Monthly Archives: December 2018

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (8 of 8)

This is part 8 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1. Click here for the introduction.


Subjective Listening Impressions: Soul

  • They sound similar which is expected for DACs/preamps that are well engineered with excellent specs.  Both are very neutral, transparent DACs. If you’re looking for euphonics, look elsewhere!
  • However, the degree of similarity surprised me. I had to listen extremely carefully to specific recordings that I know well, to hear reliable differences. And even then, the differences were subtle.
  • 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.
  • Speakers more resolving than headphones are rare, so 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 differences is to overstate them. But here they are:
    • Oppo: Earthy, Organic, Airy
    • Soul: Pure, Taught, Resolving
  • Detailed summary of audible differences:
    • HF: Oppo has a touch more air; Soul has equal extension but less air. The first impression is slightly less HF from the Soul, but on deeper listen it is all there, yet less subjectively emphasized.
      • Ultimately, “all there but less emphasized” seems truer to live acoustic music, though different from what we normally perceive as “HiFi”.
      • Is “air” a barely perceptible hiss or noise that accentuates detail through stochastic resonance? If so, it’s a double-edged sword.
      • NOTE: “air” in the recording itself, like hearing the space in a good cathedral recording, is all there with both Soul & Oppo.
    • Treble: the Soul treble is smoother, making the Oppo sound slightly grainy in comparison. Though I would not say Oppo has grainy treble. The Soul’s treble response is unique in its naturalness.
      • Also: they balance the fundamental against harmonics slightly differently; Oppo emphasizes harmonics, Soul emphasizes fundamental. Each is a only a subtle variation of difference, both have uncolored voicing, and which sounds most natural depends on the recording.
    • Mids: Oppo is earthy or slightly “dirty”, with a hint more presence that adds a sense of extra detail in some recordings, slightly veiling in others. Soul sounds more transparent and pure, normally a good thing, though with some recordings sounding “sterile”.
      • The Soul has slightly greater midrange clarity.  It never revealed a musical detail the Oppo completely obscured, but it occasionally surprised me, revealing details I had never noticed with the Oppo, though after hearing it on the Soul I was able to hear it on the Oppo.
    • Bass: Oppo has more power in the bottom octave (< 30 Hz). Soul is more controlled with better defined bass timbre and slightly more mid-bass energy.
    • Transient response: Oppo has a bit more snap which sounds faster, but it also has a bit more ring / longer decay. Soul is cleaner, which can sound a bit “dead” at first but on deeper listen it doesn’t seem slower or smeared.
      • To avoid confusion, I didn’t try the Soul’s alternative minimum phase AA filter (though I’ve tried these before on other devices; the difference is subtle, but I usually prefer the linear phase “sharp” filter).
      • I did measure the effect of the Soul’s alternative AA filter. Comparing square waves, it eliminates pre-ripple, at the expense of rippling longer & louder after the impulse.
    • Dynamics: Soul is punchier with bigger macro-dynamics. Both have excellent micro-dynamics, though the Soul sounds darker between plucks/smacks, which hints at faster decay, lower noise or distortion.
    • It took time listening to a variety of music to establish a preference.
      • Sometimes the Oppo’s earthy airiness added realism and refinement. Other times, it slightly veiled what the Soul made more clear.
      • Sometimes the Soul’s tonal purity made the Oppo sound veiled in comparison. Other times, this purity sounded sterile where the Oppo sounded organic.
    • With more listening across a wide variety of music I came to find the Soul more transparent and true to the source. What clinched it was piano and voice, which highlight the Soul’s clean, pure midrange.

Engineering: Soul

  • The Soul has several engineering features that differentiate it from other high quality solid state DACs:
    • Volume control: It changes metal film resistors in the gain-feedback loop, rather than attenuating a fixed gain, so there is no potentiometer in the signal path.
      • Advantage: lower noise and perfect channel balance at all volume settings, no loss of SNR at low to medium volume settings.
      • Sometimes with a stepped attenuator the perfect volume you want is between clicks. But this never happened with the Soul; it averages about 0.5 dB per click which is fine enough to set the perfect level.
    • Meier “FF”: The Soul’s digital and analog stages are frequency-shaped to reduce distortion and noise in the midrange and treble where the ear is most sensitive.
      • Jan calls this feature “FF” and describes it here.
      • That article is long and can be hard to understand. My simple take on it is here.
    • Power supply: The Soul has 4 switched power supplies with about 70 mF (a lot!) of filter capacitance: 1 for the digital section, 1 for the USB section, and 1 each for the positive and the negative supply lines of the analog stage. This provides near perfect DC with incredibly low noise and not even a hint of 50/60 Hz ripple.
    • DAC implementation: The Soul uses the Wolfson WM8741 DAC in mono mode (where it has a slightly higher SNR), one per channel (L and R). This chip’s analog output pins are balanced, which the Soul maintains all the way to its analog outputs. It also operates the DAC chip in maximum oversampling mode and enables the user to select which digital filter to use (sharp vs. slow).
      • Note: the Oppo uses the ESS9018 which has the ESS Hump, an anomaly that increases distortion at the low to medium levels used by most music.
    • I believe the above engineering features make the Soul sound subtly different from other top quality solid state DACs, and are the primary contributing factors behind my subjective listening observations.
  • These features give the Soul a higher level of attention to engineering detail. From an engineering perspective, it’s the right thing to do if you want the best sound at any cost. As an engineer myself I believe in these kinds of features.
  • Yet a music lover asks: does this get me closer to the music leading to greater appreciation and enjoyment? Possibly… yet in general not necessarily. With the Soul, I think it does.
  • For example:
    • Years ago I built a stepped attenuator to replace my preamp. It sounded better than any active preamp I had heard. It revealed subtle musical details that even this very fine preamp (Rotel RC-990BX) veiled.
    • I enjoyed it for over 10 years until I replaced it with a dedicated DAC (the Oppo), which incrementally increased transparency.
    • Back then, the difference between my preamp and the attenuator were of a similar nature to what I heard from the Oppo to the Soul: incrementally improved purity and clarity.
  • At this level of engineering and quality the equipment measures as perfect as engineering can make it. Reliably hear-able sonic differences may (or may not!) exist, but if they do, they are subtle and which is “best” is subjective.

