The unit arrived today!
First off, I really love the look and feel of the amp. Clean lines on cool sturdy aluminum... Really, the unit screams quality. I like it a lot. I'll try to take some nice pictures when I have some time.
Well I've spent the past two hours or so just comparing its headphone out, its analog out, and the analog out of my guzzler USB DAC. The amp used was a maxed-out PIMETA with 4 buffers on each channel and OPA637/627 opamps powered by an Elpac WM080 power supply.
Interconnects used were identical pairs of 1m MagWires AllClears (twisted pair of 26ga teflon-insulated silver plated solid copper wires, terminated with Canare RCA plugs).
Headphones used were Grado SR-225s and AKG K501s. I have a pair of ER-4S as well but didn't use it because I couldn't be bothered to take it out of my drawer and stick it into my ear canals. Sorry.
One thing that surprised me was the fact that the output volume pot controls both the headphone and all analog outs. There apparently is no way to bypass it, but the pot seems to have excellent tracking from extreme to extreme (tested with my headphones) and this means that the unit can serve as a preamp connected directly to a power amp.
Anyhow... onto the sound...
UA-25 Line Out vs. guzzler USB DAC (gUD) Line Out
The first thing that stands out is that UA-25 definately has more pronounced upper end compared to the gUD, but at the cost of sounding relatively shrill. At times the harsher upper end can be a bit fatiguing. The gUD is much more smooth and not fatiguing at all, with a beautiful midrange, but in comparison to the UA-25 it can sound a bit recessed and less airy in some songs. While at times a bit metallic sounding, the UA-25 seems to have a bit better soundstaging due to the more aggressive and pronounced upper end. Unfortunately, I didn't spend too much time in this comparison.
UA-25 Line Out vs. UA-25 Headphone Out
The biggest surprise here was that the headphone out actually sounded very very good, with extended, deep bass and lots of high end energy. Like the line-out, the midrange is perceived to be a bit recessed due to the pronounced lower and upper ranges; for example, on some songs the line out to amp setup, my focus would be on the vocals and the texture of the midrange, but once I plugged my phones directly into the UA-25 I started focusing on all the new detail presented to me. Then, when I switched over again, I found that the line out to amp setup also provided the same level of detail but just more subtly. Right now I'm trying to make myself get used to the headphone out section of the UA-25 so that I can do a fairer comparison later on, and the extra detail is turning out to be very pleasing. Perhaps it's burning in and the harsher edges are smoothing out a bit. But still, sometimes I miss the smoother midrange and less emphasized upper end of the properly amped setup.
On the K501s, it was hard to gauge the bass quality on the headphone out due to the recessed bass response, but when I switched over to the SR-225s with reversed bowl pads, the differences were clear: surprise once again, the UA-25's headphone out had more, deeper bass than its line-out amped by a maxed-out PIMETA. I didn't take too much time to evaluate the bass as I wanted to go back to the K501s, but the UA-25 headphone out may have been a tad loose compared to the properly amped bass.
What's so surprising here is that the entire unit is drawing all its power from the USB bus! The power draw is rated at 480mA, the unit must be ultra efficient to power all the D/A and A/D and have juice left over to power headphones so well. And, although it's powered by USB, the unit is dead quiet. Very little audible difference compared to the battery-powered gUD. Anyway, regarding the amp section, perhaps it's a new or proprietary digital design that has drastically improved efficiency. This is IMO miles better than the headphone out implementations of the OptoPlay. One thing is that the headphone out gain seems to be rather low; for some low volume classical recordings, I found myself maxing out the volume knob to get satisfactorily loud volume. For most recordings, I would be about halfway to 3/4. Oh, another observation is that the line out of the UA-25 is a good bit louder than the gUD's line out.
Anyway, I think these kind of sum up my initial impressions, overall I'm very very impressed with the unit. I'm going back to listening to my K501s directly out of the unit!
I'll post more impressions later, hopefully when the unit has burned in further.
