I’ve used an Oppo HA-1 as my DAC, preamp and headphone amp for 4 years. I love its clean neutral sound, driving HD-580 and LCD-2 Headhpones, and my Magnepan 3.6/R speakers. Yet I wonder whether better sound could be had.

Jan Meier built 2 Soul prototypes, each of which resembles a science project but is solid, if not elegant, and electrically and sonically equivalent to the production unit. If you contact him you can arrange to borrow the prototype. Jan has a generous policy of not charging to borrow it (though you have to ship it back to him in Germany), and he has a 14-day return policy for the final product.

It may seem unfair to compare the Oppo with the Soul, as the Soul costs several times as much. But the Oppo is what I have, and I’ve always believed, based on comparison with other headphone amps and DACs, that it punches well above its weight class.

From a distance, the Soul & Oppo sound similar, which is expected for DACs that are well engineered with excellent measurements. Both are neutral and transparent. However, the degree of similarity surprised me. I had to listen carefully to specific recordings that I know well, to hear reliable differences. Even then, the differences were subtle--but real, as I confirmed with blind tests. The differences were easier for me to hear on speakers. I suspect this is because my speakers are more neutral and resolving than my headphones. Audeze LCD-2 are great headphones but not as resolving as Magnepan 3.6/R. Most people, especially those with revealing headphones that are harder to drive (like the HD-800), will hear differences more easily on headphones than on speakers.

To characterize the subjective differences is to overstate them. But to summarize:

• Bass: the Oppo has deep, clean bass yet the Soul in comparison makes the Oppo sound just a touch wooly. With excellent recordings of bass drums and violins, the Soul reveals more of the percussive attack/decay and complex timbre.
• Midrange: the Oppo is neutral and clear, yet the Soul in comparison makes the Oppo sound just a touch veiled. Excellent recordings of piano and acoustic instrument ensembles take on greater clarity and slightly more pure voicing.
• Treble: the Oppo is extended and linear, yet the Soul is just a bit smoother and more natural, making the Oppo in comparison sound like it has a hint of grain.

I'm not a big fan of DSP, which is too often used as a gimmick. But I have come to use the Soul's DSP features under certain conditions.

• Tone controls: anyone with a big enough music library inevitably has poor quality recordings. The problem is often unbalanced frequency response, in which case the Soul's tone controls can improve things. Implemented in DSP, I find them more transparent than conventional analog tone controls.
• Headphone crossfeed: the Soul has Meier's crossfeed, which narrows L-R separation and is useful when listening on headphones to recordings having artificial hard L-R separation. It has 5 levels from subtle to obvious.
• Speaker crossfeed: the Soul has a separate function to widen L-R separation for speakers, with adjustable levels to tune for different speaker distances & angles.
• Headphone notch filter: many headphones have a narrow response peak. The Soul has an adjustable notch filter to tame these headphones. The filter is always -6 dB, Q=2, and the knob adjusts the frequency between 6 and 11 kHz. This is particularly effective with the HD-800.
• Digital filter: the Soul has a switch to select the standard "sharp" (linear phase) versus "slow" (minimum phase) digital filter.
• Redbook emphasis: the Soul implements this and provides a manual override. This is more of a historical curiosity since this obscure feature of the CD Redbook was so rarely used. But if you have old CDs from the 80s or 90s that used it, it's nice to have.

The Soul has excellent build quality, among the top tier in consumer & professional audio. It also has excellent support from Jan Meier, who stands behind his products and is willing to customize some aspects to suit individuals.

The Soul has some unusual engineering features that contribute to its excellent sound quality:

• Volume control is a 64-step switched attenuator in the analog gain-feedback loop. When you turn down the volume it reduces the gain rather than attenuating a fixed gain ratio. This reduces noise and distortion, especially at the low to medium volumes we actually use when listening.
• Switched power supplies - providing dead flat quiet stable DC to all internal components (digital and analog) without even a hint of 50/60 Hz ripple.
• Frequency shaped feedback (FF), which "unloads" both digital and analog gain stages for cleaner more transparent sound.
• The Soul uses dual DAC chips (Wolfson WM8741), one per channel (L and R), each in mono mode, in which this chip has slightly better performance.