Functionality: Tie (different trade-offs)

  • The Soul has more DSP features: adjustable filters, EQ, channel mixing, etc.
    • I already have a digital parametric EQ (DEQ2496) supporting any number of bands. With this I can fine-tune the output more precisely than the Soul’s tone controls.
    • However, that fine-tuning comes at the cost of complexity: I spent hours carefully crafting each set of EQ with measurements and listening, then saved it as a named setting.
    • If I’m listening to the occasional music that is imperfectly mastered, the DEQ2496 is too cumbersome to EQ it on the spot.
    • The Soul’s controls are much simpler: 4 tone controls equally spaced at 2-octave intervals, digitally implemented.
      • Note: the tone control spacing is not equally spaced from the factory, but Jan Meier customized them for me with corner frequencies at 80, 320, 1250, and 5000.
    • No recording is perfect and I normally listen to how it naturally sounds, however imperfect. Yet some are more than imperfect, flawed to the point of distracting from the music.
    • Here, I use the Soul’s controls to apply a mild correction to get past the imperfections and closer to the music.
    • This also applies with headphone listening. The Soul’s cross-feed gives a nice correction to music sources that have artificial hard L-R stereo separation.
  • The Oppo has more types of inputs and outputs, both digital and analog.
    • The Oppo has Bluetooth and handles a wider range of digital formats (DSD, and additional PCM sampling frequencies).
    • The Soul doesn’t have unbalanced inputs or outputs, so you’ll need an unbalanced → balanced converter for unbalanced RCA audio sources.
      • In my case that’s OK because none of my unbalanced sources are reference quality (game box, computer).
    • With the Soul you’ll need balanced cables for your headphones and if you use its line-outs you’ll need XLR cables for your power amp.
  • USB
    • My Android phone or tablet never worked with the Oppo’s USB input (it was made for Apple devices).
    • But, they do work with the Soul, and apps like USB Player Pro stream the bits without modification, so a mobile device becomes a fully transparent audio source.

Build Quality, Durability, Support: Soul

  • Both have great build quality.
  • Both get warm during use, but the Oppo much warmer than the Soul–possible longevity disadvantage? The Oppo volume control has a reputation for failing.
  • Support: Meier sets an example for the trade with his engineering expertise and enthusiasm for music and engineering. He is responsive and direct with questions and feedback. I’ve never seen better support.
  • The Oppo is built better than most consumer gear, both internal (big toroidal power supply, high quality opamps, etc.) and external (case, knobs, etc.).
  • But the Soul has the edge here as it levels up to professional hand-selected parts and is built by Lake People in Germany.
  • I’ve owned Meier’s Corda Jazz for several years of daily use. It shows no signs of wear; the switches, knobs, case, etc. all like new. It’s at least as solidly built as the Oppo, and the Soul is a step up from there.
  • Ten years from now, which is more likely to still be running like new? Probably both, but if I had to pick one or the other, no question it’s the Soul.

This has been a fun and educational week, though my ears and brain will take time to recover from all the critical listening. Good consumer gear has gotten very good indeed, raising the bar. From objective measurements alone, it can be indistinguishable from the best of the best. Yet even someone with an “engineering-first” attitude (myself included) must admit that even gear whose measurements show all forms of distortion below theoretically audible thresholds, still can sound different. We measure much of what we hear, even most of what we hear, but we don’t necessarily measure everything we hear, and the quirks of perception acuity can sometimes surprise us.

The Oppo HA-1 is no longer made, so it’s hard to recommend despite being a fine piece of kit. But if you can find one on eBay, it’s hard to find its equal in sound quality under a kilobuck, and it’s super flexible having many inputs and outputs. However, if you want a DAC, line stage and headphone amp that is among the best available at any price, I recommend contacting Jan Meier and listening to the Soul. Sadly, some expensive high-end gear is just audiophile bullshit. The high price is mainly about fancy cases and knobs, low production numbers, and social signalling exclusivity. It’s great to see engineers like Meier bust that stereotype, justify the price with real engineering features, and demonstrate that well engineered and built equipment really can sound better (even if only slightly, since the bar is so high) and get us closer to the music.

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (7 of 8)

This is part 7 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1. Click here for the introduction.

Fri 12/28; speakers, direct, no EQ

  • Beethoven; Early String Quartets; Emerson; Deutsche Grammophon
    • both great, only slightly different but equally good
  • Brahms; Cello & Piano; Pratt, Bailey; Telarc
    • Oppo: slightly darker, muddier
  • Dvorak; Takacs; 96/24: a bright recording with slightly excessive midrange echo
    • Same differences observed; the Oppo is kinder to this flawed recording
  • Moussorgsky; Pictures at an Exhibition; Oue, Minnesota; Reference Recordings: a superb recording in every way; natural tonality, huge dynamics, depth, detail
    • Soul: slightly more resolving, but a touch sterile
    • Oppo: familiar organic tonality, but details are slightly veiled especially with entire orchestra playing (higher IM distortion?)
  • Schubert; Impromptus; Lupu; Decca: an incredible solo piano recording, and sublime performance.
    • Soul: more like a real piano; could be the best I have ever heard?
    • Oppo: slightly more earthy and veiled (still very nice)
  • Tabula Rasa; Fleck, Bhatt; Waterlily 88/24; Tracks 2-6
    • Soul: tighter mids and upper bass, incredible micro-detail of finger/hands hitting the skin of the drums. Plankton!
    • Oppo: fast transients, slightly looser bass, more organic sound
  • Rebecca Pidgeon; The Raven; Chesky: mostly forgettable music, but has a couple of good tracks. Recording is not as good as it first sounds
    • Soul & Oppo indistinguishable; both reveal the flaws in this recording. The Raven is a beautiful song, but I don’t know why audiophiles use this recording. It sounds artificially enhanced; probably sounds great in the car.
  • Red Stick Ramblers; Bring it on Down; Tracks 2, 11: a fantastic recording, sounds great at first and gets better with deep listening
    • Soul: tighter, more clarity, image depth, sweetness – WOW. I used to think the bass in this recording was less than perfectly tight and controlled, but the Soul changes that!
    • Oppo: veiled especially in bass & mids; slight emphasis on air & transients (above treble range). Sounds great, but not on par with the Soul.
  • Gillian Welch; Harrow & Harvest; same as above, dynamically compressed but otherwise great recording with very subtle voicing & details
    • Soul: tight, pure, dynamic, microdetail/plankton
    • Oppo: a touch more upper midrange edge to voices, softer
  • Pizzarelli; Kisses in the Rain; Telarc
    • Soul, Oppo: very similar, virtually indistinguishable
  • Phil Woods; Little Big Band; Chesky: great performances, a good recording but a bit thin sounding
    • Soul: cleaner, tighter
    • Oppo: slight emphasis on harmonics accentuates timbral differences of the saxes and trumpet
  • Observations
    • Again, a day of listening for musical engagement and enjoyment rather than criticism.
    • Yesterday I found that I enjoyed and preferred the Oppo on about 2/3 of the recordings I listened to across a variety of genres.
    • This was contrary to my expectations. I know the Soul is designed and built to higher engineering standards and wanted to like it more. But my ears told me a different story.
    • Last night I was almost convinced that the Oppo was a keeper and I’d be passing on the Soul. The Soul might be a “better” amp but I found the Oppo more engaging and enjoyable to listen to.
    • I almost boxed up the Soul. But I told myself, you have another day, might as well use it.
    • Today those tables started to turn. On almost all the recordings I had a slight preference for the Soul. Perhaps it takes some time to get comfortable with a different sound.

