SMSL 2021

SMSL SP400 Review

The SMSL SP400 is a THXAAA-888 Headphone Amplifier offering up to 6 watts of balanced output power into 32 Ohms. It is priced at $629.99

Disclaimer: The SMSL SP400 sent to us for this review is a sample in exchange for our honest opinion. We thank Shenzhen Audio for giving us this opportunity. 

To read more about SMSL products we have reviewed on Headfonics click here.

Note, this review follows our new scoring guidelines for 2021 which you can read up on here.

This is the one little powerhouse of a desktop amplifier we've all been waiting for. It is so small and sleek, so attractive and so absurdly powerful.  I enjoyed my time with it and am so happy with what SMSL has cooked up lately. They stepped their game up, big. And I think we all noticed.
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Reader's Score

The SMSL SP400 is a monster Balanced Desktop Amplifier that runs for about $629.  Wow, SMSL has been on a roll lately, chalk up 2020 to be horrendously everywhere but in the audio field. 

This SP400 has a plethora of functions and details that need to be sifted through, so let’s get right on that bandwagon and see what she can do on the test track.


Packaging & Accessories

Not much here except a standard cardboard box with SMSL’s logo on it.  I don’t think we need much else for a desktop amplifier, not like we need a case or something…hmm! Trademark on amplifier cases is owned by me (kidding). 

We get a power cable and nothing else.  I am sad to see no interconnects but at this point in life, don’t we all already have some RCA or XLR cables?  If not, you can grab em for $10 over at Monoprice. 

I am so, so happy when companies use an internal power supply rather than a wall wart with specific requirements that cannot be used anywhere else but on this amp and few others interchangeably. Just a nice standard 3 prong input for the power.

Takes the stress out of sifting for the proper adapter in my drawer of nonsense.  God forbid I accidentally place that specific voltage adapter somewhere near others and forget, then come back later and have to check every single one to make sure it’s the correct one.  And of course, it will be the dead last one I check that was the winner.  Everything should be 3 prongs.



Oh dear, my goodness. Talk about amazing build quality, no different than SMSL’s M400 DAC. The top side is a lovely piece of glass that is highly reflective, although, very prone to finger smudges. 

The sides are aluminum and the amplifier weights a pretty penny, no doubt, for being so small. The volume knob feels fantastic in the hand and I cannot stress how lovely the connections feel when I plug it in and out. No jiggle or wiggly bits. 

The construction of this amplifier is top-tier.  The LCD screen is interesting, as it has a function to dim the luminosity.  That is great for when I want to keep the amplifier on at all times. 

Oddly, no power button. This stays on as soon as you plug it in your wall outlet. Truly spectacular build quality and visuals here, this is one damned fine-looking little amplifier if you ask me. 



Holy Smokes, Batman.  This amp runs at 12W into 16Ω and 6W into 32Ω via its balanced output.  Do you know how much that is?  I was super happy about my Heron 5 from Airist Audio piping 4w.  But this…this eats Planar’s for breakfast with plenty of power left over to run a small island nation. 

The SP400 headphone amplifier shouldn’t have any issues with the older Hifiman HE-6, which I powered with their EF-5 amplifier previously.  Sure, it probably needs more than that, but the power you get in balanced is like a nuclear blast.  Unbalanced it runs far lower at a mere 3W into 32Ω.  Haha. Jeez.  Might as well be a little supernova contained in a bottle at this point.

I had to go purchase an XLR balanced to 2.5mm female balanced adapter, so I can use my 2.5mm connections with the SP400.  Just to name a few: the Sennheiser HD800, Empire Ears Nemesis, Shouer Tape, and the Audeze Euclid were all tested in Balanced cable mode for this review. 

Maybe I am a little sour over it, but I didn’t want to pay another $20 on top for this adapter just to use it in balanced mode. I have a few other lower-tier balanced amps but nothing worth mentioning.  I also have the balanced Little Dot LD H1 (double the price of this SMSL SP400) to test with.


Sound Impressions


This SP400 is stark clinical neutral in tonality.  It is crystalline in feel and texture from top to bottom.  If you are interested in a very powerful and very clinical-sounding amplifier, this is probably the one you should be interested in looking into for the $600-700 range. 

This amplifier sounds similar in tone to that Little Dot H1 I mentioned, lacking warmth and instead of ejecting a sense of lightning-fast decay.  Hardly anything lingers for more than a fraction of a second and due to that, the texture of the entire experience sounds hyper-slick, very fast.  

This is a different type of sound than let’s say the Feliks Euforia 20th ANV, which is more roughly 4x the price.  That Feliks Euforia 20th ANV puts out a more natural and middle ground tonality, whereas this SP400 from SMSL feels much rawer. 

Both of them are quite different from very warm amps like the Heron 5 or the Burson Conductor 3 Performance that feel a bit warmer and slower.

Bass Fidelity

As mentioned, the SP400 is highly neutral sounding and very clinical.  The low end is decay factor is supremely fast and feeling like an Objective2 on steroids. 

The purity factor is sublime for a $629 amplifier. I expected far less and received far more quality than I had thought it would provide. Testing indicates that the headphones like the HD800 run better in balanced mode and offer more of a stern, solid approach to the low end. 