The net effect of these engineering features makes the Soul the quietest (lowest noise) DAC or preamp I have heard or measured, especially at the low to medium volumes we actually use for listening. That lower noise contributes to absolute black musical background at all volume levels, accentuating low level subtle detail and transparency.

Now the Soul has some limitations:

• Balanced only: no unbalanced inputs or outputs.
• PCM only: no DSD. And sampling rates are limited to 32, 44, 48, 88.2, 96 and 192 kHz. No 176.4 or frequencies above 192k.
• Analog output is max 8 Vrms for a full scale 0 dB digital signal. It has a gain switch, but it's digital so using high gain (+6 dB) risks digital clipping. It would be nice to have an analog high/low gain switch like Meier has in his other amps. This could match the output to a wider range of headphones, or to boost analog gain if you have DSP in front of the Soul attenuating the digital signal (for example, digital parametric EQ).
• Appearance: the Soul looks and feels something like military signal processing equipment. Its build and part quality is impeccable, but no fancy displays, swanky enclosures or knobs. I enjoy this "black box" look and feel, but others might not, especially at this price point.

The Soul is the most transparent DAC and preamp that I have heard. It gets me closer to the music, making listening sessions more engaging. As an engineer, I like the non-nonsense industrial build quality that says "tools, not toys". I don't think more transparent, higher fidelity is possible at any price. For sound quality alone I would give it 5 stars (6, if I could!). But to be a realist I have to take it down 1/2 star due some of its limitations, which mean it won't be for everyone, and at its high price the field is wide open with so many other options to explore.

I've used the JDS Element with my Audeze LCD-2 and Sennheiser HD-580 headphones for a few months now. For comparison, I also have an Oppo HA-1, and have had several other headphone amps over the years (Corda Jazz, Headroom Maxed Out Home, Wheatfield HA-2, among others).
First and most important: the Element has excellent sound quality. If you didn't know it only cost about $300, you might call it reference quality. A side-by-side comparison with the HA-1 and Corda Jazz reveal the Element is not quite as refined, lacking their richness in the bass and sweetness in the mids and treble. However, this difference is subtle; the Element is a great sounding amp. Everything is there and well presented, even if that presentation is just a tad less engaging in comparison.
The volume knob is an analog pot, but it's among the best I've used. Wide range, smooth, linear response, perfect L-R balance at all levels.
It has high and low gain settings, which combined with the wide range volume knob and max power output > 1 watt enables it to drive almost any headphone on planet Earth (except for electrostats, which always require a stepup transformer).
It can function as a pure analog headphone amp (unbalanced RCA inputs). Or you can use it as a DAC+amp with the USB input from any computer (Windows, Mac or Linux).
The build quality is good, but less than the thunking solid build of megabuck gear. That's how you get mebabuck sound quality on a budget, and it's an entirely reasonable tradeoff.
It's not a preamp, has no switchability. Strictly one input, one output. Simple, but not very flexible.
I saved $50 getting a b-stock unit which apparently is not cosmetically perfect, but still it looks great. I can't find whatever cosmetic blemish caused them to drop the price.
I've reviewed it in more detail here: http://mclements.net/blogWP/index.php/2017/03/18/review-jds-labs-element/

Based on solid reviews everywhere, I bought an HA-1 and have been super happy with it.
 
I use the unbalanced 1/4" headphone jack to drive Sennheiser HD-580s. I use the balanced headphone jack to drive Audeze LCD-2F. I use the balanced line stage to drive an Adcom 5800 to Magnepan 3.6/R speakers. I use the unbalanced line stage to drive my Tascam digital recorder.
 
