This is a review and detailed measurements of the SMSL HO100 balanced (input) stereo headphone amplifier.

The HO100 matches its companion DO100 DAC. Switches feel nice and so does the volume control. While 4.4mm penticon connector is provided, its output is no more than 1/4 HP jack (so it is for convenience, not power).

What is unusual and welcome at this price point is inclusion of AC adapter and XLR inputs:

As with other SMSL products I have tested, it comes with regulator and safety certification:

SMSL HO100 Measurements
As usual we start with 4 volts in (for XLR input) and adjust the output to get 4 volts out. This meant using medium gain at max volume. Output is the same with either HP connection:

The only artifact visible is at -130 dB (15 dB below threshold of hearing) so definitely transparent to the source. This lands the HO100 in our top 10 best headphone amplifiers ever measured:

Noise performance is excellent at max level of 4 volts and near that at 50 mv out:

I tested the frequency response with the amplifier with 4 volts in/out with load of 600 ohm. This caused an odd muting at 14 Hz:

I tested it at 2 volts out and it did not do this. I am assuming it is the protection circuit kicking in.

Multitone test shows impressively low intermodulation distortion:

Sweeping at all frequencies we likewise get excellent performance:

Power is one of the most important measurements of the headphone amp so let’s see what we have at 300 ohm:

My minimum requirement for desktop amps is 100 milliwatts and the HO100 clears that mark. It also delivers over 1 watt at 32 ohm:

Notice how the noise penalty is marginal for medium gain yet you get a lot more power than low gain. So that would be the setting I would use instead of low.

Sweeping for a range of loads we get sable and nice performance:

Finally channel matching is excellent as I adjust the volume but your sample may not be as good:

SMSL HO100 Listening Tests
I started in reverse order of my usual testing with Dan Clark Stealth. This is a very difficult and power hungry headphone to drive. HO100 could only drive it to medium volume before getting distorted. Switching to Drop Ether CX bettered performance where I could get to ear bleeding levels before I could hear distortion. Best performance was with the Sennheiser HD650 where the HO100 drove it with authority to any volume level you wanted. Dynamic rand and detail was superb as is the case when the amplifier is not the bottleneck.

Conclusions
The SMSL HO100 delivers on three of my favorite features: balanced input, included AC adapter and low cost. Distortion and noise performance are excellent landing near the top of the class at any price. Power output though is good to excellent depending on your headphone impedance, leaving room for pricier offerings from SMSL and others. The only odd thing in the measurements is the muting at subsonic frequencies (which I could not force with music content with very deep bass).

I am happy to give a recommendation to SMSL HO100.