Fosi Audio GR70 Tube Headphone Amplifier

Vintage Charm Meets Modern Versatility

The headphone amplifier market has exploded with solid-state options in recent years, but there’s still something uniquely engaging about a well-designed tube amplifier. The Fosi Audio GR70 strikes an impressive balance—delivering the warmth and character of vacuum tubes while incorporating modern features and user-friendly design that makes tube technology accessible to everyone.

What makes the GR70 particularly intriguing is its high/low-impedance switch, which isn’t just a simple gain adjustment but allows the amplifier to interact differently with various headphone impedance curves. This creates a genuine tube-driven sound signature that can be tailored to different headphones—something I’ve documented with measurements in this review to show exactly how this can nicely change the sound signature of some high-impedance headphones.

Unlike many tube amplifiers that are either too colored or too sterile, I feel that the GR70 strikes a sweet spot that enhances music without overwhelming it. For headphones like the 300Ohm Sennheiser HD600, this interaction creates a naturally pleasing bass response without requiring EQ—something I’ve come to appreciate over months of daily listening.

I’ve been enjoying it for several months and decided it was time to share my experiences with this excellent tube amplifier but before we get into the sound lets look at the unboxing experience.

Unboxing and Build Quality

The Fosi Audio GR70 arrives in a sturdy, well-designed box that protects the delicate tube amplifier during shipping.

Opening the box reveals the amplifier securely nestled in protective foam, along with accessories and documentation.

Inside, you’ll find the GR70 carefully packaged:

The complete package includes:

• Fosi Audio GR70 Main Unit
• 2× 6AC7 Vacuum Tubes
• 2× 5654 Vacuum Tubes
• Tube Protection Covers
• 12V/3A Power Supply
• User Manual

The tubes come separately packaged for protection during shipping:

The power adapter is a standard 12V/3A unit suitable for global use:

Build quality is impressive for the price point. The GR70 features a CNC-machined aluminum alloy body that feels substantial and durable. The chassis provides both aesthetic appeal and functional heat dissipation for the tubes.

Design and Connectivity

The GR70 features a classic yet modern design that showcases its tubes while maintaining a clean, elegant aesthetic.

The front panel includes:

• Large volume control knob
• Bass and treble adjustment knobs
• High/Low impedance mode switch
• 6.35mm (1/4") headphone output
• 3.5mm headphone output
• Power button with LED indicator

The top view highlights the four vacuum tubes, which not only provide the amplification but also serve as an attractive visual element, especially in low-light conditions when their warm glow is most visible:

A closer look at the tubes shows the quality of construction:

The rear panel offers simple but functional connectivity:

• RCA stereo inputs
• RCA stereo outputs (for use as a preamplifier)
• DC power input (12V)

This layout provides everything needed for a headphone amplifier while also allowing the GR70 to function as a tube preamplifier in a larger system—a welcome bonus feature.

Technical Architecture

The GR70 employs a two-stage tube amplification design that contributes to its distinctive sound character:

Quad Tube Configuration

The amplifier features four vacuum tubes in a thoughtfully designed circuit:

• 2× 5654W tubes: These handle the voltage amplification stage, providing the initial signal boost and contributing to the amplifier’s tonal character.

• 2× 6AC7 tubes: These power amplification tubes deliver the current needed to drive headphones effectively.

This two-stage approach allows each tube type to focus on what it does best, resulting in a balanced, transparent sound with the warmth and texture that tube enthusiasts seek. The 5654W tubes (sometimes labeled as “5654” on the packaging) are known for their smooth, detailed sound, while the 6AC7 tubes provide excellent power delivery.

Dual Impedance Modes

One of the GR70’s standout features is its high/low impedance switch, which alters how the amplifier interfaces with headphones of different impedances:

• Low Impedance Mode (16Ω-120Ω): Optimized for most modern headphones, IEMs, and portable over-ears
• High Impedance Mode (120Ω-300Ω): Tailored for traditional audiophile headphones like the Sennheiser HD600/650/800 series

This isn’t simply a gain switch—it actually changes how the amplifier’s output impedance interacts with the headphone’s impedance curve, creating different frequency response profiles.

