There’s something special about tube amplifiers. That nice, warm, glowing sound they have makes music sound so real—like you're listening to old vinyl rather than some cold digital thing. Fosi Audio's GR40 packs all those good vibes into the modern era, combining tube warmth, solid-state convenience, and Bluetooth versatility in one tidy package. The matte black aluminum casing looks pretty sweet on your workspace, and the glowing orange tubes provide that nice retro touch without going too far. But wait, the best part? It's not just a tube amp. It's a high-res DAC (32-bit/384kHz & DSD256), a Bluetooth receiver (with aptX HD support), and it has a couple of bass and treble knobs so you can customize the sound however you like.
Fosi Audio GR40 Review
Let me confess, I’m a crazy tube lover. From my childhood I used to listen to vinyl with tube amps. When the deck set arrived with solid-state amps, I switched to them but missed those good old days. But now tube amps are softly conquering the audiophile world, and that makes me happy. Now I have three desktop tube amps and one tube DAP in my collection.
What is really good about the Fosi Audio GR40? It’s nothing but those nostalgic mellow tube sounds it is capable of providing. The overall sound is warm, soft and has a lot of tube texture in it. Moreover, you can spice up your music with bass & treble knobs, and that plays a fantastic role! You may or may not like it, but it’s there for you.
DISCLAIMER
FOSI AUDIO was kind enough to send me the GR40 to review. I am not affiliated with Fosi Audio, and of course, I was not asked for any copy-checking before publishing. All opinions shared in this ‘Fosi Audio GR40 review’ are my subjective thoughts.
One thing more: The review is based on my personal experience and what I hear using different sources. Your experience may vary because of personal preference and physical differences like the shape and depth of the ear canal.
For your personal purchase, check out HIFIHeaven or HIFIGO for their fantastic products and promotions (non-affiliated links).
SPECIFICATION
● Input: USB-C, Optical, Coaxial
● Output: 2X RCA, 3.5mm stereo headphone
● Frequency Response: 20Hz-20kHz
● Dynamic Range: 104dB
● Sampling Rate: 32bit/384KHz, DSD256
● Headphone Impedance: 32Ω-300Ω
● Headphone Output Power: 300Mw@16Ω – 250mW@32Ω – 50mW@250Ω – 40mW@300Ω
● Channel: Stereo (2.0)
● Bluetooth Version: 5.1
● Lossless Codec: SBC, ACC, aptX, aptX HD, aptX LL
● Voltage Range: DC 12V, 1.5A
BOX CONTAINS
● The GR40 main unit
● Two 5654W vacuum Tubes
● Power supply unit
● Bluetooth Antenna
● User Manual
DESIGN & BUILT
● Full aluminum alloy chassis with a matte-black finish and signature orange accents
The GR40 might be considered entry-level in terms of price, but its design and build feel far from cheap. The body is made entirely of aluminum alloy, finished in matte black. It’s compact, solid, and feels reassuringly heavy for its size. Fosi’s signature orange details give it a bit of personality without looking flashy.
● Rubber feet underneath to reduce vibration
Rubber feet underneath help the unit stay put and reduce vibration. The only plastic parts here are the Bluetooth antenna and, naturally, the glass vacuum tubes themselves.
● Sturdy, feels more premium than you’d expect at this price
Everything else feels metal and sturdy. It’s a minimalist look, but with enough flair to stand out. It’s the kind of device you don’t mind showing off beside your gear.
INTERNALS
● DAC: ESS9028Q2M Sabre chip
Under the hood, the GR40 uses a respected ESS Sabre ES9028Q2M DAC chip—a chip known for its high performance, low distortion, and solid dynamic range. This gives the digital section a clean, well-resolved sound.
● Amplification: Dual GE 5654W vacuum tubes
For amplification, the GR40 uses a pair of classic 5654W tubes from GE (General Electric), which are known for reliability and smooth sonic character. The tubes add that classic analog warmth many listeners crave.
● Bluetooth: Qualcomm QCC3031 module
Bluetooth connectivity is handled by a Qualcomm QCC3031 module. This chip ensures stable transmission and decent codec support, including aptX and its variants.
OPERATION
One of the nicest things about the GR40 is how simple it is to operate. The orange volume knob controls both power and source switching—press and hold to turn it on or off, and quick presses to switch between USB, optical, coax, and Bluetooth. There’s a small LED ring that lights up when the device is on, and subtle lights beneath the tubes add a cozy visual touch, especially in low-light environments.