Next (and finally), conclusion and summary

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (6 of 8)

This is part 6 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1. Click here for the introduction.

Thu 12/27; speakers, direct, no EQ

  • Volume setting: the Oppo’s balanced analog output is louder than unbalanced
    • Now that I’m using balanced outputs from both Soul & Oppo, I need to re-check the matching levels
    • Soul click 31 / 12:00 → Oppo -16.5dB (1.5 dB different from unbalanced outputs)
  • Continue listening to a variety of different music, not to hear differences but to see which is the most enjoyable and compelling presentation.
  • Lyle Lovett; Joshua Judges Ruth; a great recording, far better than most popular music. Big natural dynamics, deep bass that is tight and not emphasized. The extreme highs are slightly off sounding, but not distractingly bad. Tracks 1 and 2.
    • Soul: super clean but a bit sterile.
    • The Oppo’s earthier presentation with a touch more air sounds ever so slightly less detailed (though the detail is there if you listen for it) but somehow fits this music and recording better.
  • Beethoven Kreutzer Sonata; Ashkenazy & Perlman; Decca: an astoundingly awesome performance and incredible recording. Essential listening for all classical music fans.
    • Soul: the tone is more round and pure, more noticeable on piano than violin. But perhaps a touch too pure.
    • The Oppo tone has a touch more air. This could be perceived as grain or veil, yet with this particular recording the effect sounds more refined and natural.
  • Chieftans 7; this classic recording is really better than it sounds! Its voicing has an edgy midrange presence. So much that a while ago I reburned the CD with a mild parametric EQ, -2 dB centered at 1 kHz, Q=0.67. Also added a mild boost to the lowest bass (20 to 40 Hz). This tames the edge, brings out the rich details behind it, and makes this fantastic traditional music more natural and enjoyable.
    • Soul: slightly greater apparent resolution, easier to hear the individual instruments.
    • Oppo: slightly warmer and softer; everything is there including all the detail but it doesn’t come to you; you have to listen for it. Overall, a more organic sound.
  • Mozart Requiem; Levin; Dorian: a great recording: detailed, natural voicing, dynamic
    • no significant differences noted (listener fatigue, or just getting into the music?)
  • Arnesen Magnificat; 2L; 96/24: a high-res recording with incredibly deep organ bass (to 20 Hz and below), but multi-miced so the image is amorphous and unrealistic
    • Oppo: bass slightly deeper, yet less tight & controlled
  • Ayreheart; Barley Moon; 192/24: this is a great recording, very detailed and dynamic but the midrange is a touch edgy
    • Soul: voice is slightly more pure and natural
    • Oppo: softens the midrange edge, bass has more depth yet not as tight, slightly less dynamic
  • Vivaldi; Recorder Concertos; Naxos 8.557215: this is one of those rare great recordings from Naxos; excellent natural voicing with layers of detail
    • Soul: slightly tighter, more dynamic and rounder tones
    • Oppo: more organic, refined
  • Doug LacLeod: One Eyed Owl: same track from Wed, this time on speakers
    • Soul: clarity, tighter bass, vocal purity, punchy dynamics
    • Oppo: more air, bass depth, vocal refinement
  • YoYo Ma/Edgar Meyer; Appalachia Waltz; Sony: a superb recording and performance, though a bit on the subdued side
    • Soul: rounder, fatter timbres, more detail, sweeter
    • Oppo: more earthy & organic voicing
  • Schubert Violin/Piano; Lupu/Goldberg; Decca: a beautifully voiced recording, though with some technical imperfections
    • Soul: pure, tight, sweet
    • Oppo: more complex timbre, woodier
  • Chopin Etudes; Earl Wild; Chesky: an otherwise good recording that suffers from an excess of echoey resonance, sounds like the result of less than ideal room & mic placement
    • Soul: so pure, a bit too round, aggravates the resonance
    • Oppo: a hint of extra air helps the timbre, tames the resonance
  • Mapleshade; Boogeyin! A La Carte Brass; tracks 1 & 2: this is an “in your face” direct to analog 2-track recording of some “in your face” music
    • Soul: tighter, cleaner, huge effortless natural dynamics
    • Oppo: smoother, more refined, dynamics not as big as the Soul
  • Ian Shaw; World Still Turning; track 1 “Alone Again”: a fantastic recording of voice & piano. Big dynamics, incredibly lifelike voicing, great detail without brightness
    • Soul: pure, clear, punchy, solid state
    • Oppo: air, refinement, less dynamic, like that tubulicious SET OTL sound
  • Listening to tracks today on speakers, I was going for musical enjoyment rather than critical listening for differences.
  • My perceptions of the amps were technically the same as before, but from an enjoyment and engagement perspective everything changed.
  • I listened for hours with no fatigue, really enjoyed this session.
  • I love this music and both DACs revealed each piece slightly differently. Instead of judging them, I just opened my mind and took it all in.
  • About ¾ of the way through I had a flashback to a headphone amp I owned almost 20 years ago, the Wheatfield HA-2 designed by Pete Millet. It was an OTL SET amp I used to drive my HD-580, which at 330 ohms were perfectly suited to an OTL tube amp.
    • My epiphany was that the differences between the Soul & Oppo is of a similar character.
    • The Soul has a “solid state” sound while the Oppo is more “tubey”.
    • Of course, both are solid state with none of the euphonic distortions of tubes. The Soul and Oppo are more similar than they are different. But what differences they have, are of a similar nature to solid state vs. tubes, though they’re much more subtle in magnitude.
  • At the end of today I realized that I really didn’t prefer one of these amps to the other overall. They are both excellent, each in its own slightly different way. If I owned both I would use them on different days, depending on my mood, the music I was going to listen to and my reasons for listening.

Next, subjective listening notes part 7 (day 6)

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (5 of 8)

This is part 5 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1. Click here for the introduction.

