All posts by Mike Clements

Fractional Octaves

I’ve been working with parametric EQ settings lately; here’s a quick cheat sheet.

Overview

We perceive the frequencies of sounds logarithmically. Each doubling of frequency is an octave. Thus, the difference between 40 and 80 Hz sounds the same as the difference between 4000 and 8000 Hz. Even though the latter difference is 10 times greater, it sounds the same to us. This gives a range of audible frequencies between 9 to 10 octaves, which is much wider than the range of frequencies of light that we can see.

Ratios

Two frequencies 1 octave apart have a frequency ratio of 2:1; one has twice the frequency of the other. A half octave is halfway between them on a logarithmic scale. That is, some ratio R such that f1 * R * R = f2. Since f2 = 2 * f1, R is the square root of 2, or about 1.414. Sanity check: 40 * 1.414 = 56.6, and 56.6 * 1.414 = 80. Thus 56.6 Hz is a half-octave above 40, and a half-octave below 80. Even though 60 Hz is the arithmetic half-way point between 40 and 80 Hz, to our ears 56.6 sounds like the half-way point between them.

More generally, the ratio for the fractional octave 1/N, is 2^(1/N). Above, N=2 so the half-octave ratio is 1.414. If N=3 we have 1/3 octave ratio which is 2^(1/3) = 1.260. Here is a sequence taken to 4 significant figures:

  • 1 octave = 2.000
  • 3/4 octave = 1.682
  • 1/2 octave = 1.414
  • 1/3 octave = 1.260
  • 1/4 octave = 1.189
  • 1/5 octave = 1.149
  • 1/6 octave = 1.122
  • 1/7 octave = 1.104
  • 1/8 octave = 1.091
  • 1/9 octave = 1.080
  • 1/10 octave = 1.072
  • 1/11 octave = 1.065
  • 1/12 octave = 1.059

The last is special because in western music there are 12 notes in an octave. With equal temperament tuning, every note has equally spaced frequency ratios. Thus the frequency ratio between any 2 notes is the 12th root of 2, which is 1.059:1. Every note is about 5.9% higher in frequency than the prior note.

Example

Suppose you are analyzing frequency response and see a peak between frequencies f1 and f2. You want to apply a parametric EQ at the center point that tapers to zero by f1 and f2.

First, find the logarithmic midpoint. Compute the ratio f2 / f1 and take its square root to get R. Multiple f1 by R, or divide f2 by R and you’ll have the logarithmic midpoint.

For example if f1 is 600 Hz and f2 is 1700 Hz, the ratio is 2.83:1, so R = sqrt(2.83) = 1.683. Double check our work: 600 * 1.683 = 1010 and 1010 * 1.683 = 1699. Close enough.

So 1,010 Hz is the logarithmic midpoint between 600 and 1700 Hz. We center our frequency here and we want it to taper to zero by 600, and 1700. That range is a ratio of 1.683 on each side, which in the above list is 3/4 octave. So now we know the center frequency and width of our parametric EQ.

Room EQ Wizard – A Great Tool!

Today I learned how to use Room EQ Wizard to tune my audio room. I had already done room tuning on my own and was happy with the results. But REW enabled me to get it even better.

Here’s the final FR measured from the listener position, 1/6 octave smoothed. Note this is 2 dB per division:

This shows a linear 1.5 db / octave slope from 40 Hz to 18 kHz. Deviations are +/- 4 dB of slope. I’m quite happy with this. I didn’t fix every little bump, but applied a few strategically located bands. The parametric EQ to get here is pretty mild. Each EQ band has amplitude of 4 dB or less, and widths range from 1 to 1/4 octave on each side of the center freq. In other words, gentle corrections and slopes. I’d rather have a few little bumps in the response, than perfectly flat response with bloated phasey sound from extreme EQ settings. Don’t let the cure be worse than the disease!

What I had before was pretty good, but this is better. Here’s how they compare:

Grey: no EQ; Blue: prior EQ; Red: current

Overall, this smoothed response throughout the range. During test listening I can switch curves instantly while the music is playing. My ears like the difference, especially noticeable on good acoustic music recordings.

Equipment & Details

  • Test audio files created by REW version 5.2 beta 4, burned to DVD-A
  • Oppo BDP-83 toslink PCM output
  • Behringer DEQ2496 digital EQ, toslink input and output
  • Oppo HA-1 DAC-preamp, toslink input, XLR output
  • Adcom 5800 amp (27 years old!), XLR input
  • Magnepan 3.6/R speakers (18 years old!)
  • Room treatments (floor-ceiling tube traps, RPG acoustic foam, etc.)
  • Rode NT1A mics
  • Zoom H4 portable recorder
  • Mic compensation curve
  • Recorded from the listener position

Here are the rest of the REW plots:

Total distortion averaged about -50 dB (0.3%); higher in the bass, lower in the treble. That seems surprisingly low, considering it’s measured at the listener position and includes all distortion from the power amp, microphone & recorder. Many headphones, even some tube amps, have more distortion than this.

The bad news is that distortion at 40 Hz is about 10%. Yikes! But it’s down to 1% by 60 Hz, and higher bass distortion is typical of speakers, the exception being planar magnetic headphones.