The bass and treble knobs have clear midpoints at 12 o’clock, making it easy to find your neutral baseline. One drawback is the noticeable background hiss when using sensitive IEMs, but this can be mostly resolved with a 30-ohm impedance adapter. Bluetooth pairing is fast and reliable, though for serious listening, USB is the way to go.
SOUND IMPRESSIONS
Plugging in the GR40 for the first time, the initial reaction might be underwhelming—especially if you leave the tone knobs at neutral. The sound can feel slightly flat or even sterile. But once you start using the tone controls and let the tubes warm up, everything changes. The GR40 opens up into a much more musical and emotionally engaging experience.
Used with IEMs and some planar headphones, the GR40 showed it could handle both efficient and power-hungry gear. It may not be dead-silent with sensitive IEMs (you will hear some background hiss), but with mid-to-high impedance headphones, it performs beautifully. And yes, the tone knobs aren’t just for decoration—they’re part of the fun.
LOW FREQUENCY
• Drums and bass guitar had a solid presence
• Warm, deep, but never bloated
• Tone controls help dial in more sub-bass if you want
The bass from the GR40 is warm, round, and just a little soft at the edges—but in a good way. It’s not a tight, clinical bass like you’d get from a THX amp or studio DAC, but it feels rich and satisfying. Kick drums thump nicely and basslines have real weight. If you want a bit more slam, you can easily dial up the bass knob without muddying the rest of the mix. It’s not overpowering—just musical and full-bodied.
MID FREQUENCY
• At the 12 o’clock tone knob setting, vocals sounded clean, not recessed
• Mids felt lush, not coloured too much by the tubes
• With instruments, midband detail stayed intact
The mids are where the GR40 really shows its tube charm. Vocals sound natural and slightly forward, with a nice sense of presence. Guitars, pianos, and strings all come through with a pleasing richness and texture. These frequencies are well-supported, never thin or recessed. You get a sense of body in the music that makes everything feel more “alive,” especially with acoustic or vocal-driven tracks.
HIGH FREQUENCY
• Highs can be a touch laid-back—without tone adjustment, it feels smooth
• Dialing treble up a bit helped bring more sparkle back
• No harsh tizz, good for long listening
Treble is smooth and relaxed. There’s a bit of roll-off, but that’s part of the tube sound. It won’t sparkle as much as solid-state amps aimed at resolution, but it won’t ever hurt your ears either. This makes the GR40 perfect for long listening sessions. Even tracks with aggressive cymbals or high-pitched vocals stay comfortable. You can add a little zing with the treble knob if needed, but even then, it avoids sibilance.
STAGE & SEPARATION
• Soundstage is decent, not massive
• Instruments remain distinct, no smearing or crowding
• Particularly with tone controls, spatial clarity improved
Soundstage on the GR40 is not huge, but it feels natural and intimate. You won’t get a super-wide or deep holographic image, but you’ll get enough space to let vocals and instruments breathe. The layering is decent—especially with more dynamic or acoustic recordings. Stereo separation is clean, and imaging is accurate enough to enjoy live recordings or complex arrangements.
TECHNICALITIES
• Dynamics – Punchy, But Not Overwhelming
• Speed – Smooth, Not Lightning-Fast
• Analytics – Warmth Over Absolute Detail
DYNAMICS
The Fosi GR40 imparts a nice quality of musical dynamics, especially if you tinker with the tone knobs. It has that warm tube sound that gives music a slightly rounded edge. The quiet parts sound intimate, and when the music gets fuller, the amp opens up with good energy. It provides you with that pleasant, smooth boost that just makes you wanna nod your head to the rhythm.
SPEED
Tube amps, especially the entry-level ones like the GR40, aren’t exactly speedy—and that shows here too. The transient response is a little soft, so the attack and decay aren’t super-sharp. Notes will bloom slightly before they decay, particularly in bass and midrange. That makes the GR40 more appropriate for smooth genres like classical, lo-fi, ambient, and soulful vocal songs.
ANALYTICS
Let’s get real—the GR40 isn’t an analytical amp. This isn’t an audio device that’s meant to dissect every final micro-detail in a song. Instead, it’s musical, with a sound that’s more rounded and cohesive than hyper-detailed. The resolution is nice, due to the Sabre ES9028Q2M DAC chip inside, but the tube stage introduces some warmth and softness, which blurs some of the finer details.