Wed 12/26; LCD-2 headphones; direct, no EQ

  • New config for faster switching
    • Oppo BDP-83 coax output to HA-1
    • Oppo BDP-83 toslink output to Soul
    • Or reverse of the above; coax and toslink output levels match
    • Level matched using white noise & SPL meter (as before) to < ½ dB
    • Simply replug the headphones back & forth, nothing else
    • Both amps continually playing the same signal
  • Many of the above tracks played repeatedly… also
  • Bruce Katz; Three Feet off the Ground: an excellent Bernie Grundman master
  • Clementi; Demidenko; Helios
  • Doug MacLeod; Brand New Eyes; One Eyed Owl
    • This is a superbly recorded track deep, tight bass, light fast transients and near perfect natural vocal reproduction
    • Oppo & Soul almost the same, but the Soul had slightly deader space between the notes, tighter bass
  • Michael Hedges; Aerial Boundaries: fast transients with extreme HF
    • Soul & Oppo: equal speed, crisp transients
  • Tuck & Patty; Love Warriors; Little Wing
    • This is a nice recording, uncompressed and natural sounding
    • Soul & Oppo: sound the same, bass & voice have same timbre, bass plucks are equally fast & light
  • Julian Bliss Quartet; Hyperion:
    • Almost the same
    • Oppo slightly more air, Soul a touch more mid bass
  • Gillian Welch; Harrow & the Harvest: compressed but very detailed with subtle timbres
    • Both Oppo & Soul capture the very delicate shades of timbre in the voices, the guitar work and micro-detail of breathing & movement
  • Ronnie Earl; Maxwell Street: crunchy & compressed, how well do they portray a bad recording?
    • Oppo & Soul sound the same.
  • Dream Theater; Systematic Chaos: dynamically compressed but otherwise clear with full, wide bandwidth: how well do they rock out?
    • The Oppo has slightly more air, but the difference is so small I can’t be sure
    • Otherwise both sound the same: the bass hits down to 20 Hz, the midrange tonality, the layers of background detail, all identical.
  • Also played several tracks from Steven Wilson’s Yes re-mix
  • These are so similar that even for a picky detail-oriented guy like me, even if I could tell them apart in a blind test (not sure I could), I could love either one.
  • This is beyond splitting hairs. That said…
    • The Soul seems a bit more tight, pure, punchy
    • The Oppo seems to have more depth & breadth

Wed 12/26; speakers, direct, no EQ

  • New config for more fair comparison
    • Both players running as above (Oppo from coax, Soul from toslink, or vice versa)
    • Both preamps running in balanced mode (no more unbalanced Oppo output)
    • Swap the balanced XLR outputs to the power amp
    • Balanced cables = quiet hot swap, no need to power off amplifier
    • This swap is about as fast as before
  • Several of the above tracks played again, plus:
  • Tabula Rasa; Fleck, Bhatt, Chen; 88/24
  • Bourbon & Rosewater; Meyer, Bhatt; 88/24
  • The Oppo’s balanced output is a slight improvement; a bit of the veil is lifted, the bass tightens up a smidge and it’s dynamically punchier.
  • Some of the differences I was hearing were limitations of the Oppo’s unbalanced line out.
    • As mentioned earlier, the Oppo’s primary signal path is internally balanced; the unbalanced inputs and outputs have an additional conversion
  • The Soul still sounds slightly different from the Oppo; it’s more pure and tight where the Oppo gives the impression of breadth & depth.
  • But much like the headphone observations above, the Oppo’s balanced output shrinks these differences.
  • NOTE: from this point forward, all speaker comparisons were done in this way using exclusively balanced outputs from both devices.

Wed 12/26; LCD-2 with EQ

  • Now that I know what the Soul & Oppo sound like, how they’re different, it’s time to listen for enjoyment across a variety of music and see which I want to live with.
  • Listened to the first 2 albums of the Steven Wilson Yes remix
    • Not exactly audiophile material, the original recordings are limited and flawed.
    • But it sounds way better than the originals, which I could never listen to because they gave me a headache.
    • This good music deserved a better recording, and now it has one.
  • Used the Soul’s first notch of crossfeed to fix some of the absolute hard L-R separation. Very nice, a subtle effect that doesn’t eliminate it but makes it less annoying.
  • NOTE: this crossfeed seems more transparent than the one on my Jazz amp. It does the same thing, but with less impact to tone and resolution.

Next, subjective listening notes part 6 (day 5)

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (4 of 8)

This is part 4 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1. Click here for the introduction.

Subject Listening Comparison, continued…

Tue 12/25; speakers; direct, no EQ

  • Beethoven; Piano Sonatas Op 13; Apassionata; Brendel; Decca
    • Soul: purity, timbral accuracy
    • Oppo: +HF, more earthy, dirty tone
  • Chopin Op 58; Mapleshade, Gampel (Fazioli piano): this is a good but slightly flawed recording of a huge “in your face” sounding piano with a bit too much midrange presence.
    • Soul: bright, slight edge on HF, some distortion in upper RH dynamic peaks (sounds like analog tape overload)
    • Oppo: virtually indistinguishable
  • Schubert; flute/piano songs D911; Naxos/Grodd: one of few truly excellent Naxos recordings
    • Soul: slightly rounder flute tone; less air, but extreme HF information is there (lip/air overtones & light whistletones)
  • Oppo: a touch more air
  • Doppler; Andante & Rondo; Rampal, Arimany, Ritter; Delos
    • both excellent: tone, dynamics, voicing, virtually identical

Tue 12/25; headphones LCD-2; with EQ (+3 dB @ 4500 Q=0.67)

  • Volume test
    • Soul click 31 / 12:00 noon → -7.5 dB on Oppo
    • set to -8 dB by ear, fine tuned to -7.5 dB with SPL meter and white noise
  • Doppler (from above): indistinguishable
  • Taheke track 13 (from above): indistinguishable
  • Chieftans 7: indistinguishable
  • Notes
    • Are the Magnepans are more revealing than the headphones? Probably.
    • Is the Oppo’s headphone amp slightly different sounding than its line level outputs? Certainly.
    • Either way, the Soul & Oppo seem indistinguishable on headphones. Both are excellent!
  • Before today, I’d say the HA-1 is the best sounding headphone amp I’ve ever heard; the Soul is its equal.

Tue 12/25; speakers; direct, no EQ

  • Brahms piano quartet; Belcea 96/24: despite its high bit rate, this recording is imperfect with slightly edgy voicing and the piano sounds distant.
    • Soul: purifies the tone, smooths the edge
    • Oppo: detailed yet slight grain/edge
  • Krall; Quiet Nights; 96/24; track 4: this recording is good overall but like most of her albums, it adds an edgy presence to Krall’s voice
    • Soul: smooths the vocal edge, but all the highs, cymbal brush, still there. Three dimensional imaging.
    • Oppo: voice is just a bit over the top with edge, HF more present but dirtier. Image has depth but a touch less 3D deep as the Soul.
    • Which is more true to the original master is unknown, but the Soul sounds cleaner.
  • Krall; Girl in the Other Room; Temptation: another good but edgy Krall recording
    • Soul: surprisingly, not apparently smoother or more pure sounding
    • Oppo: bass during solo has slightly greater perceived depth
  • Mokave; first album; Audioquest; tracks 1, 3, 5: this is a near-perfect recording!
    • Soul: rounder, more pure piano tone; smoother extreme HF transients, may be slightly rounded off. A touch more mid-bass, less bottom depth.
    • Oppo: extreme transients sharper, slightly accentuated.
  • Lily & the Rose; Binchois, Kirkman; 96/24; tracks 16-17: a superb recording from Hyperion
    • Soul: slightly more pure midrange voicing, a tiny tad less sibilant
  • Monteverdi; book 7; Naxos: one of few truly excellent Naxos recordings
    • Disc 1 track 2
      • Soul slightly more distinct and pure
      • Oppo more emphasis on harmonics / overtones
      • Differences very slight, nearly identical
    • Disc 2 track 3
      • Soul: a very thin slight veil lifted from the music
  • Schubert; Schiller-Lieder vols 3 & 4; Naxos: very good but too much midrange edge on the voices (why do mastering engineers feel this is necessary!?)
    • Soul: may be smoother but so slight I can’t be sure; virtually identical
    • With this recording having a bit of edgy midrange presence, I expected to hear the Soul smooth it to a more natural presentation. Against my expectations, I was surprised to find it virtually identical to the Oppo.