You can really feel the bass stiffen up with more power on the HD800. And no joke, the SMSL SP400 puts out just shy of 900mW into 300ohm loads.  I have no doubt though that this SP400 sounds excellent for the price. 

Bass Response

This is not a basshead amplifier. If you own basshead headphones, you can really maximize their quality potential, so long as your DAC is sufficiently good enough to be paired logically with this SP400. 

I decided to run the Audeze Euclid IEM in Balanced and Unbalanced and found that the balanced mode offered a noticeably more firm approach.  However, this type of experience was reversed with easier-to-drive sets like the Shouer Tape and the Empire Ears Nemesis.

I found that lots of power sterilized the depth and rumble factor sometimes and I preferred to run the easier to drive headphones in unbalanced mode. Sometimes, too much power doesn’t work out for bass-focused products that are not Planar in design. Otherwise, the snap factor is moderate and enough to keep me engaged.  Thankfully, the SP400 is never painful or wince-worthy.

More and more products are finally getting hand-tuned to sound clinical in tone, but more reserved in physical slam. Clinical and Accurate tonality can be enjoyed with light to moderate slam effect, it doesn’t need to be harsh to still be Clinical.

Beyond this, I found the SP400 to respond moderately well to bass EQ.  You can squeeze out a bit more, but due to being so clinical, the +5dB and up tier does little for quantity fact.  And the amp is so clean, that the extra dB’s don’t get warped, so basically, you are dialing in more and receiving almost no effect after +5dB.  At least not in my tests.



Side by side with the other amplifiers I have, the SP400 is the most moderate and I feel like that is the purpose of it. The vocal experience is not recessed, but it is not very forward. 

This, combined with a clinical tonality + an enjoyable slam factor that isn’t at all harsh, makes for an interesting rare sound signature.  I can enjoy clinical-sounding headphones like the HD800 while never wincing at sudden vocal cues in Jazz. 

I like old Jazz tracks and I can enjoy them with very clinical DAC’s and headphones, but without ever feeling like my ears are taking a beating. The vocal experience on the SP400 is fantastic in purity, in fact, I would say it is one of the best for the price I’ve come across and it is clear that SMSL is aiming for that.

High Purity

The SP400 purity factor is off the charts good for the price. Crazy good. Running my old ATH ESW series headphones through it yielded incredible effortlessness in the sound signature coming at me.  By that, I mean the amp is so clinical in tone + being so fast on decay, those vocal-centered headphones still sound amazing with it. 

The colorless appeal jives with Jazz standards very much. I really enjoy this. It is not common to get such purity alongside a refreshingly nice physicality impact factor. It really lets me enjoy Sinatra with headphones I have been terrified to use with neutral-sounding amplifiers like this. 

For those unaware, as mentioned, in the past, “very clinical” sounding amplifiers often offered harsh slam and agonizing treble.  Not always, but in general, I felt like the combination of an HD800, which is hyper clinical and a very clinical amplifier was a no-no for me on a personal level.  And that is because the physicality and wince factor were simply too much to bear.

This is no longer the case, as more clinical tones are offered but without harsh physical strike impact levels.  This is a big problem in the upper mid areas for me as a listener when singers get louder all of a sudden but don’t quite reach into the treble area.  This type of “problem” is dying out.  The SMSL SP400 handles this type of thing wonderfully.



The top side of the SP400 is bright, but not painfully so. I call it engaging. Swapping back to the Burson, or my Heron 5, or any number of other amps I have, showcases the HD800 and other bright headphones to sound noticeably dimmer on those others. 

Connecting to the SP400, that top side brightens up, but quality amps itself as I have found more power equals better treble with the HD800.  This is again, not the case with some sensitive IEMs though so be careful. 

I really enjoyed this amplifier with my Grado Hemp, as I’ve found that headphone to offer a gentle sheen and brightness that is similar to my favorite old PS500 original (none-e) variant. 

I consider this SP400 as an “upper mid-tier” amplifier, which to me mid-tier means $300-999.  I consider $100-299 to be budget and that changed a lot recently due to the new standard budget for Apple type products changing the market with their expensive audio products. 

To generalize, was my aim there.  At $629, I consider this SP400 still in the middle tier pricing.  And with that in mind, the treble experience sounds more like the lower end of the Summit Tier, which to me is $999 and up. This amplifier’s topside is stunning. 

Treble Impact

As with the bass side offerings of this amp, the top side is lightning fast as well. The decay factor feels so fast and crystalline in the presentation. It does not ring out, which is a trait that some musical amplifiers and products have where some tones linger for an audible amount of time. 

But, in this case, you can hear the decay factor just drop out suddenly and that makes the amplifier sound very quick and pure. The physical aspect of this SP400 is again, moderate and thankfully so. If this were 10 years ago, this amplifier would be making my ears bleed with physical strike and wince factor, or, that physical slam effect.

We can endure this on bass way better than we can treble. Piercing treble is bad, even if the treble was recorded that way originally. Nobody wants that, right? The SP400 is engaging enough to stay interesting but lacking any harshness to it.  I am so happy to hear this new trend in these types of products, I hope the future years stick with this type of sound.


