SMSL SP400 Review (headphone amp)

[JohnYang1997]

@SHENZHENAUDIO I almost don't dare to ask the question... it has output relays... is it DC protected ?

All THX based amps are well protected.

 

[JohnYang1997]

What about Gustard H16? Ticks all the boxes and feels more solid. Despite being cheaper.

Not trying to knock on Gustard but there's no way to toggle between preamp and headphone out. Ouch.

 

[JB5150]

Not trying to knock on Gustard but there's no way to toggle between preamp and headphone out. Ouch.

I don't recall a toggle on SP400.

 

[iec 60318-4]

Left vertical axis is for the red line that represents the output level, where 0 dB means the max. volume knob (max. output voltage).

The 24 sec. from the bottom horizontal line represents the time took for Amir to lower the output power from 0 dB to -80 dB.

Right vertical axis is for the blue line that represents the deviation in dB from the absolute 0 dB, where 0 dB means the difference between the left/right channels. Basically, the blue line should remain within the +/-0.5 dB to be on the safe side, but on SP400 this is pure 0 dB due to the relay volume adjustment, which translates to "perfection".

I wrote an explanation: https://www.audiosciencereview.com/...mbalance-vs-volume-position.19189/post-629699

Thank you both for the kind explanation!

 

[Lero]

Noted but is that subjective or objective?

Of course no issues with having more power headroom but my point was: is there are headphone that objectively needs more power that SH-9 delivers

600ohms headphones (beyerdynamic dt 880 600ohms) some magnetic planar headphones. More power means headroom,and its always a good thing. Sometimes you need a couple of watts at relatively low impedence.

 

[YSC]

600ohms headphones (beyerdynamic dt 880 600ohms) some magnetic planar headphones. More power means headroom,and its always a good thing. Sometimes you need a couple of watts at relatively low impedence.

Sometimes I got confused about this.
Say if I have a 4W amp and use HE6 or some 600ohms phones, it got way too loud in max output, so I turned the volume knob down which attenuates the whole 4W output into whatever wattage (say 1W?), so in my understanding the max allowed swing won't go back to remotely near 4W when the knob was kept at that position, and if so, why and where is the extra headroom??

 

[blueninjasix]

Not trying to knock on Gustard but there's no way to toggle between preamp and headphone out. Ouch.

Could you just switch the power amps off if you wanted to use headphones instead of speakers?

 

[xema]

Could you just switch the power amps off if you wanted to use headphones instead of speakers?

You can, but it's not very convenient, and the headphone is still singing.
It will be even worse if the headphone output is directly connecting to the line outputs, the overall load impedance will be twisted.

 

[blueninjasix]

You can, but it's not very convenient, and the headphone is still singing.
It will be even worse if the headphone output is directly connecting to the line outputs, the overall load impedance will be twisted.

Would these problems be avoided with the Gustard if I unplugged the headphones whilst listening to speakers, and switched off the power amplifiers whilst listening to headphones? Is that the extent of the inconvenience or is there still a technical issue with impedance that I should be aware of?

 

[xema]

Would these problems be avoided with the Gustard if I unplugged the headphones whilst listening to speakers, and switched off the power amplifiers whilst listening to headphones? Is that the extent of the inconvenience or is there still a technical issue with impedance that I should be aware of?

If you do so, there are no more problems at all.
But it's very tossing to me.

 

[Music1969]

More power means headroom,and its always a good thing.

Of course, and I said the same thing.

The key is, when you have more power than the already powerful SH-9, does it objectively make a difference ?

You mention the impedance (600 ohms) but what is the sensitivity?

Can you already get to ear bleeding SPL's with the SH-9? If so, then why would you want to get even more ear bleeding ?

 

[Francis Vaughan]

Sometimes I got confused about this.
Say if I have a 4W amp and use HE6 or some 600ohms phones, it got way too loud in max output, so I turned the volume knob down which attenuates the whole 4W output into whatever wattage (say 1W?), so in my understanding the max allowed swing won't go back to remotely near 4W when the knob was kept at that position, and if so, why and where is the extra headroom??

This is a bit of an electronics 101 topic, but useful to cover.
Power is work done per unit time. The amp capable of delivering that power, but that isn't to say it actually does.
Amplifiers used to drive headphones and loudspeakers are designed as voltage sources. That is to say that for a given input (and volume knob setting) they deliver a defined voltage at their output. Even with nothing connected to the output this voltage is present.
When you connect a load (headphones, speakers, etc) that load has a resistance and for a given voltage, current will flow into the load. That current is what makes the magnetic bits do their work, and how work is done. The actual power delivered can be calculated from the load's resistance and current flowing in the load. More usefully, it is also the same as current times the voltage (this is from Ohm's law).
The load's resistance is not just the resistance of the wire in the coils, the magnetic machine reflects the mechanical load back into the aggregate, and the resistance to current flowing includes the resistance to moving the air. (Usefully this property makes it is possible to mathematically transform mechanical systems into equivalent electrical systems which makes all sorts of analysis easy.)