Next, subjective listening notes part 5 (day 4)

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (3 of 8)

This is part 3 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1. Click here for the introduction.

Subjective Listening Notes

Before we dive into my notes, I must say that these are my personal subjective observations. I reliably detected these differences in level matched blind tests, so they are real. But I don’t claim they relate to advantages or flaws in any measurable engineering sense. I do my best to describe these differences as a neutral observer without judging which is “better” or “worse”.

For example, even terms like “pure” and “dirty” aren’t necessarily praise or criticism. “Pure” can be good, meaning free of distortion. Pure can be bad, meaning the reproduction of a natural sound is more pure than it sounds in reality (such as its complex timbre sounding filtered or simplified).

The Soul and Oppo are both high quality well engineered DACs with no obvious measurable flaws, they both have a neutral solid state sound without obvious euphonics or colorations. So the differences are necessarily subtle. We’re splitting hairs here, but that’s what high end audio is all about!

Also: I mention the recordings used but I only rarely give CD catalog numbers. You can probably find the exact recordings from the descriptions, but if you can’t, contact me and I’ll be happy to provide them.

Sun 12/23; speakers; direct, no EQ

  • Level testing
    • Set by ear using music and white noise without emphasis (equal energy all freqs 20 Hz – 20 kHz).
    • Tested with SPL meter @ listening position: subjective level matching was about ½ dB off.
    • Soul 12:00 Yellow (click 31 from minimum) = Oppo -14.5 to -15.0 dB (unbalanced line output).
    • Soul clicks measured as SPL, average 0.5 dB per click around the center position
      • From 10:00 to 3:00 position, clicks 19 through 51.
      • Different from manual, which says 0.8 dB per click.
      • Perhaps the manual averages all clicks, which gives bigger number because the first few clicks are bigger jumps.
  • Brahms Clarinet Trio; Ax, Ma, Stoltzman; Sony; track 1:
    • The Soul resolves the instruments so you can hear slightly better what each is doing even during dynamic crescendos.
    • The clarinet and piano are voiced ever so slightly differently through the Soul, just a touch more pure to my ears.
  • The Elfin Knight; Frederiksen; track 3:
    • The Soul resolves the flute & string instruments slightly better especially when they’re in the background with other instruments playing.
  • Soul’s bottom octave (< 30 Hz) sounds weaker.
    • It’s there if you turn it up, but sounds attenuated relative to the Oppo at the same overall volume level.
    • This is perception, not measurement. Both Soul & Oppo have ruler flat frequency response, so this perception is probably related to something else going on in the sound.
  • Taheke; McGee/Krutzen; track 13: the harp’s lowest 25 Hz tones subtly push the air in the room from the Oppo, yet are less noticeable from the Soul.
  • Roots and Sprouts; Abou-Khalil; track 2: the double-bass solo is more audible from the Oppo, and the Soul portrays it with slightly less depth, but more subtle timbre.
  • Barley Moon; Ayreheart; 96/24; track 4: when the drum enters about 50 seconds into the track, it sounds slightly deeper and more compelling from the Oppo.
    • Soul’s mid-upper bass is slightly emphasized relative to the Oppo.
    • Soul’s midrange is slightly more pure than the Oppo.
  • Dowland First Booke of Songes; Grace Davidson, David Miller; Hyperion 96/24; all tracks: at first the Soul and Oppo sound identical, but deeper listening reveals that Soul renders Davidson’s voice as ever so slightly more pure, with sibilants just a hint softer.
  • Soul’s extreme HF (> 10 kHz) is slightly less than the Oppo.
    • ?? initial impression, more listening to confirm
    • Is it possible that this relative attenuation (however slight) contributes to the observed midrange purity?

Mon 12/24; speakers; direct, no EQ

  • Soul’s extreme HF (> 10 kHz) is slightly less than the Oppo.
    • Confirmed. The Soul isn’t lacking these frequencies, but they sound very slightly attenuated compared to the Oppo. Which sounds “best” depends on the recording.
  • Tarab; Abou-Khalil; tracks 2-4: the top overtones of the instruments are slightly more evident with the Oppo. The difference in tonality is so subtle, it’s like the difference in live listening just a few feet further away.
  • Eeg & Fonnesbaek; tracks 1, 3, 6: this great recording has a bit of edge on Eeg’s voice. This is more apparent on the Oppo than the Soul. The Soul sounds slightly more natural, yet still with more “edge” than reality. Which is more true to the slightly edgy original master is unknown.
  • Vivaldi Concerto for violin, flutes, oboes, bassoons; RV577; McGegan, Philharmonia Baroque; tracks 7-9: this fine recording is on the airy side of reality. The Oppo slightly accentuates this airiness while the Soul slightly de-emphasizes it. Unknown which is more true to original master but the Soul gives a more natural presentation for this excessively airy recording.
  • Soul’s bottom octave (< 30 Hz) is weaker.
    • Eeg & Fonnesbaek; tracks 1, 3, 6: the bass on the Soul sounds a tad tighter, perhaps just a hint more speed, grip & control, yet not quite as much depth and richness as the Oppo.
    • Saint-Seans Symphony 3; Stern, Kansas City; tracks 4, 6: about 15 seconds into track 4 the organ hits a deep soft 20-30 Hz tone that pushes the air in the room. Both Soul & Oppo portray this, but the Oppo has a touch more depth and energy.
  • Soul’s midrange is ever so slightly more pure than the Oppo.
  • The Elfin Knight; Frederiksen; several tracks: the Soul has a touch more midrange purity. This could be related to its relatively attenuated HF, but the impression is that it is slightly more damped, as if the brief pauses of silence in the music are quieter.
    • Note: usually, a perceived attenuation of HF (however slight) relates to less clarity, not more. The Soul’s character is enigmatic.
  • After a few hours of the above, listener fatigue set in… resume later

