Today’s review is about the Fosi Audio GR70, an affordable vacuum tube headphone amplifier and preamplifier.
The review sample was kindly provided free of charge in exchange for an honest review. I didn’t receive monetary or any other kind of compensation and I don’t use affiliate links.
The price of the GR70 is $229.99 and you can buy it directly from Fosi Audio or authorized dealers around the world.
Fosi Audio
Fosi Audio is a relatively new audio brand from China that has become quickly known for producing simple and easy to use audio gear at very affordable prices. Their broad catalogue includes DACs, amplifiers, speakers and headphones addressed to audiophiles with limited budget but without skipping on features, audio performance and manufacturing quality.
You can also read some of our previous Fosi Audio reviews here.
Fosi Audio GR70
The Fosi Audio GR70 is an OTL (Output Transformer-Less) headphone amplifier and preamplifier that uses two 5654W input tubes and two 6Ж4/6Ж8 output tubes that are user replaceable. The 5654W is NOS USA made miniature pentode tube and the 6Ж4/6Ж8 is a NOS USSR made power tube that is distinguished by its unique metal outer shell.
Design and Build Quality
The chassis of the GR70 is made from high quality aluminum alloy in a smooth black finish. Build quality and attention to detail are outstanding, the GR70 has a modern industrial design with smooth lines and rounded corners that are pleasing to the eye and the touch. The amplifier comes with extra protection covers for the input tubes and has smartly designed side ventilations for efficient cooling and thermal stability during operation.
Connectivity and Interface
The GR70 features a stereo RCA line input and a stereo RCA preamplifier output on the rear panel, together with 3.5mm and 6.35mm headphone outputs at the front panel. The device is powered by an external 12V/3A power supply, which is included in the package.
A large aluminum volume/power on-off control knob in orange color is positioned at the top center of the chassis, while bass and treble controls are located on the right side of the front panel, also made from aluminum. A high/low impedance mode switch is to be found on the far left, next to the headphone outputs. A simple, yet ergonomic and practical layout.
The Bass and Treble Controls
The bass and treble controls are very useful for fine-tuning the sound and work without significantly reducing the amplifier’s headroom or introducing noticeable distortion, provided they are used without exaggeration.
The tone controls are effective both for the headphone and line outputs, adding great versatility to the GR70 that can match with a great variety of headphones and speakers.
Output Power and Noise
Vacuum tube amplifiers like the Fosi Audio GR70 are typically more powerful at high Z loads than lower Z. The GR70’s output power is rated at 90mW/32Ω in low Z mode and 380mW/32Ω in high Z.
The GR70 is a versatile amplifier that can drive both high impedance headphones, like the SIVGA Peng, and lower Z headphones like the Fosi Audio i5. That said, planar magnetic headphones of relatively low sensitivity will not pair well with the GR70.
Noise floor is especially low for an OTL vacuum tube amplifier, there is no audible background hiss with headphones and even with some not too sensitive earphones. The tubes are very sensitive to external EMI and the chassis gets very hot, so the amplifier must be placed in a well ventilated area and far away from EMI sources.
About The Output Impedance
OTL vacuum tube headphone amplifiers typically have a high output impedance, making them best suited for use with higher impedance headphones. However, the Fosi Audio GR70 is equipped with an impedance matching switch, allowing it to pair with lower impedance headphones as well.
Fosi Audio does not specify the amplifier’s output impedance, but it must be around 120Ω in high Z mode, dropping to approximately 20Ω in low Z mode. In high Z mode, most suitable headphones will operate without significant changes to their frequency response.
However, headphones with an impedance curve fluctuating considerably across the frequency range may exhibit audible changes in their tonality. Usually, a boost occurs in the region where the headphone’s impedance rises, which is often in the bass frequencies.
Well known examples with such a behavior are the Sennheiser HD600 and HD650 headphones. Some listeners may enjoy this subtle increase in bass presence, while others can compensate for it using the bass control or simply switch to the low Z mode.