REAL WORLD TESTING
SENDY AUDIO AIVA 2
Aiva 2 has recessed mids, and the sub-bass along with the mid-bass lacks a bit of warmth. The overall tonality is kinda bright to my taste. Moreover, with 32 ohms impedance and 96 dB sensitivity, it is somewhat of a hard-to-drive headphone. First of all, this Fosi Audio GR40 is capable of driving this easily, no problem at all. And when it comes to the sound, mids still remain recessed. But with the bass knob in the 3 o’clock position, the lower frequencies show a very good amount of warmth, which is very enjoyable. And that warmth didn’t overwhelm the mid or upper frequencies.
EDIFIER D12
The one and only speaker system I have, cause I’m not a speaker guy. On a budget, this is a fantastic pair of active speaker systems for a small room with 70 watts (RMS) of output power. When I connected it to the GR40, oh man, the story is different! I never thought they would sound this sweet. The first thing I notice is the dynamic details it increases. I mean, it’s not analytical and too detailed. And the bass got a bit of ‘womph,’ especially the mid-bass, which shows some good body in it. The top end also sounds sweet.
ARTTI T10
A budget planar driver IEM with great details. It’s not very hard to drive, but it lacks a bit of warmth in the lower region. When I plugged it in with the GR40, my experience was different. The bass knob shows its bold capabilities here. Without overpowering the mids, the lower region becomes present with a good body and warmth.
QoA ADONIS NEW
It’s dark and bass-heavy. By ‘dark,’ I mean the shyness of the upper mids and the treble spark is missing here. And the treble knob of the GR40 comes as a great salvation. Without giving any fatigue, the treble presents with a good hint of spark and shine. And I really enjoy the airiness in it. The overall sound from Adonis New becomes kinda ‘warm-balanced,’ where I definitely enjoy the bass and also the open mids with shiny treble section.
COMPARISON
● VS. xDuoo MT-604
MT-604 is only an amp without any DAC inside. You need to buy MU-604 or use any other DAC from your collection. I will concentrate on sound only other than build, input-output etc. First of all the MT-604 is a four 6J1 tubes and transistors hybrid set-up, while the GR40 uses two 5654W GE tubes. Sound from the MT-604 is kinda balanced with a lodas of details in the mids and treble, has good mid bass speed, while the sub-bass is sweet but lacks some rumble. On the other hand GR40 delivers almost true tube mellow sound with a good hint of warmth in the lower region and lacks a bit of details in the mids and upper frequencies compared to the MT-604.
The output power is also different. The GR40 is able to deliver 250mW @32 ohms, where the MT-604 can deliver 2000mW @32 Ohms. The MT-604 has only balanced output no single ended, Bluetooth, not even RCA. On the other hand, the GR40 has single ended output, Bluetooth, RCA and coaxial but lacks any balance output.
The MT604 is a fully balanced AMP without any DAC inside but GR40 has built-in DAC, Bluetooth connectivity but with only single ended output.
VS. KAEI Design TAP 2
TAP2 is one of my favorite amps in my collection. They sounds so sweet and organic. It has two different mode- solid state & tube. Like the MT-604, it has no DAC inside, so you need to connect it to an external DAC. Though they implemented the fantastic JAN 645 tubes poorly with no shock absorber, but the sound it produce is fantastic. With the bass & treble knobs in 12 o’clock position, the GR40 won’t beat the TAP2. But those knobs are the main strength of the GR40. In terms of portability and battery installed inside, I will prefer the TAP2 over the GR40 anytime. Even the power output is also higher in TAP2 with 1000mW @32 ohms.
CONCLUSION
The Fosi Audio GR40 is a really cool all-in-one tube amp and DAC that warms up your desktop and fills it with sound. It’s well constructed, extremely easy to use, and has that vintage vibe that just makes you wanna sit and listen to your favorite music. The tone controls have that nice, hands-on feel.
But hey, it’s not gonna be ideal for everyone. If you’re listening on sensitive IEMs, this might not be your first pick. There’s a slight bit of background hiss unless you grab some impedance adapters, and that’s just a hassle. And if you’re into really fast, detailed, or analytical sound, the GR40 might be too laid-back or relaxed for you.
WHATEVER YOU’RE THINKIN’ OF BUYING—DAC, DAP, IEM, HEADPHONE, AMP, WHATEVER—JUST MAKE SURE YOU GIVE IT A DEMO RUN FIRST. TRY IT OUT, LISTEN TO IT WITH YOUR OWN TRACKS. SERIOUSLY, ONE QUICK AUDITION CAN SAVE YOU A BUNCH OF MONEY AND REGRET LATER ON.
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