Same Day, Hours later…

  • Measured FR to see if it shows any hint of my above observations–probably not, buy why not check?
    • Recorded warble tones from Stereophile test CD #2, analog line-level balanced XLR outputs of each device (Soul, Oppo) to Tascam recorder.
    • Frequency response matches within 0.1 dB from 20 Hz to 20 kHz (1/3 octave spacing) except at extremes
      • Matched levels at 1 kHz.
      • 20 Hz: Soul is +0.05dB relative to Oppo (-1.35 vs. -1.4 recorded on Tascam)
      • 20 kHz: Soul is -0.25dB relative to Oppo (-1.35 vs. -1.1 recorded on Tascam)
    • These differences should be inaudible
    • NOTE: these levels are relative to each other, not absolute (the Tascam doesn’t have perfectly flat response).
    • HD unmeasurable; both below -90 dB
  • As expected.
  • The above subjective listening impressions are subtle.
    • Subtle changes near the threshold of hearing can be perceived differently from what they actually are (slight difference in loudness perceived not as loudness but as sounding “fuller” etc.).
  • In light of this, how to explain the differences I’m hearing?
  • They’re not psychosomatic; I can differentiate them blind.
  • It sounds as if the Oppo has a slight touch of extra frequency content in the upper mids to treble, and a hint more low bass energy.
  • Sometimes it sounds like a touch of extra detail, other times it sounds like a touch of glare or grain; depending on the music.
  • Could it be a slight difference in frequency response? Could it be harmonic or intermodulation distortion?
  • Unlikely, the measurements are so similar.
  • But subjectively, that describes what it sounds like.

Next, subjective listening notes part 4 (day 3)

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (2 of 8)

This is part 2 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1. Click here for the introduction.

Before diving into the listening sessions, let me summarize a few things:


  • Both have DAC, preamp and headphone amp.
  • Both operate natively in balanced differential mode.
    • Technically speaking, the terms “balanced” and “differential” are two different things–often, but not always, used together.
    • Here, I use the word “balanced” to mean both, as is commonly done in audio circles.
  • Both have digital inputs (toslink, coax, USB) and analog outputs.
  • Both are well engineered and built.

Functional Differences: Summary

  • The Soul has DSP features; Oppo doesn’t.
  • The Oppo has additional inputs and outputs that the Soul doesn’t have.

Functional Differences: Details

  • Oppo also has unbalanced inputs and outputs (line level & headphone); Soul doesn’t.
    • The Oppo’s internal signal path is balanced.
    • Its unbalanced inputs and outputs go through an extra conversion.
    • This is completely internal and automatic: just plug it in.
  • Soul has multiple digital inputs (3 toslink, 3 coax), Oppo has only 1 each.
  • Soul has DSP: selectable DA reconstruction filter, L-R balance, EQ, channel mixing. Oppo doesn’t.
  • Soul has digital output (to use its DSP with another DA converter), Oppo doesn’t (it doesn’t have any DSP effects to do this with).
  • Soul has a high (120 Ohm) impedance headphone output — in addition to a standard low (< 1 Ohm) impedance output. Jan describes the reason here. Summary:
    • The low Z output is normally used with most headphones, especially high impedance and planar magnetics.
    • The high Z output can dampen oscillation (e.g. tame a “hot” response) for certain headphones having low impedance.
  • Soul has a ground lift switch. You shouldn’t need it but it’s nice to have. Oppo doesn’t have one.
  • Soul has switchable high/low gain for its analog input. Oppo has switchable high/low gain for its headphone output, which applies in the final analog stage to all inputs both analog & digital.
  • Soul is custom built gear and Jan will make whatever individual adjustments you want to your unit: changing the analog gain, custom DSP, whatever. Just ask him!
  • Oppo has Bluetooth input, Soul doesn’t.
  • Oppo has AES/EBU digital input, Soul doesn’t.
  • Oppo has mobile USB input (Apple only), Soul doesn’t.
  • Oppo USB input accepts PCM and DSD, Soul is PCM only.
    • The Soul handles the following sampling freqs: 32, 44.1, 48, 88.2, 96, 192.
    • Oppo handles a few sampling frequencies that Soul doesn’t: 176.4, 352.8 and 384.
    • These aren’t used much, but if you have a source using them, Jan recommends resampling them to a frequency the Soul handles. This can be done in software on a PC. Just make sure you use good software that converts properly in 24 bit or greater with frequency-shaped dither.

Functional Differences I care about

  • Unbalanced analog inputs and outputs are nice to have (though not essential).
  • An unbalanced headphone output is nice to have (though not essential).
  • Channel mixing crossfeed is nice to have when headphone listening to recordings having absolute L-R separation (though not essential).
    • Because this feature slightly changes tonal balance and resolution, I use it only when hard channel separation is annoying.

Other Equipment

  • Disc player: Oppo BDP-83 playing CDs and DVD-Audio, using Toslink and Coax PCM output direct to preamp (Soul or Oppo). Varying bit rates from 44-16 to 192-24.
  • Headphones
    • Audeze LCD-2 Fazor, version 2016 upgraded drivers
    • Sennheiser HD-580 with fresh ear & headband pads
  • Speakers
    • Adcom 5800 amp
    • Magnepan 3.6/R speakers
    • Tuned listening room (floor to ceiling tube traps, acoustic foam, etc.)
  • EQ: Behringer DEQ-2496
    • Not always used; details below

The equipment with electrons flowing through it.

The Oppo looking all shiny & black (hard to believe it’s had 4 years of regular duty).

The Adcom 5800 pushes the electrons through my Magnepans. Hard to believe it’s over 25 years old, still going strong (I have it tested every few years).

It’s amazing how 21″ diameter floor-to-ceiling tube traps clean up the bass response (yes they’re home built)! The dark stuff on the wall is 4-layers thick of RPG acoustic foam strategically located to clean up the midrange response.

These Magnepan 3.6/R have given me over 15 years of musical enjoyment. Being dipoles, they are very sensitive to room setup, but when set up right they are downright magical. With the room treatment and positional setup, at the listener position they measure within 3 dB of flat from 30 Hz to 20 kHz. I love their midrange voicing, so natural and free of resonances; with extended and detailed yet silky smooth treble, distortion lower than most headphones, and bass having the taughtness, control and timbral accuracy that is unique to planar magnetics.

