THX AAA 789 Review: Measuring Up Three Years Later

The Drop THX AAA 789 arrived at a specific moment in the hobby , when measurement culture was beginning to displace mythology as the default framework for evaluating source gear. Three years into this hobby, reading ASR threads from 2019 and 2020, it’s clear the 789 wasn’t just a product; it was a statement. An amplifier that measured as well as anything at several times its price, available to anyone willing to wait through Drop’s queue.

That context matters when evaluating the 789 today. The headphone amplifier landscape has shifted considerably since its debut, and what made it exceptional in 2019 occupies a different position in 2024. That’s worth understanding before deciding whether it belongs in your stack.

What to Look For in a Desktop Headphone Amplifier

Output Power and Headphone Matching

The most common mistake first-time amp buyers make is treating output power as a prestige metric rather than a matching requirement. More watts is not better , appropriate watts is better. Dynamic headphones like the Sennheiser HD600 are relatively easy to drive; most competent desktop amps handle them without strain. Planar magnetic headphones are less forgiving. The HiFiMan Sundara, for instance, benefits meaningfully from current delivery that budget dongles simply can’t provide.

The practical question is: what headphones are you driving now, and what do you expect to add in the next two years? An amp that handles your current headphones adequately but clips on your next purchase is a wasted investment. Size the amp to your collection’s ceiling, not its floor.

Balanced vs. Single-Ended Outputs

Balanced output (typically 4.4mm pentaconn or XLR) matters in specific circumstances and is irrelevant in others. The theoretical advantage , lower noise floor, doubled voltage swing , is real in measurements. Whether it’s audible in practice depends heavily on the sensitivity of your headphones and the quality of the single-ended stage you’re comparing it against.

For most entry-to-mid-tier headphones, the practical difference between a good single-ended output and balanced is narrow. Where balanced matters more is with high-sensitivity IEMs (where background noise becomes audible) and with planar magnetics driven at high gain. If balanced compatibility is part of your future stack planning, factor it in now , retrofitting it later means buying a new amp.

Noise Floor and Measurements

THX AAA technology made measurable noise floor and distortion figures accessible to enthusiasts who previously had to spend significantly more to approach those numbers. ASR’s database makes this concrete: amplifiers with SINAD above 110 dB are objectively quiet enough that any audible coloration comes from the headphone or source, not the amp.

The honest version of this point is that above a certain measurement threshold, amplifiers stop being the limiting factor. The floor has risen. Exploring the full range of headphone amplifier options before committing reveals how competitive the mid-tier has become , several manufacturers now hit benchmark measurements at accessible price points.

Input Flexibility and Stack Integration

Desktop amps live inside stacks, and their input options determine what source gear they’ll work with cleanly. Single-ended RCA inputs cover most DAC pairings. Balanced XLR inputs allow you to run a fully balanced chain when your DAC supports it , the SMPS200, Topping E50, and similar units output balanced XLR, and using that connection preserves the noise advantage from source to output.

The practical takeaway: match your amp’s inputs to your DAC’s outputs, and plan the stack before buying either component individually.

Top Picks

DROP + THX AAA 789 Linear Headphone Amplifier Desktop Amp Balanced XLR

The DROP + THX AAA 789 Linear Headphone Amplifier Desktop Amp Balanced XLR is where THX AAA technology entered the enthusiast conversation at scale. Before the 789, the THX AAA circuit , a feed-forward error correction topology developed by Benchmark , existed only in considerably more expensive hardware. Drop’s collaboration brought it to a price point that a significant portion of the hobby could access. ASR’s measurement of the 789 showed SINAD figures that were, at the time of release, among the best measured at any price point. That’s still true in relative terms.

On the Sennheiser HD600, owner reports consistently describe the 789 as transparent to the point of having no audible character of its own. That’s precisely the design intent , THX AAA aims for a straight wire with gain, and the 789 delivers it. Verified buyers note the single-ended output is notably clean, and the balanced XLR output measures better still. For a HD600 user making the jump from a dongle DAC, the improvement is real. For the HiFiMan Sundara specifically, the current delivery makes a more meaningful difference , planar magnetics are more source-dependent than dynamic drivers in practice, a reality that owner consensus across Head-Fi and r/headphones confirms consistently.

The competitive picture has changed since 2019. The SMSL SP200 and Topping A50s occupy similar measurement territory at similar or lower price points, and both are available through conventional retail rather than Drop’s queue model. The 789’s main disadvantage is structural: it’s Drop-exclusive, stock appears intermittently, and you may wait weeks between availability windows. If you need an amp on a specific timeline, that’s a real constraint. If you can wait, and the 789 is in stock when you’re ready to buy, the measurements still justify the purchase , it remains a genuine reference-class amplifier at mid-range pricing.

Check current price on Amazon.

