Erzetich Bacillus II+ Review – Sonic Storm in a Compact Form

Can you guess the character? He’s bald, walks barefoot (including at audio shows all around the globe), wears a Scottish kilt, and paints his beard with either pink or purple ink. If you’re thirsty, he might even offer you a drink or two, but prepare yourself for a strong kick (in the gut). Of course, that’s our esoteric headphones and headphone amplifier builder – Blaž Erzetič. Ever since reviewing his first-generation Bacillus headphone amplifier five years ago and then going on a quest to try out all of his hand-crafted headphones, I started having a higher respect for boutique crafts of all sorts, not only for audio-grade man toys. Although chatty and smiling, Blaž takes his work very seriously and over the years crafted what I believe are some of the best headphones money could buy. Erzetich Charybdis is still the king of soundstage and stereo separation, thanks to an oversized driver assembly, and it still offers some crazy technicalities for the price. If a head-fi enthusiast would ask me what’s the best-kept secret of today, it’s the Charybdis, which could be compared with pricier headphones, something I already did in my Erzetich Charybdis review.

Over the years, I had the pleasure of meeting Blaž and his crew at the High-End Show in Munich, and after trying his newest amplifiers, such as Deimos and Scylla, that looked so unconventionally cool and different from anything I’ve laid my eyes on, I swore that one day I would be rocking a similar amplifier on my table. Whether it was fate or pure coincidence, out of the blue, Blaž comes out of the shadows and asks if I’m willing to have a look at either his brand-new single-ended only Bacillus II or the balanced and single-ended Bacillus II+, which doubles the power output. How could I refuse to make acquaintance with one of his newest family members?

Since I’m more into current-hungry planar magnetic headphones than into high-impedance voltage-driven dynamic headphones, choosing the much more powerful Bacillus II+ made more sense to me since it wasn’t made only for their extensive lineup of headphones, but for a wider selection, whether dynamic or planar-based, single-ended or balanced.

We’re talking about an unusual Class-AB amplifier that rocks three encapsulated linear transformers, Obbligato Premium Gold audio capacitors, quad dual operational amplifiers for the input stage, and quad dual amplifiers for the current buffer stage. A substantial capacitance for the filtering stage is also in there, and of course, it features the same wacky looks of its bigger brothers. The mid-sized Bacillus II+ won’t occupy lots of desktop space, but it will cost you a pretty penny, going for €1,999 in Europe and for about ~$2,150 everywhere else.

Design & Build Quality

Ever since getting to know the Erzetich Scylla amplifier, experiencing its unconventional look characterized by a striking trapezoidal shape, spotting a remarkably unique volume knob that resembled a fusion of racing rims from a sports car and the four wide windows that added to its aesthetic appeal and also effectively channeled the hot air radiating from within, it became clear to me that Blaž is as dedicated to creating an eye-catching visual experience as he is to achieve exceptional sound quality

In the following year, he presented a redesigned Deimos amplifier that rocked similar design cues, having the same volume knob mimicking racing rims, a similar trapezoidal shape, and acrylic panels surrounded by aluminum thingies. The only difference is that it had three windows on top instead of four; it wasn’t as wide, it had a more desktop-friendly footprint, and of course, a wallet-friendlier price as well.

In 2024, the time has come to update the smallest members of their amplifier family, and thus Bacillus II and II+ surfaced online, having similar bodies and shapes, this time around dropping aluminum finished with wood accents that followed the heritage left by Bacillus Tilia, which used linden wood all around. The monochrome screen of the bigger brothers is no more, but this time around, the volume knob will show exactly how much power there is left on tap.

Although the amplifier is even smaller and occupies less desktop space, it still weighs more compared to similarly sized amplifiers coming from Asia. At 3 kilos (6.6 lbs), I’m pretty sure a properly made desktop headphone amplifier is still in there, disguised as a minimalist headphone amplifier. Except for having acrylic panels on its front and sides, everything else is made from metal, and it’s always nice having strong metallic feet with rubber inlays that will absorb the micro-vibrations coming from within.

I must admit that while it looks unusual compared to anything else out there, its price reflects the attention to detail and the overall build quality of the unit.

Last but not least, it’s important to mention that the repairability score of this unit is quite high. We have a total of three chambers; the power supplies are shielded from the rest of the electronics, and the three printed circuit boards are easy to access. Four out of eight operational amplifiers are socketed on DIP8 sockets, and in case one of them goes bad, you can easily swap it with home tools. If you are a tinkerer of all sorts, you might even swap those op-amps with better-sounding ones; you can tune it to your liking, and that’s what makes it a bit more interesting in my eyes. The layout is simple, straightforward, and beautiful to look at; the signal path is easy to follow, and the attention to detail is remarkable.

