SMSL DO400

The Venerable DO400 - Seamlessly Powerful, beautiful, and feature rich

The SMSL DO400 has been on the market for more than 18 months, making it an almost venerable, battle-tested veteran amplifier in the fast-moving headphone amplifier industry. While many competitors have come and gone, the quality of this amplifier has made it a standout in the market. The DO400 expanded SMSL’s lineup with a proper high-end look and that combined with a top-performing DACs and a very high-powered amp, making it a formidable all-in-one device that is still relevant today.

At the most basic level, what you get with the DO400 is a clean, neutral, and extremely powerful amplifier, and while I have had the pleasure of reviewing many other clean and neutral amplifiers, in the past 2 years, the DO400 still stands out in its segment. I do especially like the look of it in my setup, and it is now my new reference ’neutral’ standard headphone amplifier:

So, there are cheaper amplifiers like the SMSL C200-Pro, Schiit Midgard or even the FiiO K11 that also offer that clean neutral sound, but as you will see in this review, the DO400 is more powerful, has better features (e.g., XLR as well as RCA), better looks, and is built to last and has been well proven in the community over the past 18 months.

For example, the devil is always in the details in headphone comparisons. I have an upcoming review for the new FiiO K15 which itself is a powerful feature-rich amplifier, which also supports 3000mW into 32 Ohms (using its ultra-high gain mode). However, when you drill into the other power specifications, you see that the K15 only supports 1400mW into 16 Ohms, while the DO400 delivers an impressive 6000mW into 16 Ohms.

So even against some of the best recent releases, the DO400 has stood the test of time, and for the price it is unmatched in terms of raw power.

Unboxing and Build Quality

The SMSL DO400 comes in a simple cardboard box with some simple branding :

Opening the box reveals the amplifier nestled securely inside:

Taking everything out of the box, you get a manual, power lead and a usb cable as well as the DO400:

The SMSL DO400 is entirely made of metal, giving it a premium feel and solid build quality. It was the first SMSL amplifier to use a new design which makes the screen seem to “float” and puts it on the corner in the left-hand side of the front. This was quite a big change compared to previous models such as the DO300 but means you still have a very modern- looking headphone amplifier and one that does not look dated.

Connections

There are also three headphone outputs on the front: XLR and 4.4 mm, both balanced, and 6.3 mm, which is single-ended:

The back is home to various of input and output connections nicely layed out.

Looking at these in details, I really like that you get both XLR and RCA outputs, as you will see in the measurements later, both of these are exceptional:

And with the inputs, you get a good set of options:

1. USB (but not the more modern usb-c, but you do get a cable in the box)
2. Optical
3. Coaxial
4. AES through a single XLR connector
5. I2s through an ‘HDMI’ connector
6. Bluetooth (only 5.1, but as you will see, it does support the major high-res codecs)

The DO400 officially supports Windows from version 7 onwards (drivers are required), as well as macOS (10.6+) and Linux. I mainly used the D400 though optical (from my WiiM Pro) and Usb from my Macbook Pro without any issues.

The volume knob has notches in relief which help you grip it as well as providing a bit of design variance. The knob itself is actually a digital stepped wheel, with discrete steps for volume adjustment as well as navigation through the menus of the firmware. It has a nice bit of weight to it that makes turning it (and therefore adjusting volume) with precision easy.

Overall, I think this is a well-equipped and fantastic looking headphone amplifier.

Features

While the most important internal features are the DAC implementation, the DO400 has plenty of good internal features:

DAC Chip and Architecture

SMSL opted for the ESS Sabre ES9039MSPRO DAC chip, which supports up to eight channels, meaning you can have a truly balanced setup with a single chip. On top of the “usual” 32 bit / 768 kHz PCM and DSD512 support, it also supports MQA should anyone still care about that format.

The connection is managed by a latest-gen XMOS XU-316, while eleven TI OPA1612A take care of amplification.

The 11 TPA6120A amp chips are the reason behind the massive output offered by the DO400, which goes up to an astounding 6W at 16Ω. That’s more power than most discrete headphone amplifiers, which puts the progress done in recent years in this department into perspective.

Bluetooth Codec support

While the DO400 ‘only’ supports Bluetooth 5.1,it offers all the codecs you would want with the SBC, AAC, aptX, aptX HD, and LDAC codecs. This makes the DO400 extremely flexible, and you can stream high-def audio from most Android Smartphones and DAPs. Though, from an iPhone’s you can only stream the SBC or AAC formats, but to support Airplay, for example, would require Wifi which itself would bring more internal components, with more expensive and possible compromises the noise handling.

Screen

The SMSL DO400 has a full-color LCD screen that provides clear information about the current settings and status. The front also hosts the volume knob, which also acts as the power button and as the way to interact with the menus, as well as a status LED that turns on when the unit is turned off:

Lets have a look at the different menu options:

Menu options

Output can be selected from the menu:

You can choose between headphones, unbalanced, balanced, or “all line out” (which includes both balanced and unbalanced).

You can also select whether line out should be variable (so you can control the volume from the DAC: quite useful, e.g., for active speakers) or fixed.

The inputs are obviously selectable via the menu and the remote control:

And here is a selection of the other configuration menus options:

There are menu options for other more fine-grained controls. For example, the DPLL ‘bandwidth’ value, which can be set through the menu to values between 0 and 15, helps combat jitter and stuttering. By default, it is set to 5, but it can be raised if you encounter issues.