Buying Guide

Understanding THX AAA Technology

THX AAA , Active Acoustic Architecture , is a feed-forward error correction topology. The technical goal is to cancel distortion before it reaches the output stage rather than correcting it after the fact. The practical result, visible in ASR’s measurements of multiple THX-equipped amplifiers, is SINAD figures that exceed what most competing topologies achieve at equivalent price points. This matters if your evaluation framework is measurement-first. It matters less if you prioritize features or availability over benchmark performance.

The 789 was the first widely accessible implementation. That historical position gives it a particular weight in hobbyist discussions that newer THX-equipped amplifiers haven’t fully displaced.

Stack Planning: Matching Amp to DAC

The 789 accepts both single-ended RCA and balanced XLR inputs. If your DAC outputs balanced XLR , Topping E50, SMSL SU-9, and similar units , running the full balanced chain is worth doing. The noise advantage compounds from source to output. If your DAC outputs single-ended only, the 789’s single-ended input stage is still competent; you’re not wasting the amp by using it single-ended.

The practical planning question: buy the amp and DAC together, not sequentially. A mismatched stack , balanced amp, single-ended DAC , leaves capability on the table.

Headphone Compatibility

The 789 has enough output power to drive nearly any headphone in the entry-to-mid tier without difficulty, including power-hungry planars. The HD600 is an easy load; the 789’s measured performance is well above what the HD600 needs to reveal everything the headphone offers. The HiFiMan Sundara and similar planars benefit more from the 789’s current delivery than dynamics do , the performance gap between the 789 and a weaker source is audibly larger with planars. This is consistent with owner reports across the community and worth weighing if planars are part of your collection plans.

Gain Settings and IEM Compatibility

The 789 has a three-position gain switch: low, medium, high. For sensitive IEMs, low gain is the appropriate starting point , the noise floor on low gain is quiet enough that background hiss is not a common complaint in owner reports. For full-size headphones, medium gain covers most use cases. High gain is reserved for genuinely difficult loads. Matching the gain to the load is basic practice, but the 789’s flexibility here is a genuine practical advantage over single-gain implementations at the same tier.

Availability and Alternatives

The Drop-exclusive distribution model is the 789’s most legitimate current disadvantage. The SMSL SP200 and Topping A50s measure comparably and are available through Amazon and other conventional retail channels. For buyers who want THX AAA technology with fewer availability constraints, those are the rational alternatives. That said, the 789’s community reputation and resale market are well-established. Exploring amplifier options at this tier reveals the full competitive landscape , the 789 competes on measurements, not convenience. If availability lines up, it remains a strong choice.

Frequently Asked Questions

Is the Drop THX AAA 789 still worth buying in 2024?

Owner consensus and ASR measurements both support a yes , with context. The 789’s SINAD figures remain competitive with current mid-range amplifiers, and its output power handles everything from the HD600 to demanding planars without strain. The caveat is availability: stock appears and disappears on Drop’s schedule, and alternatives like the SMSL SP200 and Topping A50s offer comparable measurements through conventional retail. If the 789 is in stock when you’re ready to buy, the performance case is solid.

How does the 789 compare to the SMSL SP200?

The SP200 and 789 measure within a narrow margin of each other , both use THX AAA topology, and both hit SINAD figures that place them among the better-measured amplifiers in the mid-range tier. The practical difference is distribution: the SP200 is available through standard retail without waiting on Drop’s queue. Owner reports across Head-Fi suggest most buyers find the two sonically indistinguishable on matched headphones. The 789 holds a slight historical and community-reputation edge; the SP200 wins on availability.

Does the THX AAA 789 work well with planar magnetic headphones?

Yes, with a useful nuance. Planar magnetics , the HiFiMan Sundara being a common pairing , are more current-hungry than most dynamic headphones, and the 789 delivers enough power to drive them without clipping. More importantly, planars tend to reveal source quality differences more readily than dynamics. Owner reports and community consensus suggest the gap between a weak source and the 789 is more audible on planars than on something like the HD600.

Do I need a balanced DAC to use the 789 properly?

No. The 789 accepts single-ended RCA inputs and performs well with them. Balanced XLR input is available and does produce slightly better measurements in a fully balanced chain, but the single-ended stage is not a compromised implementation. If your DAC outputs single-ended only, the 789 remains a fully capable amp.

What headphones does the 789 NOT drive well?

There are very few entry-to-mid-tier headphones the 789 struggles with. It has sufficient output power for the HD600, HD650, HiFiMan Sundara, and most planars in the consumer tier. Where it reaches limits is at the extremes: very high-impedance vintage dynamics (above 300 ohms at low sensitivity) may prefer higher-voltage amplifiers, and some ultra-low-impedance IEMs can expose residual noise on higher gain settings. For the vast majority of headphones the article’s audience is likely to own, the 789 is more than adequate.