The headphone balanced output runs in pure class A, beefy powerful, with adjustable gain so it can drive almost any headphone. It gets quite warm. The headphone unbalanced output sounds equally clean, though 6 dB quieter (half the voltage, 1/4 the power). The line stage is as impressive as the headphone output. The HA-1 is neutral (neither bright nor warm) and smooth yet detailed. No euphonics, just incredibly detailed yet natural sound for those who want to hear exactly what's in the source without any editorializing.
 
I would have liked to have had a headphone crossfeed circuit like the ones that Meier amps have, but given all the HA-1's other strengths, I can live without that.
 
I bought a refurbished unit directly from Oppo. Saved $250 on the price, it looks and runs like new, with same warranty.
 
Details http://mclements.net/blogWP/index.php/2015/09/06/review-oppo-ha-1/

The overall sound is similar to the Audeze LCD-2F - slightly on the warm side of neutral, but without the ultimate resolution the LCD-2F has. And easier to drive so you can plug them into your phone. The bass is all there but it's not bloated or enhanced like it is with so many IEMs. The treble is all there but at natural levels so they don't sound bright. The mids are just slightly recessed but still sound natural with classical and acoustic music. And the entire frequency range is integrated smoothly with no apparent peaks or troughs. That's quite a lot considering the price is around $100. Can't find much if anything at twice the price that has sound as good as this. They don't have reference level of detail, but enough for serious listening, yet still enjoyable and not fatiguing for long listening sessions.
I went through several different IEMs before I finally found these. Etymotics had crystal clear mids but no "air" with a rolled off top end. The other IEMs had muddy bloated bass. The GR07 classics were just right.
I can listen to these all day - and sometimes do.

I agree with pretty much everything the other reviews said, so I'll add how this compares to my Headroom Maxed Out Home (MOH).
 
First, a little on the MOH I'm comparing with the Jazz. The MOH is a great amp: toroidal power supply, OPA627s biased class A, quiet background (high S/N ratio), linear, neutral sound. Power limited to about 250 mW and 250 mA which is plenty to drive just about any headphone short of a a HiFiMan HE-6. The MOH sound is smoother and darker than most solid state amps, yet despite being slightly dark it has incredible detail; it is not veiled. I used to be a tube guy (Wheatfield HA-2 driving HD-600s, a match made in heaven) and the MOH is the amp that converted me to solid state. In its day, the MOH cost a kilobuck or more depending on options and was on the Stereophile class A list.
 
The Corda Jazz is very similar to the MOH; both are excellent amps and the differences are subtle. The Jazz sounds more neutral: not as dark, but not bright either. It's hard to describe, but "sweeter" seems to capture it. Sweet as in the sound of the strings in the 2nd movement of Rachmaninoff's 2nd played by Earl Wild on the 1960s RCA Victor LP. The Jazz is not as sickly sweet as that, the Jazz is too neutral for such euphonics but compared to the MOH its voicing hints in that direction just not nearly as far. Both are very neutral amps overall, but the Jazz leans sweet where the MOH leans warm.
 
Compared to the Jazz, the MOH bass is a just a touch emphasized and wooly. Put differently, the Jazz bass extends just as deep but seems a touch more crisp. The MOH bass adds to its overall warmth and this is almost surely a contributing factor to the Jazz sounding less warm. It's almost impossible to describe this difference without exaggerating it; it's a subtle difference and I can't say I have preference; it's like two equally amazing yet slightly different presentations of reality. Both sound great and which sounds better depends on the recording. My LCD-2 (2014 Fazor model) headphones have deep linear bass extension and don't need the exaggerated bass so the Jazz pairs nicely.
 
In the extreme high end (15 kHz +) , the MOH rolls off just a hint while the Jazz seems to maintain perfect ruler flat response. These frequencies are well above what would constitute "brightness", contributing more to a sense of space than of sound, they aren't in many recordings and even when they are they're hard to hear. I can't be completely sure this is exactly what I perceived though there was definitely a subtle difference and I believe this is what it is.
 