Controls & Connectivity

Since the Bacillus II+ is a headphone amplifier only, forget about getting a remote control or a preamp functionality, and since the unit is so simple inside and out, I don’t see the point in using a remote. On the front panel, a massive volume knob can be spotted exactly in the middle, surrounded by input and output selectors; an On/Off button sits on the left, and the usual 6.35mm (1/4″) and 4-pin XLR headphone outputs are located on the right side. Flip it around, and you’ll find a pair of RCA and XLR inputs along with the AC socket. Do note that for maximum performance, it doesn’t use a universal power supply; it is either a 110V or 220V unit, so make sure you order the right version.

One important aspect that many of you overlook, but these guys didn’t – the quality of the RCA and XLR inputs, of the AC socket, and course of the 6.35mm and 4-pin XLR socket. All of them are of a higher quality than usual, for example, that’s a Furutech AC socket and those are CMC RCAs and Neutrik XLRs, and all of the internal wiring uses silver-plated oxygen-free copper, used to preserve signal purity and keep the internal resistance as low as possible.

Under its Hood

If you follow the classic analog amplifier designs, where a short signal path is always desirable, achieving the goals with as few components as possible, then the Bacillus II+ is definitely that kind of unit. The signal path is easy to follow; the printed circuit boards are not overly populated with hundreds of components. Everything seems easy to follow and easy to understand, even for people who aren’t into electrical engineering.

Right after the AC socket, you can spot the first and second stages of the power supply design. Two encapsulated linear transformers (also called Printed-Circuit-Board transformers) will be powering its amplifier modules independently; the filtering stage follows next with a decent capacitance storing current for stronger dynamic swings. The third encapsulated transformer powers the auxiliary circuits, like the input and output switches, the On/Off button, and the volume pot. Speaking of which, this is a four-pole Alps blue velvet pot, already suggesting that a fully balanced circuit follows.

A gain stage wasn’t implemented for the cleanest possible signal, but we still have two OMRON relays at the input and another two at the output stage for perfect channel balance. As for the dual-mono amplifier modules, the input stage is first cleaned up by an Obbligato Gold Premium audio capacitor (1.0uF and 630V), followed by a DIP8 socketed LME49720NA op-amp that splits the unbalanced (RCA) signal into a balanced one. A couple of resistors later, we see the next socketed LME49720NA op-amp that handles the input preamp stage. The final stage consists of two incredibly powerful LME49600 high-current audio buffers that provide up to 250mA each (1A in total), from which planar headphones will greatly benefit.

And just like that, from the AC input to analog output, this little amplifier proves that you don’t need a complicated circuit to craft boutique headphone amplifiers.

The Elephant in the Room

While the Bacillus II+ stands as a solid amplifier, consuming just 9 watts and operating in Class AB mode, capable of driving an impressive array of low and high-impedance headphones, there’s an intriguing quirk to note. On paper, it offers merely 314 mW of power per channel through its balanced output and half that through its single-ended output. At first glance, it doesn’t seem like a powerhouse, likely one of the less impressive amplifiers I’ve encountered recently. Yet, the curious part is that my second hardest-to-drive headphones, the T+A Solitaire P were comfortably powered around the 12 o’clock volume position, a spot where my ears began pleading for a little relief. With the Sennheiser HD800S, the volume dialed down to 10 o’clock. I’m not quite sure whom to praise and whom to criticize, but good grief! It managed to drive nearly every headphone in my collection, save for the notorious HiFiMan Susvara and its sibling, Susvara Unveiled.

Perhaps some headphone amplifier manufacturers are embellishing their power ratings, or maybe Blaž is offering a little magic from the dark side to tame this beast of an amp, soothing its wild nature and trimming down those numbers in the process. The true essence of the situation remains somewhat elusive, but it becomes clear this amplifier can deliver ear-bleeding volume levels with most of my headphones while maintaining a brisk tempo and outstanding dynamics. Not all watts are created equal; some carry a heavier current drive, and this unit appears to be one of those. An essential feature is its avoidance of output capacitors, resulting in a grain-free sound without compromising the resolution of my DAC.

The Headbanging RIG

The Erzetich Bacillus II+ took center stage in my office, receiving signals from the Gustard X26 III DAC. I tested every imaginable IEM, along with portable and desktop headphones, eager to explore the upper limits of power and, of course, to assess the noise floor and channel crosstalk using notably easier loads. Whether it was high-impedance dynamic cans, low-sensitivity planar drivers, or ultra-high sensitivity IEMs, each type received individual attention through the robust 4-pin XLR output. With the Gustard X26 III being Roon-enabled, I simply plugged in an Ethernet cable, gaining immediate access to my entire music collection at my fingertips.

Alrighty, everyone, my body and soul are ready for some well-deserved music, so let’s hit some eardrums!