Remote control

There is a remote that allows you to turn the unit on and off, as well as to adjust the volume or to mute it entirely, and to select the input.

SMSL has provided a nice diagram describing the function of each button:

I really like the “FN” programmable button: you can set it to switch between line out and headphone output, or to cycle between outputs, to select Bluetooth as input, or to select phase (normal or inverted).

Sound

I tested the SMSL DO400 with various headphones I have been reviewing recently, including the Mod house Tungsten, FiiO FT7, Sennheiser HD600, Meze Empyrean II, HiFiMAN Ananda Nano, and brand new the Fosi Audio i5.

SMSL’s claim that their power supply is especially low noise is verifiable by the fact that there is no hum, buzz, nor hiss in the background even when using very low impedance earphones. The unit is entirely silent.

The DO400, like most high-quality DACs these days, offers a ruler-flat frequency response with virtually no distortion whatsoever. Its sound is clean and clear, with no audible deviation from perfect neutrality. Thanks to the super-low output impedance, there’s no variation in the sound even if you use extremely low impedance earphones.

It doesn’t have any issues driving headphones that require a lot of power, including the likes of the Modhouse Tungsten, but also worked seamlessly with my Sennheiser HD600:

I also loved its power with the Meze Empyrean II, which really brought out the best in that headphone especially when compared with a few of my slightly less powerful amplifiers:

The significant power output it is capable of makes it a unit that can drive basically anything, and this was most evident with the Mod house Tungsten which I had borrowed, but I have now sadly returned).

Finally, while I would love some PEQ to ‘fix’ the Fosi Audio i5 midrange, the level of power provided to the i5 still made it an exceptional listening experience for that headphone:

Basically, this amplifier provides a complete ruler flat frequency response with no colorisation of the sound, but mostly importantly, has all the power you could need for basically any headphone in your collection to insane levels of audio. This is why I have switched my default ‘reference’ setup to use the DO400 for future headphone reviews.

Comparisons

When comparing the DO400 with the Schiit Midgard and the SMSL C400 Pro, the DO400 holds its own impressively well:

The Meze Empryrean II was especially interested in this comparison, where the Midgard did a good job and both the Midgard and the DO400 were far better than the C200 Pro, the DO400 just have the edge, in my opinion:

I also compared the DO400 with various amplifiers initially using my Sennheiser HD600:

The DO400 also compares very favorably in terms of power to the FiiO K17 and new FiiO K15, though both are more expensive and neither has as much power:

And while subjectively, all these amplifiers have a neutral transparent sound signature, there was a clear difference in tracks that require significant power because of their dynamic range.

For example, in the Pink Floyd classic “Wish you were here” the dynamic range and power handling is required to deliver that classic track sounded amazing even with the extremely difficult to drive Mod house Tungsten.

Specifications and Measurements

So with lots of features comes lots of specifications:

But for the key ones I built up a little comparison:

Headphone Power Output Comparison Note: I am quoting the balanced output values unless otherwise stated:

While all these amplfiers have enough power for the majority of headphones, you can see that the DO400 has consistently the top (or nearly the top) power output independent of impedance, making it an easy choice as far as output power.

SINAD (dB) / THD+N Comparison

Similarly, for the SINAD values I built a little comparison of the published values:

So, the DO400 shows state-of-the-art SINAD values, so even after near 2 years this amplifier is still at reference levels. For those who chase both exceptional sound and the best specifications, the DO40O is hard to beat.

Measurements

SMSL provided some Audio Precision measurements that I thought would be useful for completeness to include in this review.

First, the XLR non a-weight XLR output :

And of course, with A-weighted it gets slightly higher values:

But even from RCA output, you get exceptional performance:

Finally, I thought this is useful to see how consistent the THD handling is at different power levels:

Note: To get an idea of how well this headphone has performed in the community, here is a link to a very long and informative discussion on Audio Science Review, What I found fascinating in this discussion that most of the comparisons actually compare the DO400 not to similar priced amplifiers but to much more expensive RME ADI-2 DAC as its performance at this price range is basically at a different level.

Rating

It earns a pragmatic 5-star rating for its combination of clean power, features, build quality, and ultimtely exceptional sound performance. Given its age, it maybe is lacking the absolute latest specifications like bluetooth 5.4, and I would love to see some PEQ capabilities coming to SMSL headphone amplifiers soon, as we have started seeing those features in the latest Topping and FiiO headphone amplifiers, but this is a minor quibble as you get exceptional performance in a relatively inexpensive package.

Pros

• Solid build
• Lots of features
• Very high power output
• Neutral sound signature
• Great value for the price

Cons

• No PEQ capabilities
• Only Bluetooth 5.1 (which is still great but not the latest as Bluetooth 6.0 is starting to appear)

Overall, with the current discount at AOSHIDA Audio, the SMSL DO400 still represents an exceptional value and remains a top contender if you are looking for an exceptional headphone DAC/amplifier.

Final Thoughts

The SMSL DO400 does basically everything you could need (except for PEQ), and it executes on its features extremely well. It offers a slew of options on all fronts: input, output, and features. Considering it offers up to six watts of power (into 16 Ohms) on the headphone output, with extremely low output impedance, this can very well be your only device to drive your headphones as no external amplifier is needed. It also has virtually no noise coming from the power supply, which is a very welcome bonus.

Many other amplifiers have come and gone in the past 18 months, but this amplifier has stood the test of time very well, it is a well-tested and a top choice for any audio setup.