Both amps have a headphone spatial processor, though the Jazz's is clearly better - more refined and subtle. The MOH dates from the late 90s and had one of the first spatial processors. It does work - improves imaging and reduces listening fatigue - though it bloats the bass (due to mono summing) and attenuates the upper mids / lower treble (due to comb filter effects). The MOH has a "filter" switch to give a 2 db boost around 1-4 kHz to counter the comb filter effects. I never used it for serious music enjoyment, only for casual listening and movies. The Jazz processor is a whole 'nuther thing. It does everything the MOH does (though it does it a little less, and here less is more), but with neither of the MOH's drawbacks and none of its own. It doesn't change the overall tonality or clarity of the music; it only opens up the image and reduces listening fatigue. Its effects are more subtle than the MOH, apparent mainly on natural recordings of acoustic music, which makes sense give what it is, and that's most of what I listen to.
 
Regarding how they work: The Headroom processor is simple: it takes some of the L, mixes into the R with level reduced and phase shifted. The Jazz processor is more sophisticated and selective. In Jan Meier's own words: http://www.meier-audio.homepage.t-online.de/crossfeed.htm
 
The Jazz has an adjustable gain switch that changes the level about 16 dB. I believe it swaps the metal film resistors that set gain across the op amps (instead of using fixed gain and attenuating the signal before or after the op amps), so it should be sonically transparent in either position. Low gain might measure cleaner (lower noise, wider bandwidth) but the differences are probably so small as to be inaudible and negligible. Any differences I thought I heard between low and high gain mode disappeared in level matched comparisons. The difference makes about a 1/4 turn of the knob. This enables it to drive pretty much any headphone under the sun. My LCD 2 headphones work well in both high and low gain modes, and after measuring the amp and speaking with Jan Meier, I use the high gain mode. Note: my LCD 2s are about 3 dB more efficient than Sennheiser HD-600s. The high gain mode is intended for normal use with full size headphones; low gain for earbuds, IEMs and high efficiency stuff made to be driven by phones.
 
The Jazz has a very nice volume knob. The way Jan describes it, the potentiometer is not in the signal path, but it triggers relays to set the gain like a ladder stepped attenuator. This should be sonically transparent with perfect L-R channel balance at every setting. It's a very nice touch and simply unheard of in an amp at this price.
 
The Jazz output is neither balanced nor unbalanced, but something Meier calls Active Balanced Ground Driving. It is compatible with unbalanced headphones, providing better noise rejection than standard unbalanced outputs. I wish more engineers would show this kind of simple, creative approach based on sound engineering. But it is not compatible with unbalanced analog audio inputs, so don't connect the headphone output of the Jazz to a preamp or similar analog input. More detail in Meier's own words: http://www.meier-audio.homepage.t-online.de/grounds.htm
 
The background noise on the Jazz is very low, but not quite as low as the MOH. On low gain at max volume it's dead silent. The loudest my ears can tolerate is around 3:00 and normal listening is around 12:00 so it's dead silent at actual listening levels. But on high gain at max volume there is an audible low level buzz/hum. The MOH gain is similar to the Jazz in high gain mode, and the MOH also has a low level buzz/hum at max volume but it's about 3 dB quieter. Either way, both amps are dead silent even at loud listening levels.
 
To summarize, both amps take you close to the sound of the live mic feed but in slightly different ways. The MOH gives a slightly warmer, darker, smoother presentation. The Jazz gives a more neutral presentation with a touch of midrange sweetness. Ultimately I think the Jazz is closer to the live mic feed, but the MOH is a slightly more "musical" interpretation of it. No "better", no "preferrered", fully equal just different.
 
Even if I ignore the modest $415 USD price, the Jazz is as good an amp as I've ever heard. The MOH cost nearly 3 times that 15 years ago and would cost even more if produced today. Given the price, it's the biggest pleasant surprise I've seen in audio for a long time and the best value I've ever seen.
 
