency part of the audio spectrum sounded balanced with no trace of harshness, demonstrating good timing during the attack part, somewhat hasty release times but also was well bodied with good feeling of harmonic content and good feeling of brightness. Here, the selection of the digital filter plays some role in the overall behavior (as already mentioned), with F2 to offer a little darker and, to my opinion, more realistic character and a better sense of air. Definitely it is worth to try both versions, but the truth is that the basic elements of performance in the high frequency region were not changed, as in both cases the converter seems able to offer very good accuracy and a sense of good scaling. Listening high resolution audio files reveal that HD25 has the capacity to cope with the demands posed by higher sampling rates, offering a higher sense of detail and, at the same time, a relaxed, non-intrusive character.

Conclusion

It seems that what we are facing here is a carefully planned and implemented product, with some special attention paid to critical details that have tangible impact on sound behavior. HD25 is not simply a typical DAC in this price region, founded on some standard recipe, but a serious effort to approach some limits in terms of sound quality, with the accuracy and neutrality attributes playing the most important role. The conclusion should be that it is a successful effort. Highly recommended!

Listening Sessions Recording

The following recordings were made with a DV-RA1000 Tascam master recorder (using 24bit/192kHz sample rate) and you can download them to have an impression of what the device under review sounded like. It is obvious that any recording of this kind could not be absolutely transparent but, according to our experience, the majority of sound attributes we listen to, during the actual listening sessions, are preserved. No need to say that you should use these samples cautiously and for informative purposes only. Do not rely on them exclusively to make any buying decisions. The file format is .wav, so expect that the zipped files will be quite large (even if the clips are about a minute or less long. You could use an ABX listening tool like the ABX plug-in for foobar2000 if you want to make some more elaborate experiments. Contact us if you have any questions.

Listening Sessions Recording TOC
File #01	Hegel HD25, 16bit/44.1kHz, USB, F1 Digital filter
File #02	Hegel HD25, 24bit/96kHz, USB, F1 Digital filter
File #03	Hegel HD25, 16bit/44.1kHz, 24bit/96kHz, USB, F2 Digital filter
File #04	Πηγή αναφοράς (dCS Puccini u-Clock/Teac Esoteric D-70

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