A Friendly Advice

Before going ALL-IN in the most overkill way, I need to tell you a secret. Writing such articles, doing dedicated photo shoots, and then crafting an equally impressive video review, shot in the highest resolution, doing color correction, and then adding tons of B-Roll material, all by myself…drains a lot of my energy and time. On average, I need six full days of work to write, film, and edit one single article and video, without counting the listening tests. All this knowledge, test results, and content go out FOR FREE. You won’t find our PayPal address around here, buy me a coffee link, or a Patreon account, everything goes out freely and I will continue in the same fashion as much as I can. The only thing I ask in return is a little bit of trust for the knowledge I’m leaving behind and of course, if that’s not too much to ask – subscribing to our YouTube channel that will motivate me to do things I like most in this complicated life. It will cost you a fraction of a second but the result will motivate me more than you can imagine. With that out of the way, let’s cheer up a little and check our test subject.

Sound Performance

I. Preliminary Sound Impressions

Since the brave-looking Bacillus II+ was gearing up for a family reunion, I immediately connected a pair of Erzetich Charybdis and a pair of Erzetich Phobos (V2024 revision), and right off the bat, a big wave of dynamics washed over my listening space. If you check any of my Charybdis or Phobos reviews, you’ll observe that these are among the easiest-to-drive planars you can buy at the moment, and it wasn’t a surprise to get them juiced up even at the 10 o’clock volume position. At 11 o’clock, I was already passing 95dB SPL and could stay there only for a couple of minutes. Since I want to preserve my hearing, it was pleasing to know that even low-powered amplifiers can drive both headphones to ear-bleeding levels without inducing any kind of distortion or coloration.

The second surprise that hit me like a storm was its neutral-ish tonality with just a smidge of warmth and tonal density that made all of my headphones ever so slightly easier to listen to. Although it never tried to juggle with the frequency response and aimed to be as honest as possible with the source material, I observed that the resolution refused to decline as most op-amp-based amplifiers would do at this point. In the world of op-amps, the LME line from Texas Instruments is regarded as one of the cleanest-sounding operational amplifiers, and I can certainly attest that a highly transparent sound was propelled forward without inducing a small dose of listening fatigue or brightness. The sound didn’t go overboard with treble sharpness as some THX-AAA amplifiers do; delivering a clean sound without becoming clinical in the long run. The Charybdis, in particular, has a slight rise in the most sensitive part of our hearing, around 7kHz – a characteristic I also observed on HiFiMan’s HE1000SE. An extra dose of treble energy could do more harm than good, but luckily, the treble excess is well-controlled, and nothing veers too far into the bright or dark extremes.

Last but not least, it’s surprising to get strong dynamics and these dopamine rush moments from an amplifier that can push only up to 314mW per channel. However, some amplifiers are endowed with “stronger watts” that can revitalize planar headphones without consuming lots of power or dissipating a ton of heat. This little puppy barely gets warm, as it dissipates only 9 watts, and yet the sound feels powerful in the lowest octaves and tightly controlled when things get messy. Dare I say, it is highly resolving without becoming harsh, making it compatible not only with Erzetich-made headphones but with a plethora of others and let’s write a chapter about that!

II. Power Output

Although I recently got a pair of HiFiMan Susvara Unveiled and the hunt for the best amplifier has already started, and yes, you can expect an overkill Susvara Unveiled review somewhere in January, I must confess that when it comes to dynamics and otherworldly transparency, I still hold my T+A Solitaire P a little higher. As much as I like the rest of my headphones for their particular skill sets, the Solitaire P ticks more boxes for this 40-year-old headbanger.

There is, however, one problem with the Solitaire P – they need lots of power to be controlled. At least this is what I believed until the Bacillus II+ hopped onto my table. These are my second hardest-to-drive headphones, and therefore not all headphone amplifiers were able to show their inner beauty. Before pressing the play button, the little Bacillus was already rubbing its hands and smiling from the shadows, as when I pressed the red button, the volume went from 10 to 12 o’clock, and again, half of the power remained on tap. Yet, I was already banging my head to 95dB loud electronic tunes. Just for the sake of science, I went a bit louder and even more so with bass-intensive music, as the bass notes drain most of the power from the amplifier. On average, about 85% of the power an amplifier provides is there to infuse life into the bass and control all of that tightly. Maybe I didn’t get the thunderous bass I was accustomed to from a 42-kilo Cayin Soul 170HA that pumps around 18 watts per channel, but the little Bacillus II+ hadn’t thrown its punching gloves just yet. The unusual part was getting a clean bass performance without distortion, which many affordable amplifiers would produce at this volume. Regardless of how loud I went with the Solitaire P—which is notorious for sucking the life out of desktop headphone amplifiers, the bass energy didn’t lower a single bit, nor did it get distorted, dumped to the side, or choke at dangerous volume levels.

And just like that, you can include in here most of the planar mafia headphones, all of Meze’s, most of HiFiMan’s except for the upper-class Susvara/Unveiled, as well as all of Audeze’s, Kennerton’s, and Sendy Audio cans.

