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SMSL SP400 Review (headphone amp)

amirm

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#1
This is a review and detailed measurements of the SMSL SP400 balanced THX headphone amplifier. It was kindly sent to me by one of their dealers, shenzhenaudio. It costs US $630 from them.

The SP400 makes an attempt at luxury with glossy plexiglass top and digital display:

SMSL SP400 Review balanced headphone amplifier.jpg


As you see, there are no controls on SP400. Everything is menu driven including input selection and gain. If you use multiple headphones with different gain requirements, having to switch that using the menu can be a bit of a hassle. Fortunately there is a mitigating factor as you will see in the measurements.

One main distinguishing feature is that there is remote control and relay based R2R implementation. This should assure very good channel balance -- something that we will check later. It costs a lot to implement this feature though and that is reflected in the cost.

Forgot the picture of the back:
SMSL SP400 Review balanced headphone amplifier back panel XLR pre-amp.jpg



SMSL SP400 Measurements
As usual with balanced input amps, let's feed the SP400 4 volts over its XLR input and ask for the same output over 1/4 HP jack:

SMSL SP400 Measurements Balanced Headphone Amplifier.png


Distortion is stunningly low at nearly -140 dB so noise dominates our SINAD to the tune of 120:

best balanced headphone review amp.png


Please don't sweat one or two dB in this graph. That said, signal to noise ratio for low 50 mv output is a bit lower than it should be:

SMSL SP400 Measurements Balanced Dynamic Range SNR Headphone Amplifier.png


most quiet headphone amplifier review.png


Frequency response is of course rule flat to beyond 100 kHz:

SMSL SP400 MeasurementsFrequency Response Balanced Headphone Amplifier.png


Measuring power into 300 ohm load shows that the different gain settings have almost no performance difference other than available amount of power:

SMSL SP400 Measurements Power into 300 ohm unbalanced Headphone Amplifier.png


Some other amps that get the best signal to noise ratio (and SINAD) tend to have a low gain mode with better performance. Here, the three modes are almost the same so I would just use the highest gain setting and be done with it. It is not like it impacts the channel balance as you will see later.

Note that the first generation Massdrop THX 789 is bested in noise department.

Using balanced headphone jack we naturally get more power:

SMSL SP400 Measurements Power into 300 ohm Balanced Headphone Amplifier.png


You have nearly 700 milliwatts which power just about any high impedance headphone.

Switching to 32 ohm load we get similar picture:

SMSL SP400 Measurements Power into 33 ohm unbalanced Headphone Amplifier.png


For consistency with previous tests, here is balanced not at 32 ohm but 50:

SMSL SP400 Measurements Power into 50 ohm Balanced Headphone Amplifier.png


We basically have a speaker amplifier with 4 watts of power with both channels driven.

Using our latest test to check for load dependency we get:
SMSL SP400 Measurements Unbalanced Impedance Sweep Headphone Amplifier.png


As you see, the amp does not care if the load is changed. It is happy to even drive 12 ohm load. To get the power at any of the tested impedances, simply follow the formula power=voltage * voltage/impedance.

Switching to balanced mode we of course get more power and some small signs of strain appear:

SMSL SP400 Measurements Balanced Impedance Sweep Headphone Amplifier.png


Max output is now at 14.4 volts but gets reduced below 32 ohm. At 6 volts there is finally some clipping.

Note that because there is no clipping elsewhere, the SP400 is "gain limited" meaning it could have been pushed harder. But decision was made to give you absolutely clean and transparent power to max volume.

Finally here, is our channel balance demonstrating the value of the R2R volume control:
SMSL SP400 Measurements channel balance.png



SMSL SP400 Listening Tests
As usual I started my testing with Drop Ether CX low impedance headphone. The SP400 was cruising, driving this headphone with ease to incredibly high levels, causing neuron displacement in one's brain if you are not careful! There was no hint of distortion and extremely clean bass to treble response. Switching to Sennheiser HD650 had similar results with exceptional fidelity.

I did hit a usability issue though that is common with R2R volume controls: they are slow to respond. You have to crank and crank the volume control to go from loud to average level. Wish they would add some kind of acceleration to the volume control and jump a bunch of steps when you rotate it fast.

Conclusions
We another state-of-the-art headphone amplifier in the form of SP-400 with some differentiation in the form of R2R volume control which also gives you volume control should you want to use it as pre-amp in far-field listening. More choices in instrument grade headphone amplifiers with superb fidelity and seemingly infinite power availability.

I am happy to put the SMSL SP400 on my recommended list.

------------
As always, questions, comments, recommendations, etc. are welcome.

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mkawa

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#7
Wow, well, might as well go set it to high gain and try the dt880s with eq again.

hard to emphasize how nice it is to have a balanced attenuator. Everything from 0 to 100 on volume is usable with zero drawbacks eg imbalance or distortion
 

Helicopter

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#8
Thanks Amir. I am surprised to see THX technology compete at this level, as I had the prior impression that Schiit and Topping had surpassed THX in headphone amp objective performance. The power, and performance at varying outputs are also impressive.
 

vault108

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#9
sp400.JPG

I was hoping you can verify the power output on the SP400 because there is no way it is outputting 12W at 16ohm on balance. SP400 on low gain can go higher than THX889 low gain but on high gain it’s way weaker than THX889 high gain. I doubt the SP400 can even output 6W at 16ohm on balance. I am testing on an 16ohm Ether 2.
 

PeteL

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#10
Great stuff, but doesn’t that seems a bit pricy? I own the massdrop 789, happy with it, honestly I do not need anything else, but if I recall rigght it was half this price? I didn’t go with torough with all the metrics, but same ballpark right? does that offers a significant upgrade? OK I see the stepped volume, personally I don’t hear channel imbalance on the 789, Am I right to think this would sound the same? Also out of curiosity? what is this brightness setting from the top picture, do we have tone control?
 

raif71

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#11
The remote sure looks like the m400. Maybe when click C, controls the dac and B controls the amp?
 
OP
amirm

amirm

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Thread Starter #13
I was hoping you can verify the power output on the SP400 because there is no way it is outputting 12W at 16ohm on balance.
I agree. I computed 6.25 watts at 16 ohm. More may be available though if you drove it at more than 4 volts input. You would then need a DAC that can go up that high which is rare.
 

gvl

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#15
I did hit a usability issue though that is common with R2R volume controls: they are slow to respond. You have to crank and crank the volume control to go from loud to average level. Wish they would add some kind of acceleration to the volume control and jump a bunch of steps when you rotate it fast.
There is the "volume mode" setting which seemingly may help with this issue? Or maybe not, I can't clearly decipher what the manual is saying, but it sounds like when enabled it jumps directly to the final volume level without intermediate steps sparing the relays.
 

JohnYang1997

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#16
View attachment 105722
I was hoping you can verify the power output on the SP400 because there is no way it is outputting 12W at 16ohm on balance. SP400 on low gain can go higher than THX889 low gain but on high gain it’s way weaker than THX889 high gain. I doubt the SP400 can even output 6W at 16ohm on balance. I am testing on an 16ohm Ether 2.
They have already posted THD+N vs power graph. It's where you draw the power. clipping, 0.1% or 1%?
https://www.audiosciencereview.com/...headphone-amplifier-thx-aaa.17881/post-595361
 

ninetylol

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#18
Great amp, but pretty pricy when you Look at the market. I dont know the manufacture costs but 399 would be more in line with the market IMHO.
 

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