SMSL SP400 Review


In this review we’re taking a look at the SMSL SP400 Amplifier from Chinese manufacturer SMSL, selling for $629 USD.


Disclaimer: ShenzhenAudio sent us the SMSL SP400 for this review, free of charge. I only covered the customs fees & taxes. All thoughts and experiences with the product are naturally my own. You can find more about it HERE.


Honestly, if you don’t know SMSL, you have to get acquainted as soon as possible because you’re missing A LOT. They’re a Chinese company that focuses on a wide range of devices, the most notable ones being DACs & AMPs. They are famous for their value for money-oriented products and that’s the main reason I found them online about 5 years ago.

Their products are always well-built and they are quite picky when it comes to components. I have never used anything from them that was not a CNC machine milled. That is some commitment right there! SMSL (ShuangMuSanlin Electronics Co. LTD) was founded in 2009 in China’s famous techno-city, Shenzhen. Today we’ll be reviewing one of their latest AMPs, the SMSL SP400. It utilizes THX’s Achromatic Audio Amplifier (AAA-888) technology. We have reviewed their wonderful SMSL SH-9 in the past, as well. 

You can find all of our SMSL reviews here:

SMSL SP400 THX AAA-888 Balanced Amplifier 

The SMSL SP400 ‘s dedicated web page can be found here:


The SP400 comes in a quite large, white rectangular box. SMSL did not include any product details or specifications on the box and aimed for a cleaner look instead. Only the model name and the brand name is present on the outer cover. As for accessories, SMSL provides all the essentials, excluding the interconnects. You get a power cable, a remote, and a user manual with the amp. I really like the remote and SMSL has been quite consistent with its inclusion. Nearly all of the SMSL products that we reviewed had remotes and I think it is quite useful. We will talk more about it in the related section.


Design & Build Quality

First of all, SP400 is not a small amp it will occupy some space on your desk. It has a square form and you can use it to your advantage by placing your DAC on top of it. For example, my hefty Gustard X16 can stay on top of it without any problems. The SP400 has a glossy, plexiglass top cover and the rest of the unit is matte black CNC’d aluminum. I like the design of the device as it looks quite handsome with the glossy top cover but I must also say that it is a fingerprint magnet. It is of course, nice to see SMSL exploring new design options. The device looks and feels durable and I must say that it is not a light device at 1.3 kilograms. Similar to the rest of their line-up, it is quite obvious that SMSL built this device to last many years, serving you and your ears. Additionally, it shares the design with the M400 DAC. Let’s talk about the device layout.

On the front, we have a 1.9-inch color TFT display. They’ve used the same display on their other devices as well, including the SH-9 we reviewed a while ago. The screen shows info about the volume level, gain, and active input. We also have a volume knob, which offers 100 precise steps. The volume knob also acts as a navigation button and it has a small button on the center of it. The detailed menu can be accessed via the included remote and we’re going to be talking about that in a short while. Overall build quality is, as with many of the SMSL products, very good and my eyes can’t spot any imperfections such as CNC milling defects or assembly issues. Everything looks perfectly in place. The volume knob feels accurate and durable, the sockets on the front feel solid and sturdy.

The main I/O options are located on the back of the device. As for the inputs, we have unbalanced RCA and balanced XLR. We also have a pre-balanced XLR output on the back. Additionally, socket quality is great and they feel quite rigid. SMSL has been quite consistent with the build quality of their products and I think that deserves praise.


Technology & THX

The SMSL SP400 utilizes THX’s AAA-888 technology. So what is THX and why is it beneficial to a headphone amp you may ask. Let’s answer those and clear the fog. First off, THX is an American company founded in 1983 by George Lucas and headquartered in San Francisco, California. The same George Lucas who created Star Wars. THX was developed by Tomlinson Holman at George Lucas’s company, Lucasfilm, in 1983 to ensure that the soundtrack for the third Star Wars film, Return of the Jedi, would be accurately reproduced in the best venues. The THX system is not a recording technology and it does not specify a sound recording format: all sound formats, whether digital (Dolby Digital, DTS, SDDS) or analog (Dolby Stereo, Ultra Stereo), can be “shown in THX”. THX is essentially a quality assurance system. THX-certified theaters provide a high-quality, predictable playback environment to ensure that any film soundtrack mixed in THX will sound as near as possible to the intentions of the mixing engineer. 