Impedance Interaction Effect

The GR70 pairs wonderfully with a variety of headphones, but lets start with the Sennheiser HD600:

To understand why this matters, let’s look at the impedance curve of the Sennheiser HD600, a classic headphone known for its uneven impedance:

The HD600’s impedance rises significantly in the bass region (around 100Hz), reaching over 500 ohms. When paired with an amplifier that has higher output impedance (like the GR70 in high-Z mode), this creates a natural bass boost through electrical interaction—a phenomenon sometimes called “impedance coupling.”

I measured this effect with several headphones, starting with the HD600:

As you can see, switching to high impedance mode with the HD600 produces a pleasant bass boost centered around 100Hz—exactly where the HD600’s impedance peaks. This creates a warmer, more full-bodied sound without requiring EQ. The difference is roughly +3dB in the midbass region, which is subjectively perfect for bringing the HD600’s somewhat lean bass into better balance.

This effect varies with different headphones depending on their impedance characteristics.

With the Sivga Peng, which has a flatter impedance curve, the difference between modes is more subtle but still present, primarily affecting the upper bass and lower midrange.

With the Meze 105 Silva while we loose SPL with the High-Z option we get relatively more sub-bass so again a nice change:

This variability between headphones is part of what makes the GR70 and tube amplifiers in general a lot of fun. I would recommend that everyone have at least one tube amplifier to try with their headphones as it can enhance certain headphones’ characteristics in a way that feels natural rather than artificially boosted.

For headphones with a low and flat impedance curve like Fosi Audio i5, the GR70’s low-z mode is ideal providing a nice headphone amplifier with a natural sound character:

I also tested the GR70 with high-sensitivity IEMs and found that in low impedance mode, there was no audible noise floor, making it versatile enough for use with a wide range of headphones.

Sound Impressions

The GR70 delivers a tube sound character that’s pleasingly musical without being overly colored. Its presentation differs from solid-state amplifiers in subtle but meaningful ways.

Bass

The bass response is where the tube character and impedance interaction are most noticeable. With the HD600 in high impedance mode, the bass gains weight and body that these headphones can sometimes lack when used with linear solid-state amplifiers. There’s a satisfying warmth and roundness to bass notes, particularly evident on acoustic bass in jazz recordings like Christian McBride’s “Night Train.”

Even in low-impedance mode with less impedance interaction, the bass remains full and detailed, with good texture and decay that brings out subtleties in bass-heavy electronic music like Massive Attack’s “Angel.”

Midrange

The midrange presentation is rich and somewhat forward, placing vocals and lead instruments slightly ahead of the rest of the mix. Female vocals in particular benefit from the GR70’s presentation—Diana Krall’s “The Look of Love” showcases a lush, intimate quality that draws you into the performance.

The tube implementation provides a subtle harmonic richness that’s especially evident with acoustic instruments. Piano notes have a natural weight and decay, and string instruments display a tangible texture that makes performances feel more lifelike.

Treble

The treble is smooth and well-extended without harshness or fatigue, even during extended listening sessions. The GR70 tames the occasional brightness of headphones like the HD600 without sacrificing detail or air.

Cymbals and high-hat details are rendered with natural texture and decay. On complex orchestral pieces like Mahler’s Symphony No. 5, the upper registers remain clear and distinct without becoming etched or strident.

Soundstage and Imaging

The GR70 creates a moderately wide soundstage with good depth. Imaging is precise, allowing for easy placement of instruments within the sonic landscape. The tube presentation adds a sense of dimensionality that makes the sound feel less confined to the headphones.

On binaural recordings like “Bubbles” by Yosi Horikawa, the spatial cues are rendered with convincing accuracy, creating an immersive listening experience.

Comparison with Other Amplifiers

To put the GR70 in context, I compared its frequency response characteristics with several solid-state amplifiers that I am also reviewing, the Burson Playmate 3 and the Aune N7 as you can see in the graph below those amplifiers (and even the Apple usb-c dongle) all provided a similar Frequency response with minimal differences:

The GR70 isn’t trying to compete with these solid-state amplifiers on technical measurements—instead, it offers a complementary listening experience that many enthusiasts will appreciate having in their collection. The tube warmth and impedance interaction provide a sound character that’s impossible to achieve with digital EQ alone.

Key differences between the GR70 and solid-state alternatives:

1. Tonal Character: The GR70 has a warmer, more organic presentation compared to the clinical neutrality of solid-state options, especially with higher impedance headphones.