Alright, but how about dynamic headphones? I still have a couple of those; most are driven even by portable DAPs. However, I still have one that can cause trouble even for a few desktop amplifiers, so why not? Let’s try the high-impedance Sennheiser HD800S on the little Bacillus II+. Since we’re testing the amplifier’s limits, I didn’t go easy on it, instead, I engaged some of my nastiest tracks. G.O.A.T. and later Playing God by Polyphia (found on Qobuz and Tidal) started dropping some bass-heavy riffs onto my head. The absolute highest volume I could reach for a couple of seconds was at the 11-and-a-half position; 12 would be way too loud, even for a headbanger. I later engaged in some of Symbolico’s masterpieces such as I Am Staring At You and Tribal Future (found on Qobuz and Tidal), renowned for their punchy bass lines that have already crippled tons of headphone amplifiers in the same office. My eyes opened wider as the bass was perfectly controlled, and this time around, I got a stronger dose of it, quantity-wise, something one shouldn’t expect from a pair of HD800S. The sound was close to perfection, and the HD800S performed the way I know them to be on much more powerful and costlier amplifiers. The resolution remained sky-high, but the treble still didn’t bother me. It’s a win-win situation if you ask me.

Alrighty, but how did the HiFiMan Susvara and Susvara Unveiled fare? Well, this is where the white flag was dropped on the ground. Although, volume-wise, both cans sounded very loud around the 3 o’clock position, the bass notes were heavily distorted, a clear sign that the amplifier was choking, incapable of delivering additional amperes. You can include every Abyss nutcracker, the DCA Stealth and Expanse, and of course, all of the RAAL headphones, as our little Bacillus would be incapable of nicely controlling their drivers.

III. Noise Floor & IEM Pairings

The first-generation Bacillus, which was about four times smaller than today’s unit, couldn’t deliver noiseless performance with noise-sniffing IEMs. Regardless of the volume level, the noise floor was higher than that of the music, and some hiss could be heard with the music on pause. But I’m ready for round two. I spot a much nicer power supply implementation and filtering stage, and while the power has increased, the noise suppression mechanics feel much stronger on the newest unit. Since some of you are still rocking IEMs even at home, let’s give this one another chance.

The manufacturer doesn’t specify the noise floor of either the 6.35mm or 4-pin XLR output, meaning I will need to test it the old-fashioned way, with some of the most sensitive IEMs in my collection. Armed with three pairs of ultra-sensitive IEMs (FiiO FH19, FA19, and Westone Mach 80) and using the most powerful 4-pin XLR output, I found that regardless of the headphone jack used, I could never detect any hiss playing in the background. With a sensitivity of almost 110dB per single milliwatt of power, the FH19 has already dethroned plenty of desktop amplifiers up to this point, including much more expensive ones like the HiFiMan PRELUDE, LAiV Harmony HP2A, Cayin Soul 170HA, Burson Soloist Voyager, hell, even the Enleum AMP-23R that I love so much, had a bit of hiss with such ultra-sensitive creations.

The little Bacillus II+, on the other hand, was completely dead silent even at full power/maximum volume, something I don’t remember experiencing even with the Benchmark HPA4. On paper, fully discrete amplifiers should be noisier, and they usually are, mostly because several op-amps are replaced by hundreds of components that generate significantly more noise, lowering channel crosstalk in the process. And that’s exactly what I’ve experienced over the years. An open circuit will generate noise and crosstalk if crucial components remain in proximity to each other, which is why high-end DAC and amplifier makers encapsulate their output stages and amp modules. Erzetich didn’t overcrowd the PCB layout, maintaining proper spacing between analog components and using a thick metallic spacer/heatsink around the power transformers.

In the end, it didn’t matter what portable headphones or IEMs I used with this guy; all of them sounded noiseless regardless of the headphone jack or volume position. You can forget about some hiss or nasty gremlins shouting in the background, as there’s only music playing and nothing else. You can use the highest sensitivity IEMs, and you’ll never hear disturbing noises, making it an extremely IEM-friendly unit. Volume-wise, the highest I could go was between 9 and 10 o’clock, and if you want to have total control over your IEMs, use the single-ended 6.35mm jack that will offer you a higher volume travel.

IV. Dynamics & Transients

Alright lads, let’s get into it and talk about the dynamics. The ugly creatures that make us go wild, jump around, and feel the blood flowing are definitely dynamics. I went back to the Charybdis since these can get rad fast and engaged some dopamine-rushing tracks that I know very well. This time around, I didn’t engage the usual electronic suspects but rather a quite unusual track that an audiophile would never use! It’s raw, it’s noisy, it’s mixed and mastered like crap, the channel crosstalk is higher than Burj Khalifa; it’s the last song a sane person would use to test Hi-Fi grade electronics. But oh man, it’s so incredibly fun, punchy, mean, and electrifying!

