Sparkos Labs, founded by audio engineer Andrew Sparks, is known for its focus on precision and high-quality discrete components. The company first gained recognition with its hand-built op-amps, designed for both audiophiles and professional use. Over time, Sparkos expanded into complete amplifier systems, including the Gemini hybrid headphone amplifier. The Gemini reflects the brand’s commitment to clean, detailed sound at a competitive price point. In addition to amplifiers, Sparkos continues to support the DIY and upgrade market with parts like voltage regulators, power supplies and op-amp adapters, many of which are also used in their own products.
Their entry into the headphone amp market began in 2019 with the Aries, premium-priced model. In 2023, the company introduced a more budget-friendly option, the Gemini. It pairs a 6922 Electro Harmonix vacuum tube input and a Sparkos Labs’ own SS2590 discrete op-amp output architecture.
Andrew Sparks has generously provided us a Gemini headphone amplifier for a review, allowing us to thoroughly explore its sonic capabilities and build quality.
At jjazzifi, I’ll be putting this exceptional amplifier through its paces, enjoying the fun of tube swapping. This is a great feature that is too often overlooked or ‘sadly’ just mentioned in other reviews. Only few realize that this is one of the amplifier’s greatest strengths: the ability to reshape its entire sonic character with just the change of a tube. That’s why I’ll be pairing it with an exciting selection of NOS vintage vacuum tubes from renowned manufacturers of the past, like Telefunken, Mullard, Philips and the original Tesla’s.
It’s like a game of chess, swap out a single piece and the entire strategy shifts. Dive into the world of Sparkos Labs with me and experience music exactly as it was meant to be heard.
Test equipment:
– Headphones: The Hifiman Arya Unveiled, Heddphone Two GT, Fir Audio M5 and the new Letshuoer DX1
– Streamer: Eversolo A8 + upgraded LHY LPS
– DAC: Chord TT2 + Mscaler
Valve Tubes used:
– Telefunken E88CC, 1967 Germany/Ulm
– Tesla E88CC 1980, (former) Czechoslovakia/Vrchlabí
– Mullard 6922 1972, Military grade Mitcham/UK
– Philips SQ E88CC 1969, Special Quality Netherlands/Heerlen
– Philips ECG 6922 USA 1984, (Former Sylvania / Pennsylvania)
– Electro-Harmonix 6922 (Installed in the box)
– Electro-Harmonix 12BH7
The Sparkos Labs Gemini:
The Gemini comes equipped with RCA input and preamp output connections on the rear, up front, it offers a 6.35mm (¼”) single-ended headphone output and a balanced 4-pin XLR connector. There’s also a power switch and a gain toggle, allowing users to switch between high and low gain, which is not a normal gain, it actually give you the option to give more or less tube sound into your stage. A 6922 vacuum tube handles the input stage. Sparkos Labs includes a user-adjustable bias control, letting you fine-tune the tubes sonic signature. You can find more information and other settings in the manual (see link below). Standard configuration is ‘high’ Bias, Which Sparkos Labs thinks, sound the best with a 6922/E88CC Tube.
One important notice about the jumper setting:
It’s harmless if the jumper is set to the wrong tube type. However, never install jumper caps on both tube type positions at once. Use either type 1 or type 2 never both, or you’ll short the heater power supply.
The gain button allows you to change how much of the tube’s sound character comes through. When set lower, the music becomes warmer, fuller and more textured; the classic tube sound. When set higher, the sound becomes cleaner and more neutral, with less of that tube flavor. It’s a simple way to choose between a smooth, rich sound or a more precise and detailed one. A very nice unique feature I have not encountered before in other amps.
The output stage features Sparkos’ discrete TOTL SS2590 op-amps alongside their own voltage regulation modules. Volume is managed via a high quality Alps Blue velvet potentiometer. The circuit is designed with zero global feedback and avoids capacitors in the signal path for maximum purity.