No voltage source is perfect, clearly there is a point where if you apply a load with too low a resistance the amplifier will run out of ability to source current. You can't connect a nail across the output an expect it to swing full voltage. Usually the limitation is either the maximum current you can pass through the output devices before damage, or the size of the power supply.

OK, so what happens when you turn your volume control up? Well the voltage on the amp's output increases. But the power delivered depends on the load. A 600 Ohm load is sufficiently high that the amp will never deliver enough voltage to push 4 watts of power into the load.
Power is also V*V/R, so V = sqrt(4W*600Ω) = 49 volts to get 4 watts. That is huge. Very rare to see headphone amps with more than a 15 volt swing. Connected to a 32Ω headphone a 49 volt drive would deliver about 75 Watts. Clearly this is not happening.

But into lower resistance loads, say a 32Ω headphone, you would only need 11 volts. Assuming the amplifier can deliver the voltage swing, it remains to see if it can source the current. This sp400 can it seem deliver enough current. So you can get that 4 watts. Unless the headphones are very inefficient you would utterly overpower them and be well past the point of permanent damage to both headphones and you.

Bottom line is that the volume knob doesn't directly control power delivery. It controls the force that pushes the current. Headphones are designed to be driven with a voltage source. Power delivered is a secondary measure, and for headphones not of much importance. It is a proxy for ability to deliver high current into low efficiency low impedance loads. If you don't have such headphones it is just bragging rights.

 

[anmpr1]

Great stuff, but doesn’t that seems a bit pricy?

Once you get past the one hundred or possibly one hundred and fifty USD price point, other factors have to come in to play. Mostly features, warranty with build quality, and can I get it easily serviced/replaced if something goes wrong? If it lasts a year or two, a $100 device is almost throw-away in those sorts of situations. Six times that price, other, non-technical aspects become important. An in country distributor willing to provide support becomes necessary, in my view. Someone you can call on the phone and quickly interact with.

 

[SmackDaddies]

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?

Pricy to whom? If I am a headphone person, with a number of different ones, and willing to spend thousands on them, no, not that pricy. If I use $20 headphones and iems, then yes, very pricy. Cost and Value are individual decisions and my answer will not be your answer

 

[PeteL]

Pricy to whom? If I am a headphone person, with a number of different ones, and willing to spend thousands on them, no, not that pricy. If I use $20 headphones and iems, then yes, very pricy. Cost and Value are individual decisions and my answer will not be your answer

Sure in the grand scheme of things it's not excessive, as you said me, personally, I , find the diminishing return curve quite steep in comparison to the basic 789, which is also really really good at half the price, I do not think they are apples to oranges, same topologies and licensing approach, similar power and performance difference into inaudible territory and not a winner on all metrics. Sure it offers a couple perks like the remote. In my views, and yes it's only me feeling that way, a remote when your head is physically attached to the amplifier is not something I have use for, and a wosrst case scenario 0.2 dB Channel imbalance is not something I can hear, maybe for some it matters. I actually prefer the immediacy of a volume pot, If it's a good one. That said, yes of course this pricing is not into boutique manufacturers, luxury acquisitions territory.

 

[Yuhasz01]

Thanks for measurements. Previous model SP200 is still an impressive device, especially for price

Agree I own one as well. This device—twice as expensive—adds itching but more power. It is a marketing exercise to improve revenue and profits. Display screen , etc just audio jewelry.

 

[Yuhasz01]

Pricy to whom? If I am a headphone person, with a number of different ones, and willing to spend thousands on them, no, not that pricy. If I use $20 headphones and iems, then yes, very pricy. Cost and Value are individual decisions and my answer will not be your answer

Yes cost and value subjective; not measurements and sound qualities. In all this there is a technical scientific component to consider. Because you pay more does not mean you get more value.

 

[chasefrench]

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:

View attachment 105689

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:
View attachment 105720


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:

View attachment 105690

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

View attachment 105692

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:

View attachment 105694

View attachment 105695

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

View attachment 105696

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

View attachment 105697

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:

View attachment 105699

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

Switching to 32 ohm load we get similar picture:

View attachment 105698

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

View attachment 105700

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:
View attachment 105701

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:

View attachment 105702

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:
View attachment 105707


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.

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

Appreciate any donations using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

Hi ASR, looking forward to your review of the Drop Ether CX flow, I have them too

 

[trl]

Agree I own one as well. This device—twice as expensive—adds itching but more power. It is a marketing exercise to improve revenue and profits. Display screen , etc just audio jewelry.

Relay based volume control is not an audio jewellery; same I can say about having more power (some folks really needs that added power for their power hungry cans).

Also, LCD and remote should become a "de facto" standard these days, especially for those listening during night hours.

It's a good thing that SP200 has a very low harmonic profile, but the extra 6dB for SP400 in SINAD improvement @ 50 mV output is definitely a great asset for the SP400 for everyone wanting to use sensitive headphones, especially the ones above 120 dB/V SPL.

Given the above, SP400 is a completely new and improved device, but sharing the same THX technology in output buffer.

L.E.: The added power on SP400 is only for balanced operation, not on the single ended usage.

 

[amirm]

Hi ASR, looking forward to your review of the Drop Ether CX flow, I have them too

Sure, will test them at some point.