In this review, I’m taking a second look at the Drop + SMSL HO150X headphone amplifier which retails for $199. 

Disclaimer: Drop sent me the Drop + SMSL HO150X free of charge to be featured on Headfonia.com.

S.M.S.L & Drop

SMSL stands for Foshan Shuangmusanlin Technology Co., Ltd. The Chinese audio manufacturer was founded in 2009. Its main claim to fame has been delivering products of great value where stellar measured performance meets a price available to many. Lately, however, SMSL has been probing higher price ranges with a number of higher-end offerings for the discerning listener wanting more.

Drop (formerly Massdrop) is a California-based e-commerce platform that caters to many hobby enthusiasts, including audiophiles. Its most well-known offering is the collaboration with Sennheiser, which birthed the venerable HD6XX, a more budget-oriented version of the HD650. I’ve personally bought from them many times, and occasionally, they offer some deep-cut choices.

Enter the HO150X

Check out Yagiz’s original review for a more detailed deep dive.

Product roadmap-wise the HO150X is stuck neatly between the SMSL HO100 and HO200. The more hopeful amongst you probably think that it’s the HO200 circuit jammed in a smaller enclosure with the unnecessary bells and whistles snipped off. Well, kind of but not really. After opening up mine and looking at the circuit, it’s evident that SMSL isn’t using the particular choice of parts anywhere else.

The SMSL HO150X is a single-ended solid-state headphone amplifier with an opamp input and a discrete (!) output stage. It’s powered by an SMPSU and takes high-voltage AC from a standard IEC input. There has been some discussion about whether the HO150X is balanced. It can take differential XLR inputs which are then summed up by an opamp input stage. Therefore we get the benefits of differential signaling like common-mode noise rejection and not referencing the input signal to the oft-noisy ground.

Is the XLR output balanced? Nope, it’s a clone of the 6.35mm output and is electrically identical. Using the balanced input adds about 3dB of extra gain from the summing circuit but I found it to sound virtually identical to RCA. Use XLR if you’re having noise issues from long line-level cabling or if your DAC is natively balanced. Some DACs are known to sound different depending on output, so use whichever works best for you.

The amplifier features three gain settings: -12dB/0dB/+12dB which I found very handy with all of my IEMs and headphones. There’s also an RCA preamp output which keeps the gain stage in the loop, so gain changes will be audible there as well. It’s also always on, so you’ll need a way to mute your desktop speakers or something else you’re running from it when using headphones.

Sound Impressions

To test the Drop + SMSL HO150X I used the Topping Centaurus and SMS RAW-DAC1 as sources and 7Hz Timeless II, Yanyin Carmen, DCA Noire X, Drop + Sennheiser HD6XX, and Moondrop Para as loads.

Drop themselves call the HO150X as the closest they have to the mythical “wire-with-gain” – a fully transparent amplifier that outputs a completely faithful copy of the input, only higher in energy. The problem with this line of reasoning is that I’ve heard it before multiple times for multiple other amps. And they all sound different! So let’s see what kind of a wire this amp is.

The SMSL HO150X is extremely quiet, so that’s a good start if it wants to pretend to be a wire. Even over 120dB/V IEMs have no hum, buzz, or hiss and the -12dB gain setting is a godsend with them. Especially when many buddy DACs from SMSL and Topping love to send 5Vrms via XLR to game those SNR numbers.

After putting the HO150X through a torture test with the 8-ohm and 80dB/mW (!!!) beast that is the Moondrop Para I can say with confidence that the amp stands up well to electric hardships. Sure it gets pretty warm but it does so even with IEMs which makes me think that the output stage is biased pretty deep into Class A. I don’t know why Drop or SMSL don’t brag about it. Sure, I’ve heard better dynamics when Para is hooked to beefier amps but this was very enjoyable. Lesser amps make that planar sound downright anemic and sometimes cower into protection whilst doing so.

The classic Sennheiser HD6XX brought a rather technical performance but never veered too much into sounding analytic. Sure, it’s more of a comfort food listen when you have it hooked up to a nice OTL but the HO150X worked very well. Being a very neutral-sounding headphone, the HD6XX together with the HO150X made the Centaurus DAC shine through the signal chain.

Long story short, I like what the Drop + SMSL HO150X is putting out. While I don’t think it’s absolutely neutral and plays a bit looser in the low end than I’m used to, the listening experience is very enjoyable. To better gauge where this amp sits on the scale of similar offerings, let’s proceed to the comparisons!

Comparisons

Topping A50 III

The A50 III is Topping’s latest mid-range amplifier. It uses a nestled feedback circuit similar to what other Topping amps have – a high-precision opamp drives a high-current output chip and keeps it in check with multiple feedback loops. Powering the whole thing is an external SMPSU brick-supply which likely was chosen to keep the amp so low-profile.

Unlike the SMSL HO150X, the Topping A50 III is a true-blood-balanced design from the nose to the toes.

Despite being single-ended only, the SMSL HO150X is able to produce 6W at 16ohms per channel while the Topping A50 III does 2.2W SE and 3.5W BAL in the same load. Of course, both are way more than any of us will need. In terms of sound, I constantly found the A50 III to be technically more capable. Cleaner bass, more focussed, if a tad narrower soundstage to name a few. Treble on both left me wanting but that’s only when compared to more costly alternatives.

Ifi ZEN CAN 3

There’s little doubt that the ZEN CAN 3 looks better than the Drop + SMSL HO150X. The casing and faceplate design is hard to mistake for anything else out there.

Like the HO150X, the CAN 3 uses an opamp input to drive a discrete output stage. In this case, Ifi have gone with many parallel transistors biased in Class A, while the SMSL amplifier uses two per channel in chunkier surface-mount packages. Both amps can take balanced signals but sum it up via ICs. Ifi offers a truly balanced output with both phases driven and an increase in available voltage and power even if it can’t really touch the figures HO150X puts out.

Ifi never promises a “wire-with-gain” and with a good reason – the ZEN CAN 3 is one of the more affordable desktop amps where a deliberate sound signature tuning can be hard. The question, of course, is what the listener prefers. While both amps are what I consider electrically silent enough for sensitive IEMs, the ZEN CAN 3 has no negative gain, you’ll have to depend on the DAC to bump the volume down. With most headphones, I found the ZEN CAN 3 warmer and generally euphonic while the SMSL HO150X is more neutral and throws a wider stage. Of course, Ifi offers crossfeed, bass-boost, and other whistles but they were very situational in my tests.

Conclusion

At its original 169$ price, I’d consider the Drop + SMSL HO150X a no-brainer purchase but now the price has been bumped up to 199$ which gives it some unsavory company. Make no mistake, it’s still a potential one-and-done purchase for many people as the HO150X is stupidly powerful, extremely quiet, and offers handy gain options for every situation.

Some lament the fact the HO150X isn’t a truly balanced design but I disagree. There’s enough power and gain on tap for everything but the very extremes and the balanced input offers all of the noise rejection benefits. I think the best argument in favor of the HO150X is the fact that you’d have to pay around 5 times its price to get a tangible improvement over it in the areas where it really matters.

Summary

Pros:

• Class-leading power output capability
• Useful gain options
• Good technicalities
• Unexpected warmth from an “engineer’s amp”
• XLR inputs give benefits of balanced signalling
• Nice, tactile switch controls

Cons:

• No true balanced outputs
• No 4.4mm output
• Preamp outputs are non-muting