Listening Evaluation
(The amplifier was left playing music for about 100 hours to let the tubes settle down)
The Fosi Audio GR70 offers some of the typical tube coloration that most users like but not in an exaggerated manner. What the tubes do is to enhance certain harmonics and overtones, instead of messing up the frequency response of the amplifier which remains relatively flat and neutral.
This is a modern tube amplifier that doesn’t lack overall transparency and delivers competitive technical performance. The GR70 doesn’t make the sound gooey, overly warm, or syrupy, nor does it smooth it out excessively by subduing the energy and brilliance of the higher frequencies. It won’t mask the tonal characteristics of the source or transform the headphones into something they are not.
Compared to a neutral and flat solid-state amplifier, what it does is add weight and elasticity not only to the bass but also to the higher frequencies, offer more natural note decay, bring out the colorful harmonics that enliven the music, infuse the midrange with soul and an organic character, and make the music sound more expressive and emotionally engaging.
More Listening Impressions
From a technical point of view, it offers punchy and impactful bass that is firm and controlled enough, maintains good clarity and definition throughout the whole frequency range, can keep up and hold pace with difficult music passages and sound airy and energetic enough on the treble.
It might not be as transparent and technically proficient as a competitive solid-state amplifier, like the Aune N7, but it compensates with its more relaxed and musical approach as well as the more organic textures and lifelike timbre.
The soundstage is where the tubes also cast their spell by making it appear more holographic, grandiose and atmospheric while also retaining good positioning accuracy and separation. The GR70 might not present the music with the same discipline of a high quality solid-state amplifier but it has the unique ability to make it sound more alive and realistic.
Comparison with xDuoo TA-84
Before the release of the Fosi Audio GR70, the Drop + xDuoo TA-84 used to be the most affordable OTL vacuum tube headphone amplifier and preamplifier. Unfortunately, Drop is out of business so the only way to source the xDuoo TA-84 is through the used market.
The most significant non-audio difference between these two amplifiers is that the xDuoo TA-84 offers greater tube-rolling potential because it uses dual ECC82 input tubes and dual EL84 power pentodes, both of which are available in a wider variety of NOS and current production options, while they are easier to source than the GR70 tubes.
Sound-wise, the xDuoo TA-84 picks up where the GR70 leaves off and adds a little more of everything. It offers more pronounced tube-like characteristics, with a more natural and organic timbre, greater overall realism and musicality, while also sounding more technical, dynamic, and impressive. However, the GR70 is more compatible with lower-impedance headphones, provides a quieter background with sensitive loads, and has the two tone controls.
In the End
The Fosi Audio GR70 is currently the most affordable OTL vacuum tube amplifier on the market without compromises on sound quality.
Except for its stellar audio performance, it is also exceptionally well made and comes equipped with tone controls and an impedance-matching switch, two features that add greater versatility and enhance its overall value.
It is an excellent choice for anyone looking to experience the tonal characteristics of an OTL vacuum tube headphone amplifier and preamplifier without having to invest a significant amount of money. At its asking price, the Fosi Audio GR70 is currently a real bargain that you can’t miss.
Authored by Petros Laskis for Ichos Reviews. All content is original and copyrighted. Republishing full reviews or substantial portions without written permission is prohibited. Excerpts up to 100 words are allowed with a direct link.
+ Tonally rich and melodic character
+ Organic and expressive sound
+ Competitive technical performance and transparency
+ Holographic and atmospheric soundstage
+ High voltage swing for high-Z headphones
+ Impedance matching switch for lower-Z headphones
+ Bass and treble tone controls
+ Preamplifier function
+ Very well made and beautiful looking
+ Easy replaceable tubes
+ Excellent value for money
- Limited tube-rolling potential
- Not enough current for planar headphones
- Tubes are sensitive to EMI
- The chassis gets very hot
Leave a comment