Addendum: I recently measured this amp, results here. It has excellent measurements to match its excellent sound quality.
http://mclements.net/Mike/mrc-blog/blog-140615.html

Edited to reflect observations after extended listening.
I just installed this sound card and was pleasantly surprised at the sound quality. The reason I bought this card is because its unbalanced analog outputs were praised by The Absolute Sound and that is how I use the card. Their praise was well deserved. It normally sells around $200, I picked up one lightly used (still in new box) for $150.
 
My computer is a Dell with i7 4770 processor, 8 GB RAM, SSD, running 64 bit Ubuntu (my primary OS) and Windows 7 (which came with the Dell).
The Juli@ worked perfectly right out of the box in Linux. It was recognized by the ALSA drivers.
Installing on Windows 7 was a nightmare - multiple reboots, duplicate drivers appearing, etc. Reminded me why I stopped using Windows a few years ago.
 
Source sounds were CDs in the computer player and WAV files ripped from CD to filesystem. Full quality with no compression or equalization applied.
Software I use for listening is VLC player with no eq or other processing.
I ran the output of the Juli@ to my Headroom Maxed out Home amp and listened on Audeze LCD-2 and Sennheiser HD-580 headphones.
Musical sources were mostly acoustic music recorded in natural spaces: chamber music, ancient/medieval, classical, both small and large ensemble.
I also listened to some jazz and hard core rock, for example Swans The Seer.
 
The sound quality is surprisingly good - even excellent. I didn't know it was possible to get sound this good from a computer. It's detailed but not bright or harsh. It has deep bass with grip and control but not bloated. It resolves complex music smoothly without any veil. Even nasty harsh sounding industrial heavy metal like Swans was resolved well - some sources make it sound like a shrieking ear bleeding cacophany but the Juli@ captured the sound with all the harshness the recording engineers intended, yet adding none of its own. And it did equally well at the opposite end of the musical spectrum that I usually listen to: the resonance and purity of Cecelia Bartoli, the grittiness of Gillian Welch, the sweetness of Les Violons Du Roy. It has neutral tonal balance and realistic voicing on natural acoustic instruments. As I listen to mostly acoustic music recorded in natural spaces, midrange smoothness, clarity and natural voicing is critical for me and the Juli@ delivers.
 
One minor shortcoming in the Juli@'s sound: when turned to max volume, it does not quite have dead silence; there is a bit of low level buzz which is most likely due to insufficient regulation of the computer's relatively noisy DC power. But it is very low level and does not impair listening because to hear it you have to turn the volume up to levels that would make your head explode if the music kicked in. I haven't measured its level but based on experience measuring these kinds of things before it's about -90 dB.
 
The Juli@ has a high output level from the analog unbalanced jacks. I used the standard Linux ALSA mixer to drop the level about 6 dB to get a reasonable volume knob position on my MOH amp, which has relatively high gain. I don't listen to music at 100 decibels.
 
The Juli@ sounds as good as any CD or DVD player I've heard at $1,000 and under and better than some above that price. But it's not surprising since the most expensive parts of any audio component are the box (aluminum panels, displays, knobs) and the power supply. A sound card needs none of that (though it does need power regulation). A $1,000 audio component probably has less than $150 of parts on its circuit board. The rest of the price is in power supply, box, and moving parts that a sound card doesn't need.
 
The Juli@ has a unique internal reversible card so you can get unbalanced analog (in and out), or balanced TRS 1/4" jacks (in and out). It also has digital outputs, coax (in and out) and optical (out only). I used only the analog unbalanced outputs. With an output impedance of 100 ohms, it's designed to drive an amp, not a set of headphones directly.
 
Overall a great purchase, one of few high end sound cards having excellent quality analog output rivaling the best audio components. I gave it a slight ding for the noise floor - even though it's too low level to impact listening. More important than the noise floor is the sound quality, which is extended, natural and clean, good enough to be a primary source. For the price, it's a tremendous value.