Listening Configuration

  • All Soul DSP features disabled and standard linear phase sinc(t) AA filter used, except where noted.
    • I normally use a Behringer DEQ 24-96 for mild parametric EQ to correct headphone & speaker room response.
    • When using LCD2, I listened both with, and without, EQ. The mild EQ I use (+3 dB @ 4500 Hz, Q=0.67) partially corrects the LCD2 response dip and makes it more neutral and resolving.
    • When using speakers, I disabled EQ. The room treatments give good clean response making the speaker EQ mild and unnecessary for critical listening comparisons.
  • Both Oppo & Soul left ON all week to ensure they were fully warmed up and stabilized.
  • The Adcom 5800 powered off at night (it draws 250 W idle), but on for at least 30 minutes before each listening session–long enough for the fans to be running.
  • Headphones: the Soul’s low Z output; the Oppo’s balanced output.
  • Speakers: the Soul’s XLR output to Adcom 5800; the Oppo’s unbalanced output to Adcom 5800.
    • This slightly favors the Soul, because the Oppo is internally balanced so the unbalanced output goes through an additional conversion. Its balanced output has slightly better specs than unbalanced.
    • While imperfect, this allows faster switching (no need to plug/unplug analog cables).
    • I figured it was probably fair enough because the cables are short (1 meter), high quality (Blue Jeans Cable), and both the Adcom and Oppo have excellent measurements for both inputs, single-ended and balanced.
    • I changed this later (described in notes) and found the Oppo’s balanced outputs sound slightly better.
  • Level matching
    • All comparisons level matched within ½ dB.
    • White noise, equal energy all frequencies, used for level matching.
    • Matching done subjectively by ear, then confirmed and fine tuned with an SPL meter.

Observed Soul notes not primarily listening related

  • Soul occasionally emits a “click” to the analog outputs (speakers or headphones). Not a huge “orgre slurping breakfast” click that could damage speakers, just a light to medium volume audible click.
    • After no music input for a few seconds.
    • Occasionally when starting to play a new disc.
    • Occasionally when hitting play after the disc was stopped for a while.
    • Seems to be a minor bug in the Soul firmware/software.
    • NOTE: if it implements a volume fade-in to avoid the click, it would have to be very fast (say, 30 ms) to avoid clipping some tracks that start immediately.
  • Soul’s volume knob
    • A better design
      • No potentiometers in signal path
      • Changes gain rather than attenuating fixed gain
    • But those relays are physically loud! Not in the output signal, but a mechanical clicking noise in the room.
      • Are the relays this loud on the production version?
      • The volume control relays on my Corda Jazz are much quieter.
      • Jan says: production unit has same relays, but the box is more solid and damps the sound
    • How long do these relays last (relay life in terms of MTBF/MCBF)?
      • Jan says: veeery long, been using them for years and yet to replace one.
    • Output profile
      • About 0.5 dB / click (from 10:00 to 3:00)
      • About -15 dB from full scale at 12:00 (click 31)
      • Larger steps per click for the first 15 or so clicks.
      • Not remote controllable; confirm that the production version is?
        • Jan says: confirmed

NOTE: my setup is a bit unusual, in that my speakers in combination with the carefully tuned listening room are higher resolution than most headphones. Normally, good headphones are more resolving than good speakers. So my observations and conclusions may also be a bit unusual.

Next: subjective listening notes, part 3 (days 1 & 2)

DAC, Preamp, Headphone Amp: Corda Soul and Oppo HA-1 (part 1 of 8)

This is part 1 of an 8 part series comparing the Meier Corda Soul and Oppo HA-1.

Click here if you want to cut to the chase and read the summary.

I’ve used an Oppo HA-1 as my DAC, preamp and headphone amp for nearly 4 years. The reason I still have it is because I enjoy listening to it so much. Yet I wonder whether better sound could be had. The Oppo is primarily designed as a headphone amp; its linestage, while very good, is a secondary feature. I listen on speakers at least as much as I do on headphones, and my speakers are more transparent than most headphones. Also its volume control is a potentiometer; a high quality Alps, but still not the ultimate in transparency and perfect channel balance that a well implemented stepped attenuator can provide.

My audio system at work uses a Meier Corda Jazz amp. I bought it over 4 years ago. It has a wonderful sound: detailed, smooth and sweet without euphonics. And some nice features, like a stepped attenuator volume control, selectable L-R cross-feed and balanced ground drive. It’s hard to find anything this nice for twice the price, and the build quality is great. So when I heard that Jan Meier recently built a SOTA DAC/headphone amp/preamp called the Soul, I was intrigued. The Soul has come out with some anticipation in the headphone audiophile community. I read that Jan brought the Soul to CanJam in Europe and it was judged “best in show”. Given some of the very nice (and expensive!) gear at CanJam, that says a lot. I also found a few people online who had auditioned it, nothing but rave reviews.

Jan built 2 prototypes of this device. The prototype resembles a science project but is solid, if not elegant, and electrically equivalent to the production version (which as of Dec 19, 2018 is yet to be released). If you contact Jan, you can arrange with him to borrow it for a listening session. When I did so, he told me he was running a pre-order to gauge interest in this device and estimate production volume (which influences the price). People who pre-order pay a deposit up front and get a lower price when the production version of the Soul is released. It turns out I contacted him just as this pre-order was ending. Jan has a generous policy of not charging to borrow the Soul prototype (though shipping it back to him cost about $65), and a 14-day return policy for the final product.

It may seem unfair to compare the Oppo with the Soul, as the latter will probably cost several times as much. But I’ve always believed, based on comparison with other headphone amps and DACs, that the Oppo punches well above its weight class when it comes to performance per dollar. Also, my goal was not a head-to-head comparison of these 2 preamps, but rather to find out whether the Soul would be a relative improvement. That is only a subtle distinction, but an important one.

Two weeks later, just before Christmas, the Soul arrived at my door having crossed an ocean, a continent, and customs inspectors. I carefully unpacked it, connected it to my system, centered/zeroed all the knobs and made a quick function test. Music! Success! I swallowed my anticipation and left both it and my Oppo HA-1 powered on overnight for listening sessions the next day.

Next: system summary and setup

Note: I joined the Soul pre-order just as it was ending. Owning Meier’s Jazz amp for the past few years told me he builds high quality products that last, and stands behind them with excellent support. If the Soul doesn’t work out for me and I decide to keep the Oppo HA-1, I can forfeit my deposit knowing it supports Jan’s efforts, or I can re-sell the Soul without loss, considering its retail price will be $1000 more than the preorder price. Either way seemed worth the risk if the Soul lives up to its promise.

DAC / DA Conversion / Linear vs Minimum Phase

Digital audio requires an anti-aliasing filter to suppress high frequencies (at or above Nyquist, or half the sampling frequency). Without this, an infinite number of different analog waves could pass through the digital sampling points. With this, there is only 1 unique analog wave that passes through them. The anti-aliasing filter is essential to ensure the analog wave that the DAC constructs from the bits is the same one that was recorded and encoded (assuming that the original analog mic feed was properly anti-alias filtered, preventing frequencies above Nyquist from leaking through).

Note: what happens if the filter is not used at all?

  • As I just mentioned, without a bandwidth limit, many different analog waves could be constructed from the same sampling points – which one is correct?
  • Without a bandwidth limit, the DAC will produce an analog wave with frequencies above Nyquist, which must be distortion, since they could not be in the analog wave that was encoded.