I’ve always been happy with the bass response in this room. 25 Hz is audible, even if attenuated. But seeing these measurements, I’ll bet that if I got a subwoofer to handle everything below 60 Hz, it might reduce overall distortion. I don’t want more bass, but tighter cleaner bass is always A GOOD THING. I’ll have to look into that!

Impulse response is -50 dB by 100 ms and near the room noise floor by about 200 ms.

Group Delay looks pretty flat too. I had to zoom the Y azis to 5 ms per division to see the curve:

The CSD looks linear (no obvious ringing frequencies) and decently fast. The room treatment certainly helps here:

Here’s the Spectrogram, again looks linear, no obvious ringing spots except down at 30 Hz. Even that decays quickly at first, then takes longer after the initial decay. That’s the tube traps at work!

This was a fun day. It’s neat to be able to get some measurements to quantify the sound I’m getting.

Balanced vs. Unbalanced Conversion

Generally speaking, balanced and differential signaling are two different things. They’re often (but not always) used together, and in audio, the term “balanced” refers to this.

Speakers and Headphones

A speaker or headphone responds to the voltage difference between its 2 input wires. It doesn’t assume either is ground, though one might be, it doesn’t matter. So connecting a speaker or headphone to a balanced output is easy. Just wire (-) to (-) and (+) to (+) whether or not the (-) is a ground (unbalanced output) or carries a signal (balanced output). If the unbalanced output has a common ground for both channels (like a headphone), you can split it to both L and R (-) in parallel.

Converting a balanced speaker or headphone output to an unbalanced connector is not as simple. An unbalanced headphone cable (a standard 1/4″ or 1/8″) has 3 wires: L (+), R (+), and a single wire that is a common ground for the L and R. You can’t connect a balanced output’s (-) wires to this ground. That would mix the channels, and allow the amp’s output stages to drive each other, which is bad because they usually have very low output impedance, so it can overdrive the output stages. Also, you can’t just ignore the output’s (-) wires and connect the headphone (-) wires together; this will give a common floating ground. In short, you need a transformer to do this conversion.

Components

If the balanced/unbalanced conversion is between components like a preamp (not a speaker or headphone), it gets more complex because unbalanced components assume the (-) is a ground, but the balanced (-) carries a signal and its ground is a separate (3rd) wire. You can’t connect the balanced (-) signal to ground; it will overdrive the upstream device’s balanced output as it tries to swing a voltage over a 0 ohm load. Also, you need to ensure the (-) wire has the same impedance to ground as the (+) wire.

So the best way to convert unbalanced to balanced between components is to use a transformer.

In short, the only case that can be wired directly, in which you don’t need a transformer, is wiring an unbalanced headphone output to a balanced headphone cable.

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.

Conclusion

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, though a contributing factor may be the Oppo’s linestage is not as refined as its headphone output.
    • LCD-2 (Fazor with upgraded 2016 drivers) are great headphones but not as resolving as Magnepan 3.6/R.
  • 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
  • The Oppo has a Sabre ES9018 DAC; the Soul has a Wolfson WM8741 DAC. Both are top quality DAC chips which I suspect are inherently transparent so that any differences we hear are in implementation and analog circuitry.
  • 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 truly unique.
      • 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 and organic, with a touch 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 and resolution.  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 sounds deeper in the bottom octave (< 30 Hz), but this perception was belied by the frequency response; 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. 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).
    • 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.
    • They sound slightly different, but it took me some time 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.
    •  At first my preference depended on what I was listening to. But with more listening across a wide variety of music I came to find the Soul more transparent and true to the source.

Engineering: Soul

  • From an engineering perspective, the Soul is “better” mainly for 3 reasons.
    • First: its volume control. It changes the gain rather than attenuating a fixed gain, and it is a stepped attenuator so there is no potentiometer in the signal path.
      • Advantage: lower noise and perfect channel balance at all settings.
      • Sometimes with a stepped attenuator the perfect volume you want is between clicks. But this never happened with the Soul; its 0.5 dB per click was always fine enough to find the perfect level.
    • Second: the Soul’s frequency-shaped internal feedback reduces distortion in the midrange where the ear is most sensitive. By analogy, frequency shaped dither is common practice in digital audio to reduce noise in the frequency ranges where the ear is most sensitive (at the expense of increasing noise outside this range). Why not apply the same principle to the amplifier’s negative feedback loop, to shape distortion in the same way? It makes perfect sense, though Meier is the only person I know of who does this in his amps.
    • Third: the Soul’s power supply; actually it 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.
    • I suspect these 3 features are the primary contributing factors to 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 raised transparency by a small incremental step.
    • 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 hearable sonic differences can exist, but 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 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 badly mastered, the DEQ2496 is too cumbersome to EQ it on the spot.
    • The Soul’s controls are much simpler: bass, midrange and treble knobs; digitally implemented.
    • No CD is perfect and I normally listen to how it naturally sounds, however imperfect. Yet some are more than imperfect, but flawed to the point of distracting from the music.
    • Here, I might 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 to music sources that have artificial hard L-R stereo separation. This can be distracting and the crossfeed gives a nice correction.
  • The Oppo has more types of inputs and outputs, both digital and analog.
    • With the Soul 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.

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?
  • 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, but we don’t 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, I don’t think you will find its equal in sound quality anywhere 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. I love 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 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:

Overview

  • 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.
  • 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.

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 4 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, 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 physical 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)