Little Black Submarines (2021 Remaster) by The Black Keys (found on Qobuz and Tidal) is the pure definition of madness. At the 2-minute and 5-second mark, prepare for a Hi-Fi flustercluck. Drums go wild, guitars are being smashed while still playing, and the guitar solos feel close and personal, yet so strong and biting. The backing vocals are casually singing a note or two. This is what I would call an electrifying track that refuses to calm down. It’s just the Black Keys doing their thing, perhaps moving further away from the blues of their early career albums and bringing some good catchy riffs and rhythms. The rest of the tracks from the El Camino album aren’t overly complex or groundbreaking, but it would be a lie to say that this isn’t enjoyable as hell to listen to. Keeping it simple is key, but they never fail once to make a good song, making this a damn good album. The little Bacillus was pushing dynamics high with the Charybdis and Phobos, and while I’ve heard stronger dynamics, those were usually coming from bigger, weightier, and pricier amplifiers.

Dynamics and massive power reserves usually go hand in hand, but that’s not always the case—and the little Bacillus II+ proves it. I’ve tested several gentle giant headphone amplifiers that provided absurd power ratings, dwarfing everything else on the test bench; yet many of them couldn’t electrify low-sensitivity headphones the same way this little thing managed to do right from the start. The case size, weight, and power reserves are sometimes meaningless, and it seems that the Bacillus II+ reinforces this truth once again.

Having a super short signal path and a good power supply implementation while bypassing a gain stage somehow unshackled the sound, not only in terms of scale but also when discussing dynamics. Blaž’s secret sauce has proven effective, delivering fast and impactful bass notes, so much so that several nearby headphone amplifiers lacked both oomph and the engaging attitude provided by this middle-ground amplifier.

The second surprise that came later on was realizing that the second-gen Bacillus is keeping a faster-than-usual pace, rhythm, and timing. The pace and timing are crucial; it’s only with them that we can get a clean sound and tight control of the drivers when things get crowded. This little amp refused to slow down with electronic music, and I never felt that it was mixing the notes the way affordable op-amp amplifiers were doing it for me. In high-end audio, everything’s about resolution and timing—miss a thing, and you’ll miss everything. All amplifiers need to be fast and nimble; otherwise, you can forget about focusing on music floating mid-air, defined contours, or getting the precise location of the notes. While it won’t outperform the craziest-sounding solid-state amplifiers that passed through my hands, it’s still a strong candidate for a wild and impactful amp that doesn’t stay in the way of the music.

V. Soundstage & Imaging

I will start by mentioning that the Erzetich Charybdis already has one of the biggest soundstages you can experience via headphones. The T+A Solitaire P has a higher see-through transparency, but when it comes to left-to-right imaging and grand scale, I’m sorry, but nothing does it like the Charybdis. The Charybdis always push the sounds outside my head, more so than the Meze Elite, Audeze LCD-4 & 5, HiFiMan HE1000 Unveiled, Susvara & Susvara Unveiled, and I find them considerably airier than any other planar headphone. From my headphone collection, the only one that can challenge them is Sennheiser’s HD800S. What was a blessing for the HD800S on its release date was also a curse, as although the HD800S mimics the sound of a near-field stereo rig, that long sound travel removes some energy, resulting in a softer overall performance. The HD800S can’t rumble with the big boys in the dynamics department, as their energy falls short even on high-end amplifiers. The Charybdis aren’t like that, playing all around my head without dropping energy levels. There’s no softness with the Charybdis; there aren’t hazy leading edges. Everything’s crystal clear, yet powerful and highly energetic.

These sound massive, more like having speakers attached to your ears, and finally, words like soundstage have meaning with this pair of headphones. But of course, it can still go bigger or smaller depending on what you are connecting this particular pair of headphones to. Go with a small USB-C dongle or a mid-level digital audio player, and suddenly, their most impressive skill is no longer as obvious. Go with desktop amplifiers that use negative feedback, and again, their bigger-than-life sound becomes lifeless, not as open and grand as I know them to be. But go with something that never stays in front of the signal, and watch them fly off and sing the way they are supposed to.

I fired up Untitled by All My Faith Lost … (found on Qobuz and Tidal), a neoclassical darkwave mixed with so much love, care, and a bit of sadness. Every little note was wrapped in a mystical fog as if the musicians were standing a couple of meters away from each other. Mixing male and female vocals with tons of acoustic instruments in between, it’s beautiful music from any point of view. The spacing between the notes is so high, that I get this weird feeling that the musicians are playing in different rooms. The sound staging capabilities of this amplifier are great, and there’s no other way to describe it. This album feels so relaxing and soothing that in the next minute, I poured myself a glass of wine and watched the rain skittle down the windows. This isn’t an album to cheer you up with colorful rainbows; instead, it makes you think and reflect. The little Bacillus II+ did proper justice to this album, and when I moved to the Sennheiser HD800S, the same feelings enveloped my listening space.