The Gemini supports various tubes, there is a jumper on the mainboard to change for the various types (you can read more about that in the manual: Manual Sparkos Labs Gemini
Tube valve support:
Type1:
6922 ECC88, 6DJ8
6AQ8 ECC85
6CG7
6N1
Type2:
12AT7 ECC81, 12AT7WC
12AX7 ECC83, ECC803S, 7025
12AU7 ECC82, 6189, ECC802S
12AY7 6072
5751
12BH7
There is soft-start heater circuit with a 40-second warm-up for the tube and DC protection for the headphone outputs. The amplifier offers an exceptionally low output impedance, under 0.25 ohms, ensuring high damping and excellent control across virtually all headphones.
The Gemini is, at its core, an unbalanced design and that’s actually one of its strengths. While balanced amplifiers often get a lot of attention, the truth is that a well-designed single-ended amp can sound just as good, in many cases even better than a poorly executed balanced one. Rather than investing in the extra circuitry required for a fully balanced layout, Sparkos Labs chose to focus on what truly matters: using top-tier components to achieve exceptional sound quality.
You still get a 4-pin XLR output, but it’s there purely for convenience, not because the amp is balanced. What’s great is, that the full power of the amp is also available through the unbalanced output, so nothing is compromised.
The chassis is straightforward and functional, built from black powder-coated metal. It may look small and compact in design, in person it feels robust and well-made. The volume knob and buttons have a solid feel.
To swap out the vacuum tube or adjust the bias, you’ll need to remove the top cover. Depending on the tube type, you may also need to reposition a few internal jumpers. Which is really easy to do, unless you want to swap out tubes every day. In general I expect you keep changing untill you find your Queen piece, and stick with it.
How does it sound?
The Sparkos Labs Gemini is not your run-of-the-mill hybrid headphone amplifier. It’s an exceptional engineered amplifier that aims to bring together the lush, organic character of vacuum tubes and the surgical precision of discrete solid-state circuitry. Where many hybrid amps stop at “warm and clear,” the Gemini dives deeper offering user-controlled gain and tube bias settings that dramatically shape the sound signature. It’s like trying to win a game of chess, endless possibilities and strategies to get where you want to be.
My starting point will be the installed Electro-harmonix 6922.
Bass
What strikes you first when listening to the Gemini is the bass control. The low end doesn’t bloom unnecessarily, nor does it feel lean, it sits perfectly in that rare zone between musical warmth and analytical grip. On planar magnetic headphones like the HIFIMAN Arya, the sub-bass extends with authority, reaching low without distortion or blur. The bass impact feels fast, tight and refined, something that’s clearly the result of Sparkos’ discrete SS2590 op-amp output stage doing its job with a high damping factor.
Treble
The top end of the Gemini feels silky, but never dull. You get detail without harshness, a kind of restrained sparkle that eases you into long listening sessions. Unlike some purely analytical amps, which can etch out cymbals with almost surgical sharpness, the Gemini gives treble a rounded character, delicate, but present.
There’s a bit of a vintage vibe to how it handles highs, like the golden days of tube hi-fi, it doesn’t mask or blur details though. It’s forgiving, but not soft. Remarkable!
Soundstage and imaging
The soundstage isn’t artificially wide, but it feels natural, with excellent widht and depth. Imaging is actually pinpoint accurate. You can follow the placement of each instrument in an ensemble with ease. There’s a slight front-row intimacy in the presentation, but without sacrificing air or spatial definition.
Low vs high gain
One of the most unique aspects of the Gemini is how its gain settings don’t just change output power, they actually alter the sonic personality of the amp. This is no simple op-amp gain switch. I tried various tubes, changed bias settings, listened to high/low gain, every change made me reconsider which combination I liked the most. So, even though it’s called a “gain” switch, it’s really a way to control how strong or subtle the tube sound was heared through the music.
Low Gain Mode introduces a purer, more transparent signal path. It leans slightly more toward neutrality, offering a more balanced, linear sound. Treble feels slightly more extended, bass a touch leaner. This is ideal for sensitive headphones or IEMs where dynamics and detail retrieval are key.
If you increase the gain, and then turn the volume down to the same level as before, you’ll hear less of the tube sound, about 10dB less of that tube influence.
High Gain Mode brings The sound becomes thicker, warmer, with added harmonic weight. Bass hits with more authority and the midrange feels fuller, almost like switching from a smoky jazz club to a big concert hall.