If there is a weaker link in the chain, the imaging prowess would be the winner. While I do not consider it lacking in any way at all, there is a noticeably smaller void when comparisons were drawn with amplifiers known for exceptional staging. 

Amps like the Heron 5 and the Burson amps, in general. Yes, the HD800 sounds more spacious and deep on those other amplifiers. So, maybe the rare HD800 case being the biggest soundstage in a headphone out there is a bad motivator for the purchase of this SP400. 

Everything else though? Damned fine if you ask me. From my Grado Hemp to my Swan Song Audio open-back headphones, to excellent Summit level IEMs. This SP400 powers through them. 

I don’t hear any difference in staging at all between the Burson and the SP400, same track, same DAC, same volume level. It isn’t until you get the niche “this is the best imaging in a specific area” headphones that you’ll likely even notice. 

For example, the JVC DX1000 is excellent with stage depth, lacks width and height, and realism. You’ll notice the depth of field difference between this SP400 and let’s say the Feliks Euforia ANV.  The old school Denon D-series woodies are cavernous sounding, you’ll notice the realism and air in stage forward if you compare this SP400 to the Burson Conductor. 

So, while the SP400 is a general great all arounder, it is not suitable for niche headphones that offer a specific “best of” in staging depth, width or height factors. Otherwise, 99.99% of the headphones are going to sound amazing on this SP400.



This is a preference-thing here and what magical combo’s I’ve found that really mesh with this SP400.  The first setup was with an xDuoo XD05+ as the USB DAC and Source. Why? Because it has a bass switch on it that really amps up the low end without needing to screw around with EQ manually in the software. 

The SP400 is so clean and pure, that I can use a lower tier DAC and still make the end result sound similar to a DAC more along the expense lines of the SP400.  And what does that mean exactly?  It means you get to save lots of money, you don’t need an absurdly good source DAC. Just a good one.

Despite the XD05+ being on the warm side on the low end, that tonality doesn’t press through to the extremely clinical SP400. You get the benefits of the extra low end without ruining the clinical tonality and this really meshes with my Sennheiser HD800…like, a lot. 

This is ideal and what I’ve always wanted.  I want the bass to be more musical and interesting, but not the mids and treble. I don’t get that with my Heron 5, it is too warm.  I don’t get that with the Burson, it is a bit too natural up top. 


Rig pairing to your preference is so vital, I should start a business of my own to help guide people on how to achieve this! Oh right…I am a reviewer, that is what I do already haha!

I have also found that the SP400 really pushed the limits of the Audeze Euclid in balanced mode. What a fantastic little IEM. It can slide either way. If you want more warmth, I’d drop in the Burson as the DAC and run headphones out to the SP400. 

If you want more clinical appeal, I’d actually be opting for the Ultrasone Panther USB DAC out and into the SP400. The tonality shifts are addictive and what keeps me going these days. This is what I do and love it. Hearing how a product can shift the entire tonality structure of a rig pairing is something I am wildly addicted to. 

Our Verdict

This SP400 from SMSL is a real winner. It is highly clinical sounding but lacking harsh impact and wince factor. It pairs lovely-like with the Sennheiser HD800 and shockingly the Grado Hemp as well. 

This isn’t something I would recommend for very warm-sounding headphones, but it is something I would think Audeze and Hifiman owners should be interested in. This SP400 eats Planar’s like cupcakes. LCD-2 and HE-500 owners are going to love this amplifier.

This is the one little powerhouse of a desktop amplifier we’ve all been waiting for. It is so small and sleek, so attractive and so absurdly powerful.  I enjoyed my time with it and am so happy with what SMSL has cooked up lately. They stepped their game up, big. And I think we all noticed.

SMSL SP400 Specifications

  • Input method: balanced x1, unbalanced x1
  • Output mode: balanced earphone X1, unbalanced earphone X1, pre-balanced output X1

Output Power

Balanced headphones:

  • 12Wx 2(16Ω)
  • 6Wx2 (32Ω)
  • 880mW x2 (300Ω)
  • 440mW x2 (600Ω)

Unbalanced headphones

  • 6Wx2(16Ω)
  • 3Wx2 (32Ω)
  • 440mW x2 (300Ω)
  • 220mW x2 (600Ω)
  • Signal to noise ratio: 133dB, A-Weighted
  • Residual noise: 1.9uV, A-Weighted
  • THD+N: -123dB, 0.00007% (1kHz, 32Ω)
  • THD+N: -117dB, 0.00013% (20-20kHz, 32Ω, -3dB)
  • THD: < -125dB, 0.00006%
  • Input impedance: 47kΩ
  • Output impedance: close to 0Ω
  • Frequency response: 0.1Hz-500kHz (-3dB)


  • Power supply voltage/power frequency: AC100-240V, 50-60 Hz
  • Energy consumption (EN60065): <40 w
  • Standby power: <0.1W
  • Size: 225X2 10X43mm(WxHxD)
  • Packing size: 354X276X80(mm)
  • Weight: 1330g

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