I’d say it’s a bit more impressive here compared to a similarly priced Ferrum OOR (with the stock power supply); it’s also grander sounding compared to a pricier Burson Audio Soloist 3 GT (standard package). Although it’s not as spacious as a HiFiMan PRELUDE ($2,500), LAiV Harmony HP2A ($2,700), and Burson Soloist Voyager ($3,500), let’s not forget the price difference.

VI. Resolution / Detail Retrieval

One of the first impressions that passed through my mind while listening to this fellow was how easily it pushed the information up without tilting its tonality towards the bright side. The Erzetich Audio team did everything they could to lower the noise floor and channel crosstalk, and let’s not forget the short signal path and its dual-mono design. We’re even getting a third transformer that deals with auxiliary systems so that the other two transformers juice up only its dual-mono amplifier modules for the cleanest power. On top of that, its low-power design drowned its distortion into the abyss, resulting in a clean and transparent-sounding amplifier that could easily brawl with some of the cleanest-sounding units out there, such as the Benchmark HPA4, Sparkos Labs Aries, Topping A90 Discrete, and even the Burson Soloist Voyager.

You know what’s a little unusual? The HiFiMan PRELUDE and the LAiV Harmony HP2A, amplifiers that I loved listening to through the HiFiMan Susvara and T+A Solitaire P, couldn’t push a similar level of micro-level information. Shocking? Maybe, but that was to be expected knowing how crowded their PCBs are.

After using several tracks that I have known for a lifetime, impressive clarity and definition surfaced, but not to the point of calling it clinical or overly sharp. Even poorly mastered music sounded clean without making me lower the volume. Driving the Charybdis – its complementary headphones was quite an eye-opening experience, as terms like clinical, boring, or bright never came to mind. Despite being an amplifier with a reference tuning, the outcome was an unstained experience that ever-so-slightly tilted towards naturalness.

In the end, the Bacillus II and II+ aren’t just fancy facelifts; in line with their bigger brothers, they genuinely seem to have a higher dose of resolution under their hood while further lowering the noise as much as possible. Although for many of you, this price point will be a tough pill to swallow, when it comes to micro and macro detail information, this unit competes with some of the best amplifiers I’ve tried so far.

Since the noise has completely disappeared compared to its predecessor, which I tested five years ago, good luck finding traces of distortion even at higher volumes, as you will be listening to the limits of your DAC and headphones. Living for quite some time with clean-sounding amplifiers, I am not that easily impressed, and yet the Bacillus II+ revealed itself as Mr. Clean, disappearing from the acoustic chain while preserving all of the good and all of the bad behind your music. I have a lot of respect for audio components that aren’t adding distortion. With such devices, I will be listening only to the truth. High transparency goes hand in hand with detail retrieval, and it wasn’t a challenge finding things in tracks that I know well, revealing themselves without taxing my mind.

VII. Is this an Op-Amp roller’s dream?

Short answer: Nope! I didn’t quite expect this. But let’s rewind everything and start from the very beginning. Under its hood, we have four socketed dual op-amps and four soldered current buffers.

From the socketed op-amps, two are converting the single-ended signal into a balanced one if the RCA inputs are used, meaning those will make zero difference sound-wise if you use the XLR inputs. The only op-amps that can make a difference are the next ones that work at the input/preamp stage. Those are the LME49720NA from Texas Instruments. These op-amps aren’t biased into full Class-A operation, and therefore, you can use any dual op-amps that come to your mind. Yep, including discrete ones from the likes of Sparkos Labs, Burson Audio, Orange Amps, LHY Audio, and many other brands that create discrete op-amps in dual variants. I had on hand four pieces of Burson’s newest V7 Classic and another four pieces of V7 Vivid and I decided to give them a spin.

I started with the Classics, and instead of swapping just two of them, I went full throttle and replaced all four LME op-amps with V7 Classics. I calmed my spirits and pressed play. I can’t explain to you how and why it happened, but the sound didn’t improve…on the contrary, the sound got brighter and a little noisier. The unusual part is that the FiiO FH19 ultra-sensitive IEMs started picking up a gentle buzzing sound, turning on and off several times per second, like transistors were switching on and off. I then went all in with the V7 Vivid op-amps and more or less, I had the same déjà vu moment, sounding a bit foggy in comparison to the stock op-amps and making it even brighter. I couldn’t believe my ears, because these very same V7 Vivid and V7 Classic discrete op-amps worked miracles in the Burson Soloist Voyager and later their all-in-one Conductor GT4. The only explanation I have is that both of these discrete op-amp offerings prefer a higher voltage intake and a hotter bias than usual – something the little Bacillus II+ is not providing, thus lowering their final performance.

