Recovery and restoration service:frontea online,corp.
‹ 2026/03/01 ›
Good evening. Spring has fully arrived.
And with it — the storm of deadlines. End of fiscal year!
(Is it okay to be writing hobby articles during the busiest season? Sweat)
Ever since building the PopoDAC, the number of audio devices I want to overhaul has exploded. ^^;
(That’s how good the PopoDAC is♪)
So starting today, I’ll be building an ultra‑wideband, ultra‑high‑resolution discrete operational amplifier.
Naturally, the target specs are monster‑class. Sweat
The circuit design and PCB are already finished, so let’s first take a look at the actual hardware. ^^
Yes — for an op‑amp, it’s monstrously huge. Sweat
(The one on the left is the CFA‑type op‑amp.)
(The PCB on the right is a VFB‑type op‑amp.)
I tried to make everything SMD, but this size is about the limit for me… ><
Size alone isn’t interesting, so let’s look at where this discrete op‑amp is aiming in terms of performance.
Name: PopoCFA Clarity
Architecture: CFA (Current Feedback Amplifier)
Supply Voltage: up to ±24 V
Output Current: up to 500 mA (max 1 A)
DC Offset: approx. –21 mV
Slew Rate: 150 V/µs
GBW: 25 MHz
THD: 0.0017%
Characteristics:
High voltage, high output, excellent separation and reproduction (razor‑sharp ultra‑fast SR), ultra‑wide bandwidth (beyond audible range), sufficiently low distortion.
Applications:
Front‑stage op‑amp with excellent soundstage separation, IV‑conversion gain multi‑amp, or even a standalone headphone amplifier.
How is it? Pretty insane, right? ^^;
And you can probably sense how broadly applicable it is.
(Though this is all based on LTspice simulation…)
“150 V/µs · 25 MHz · CFA · 500 mA · ±24 V”
In short, the goal here is to build an op‑amp that fully preserves everything from silence to the rise of each audio quantum.
The ultimate objective is an amplifier that can reproduce the hardest thing in audio:
the sound of raindrops and the trembling of air, naturally, even through speakers. ^^
“Raindrops or the sound of wind cutting through air — you can hear them through earphones or headphones, but have you ever heard them naturally through speakers?”
(Most people probably never consciously think about it — or only realize it when pointed out.)
Honestly, until developing the PopoDAC, I didn’t fully understand it either.
It took me half a century to truly perceive raindrop texture.
During PopoDAC development, after endless listening and refinement, I eventually discovered raindrop particles inside a track — and only then understood how difficult they are to reproduce.
For example, when heard collectively, rain sounds like “shhh,” “babababa,” or “pitter‑patter,” essentially noise.
But in nature, sometimes it’s noise, sometimes it’s calming, sometimes it deepens loneliness.
The reality is:
each raindrop is a particle producing its own sound, and rain noise is simply the mixed aggregate of those particles.
If reproduced at the level of individual audio quanta, the sound becomes more natural.
If resolution is low, everything collapses into a single blob of noise.
“Wait, didn’t you build the Popo BVPPC Headphone Amplifier to maximize PopoDAC’s fidelity?”
Yes.
The Popo BVPPC had excellent depth, soundstage, and instrument separation.
But one thing was lacking:
It couldn’t reproduce raindrop texture as naturally as direct PopoDAC → earphone listening.
This made me realize that beyond low distortion, high SR and wide GBW are essential.
(Meaning: unless analog circuits can follow quantum‑level transitions, rain won’t separate into raindrop particles.)
Let’s deepen the understanding of “audio quanta” (Audio Frequency Quantum, AFQ).
First: audio quanta do not physically exist.
“Is it about transmission particles?”
No.
“Is it digital bits?”
Half‑correct — but AFQ is a conceptual model that includes analog behavior.
AFQ represents the conceptual “unit particle” of rain sound detectable in analog.
Think of it as the edge between sound and silence.
Let’s look at the diagrams.
(Here the original diagrams are referenced.)
The enlarged diagram shows the waveform trapped inside a digital grid and evaluated at upper critical resolution.
Left: 96 kHz / 32‑bit grid → waveform edges remain fairly faithful.
Right: 48 kHz / 16‑bit grid → edges blur and become blocky.
“More blurry than expected.”
“Looks jagged.”
Have you ever felt:
“No matter how much I chase THD, the sound still feels cloudy”?
This is the difference in resolution.
Green = linearization (hardening)
Blue = blockiness (clumping)
Red = burying (softening)
All three degrade sound quality.
Ultimately, digital fidelity depends on the relationship between bit depth and time resolution.
PopoDAC improves this via MTL and ASRC.
To preserve this in analog, AFQ must be captured and manipulated.
For example, to fully utilize 32‑bit 96 kHz DAC output:
Base sample period: 10.4 µs
AFQ resolution needed: 325 ns
To swing a 1 Vpp quantum to ±15 V cleanly:
Required slew rate: 92 V/µs
Op‑amps are indispensable in modern audio amplifiers.
People expect them to serve as precision preamps, headphone amps, IV converters, etc.
But in practice, the most important role is error absorption.
If error absorption is excellent, the op‑amp’s inherent precision can be fully utilized all the way to final output.
Looking at typical specs:
(Here the original comparison table is referenced.)
At first glance, ultra‑low THD suggests high transparency.
But aside from the monster OPA637, few audio‑oriented ICs approach SR ≈ 100 V/µs.
(Reference: TPA6120 — another monster for headphone amps.)
TLE21xx and THS46xx are often used, but they have narrow stability margins and limited supply voltage ranges, making them difficult to use broadly.
Delivering AFQ all the way to the speaker at this precision is extremely difficult.
How does Clarity compare?
Inside an op‑amp structure, it offers:
Ultra‑fast SR (325 ns resolution),
Wide GBW (handling >96 kHz transitions),
Continuous output up to ~0.5 A.
If used as an error amplifier all the way to the output stage, it should convert PopoDAC’s raindrop quanta into audible raindrops. ^^
This is not something achievable by “pampering a normal op‑amp.”
If we slightly shift what we expect from an op‑amp, Clarity’s destination becomes clear:
“Capture AFQ — the smallest time particle defining sound contours — and reproduce each raindrop as its own sound.”
If that vision resonates with you —
If you felt an “Oh!” —
You may be on the verge of experiencing unexpected realism.
Next time, we’ll dive into the circuit explanation.
Stay tuned♪