The THX patented technology uses a unique combination of feedback and feedforward error correction to reduce the distortion, even with minimal bias current.

It dramatically reduces power supply-induced distortion and noise — a high PSRR (power supply rejection ratio). This means it can achieve impressively low noise and distortion floors.

In any amplifier, when the power supply works hard under heavy load conditions, it can sag and ripple, introducing unwanted distortion and noise in the amplifier output signal. The THX patented solution is particularly effective in blocking the power supply ripple from reaching the output. Based on my research, I think that SMSL combined this technology with their own patented 24W low-noise internal power supply to achieve such brilliant measurements. The SMSL SP400 has four THX module units under the hood, which is double the number of units compared to the SMSL SH-9. Both the THD & SNR readings are really good and my sound impressions are aligned with the performance numbers on the paper.

The design of both of the amplifiers is quite similar, the SH-9 was already a very good amp but SMSL found ways to improve the design with the SP400 and that is quite impressive in my book. I must also say that the SP400 has a very precise volume control thanks to the relays imported from Japan. The 256-step attenuator SMSL chose to use in this unit provides perfect channel matching so you won’t be experiencing any balance issues, even at very low volumes. I often use IEMs with my desktop rigs and that’s a huge relief for me. I know it is for you, too. On another note, the unit offers fully balanced circuitry, and thanks to the excellent engineering behind the product, it is marvelously clean even at the highest volume setting. It has the power to drive the 99% of the headphones on the market, without any clipping or performance penalties. 


The SP400 in balanced mode can dish out 12W into a 16Ω load and 440mW into a 600Ω load. This means that the amp is quite powerful and it can feed 99% of the headphones on the market. My Hifiman Deva and modded 58X likes the power and the amp can feed them very well. There is no clipping even at max volume in high gain mode. Using the amp in unbalanced mode results in cutting these numbers in two but even 6W into a 16Ω load is a lot of power to play with. On another note, the gain settings are quite close in terms of numbers. Here are the details of gain modes in balanced mode: L=+6.5dB, M=+9.5dB, H=+11.6dB. Additionally, I have tested the amp extensively using In-Ear Monitors and I can say that the background is silent and SMSL found a way to prevent hissing while using IEMs with the unit. That is excellent because many of the products on the market suffer from major hissing when paired with sensitive IEMs.

The output impedance is really low at nearly 0Ω too. 


Menu & Controls

There are 5 sub-sections in the menu. Input, gain, volume mode, brightness, and version info. You can select whether you want to activate XLR balanced or RCA unbalanced inputs, change the brightness of the gorgeous screen and you can change the gain setting. You can also change volume mode, SMSL states that enabling this option reduces the relay operation time and increases the service life of the relay. Lastly, you have a screen that shows the device’s HW & SW version. Firmware details, basically. It is quite easy to use the amp with the included remote control. As I said before, I really like it when companies like Topping, SMSL include a remote control for their devices. It saves you a lot of time when you just need to quickly adjust something like switching between inputs. Kudos!

The review continues on Page Two, after the click HERE or by using the jump below.

Page 2: Amp Performance, Gustard X16 & SMSL SP400, SMSL SU-9 & SMSL SP400, Budget Clash, Last Words

4.4/5 - (206 votes)

Long time Tech Enthusiast, an ambitious petrol-head, Yagiz likes his gadgets and always finds new ways into the tinkerer's world. He tries to improve anything and everything he gets his hands onto. Loves an occasional shine on the rocks.

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