Unfortunately, I do not have any other discrete op-amps on hand and can’t comment on different pairings, but the moment I installed the stock op-amps back into their sockets, the sound cleaned up, the noise floor was no longer evident, and the sound acquired a nicer tonal balance, adding some richness where there was none. Was it clever engineering to choose this particular LME op-amp that fits this circuitry like a glove? I don’t know, but right now, I won’t change a thing on the stock Bacillus II+.

Frequency Response

VIII. Bass

• Heavy bass riffs with the Charybdis and Phobos? Check!
• Strong bass lines with the Sennheiser HD800S and later with the T+A Solitaire P? Double Check!
• Bass energy ramping up on modern tunes, sustaining its energy when organ pipes come to play, and then gently rolling it off without hitting a ceiling? Triple Check!

With that being said, it won’t deliver a powerful slam into your eardrums like the Enleum AMP-23R, the LAiV Harmony HP2A, and the Burson Audio Soloist Voyager do, dropping it like a bomb. The bass is there in the right doses, and sometimes it might appear as if it’s getting some muscle mass, but it won’t compete with pricier and more powerful amplifiers that can bring the thunder with the right music and headphones. Erzetich Charybdis and Phobos didn’t need additional bass energy, nor did Sennheiser’s HD800S crave for more low-end energy; however, harder-to-drive headphones and most of the HiFiMan creations were asking for a nicer impact down low. On the plus side, regardless of what headphones I used, I didn’t experience a rise in distortion levels, except on super difficult loads such as the Susvara and Susvara Unveiled. The little Bacillus II+ has already proved that high-quality bass can be achieved even from an op-amp-based amplifier, and while sometimes I crave a stronger kick in the lowest octaves with some of the difficult-to-drive cans, let’s not forget that it isn’t a very universal headphone amplifier.

While everyone knows that the Erzetich Charybdis delivers a fabulous low-end, after engaging Limit to Your Love by James Blake (found on Qobuz and Tidal), the bass murmurs at the 1-minute mark took center stage, pushing and pulling my eardrums like it was a walk in the park. I’m thrilled to report that the Bacillus II+ wasn’t just a technical-sounding little marvel but also a fun-inducing wunderbar when your favorite tunes hit the airwaves.

IX. Midrange

Perhaps the only region that didn’t feel juiced up as much as I would have liked was the midrange. I’m reminding you that, as much as I’m trying to be unbiased, honest, and neutral, personal opinions and beliefs are hard to shake, and therefore there will always be a dose of bias. I’m also reminding you that as a former measurement believer and neutrality seeker, I no longer pursue such goals in my life; right now, I’m all about the fun times. I’m more into things that make me dance and let my imagination run wild, and that’s how I slowly started appreciating powerful tube amplifiers and discrete amplifiers biased into Class-A operation. While our little test subject is neither of those, there is still plenty of life behind those musical instruments, but not enough to grab my soul and walk me down memory lane. On the flip side, we aren’t getting the dreadfully thin midrange performance typical of entry-level ChiFi amplifiers; there’s considerably more life and meat on the bone, just not as much as I would like. Think of it as a neutrally tuned amplifier that doesn’t try to mimic tube amps or powerful fully discrete creations, instead choosing to walk its own path.

The DAC that did all of the heavy lifting these days was Gustard’s X26 III, which in many ways has a similar tonality to today’s test unit, and when stacked together, they create a sense of rightfulness, as if everything plays exactly as it should, without boosting a particular frequency range or emphasizing the sharpness of musical notes. Obviously, I’m not describing a combo that adds warmth, sweetness, and pleasant coloration, but at the same time, there’s no thinness to the sound, and the voices never felt recessed or nasal.

X. Treble

When you start grinning with headphones that have strong trebles such as the T+A Solitaire P, Erzetich Charybdis, and Sennheiser HD800S, it’s clear that the amp doing most of the heavy lifting is nicely controlling the treble output. What’s particularly interesting about the Bacillus II+ isn’t merely its treble extension or amazing clarity that it has shown previously, but rather its rendition; it never makes the treble harsh, dispelling notions of brightness or listening fatigue.

Compared to some of the fully discrete amplifiers that I’ve tested as of late, such as the LAiV Harmony HP2A and HiFiMan PRELUDE, the Bacillus II+ puts on display a stronger treble definition without gently rolling off parts of it. That’s the beauty of it and also its downfall; on the plus side, additional nuances start appearing compared to these pricier fully discrete amplifiers. On the flip side, the trebles start demanding your full attention, and a bad headphone pairing is likely to happen. For example, while it nicely controlled the treble excess with the Solitaire P, HD800S, Charybdis, and Phobos, it wasn’t the best pairing with any HiFiMan headphones. Be they the Edition XS, Ananda Nano, or even the Arya Organic, this amp didn’t bring enough sweetness to the midrange, nor did it calm HiFiMan’s spirit in the treble, tilting the final performance toward the bright side.

Some headphones would perform like clockwork, without any delays or tonal balance issues, while others might need additional tuning from the downstream equipment (streamer, DAC, and cabling) to sing the way you like it.