What’s especially smart is that the gain settings work synergistically with the user-adjustable tube bias, meaning you can finely sculpt the amplifier’s behavior to your headphones and preferences.
Vintage tube tour
Now let’s get some other big pieces on (the) board to play with. I started of with the installed Electro-Harmonix 6922 to get a base to compare to.
Listening Material:
To evaluate the tonal character, emotional nuance and spatial presentation of each tube, I curated a diverse selection of recordings spanning from vocal jazz, orchestral works and blues. Each track was chosen for its production quality, dynamic range and ability to reveal subtle shifts in timbre and microdetail.
From the golden age of jazz, Ella Fitzgerald’s Summertime and Cry me a river from clap hands, Here comes Charlie! were essential in testing vocal texture and upper-midrange sweetness. The intimate delivery of Stacey Kent on The changing lights and So nice provided a test for natural vocal tone and airiness, especially in tubes with a romantic or warm character.
Melody Gardot’s Trav’lin light and Baby I’m a fool helped assess layering and soundstage depth, while Tony Bennett’s unforgettable duet with Amy Winehouse on Body and soul brought out the emotional complexity and aging texture of male vocals, particularly in midrange-rich tubes like the Mullard or Philips SQ.
To explore dynamics and orchestral scale, I turned to John Williams’ Hedwig’s theme (from Harry Potter and the Philosopher’s Stone) and Herbert von Karajan’s legendary interpretation of Beethoven’s Symphony No. 7 in A Major, Op. 92 (Berlin Philharmonic, 1962). These recordings challenge a tube’s ability to maintain control, resolve layers and handle crescendos without smearing.
To explore grit and feeling, B.B. King’s The thrill is gone and How blue can you get offered insight into low-end warmth and expressive nuance. Joe Bonamassa’s Drive and Mountain climbing brought in a more modern blues-rock edge, with dynamic guitar work and atmospheric production that revealed how well a tube handled energy, tone decay and layering under pressure.
Together, these recordings formed a comprehensive sonic landscape, making it possible to hear each tube’s unique fingerprint, whether focused on emotional warmth, technical precision or somewhere beautifully in between.
Electro-Harmonix 6922
This is a current production tube that delivers a clean, articulate and slightly forward sound. It offers solid detail retrieval and good control, making it a practical choice, especially in systems where reliability and consistency matters. While it may not match the richness and harmonic depth of some rare vintage tubes like the Telefunken E88CC or Philips SQ (which are increasingly hard to find and often very expensive), it provides a balanced and engaging foundation that performs well across a variety of genres
Treble: Clear and extended, occasionally a touch edgy in brighter setups
Midrange: Transparent and focused, though slightly lean compared to warmer NOS options
Bass: Tight and well-controlled, with good definition
Soundstage: Decent width and depth, though less holographic than higher-end vintage tubes
I would recommend for:
Rock, Electronic, Pop, Blues, Modern Jazz
Electro-Harmonix 12BH7
The Electro-Harmonix 12BH7 is a modern production tube commonly used in driver stages, where it delivers a neutral, stable and controlled performance. It offers a clear and focused sound with an emphasis on structure and reliability, making it a solid foundation in many amplifier designs.
Tonally, the EH 12BH7 leans toward a cleaner and drier presentation, which can be beneficial in systems that prioritize precision and speed. While it doesn’t emphasize warmth or romantic coloration, this tube provides a dependable platform, especially when paired with opamps like the SS2590. The SS2590 can add a welcome dose of musicality and dynamic finesse to this tube.
Treble: Crisp and defined, though can be slightly sharp with cymbals or brighter recordings
Midrange: Honest and clear, though somewhat restrained in emotional delivery
Bass: Tight and dry, well-controlled but not overly rich
Soundstage: Moderate depth and layering, focused more on coherence than spaciousness
I would recommend for:
Rock, Metal, Hard Rock, Electro/Techno
Tesla E88CC former Czechoslovakia republic/Vrchlabí – 1980
The original Tesla E88CC, produced in the former Czechoslovakia, is known for its warm, musical tone with a smooth and engaging presentation. It brings a natural flow and airiness to vocals and instruments, making it a favorite among jazz lovers who seek emotional connection without excessive coloration. This tube really breathes that vintage sound, very vocal midrange orientated.