2. Impedance Interaction: Unlike most modern solid-state amps with near-zero output impedance, the GR70 deliberately leverages impedance interaction to enhance certain headphones.

3. Harmonic Content: The tube design adds subtle harmonic richness that solid-state amplifiers don’t provide.

4. Physical Controls: The GR70’s direct bass/treble knobs offer immediate tactile control versus digital PEQ in some higher-end models.

5. Visual Appeal: The glowing tubes provide an aesthetic experience that solid-state boxes can’t match.

The reason for choosing one amplifier over another comes down to features, sound preferences, and use case. Those seeking absolute technical precision and PEQ capabilities might prefer solid-state options, while those wanting a more " musical" presentation with physical controls will appreciate the GR70.

Specifications

Input/Output

• Input: RCA
• Output: RCA, 6.35mm headphones, 3.5mm headphones

Performance

• Rated Power Output:
  • Low Gain: 67mW/32Ω
  • High Gain: 830mW/32Ω
• Headphone Impedance:
  • Low Impedance Mode: 16Ω-120Ω
  • High Impedance Mode: 120Ω-300Ω
• Frequency Response: 20Hz-20kHz (±1dB)
• SNR: ≥93dB
• Dynamic Range: ≥94dB
• THD:
  • RCA OUT: ≤0.26%
  • PHONE: ≤0.5%
• Noise Floor:
  • RCA OUT: ≤20uV
  • PHONE: 56uV
• Cross Talk:
  • RCA OUT: -63dB
  • PHONE: -65dB
• Power Input: DC 12V/3A

Tubes

• 2× 5654W for voltage amplification
• 2× 6AC7 for power amplification

Physical

• Dimensions: 46 × 192 × 129mm
• Weight: 900g

It’s worth noting that the 0.5% THD+N from the headphone output is actually a feature, not a bug, in a tube amplifier. This represents the “good distortion” that tubes provide—primarily even-order harmonics that sound musical and pleasing to the ear, unlike the harsher odd-order distortion found in clipping solid-state amplifiers.

Rating Explanation

The Fosi Audio GR70 earns a solid 5-star pragmatic rating for several compelling reasons:

1. Unique Value Proposition: It offers a genuine tube experience with impedance matching capabilities at a very accessible price point.

2. Versatile Functionality: The high/low impedance switch provides meaningful sonic variations with different headphones.

3. Thoughtful Design: Bass and treble controls allow for quick adjustments without digital complexity.

4. Build Quality: The solid aluminum chassis and quality components ensure longevity.

5. Aesthetic Appeal: The visual experience of glowing tubes enhances the overall enjoyment.

The GR70 is ideal for:

• Headphone enthusiasts looking to add tube sound to their collection
• Owners of high-impedance headphones like the Sennheiser HD600/650 series who want something different on a budget
• Listeners who prefer physical controls over digital interfaces
• Anyone seeking a warmer, more engaging sound than typical solid-state amplifiers provide

While audiophiles focused solely on measurement performance might prefer solid-state options, the GR70 excels at what tube amplifiers should do: provide an engaging, musical experience that connects listeners to their music in a different and often more emotional way.

Conclusion

The Fosi Audio GR70 Tube Headphone Amplifier delivers exactly what many enthusiasts seek in a tube amplifier: musical warmth, engaging sound character, and the ability to enhance certain headphones’ performance through impedance interaction. Its well-implemented high/low impedance switch makes it particularly valuable for those who own both low and high impedance headphones.

The measurements I’ve provided demonstrate that this isn’t just subjective impression—there are measurable, beneficial changes to frequency response when using certain headphones with the GR70, particularly the Sennheiser HD600 with its variable impedance curve. This creates a listening experience that digital EQ can’t quite replicate.

At its price point, the GR70 represents excellent value, offering a genuine taste of tube amplification without the complexity or cost of higher-end tube designs. The inclusion of bass and treble controls adds useful flexibility, and the ability to function as a tube preamplifier extends its utility beyond headphone listening.

For anyone curious about tube sound or looking to add some analog warmth to their digital music chain, the Fosi Audio GR70 is an easy recommendation and a worthy addition to any headphone enthusiast’s collection.