Wrapping Up

Let’s be honest, the only complaint we could chant in unison would be the weak power output. It’s indeed not looking spectacular on the spec sheet, and right there, it already limits several headphone choices. All of the Abyss headphones won’t work with flying colors, and the flagship DCAs won’t get the proper treatment; the same goes for HiFiMan’s upper class and some of the RAAL offerings. But let’s not forget that it was mainly created to drive their line of headphones, which it gracefully did. It was me who decided to give it a spin with the rest of my headphone collection and pit it against several IEMs, and I’m happy to offer a complete evaluation for those who rock more than a single pair of headphones.

This isn’t a universal headphone amplifier because (sometimes) you’ll be limited by the power output. And while it doesn’t sweeten things up in the midrange and doesn’t offer strong doses of dopamine and oxytocin with bright-tilted and/or hard-to-drive cans, it worked great and exceeded my expectations when driving the Sennheiser HD800S and the T+A Solitaire P. It was a perfect match with ultra-sensitive IEMs, fully juicing their diaphragms without introducing a higher noise floor into the mix.

I could never believe that a third of a watt could decently drive the Solitaire P and the HD800S while still retaining 50% of power on tap. It didn’t make any sense, but it’s a clear sign that not all watts are made equal… and I know, it sounds crazy, but it’s exactly what happened with the little Bacillus II+.

Some might raise an eyebrow knowing I’m discussing an op-amp-based headphone amplifier that has a simplified layout, a short signal path, and a limited number of components. But that’s the beauty of it. Sometimes less is more, and if all you need is an amplifier that does proper justice to your Thalia, Mania, Phobos, and Charybdis headphones, then Blaž’s middle child deserves your full attention.

If you’re already plotting for a Christmas gift for yourself, let me ease the wallet damage for you. I teamed up with Erzetich Audio for a 15% discount for the first five customers for the Bacillus II and Bacillus II+. Use the SNEWS2025 promo code at checkout, and a 15% discount will be automatically applied.

If you take the plunge and have some burning questions, feel free to drop me a line in the comments section below. That’s all for now; Sandu’s signing out!

One more thing. This was the most challenging year we’ve had so far. I want to thank you all for following, sharing, reading, and watching our content; it means a lot to me! Thank you for your love and support; it will be repaid back in some way or another. Wishing you a Merry Christmas and a Happy New Year; may it be filled with adventures and good fortunes!

Stay tuned for more; the best is yet to come!

PROS:

• Rad design choices and an unusually cool look
• Rock-solid build quality that’s easy on the eyes and hard on your ears
• Bonus points for the space-saving footprint and vibration-absorbing metallic feet
• Simple-to-understand PCB layout, short signal path, and great component selection
• Fully balanced input-to-output Class-AB amplifier
• Sounds honest, transparent, and close to the source material
• Great sound staging capabilities
• Fast pace, rhythm, and timing
• Still able to drive plenty of headphones, even if the spec sheet might suggest otherwise
• A very IEM-friendly unit thanks to its noiseless performance
• Tames treble excess and nicely controls it
• Highly dynamic sounding on all accounts
• Handcrafted in Europe using some of the best connectors

CONS:

• Won’t be able to juice up every single headphone, hungry planars desire for more current intake
• Its honest and neutral tonality won’t work great with bright-tilted headphones
• It’s not a Mjolnir-wielding hard-pounding amplifier

ASSOCIATED EQUIPMENT:

• HiFi Racks: WoodYard Suspended Triple & Baby Modular
• Digital Transport / Roon Server/ Streamer: Rockna Wavedream NET 4 TB
• Network Switch: Ansuz PowerSwitch D3, Matrix Audio SS-1 PRO
• DACs: Rockna Wavedream Signature XLR, Gustard X26 III, Gustard X30, LAiV Harmony DAC
• Preamplifiers: Chord Electronics Ultima PRE2
• Power Amplifiers: Chord Electronics Ultima 3 (X2)
• Headphone Amplifiers: Erzetich Bacillus II+, Cayin Soul 170HA, HiFiMAN PRELUDE, LAiV Harmony HP2A, Burson Soloist Voyager, Conductor GT4
• Loudspeakers: Raidho TD 2.2
• Headphones: Erzetich Charybdis & Phobos V2024, HiFiMan Susvara & Susvara Unveiled, T+A Solitaire P, Meze Elite & 105 AER, Kennerton Rognir, Sennheiser HD800S & many more
• Interconnects: Crystal Cable Art Series Monet XLR (X2)
• Speaker cables: Crystal Cable Art Series Monet
• Power Cables: Crystal Cable Art Series Monet (X4), Roboli Stars (X2)
• Ethernet Cable: Crystal Cable Art Series Monet
• HDMI Cable (I2S): AudioQuest Dragon
• Balanced Isolation Power Conditioner: KECES IQRP-3600