Treble: The treble is soft and gently rolled-off, offering a natural and non-fatiguing top end
Midrange: This is where the Tesla truly shines. The midrange is lush, realistic and full of character
Bass: Bass response is full and deep, with a warm and slightly rounded character, slightly loose
Soundstage: The soundstage is wide and atmospheric, with an almost holographic layering
I would recommend for:
Vocal Jazz, Acoustic, Classical chamber music, Folk
Telefunken Germany/Ulm E88CC – 1967
The legendary Telefunken E88CC is praised for its ultra-precise, detailed and neutral sound. It excels in clarity, articulation and control. It is ideal for those who want to hear every nuance in piano strokes, brushwork and vocal inflections; provided the system has enough warmth to avoid sounding sterile. Be aware though, tubes in this quality as shown here, will be around 200/300 euro and above a piece. Worth every penny though.
Treble: The treble is bright, open and highly extended. It brings out the sparkle in cymbals and string overtones with a fine, airy finesse but never harsh or edgy
Midrange: The midrange is exceptionally clean and articulate. Vocals are presented with a striking sense of presence and separation
Bass: The low end is tight, fast and well-controlled. While it may lack the warmth or bloom of more romantic-sounding tubes, it excels in rhythm and accuracy
Soundstage: Rather than wide or lush, the soundstage is focused and pinpoint-accurate
I would recommend for:
Classical, Modern Jazz, Electronic/Ambiant, Studio monitoring
Mullard 6922 Military grade Mitcham/UK – 1972
The Mullard 6922 is the epitome of romantic, lush tube tone. Rich mids, smooth treble and a rounded low end make it perfect for late-night jazz, smooth vocals and acoustic intimacy. It doesn’t aim for analytical precision, It is really tuned for enjoyment over analysis
Treble: The treble is gentle and sweet, with a slightly rolled-off character that avoids any harshness or fatigue
Midrange: Rich, organic and unmistakably ‘tubey’, the midrange is where the Mullard truly lives
Bass: The bass is full-bodied and rounded, with a soft bloom that complements the tube’s lush midrange
Soundstage: The soundstage is wide and dreamy, with a slightly soft focus. Imaging is less about pinpoint precision and more about creating a warm intimate atmosphere
I would recommend for:
Vocal Jazz& Ballads, Blues, Soul, Lounge, Vinyl playback in general
Philips SQ E88CC Special Quality Netherlands/Heerlen – 1969
The Philips SQ (Special Quality) is known for beeing one of the finest E88CC tubes ever created. With its refined, balanced tone and holographic imaging, it brings music to life in a natural, emotionally engaging way. A gold standard for jazz lovers who want both musicality and fidelity.
Treble: The treble is delicate, silky, and nuanced, never harsh or overly bright. Subtle details are rendered with finesse.
Midrange: The midrange flows effortlessly, offering a highly textured and lifelike presentation
Bass: Bass response is tight, well-controlled, while not the deepest the focus is musical and tuneful
Soundstage: One of this tube’s greatest strengths. The soundstage is wide and deep, with a holographic three-dimensionality that allows instruments to occupy distinct positions in space. Imaging is precise, but never artificial
I would recommend for:
Vocal Jazz, Classical (any form), Acoustic, Blues, World Music
Philips JAN ECG 6922 USA 1984 (Former Sylvania / Pennsylvania) – 1984
This American-made tube under the Philips ECG brand offers a clean, neutral sound with solid reliability. It lacks the depth and emotional nuance of vintage European tubes, but performs adequately in more clinical systems that need taming. More functional than magical, but used a lot in hifi devices in the 80s and 90s
Treble: The treble is clear and present, occasionally a bit harsh with lesser recordings
Midrange: Tonally accurate but emotionally flat at times
Bass: Bass is tight and well-controlled, though somewhat lean
Soundstage: Imaging is accurate enough, but there’s little sense of air or spatial layering
I would recommend for:
Rock, pop, General use (all-rounder)
– Not balanced (eventhough not needed) for some a deal breaker
– Opamps are surface mounted