Pragmatically, one might ask what is the problem, since the difference is all in frequencies above Nyquist, which we can’t hear? The problem is aliasing. Passing these high frequencies will cause the D-A conversion process to mis-interpret samples, creating an analog wave with spurious noise in the audible spectrum through a phenomena known as aliasing. So you get distortion in the audible spectrum – not just at supersonic frequencies. Intuitively, this effect is similar to watching a wheel spin in a movie; it sometimes appears to spin backward when it’s really spinning forward, because the frame rate (typically 24 / second) captures it at just the right moments. The wheel is spinning faster than “Nyquist” for 24 frames per second, which is aliased into the illusion of motion in the opposite direction happening slower than 24 frames per second.

So the DAC definitely needs a low pass filter to suppress frequencies above Nyquist. The question is – what kind of filter?

Audiophiles debate about whether linear or minimum phase anti-aliasing filters are ideal for sound reproduction and perception. Linear phase has the lowest overall distortion, but its symmetric response around transients (a bit of ripple just before and after a transient pulse), often called the Gibbs effect, means there is a “pre-echo” or “pre-ring”. In the diagram below, the red line is the signal and the black wave is the analog wave constructed from it using a linear phase filter.

If the X axis is t for time, this black curve is the function sinc(t). It is symmetric before and after the transient, which means it starts wiggling before the transient actually happens. This is unnatural; in the real world, all of the sound happens after the actual event. This pre-ringing is an artifact of linear phase anti-aliasing filters. Many audiophiles claim this is audible, smearing transients and adding “digital glare”.

Here’s what the audio books don’t always tell you. According to the Whittaker-Shannon interpolation formula, this sinc(t) response represents the “perfect” reconstruction of the bandwidth limited analog signal encoded by the sampling points. The pre-ring is very low level, and it rings at the Nyquist frequency (half the sampling frequency). That is at least 22,050 Hz (octaves higher if the digital signal is oversampled, as it virtually always is). This makes it unlikely for anyone to hear it even under ideal conditions of total silence followed by a sudden percussive SMACK.

NOTE: I say “unlikely” not “impossible” because even though humans can’t hear 22 kHz (let alone frequencies octaves higher), it is at least feasible that somebody could still hear the difference. Under the right conditions, removing frequencies we can’t hear as pure tones causes audible changes to the wave in the time domain. That doesn’t make sense mathematically, but human perception of the frequency & time domains is non-linear and not as symmetric as Fourier transforms.

Some audiophiles suggest minimum phase filters as an alternative to solve this problem. But this cure may be worse than the disease. Minimum phase filters have an asymmetric response around transients with no pre-ringing. A picture is worth 1,000 words, so here’s what that same impulse looks like when a minimum phase filter is used.

You can see that the impulse strikes instantly without any pre-ringing. Well it actually rings louder and longer than the linear phase filter, but that ringing happens after the transient.

This has the added benefit that the ringing is masked by the sound itself for the simple reason that loud sounds psychoacoustically mask quiet ones. So what’s not to like here?

The problem is, minimum phase filters actually have more distortion (more ringing, more phase shift) than linear phase. So you get more distortion overall, but it’s time-delayed so you get cleaner initial transients with more distorted decay. And the phase shift caused by minimum phase filters happens all the time, not just in transients. So it seems you can have clean transients, or good phase response, but not both. Choose your poison.

At this point a purist audiophile might hang his head in sadness. But there’s a better solution to the digital bogeyman of pre-ring: oversampling (or higher sampling rates). The phase distortion and ringing of any filter is related to its slope, or the width of its transition band. Oversampling further increases the frequency of the pre-ring (which was already ultrasonic), makes a shallower slope, wider transition band, reducing distortion.

For example consider CD, sampled at 44,100 Hz. Nyquist is 22,050 and some people can hear 20,000 so the transition band is from 20,000 to 22,050. That’s very narrow (only 0.14 octaves) and requires a steep filter with Gibbs effect pre-ring at 22,050 Hz. Oversample it 8x and Nyquist is now 176.4 kHz, so your transition band is now 20k to 176.4k, which is 3.14 octaves (actually, you’d use a lower cutoff frequency, but it’s still at least a good octave above 22,050 Hz). Absolutely inaudible; go ahead and use linear phase with no worries.

In short, use higher sampling frequencies (or oversample) not because you need to capture higher frequencies, but because it gives you a more gradual anti-aliasing filter which means faster transient response without any time or phase distortion.

This idea is nothing new. Most D-A converters already oversample, and have been doing so for decades. The pre-ring or ripple of a well-engineered DAC is negligibly small, supersonic and inaudible. However, some people prefer minimum phase filters! How can we explain that? Minimum phase filters have no pre-ripple, yet they also have phase distortion, they ring louder and longer, and in some cases they allow higher frequencies to be aliased into the signal.

First, if this preference comes from a non-blind test, we can’t be sure they really heard any difference at all. Maybe they did, maybe they didn’t. A negative result from a blind test doesn’t mean they can’t hear a difference, it only means we can’t be sure they hear a difference.

Along these lines of non-blind testing, Keith Howard wrote a good one for Stereophile a few years ago:
I love their experimental attitude: test and discover! But when they talk about how hard it was to tell the filters apart, it is kinda funny thinking about a bunch of middle-age guys wondering why they can’t hear a supersonic ripple well above the range of their hearing. Especially when most of them understand math & engineering well enough to know why.

Second, consider whether this preferences comes from a blind test. Blind tests only reveal whether people can hear differences; they don’t qualify exactly what differences they were hearing. Perhaps people who prefer minimum phase filters are simply finding some of these distortions to be euphonic. This seems reasonable, given that preferences for vinyl records and tube amps are also common. However, it could also be that some DAC chips implement one filter better than the other.

This topic has been endlessly debated in audiophile circles for years. Here’s an article showing some actual measurements:

A couple years later he followed up with a listening test:

So what do I think about all this? Like the Stereophile reviewers, listening to music, I find it difficult to hear a difference between the “sharp” (linear phase) and “slow” (minimum phase) filters. Test signals highlight the differences (I can hear the difference clearly with a square wave) but I don’t enjoy listening to test signals, and since they’re not natural sounds, even if you can tell them apart there’s no reference for what they should sound like. I know the sharp filter (when properly implemented) is correct from a math & engineering perspective, as long as it is properly implemented. The sharp filter in my DAC is only -6 dB at Nyquist, so it might not be properly implemented, though its slope is very steep at that point, so it’s probably not leaking supersonic noise which can be aliased into the audible spectrum. Since there’s essentially no audible difference, I prefer the sharp filter in my DAC. I made some measurements and this seems justified from a technical perspective.