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SevenHertz 71 - headphone amp & HiRes DAC

7 Hz and...some other Hz extra

[Italian Version Here]

Product: 7Hertz 71 - headphone amp & DAC
Manufacturer: 7Hz - China
Distributor: Linsoul - China
Approx. price: ±€35
Reviewer: Lucio Cadeddu - TNT-Audio Italy
Reviewed: september, 2025

Foreword

I continue my exploration of small headphone amplifiers & DACs (the so-called dongles) with a product that enjoys a certain reputation among headphone listening enthusiasts, the SevenHertz 71, or 7Hz 71 for short. 7Hz is distributed by Linsoul, a Chinese company that distributes numerous ChiFi brands. 7Hz specializes in headphones and accessories like this 71 dongle that I am testing, which is the cheapest headphone amplifier in their catalogue. There is also a more expensive model, the Artemis 39, for €140, with an ES9039Q2M chip and a built-in battery. The headphones, all in-ear type, also cover various price ranges, all the way up to the Auroras that sell for €360.

A closer look

The 7Hz 71 DAC & headphone amp is based on the AKM AK4377 chip which functions both as a DAC (up to 768kHz in PCM and DoP256, DSD128) and as a headphone amp with an output power of 60mW on 16Ω, while 7Hz manual refers of 70mW on 32Ω. Now, that's a huge difference that I can hardly believe. The chipset uses the well-known Velvet Sound technology from AKM which should guarantee a natural and non-aggressive sound. This is a chip that is normally used in devices of a much higher price class. For further technical data I recommend referring directly to the official datasheet of the chipset manufacturer.

The unit, made of CNC-machined aluminium, has no sharp edges and is truly compact: 4.5 x 1.5 x 0.7 cm. It uses a short connection cable, terminated by a gold-plated USB-C connector and made of 32 OCC copper wires. It has no volume control and no balanced S-line input, only an unbalanced 3.5 mm minijack. It works without additional drivers on all operating systems and all phones. As usual, on some smartphones you may need to enable the OTG output from the settings menu. A standard USB to USB-C adapter is missing, should you wish to use it with a computer that only has standard USB ports. The cable for Lightning ports (Apple) is also missing. A very bright LED changes colour depending on the type of file being transferred. The package includes only a tiny leaflet with the technical specifications. Considering this is a very inexpensive product, this is hardly surprising.

Listening notes

I ran the tests - as usual - with both smartphones and computers, including Linux operating systems, and I encountered no compatibility or interface issues. There's no need to install proprietary software or apps.

I compared the 7Hz 71 with both the iFi GO link Max and the Fosi Audio DS2, as well as other no-branded devices, such as those found on AliExpress for around €10 (a comparison review will follow soon).

The power output of approximately 60mW on 16Ω is certainly not devastating; on the contrary, I advise against using low-sensitivity headphones (say, below 100 dB/mW) or high-impedance headphones, as the sound level may not always be satisfactory. The timbre, however, is overall good, balanced across the various frequency ranges, with a touch of velvet (could this really be the Velvet Sound of the AKM chip?) that gives the reproduction refinement and warmth.

It's certainly not the sound you'd expect from a budget device, which generally sound aggressive, with prominent bass and highs. The bass is very deep, articulated, and controlled, perhaps even slightly prominent, while the mid-high range appears a little less incisive and dynamic than its higher-priced competitors. This timidity in the high range pairs well with tonally unbalanced headphones, as are often the low-cost ones with which this device will realistically be paired.

Overall dynamics also suffer from this quiet setting, and occasionally, especially with certain musical genres, I would have preferred a little more punch, which both the iFi GO link Max and the Fosi DS2 possess in abundance. However, the timing is essentially correct. Naturally, the dynamics are superior to what you'd typically find when simply using a smartphone's headphone output. The sensation of sound inside your head is unavoidable, and more expensive dongles manage to do better in this respect as well.

Remarks & Complaints

Manufacturing & Finish.
The device is minimal, but it seems sturdy and reliable. The connection cable is not replaceable; if it gets damaged, you have to replace the whole thing. It's very well made and easy to use. A volume control would have been helpful, since adjusting listening levels from your phone, especially when using it on a portable device, isn't always convenient. A standard USB to USB-C adapter and a Lightning adapter are missing. The power, as mentioned, might be barely sufficient when used with low-sensitivity or high-impedance headphones. The device gets a bit warm, but it doesn't seem to drain much battery life from your smartphone. It's in line with competitors I've tried and am currently testing.
Sound.
For the price, it's surprising: refined, balanced, with a slightly prominent low end and a somewhat rounded mid-high range. I would have preferred a livelier, more dynamic sound and a higher output level, which somewhat compromises its application range.

Conclusions

It's an affordable DAC/headphone amplifier, and it's well-crafted for the price. It has a refined sound, generally smooth and rounded in the high range, which, with many modern recordings, is a real treat. It's not a powerhouse, but with easy-to-drive and good sensitivity headphones, it proves to be adequate. It can handle very high-resolution files, even DSD, and this is a significant plus that brings the quality/price ratio of this affordable dongle to a very high level.

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© Copyright 2025 Lucio Cadeddu - direttore@tnt-audio.com - www.tnt-audio.com

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