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SMSL SH-9 THX Headphone Amplifier Review


Staff Member
CFO (Chief Fun Officer)
Feb 13, 2016
Seattle Area
This is a review and detailed measurements of the SMSL SH-9 THX headphone amplifier. Product was just released and will be available shortly. The sample I have was kindly sent to me by SHENZHENAUDIO and costs US $290.

The market for high performance headphone amplifiers has become quite crowded but SH-9 sets itself apart with a gorgeous digital display and volume control:

SMSL SH-9 Review THX headphone amplifier.jpg

A 256-step attenuator using resistor ladder implements volume control which should result in perfect channel matching (see measurement later). Typical of these systems, the volume control changes a bit slow compared to analog counterparts. The chatter of relays controlling which resistors are being used is rather quiet which is a relief.

The back side shows that there is only input and no pass through/pre-amp mode:
SMSL SH-9 Review THX headphone amplifier Balanced XLR Input.jpg

As you will see in the measurements, the 4 pin XLR output is NOT balanced in the sense that it doesn't provide higher output level compared to 1/4 headphone jack. There is balanced input as you see which is important to keep possibility of ground loops low. As long as there is enough output, it does not matter whether there is or is not a balanced (properly called differential) output.

SMSL SH-9 Measurements
As usual, we measure the unity gain of the unit with feeding it 4 volts and getting 4 volts out. The low gain mode is 0 dB which makes this task easy:

SMSL SH-9 Measurements.png

This is superb performance and provably transparent to the source. Ranking is way up there:

best headphone amp.png

Noise performance is excellent as well:

SMSL SH-9 Measurements SNR.png

50 mv output which forces the output attenuation is good but not state of the art:

most quiet headphone amp.png

Frequency response is dead flat as we like to see:
SMSL SH-9 Measurements Frequency Response.png

Here is our power versus distortion starting at 300 ohm:

SMSL SH-9 Measurements power into 300 ohm.png

My threshold for desktop products here is 100 milliwatts and the SH-9 sails way past that to 226 milliwatts. So it should have no trouble driving high impedance headphones.

Switching to the other extreme at 33 ohm we get similarly excellent performance:

SMSL SH-9 Measurements power into 33 ohm.png

Using the XLR output with balanced load shows that there is no more output to be had:

SMSL SH-9 Measurements power into 50 ohm.png

As you see the drop THX amp in balanced mode can produce much more power. But SH-9 beats it in noise department and by good bit.

Finally, the key differentiation for SH-9: channel balance:

SMSL SH-9 Measurements channel balance.png

SMSL SH-9 Headphone Listening Test
I tested the SH-9 with my killer load: the Ether CX with its 25 ohm impedance and low efficiency. The SH-9 had no trouble whatsoever driving it with authority with no sign of distortion to any loudness level you wanted. Likewise it took the Sennheiser HD-650 headphone's neck and drove it to skull resonating levels with ease.

dfjk jkkk jdkfj kkkkk.... what ...

Oops, sorry about that. A screw had come loose inside my head as a result of the above testing. Tightened that and all is well now.

The SMSL SH-9 Joins other instrument grade, ultra-fidelity headphone camp. It mates nicely to its DAC sibling, the SMSL SU-9. It has an attractive, high contrast high resolution display with accurate digital volume control. The cost is also quite attractive. Misses are few which is lower power due to lack of balanced output, soft control buttons, and slow volume control.

I love differentiation without sacrificing performance which SMSL SH-9 brings. I am happy to put the SH-9 on my recommended list.

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/
Isn't it the cheapest transparent DAC + Headphones amp with balanced output tested here?

Thanks Amir!
Isn't it the cheapest transparent DAC + Headphones amp with balanced output tested here?

Thanks Amir!
There is no DAC in this! It looks like it has a DAC due to the display but there is none in there.
It looks like a tweaked, lower-noise version of the SP200, which is also not balanced, or am I being crazy here :)

To me, weirdest thing is the 50mW not being on L30/A90 level despite the low noise, hmm... 88dB is not too shabby though of course.
Thanks for the review, @amirm
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Amir, did you measure IMD? If not, do you still have it and would you do it, please?
Amir, did you measure IMD? If not, do you still have it and would you do it, please?
I did not. It is not a DAC so I usually don't measure that.
Ouch... No XLR output! It would have been a perfect match for my newly acquired SU-9 but It would force me to use it via RCA if I want to keep it connected to both speakers and headphones. What a pithy... With that display and stepped attenuator would have been a perfect pre. ‍
Man, if only other absurd-performance-for-less-than-$1000 amps had a stepped-attenuator volume control. My ideal headphone amp is basically a Topping A90, but with the lower noise and distortion of the A50 (while still having the gobs of power available if needed), with a stepped-attenuator instead of the pot. The Gustard H16 is pretty close, but it seems like its performance will be around the Drop THX 789 mark and I hate the flush wheel and the fact that you have to fiddle with menus to change inputs, gain, and possibly whether the pre-outs are engaged. Gustard, why did you not use switches instead?
I did not. It is not a DAC so I usually don't measure that.

How about phase? Music reproduction is a lot about timing. Maybe this is one of the more differentiating aspects?

Thanks for the review Amir, and for the sample, shenzenaudio. This looks like a nice clean product. I like the mains in, the simplicity of no preamp passthrough, the display, and I am impressed by how well the relay-stepped volume attenuator does with channel balance. I agree with others who have stated the relay-stepped attenuator really adds value. I don't mind the clackiness of the relay-stepped volume attenuators Schiit uses, but I am fine with that being their thing and the SMSL product being less noisy. Of course, the rest of the performance is easy to endorse as well. Great job to everyone at SMSL wo contributed to this!
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