Headphone AmpOverreview

Monoprice Monolith THX 887

The THX amp that measured like a world record — and whose volume pot kept owners on the phone to Monoprice.

45 sourcesUpdated
$175Typical US price

Score aggregates reviewer opinions across topics.

Monoprice Monolith THX 887

The short version

What to know.

A balanced desktop headphone amplifier built on THX's AAA 887 circuit, now discontinued and sold used. The one independent bench found distortion at or below −140 dB, flat frequency response and nearly 4 W into 50 ohms from the balanced output, and every video reviewer heard it as clean and uncoloured. Opinion splits elsewhere. Balanced users drove almost anything, while single-ended users often found it weaker than cheaper amps. Most hear a black background, but some owners hear channel imbalance or residual sound at the bottom of the volume range. A minority hears it as dry and clinical, and a smaller one as smoothed-over. The sharpest split is durability: reviewers called it solid, while long-term owners report volume pots turning scratchy within months and some outright failures, mostly fixed under Monoprice's three-year warranty. The near-invisible knob markings and soft-feeling buttons are the common niggles.

About this product

The standalone balanced headphone amplifier sold as the "Monolith by Monoprice THX AAA Balanced Headphone Amplifier featuring THX AAA 887 Technology" (Monoprice #39359), launched in November 2019. Not the Monolith THX AAA desktop DAC/amp (a 788-based combo), not the Monolith THX portable, and not Drop's THX AAA 789, which shares its layout. Monoprice has since delisted it.

The Monolith THX 887 was Monoprice's late-2019 answer to Drop's THX AAA 789: the same layout, the same three gain steps and the same mix of 4-pin XLR, 6.35 mm and 3.5 mm outputs, with XLR and RCA inputs and a pass-through for powered speakers, in a slimmer brushed-aluminium box with a three-year warranty. Audio Science Review measured it before launch and found it beat the 789 by a couple of decibels on noise and distortion.

Nobody argues with the bench. The arguments are about everything around it: how much power you really get from the single-ended jacks, whether it sounds transparent or clinical, and whether the volume pot, the knob markings and the buttons belong on a $400 amp. Monoprice has since dropped it, so today it is a used-market buy.

A good fit for

  • Balanced headphone owners who want lots of clean power, planars included
  • Listeners who want an uncoloured amp that measures as well as anything at the price
  • Used-market buyers who want a top-measuring THX amp for under $200
  • Desks that need a 3.5 mm jack and an XLR input in one box

Think twice if

  • You'll only use the single-ended jacks with hard-to-drive headphones — cheaper single-ended amps match it there
  • You need a preamp output for powered speakers, a remote, or a manual
  • You want an amp with some warmth or flavour of its own
  • A volume pot that may need cleaning or a warranty claim within a year would put you off
  • You want to buy new with warranty — Monoprice no longer sells it

The reviewer consensus

Agreement &
disagreement.

5 / 7topics divide opinion
featuresergonomicspowernoisetransparencybuildvalue
2 Mostly agree5 Divided opinions

Common ground5

  • Measures on a par with the best headphone amps Audio Science Review had tested: distortion at or below −140 dB and a flat frequency response
  • Practically the same amp as Drop's THX AAA 789 in layout and features
  • Generous connections: XLR and RCA in, 4-pin XLR, 6.35 mm and 3.5 mm out, three gain steps

Points of contention5

  • Is it powerful? Balanced users drove almost anything; single-ended users often found less than cheaper amps
  • Is it silent? Most hear a black background; some hear imbalance or bleed-through at the bottom of the knob, or hiss on high gain
  • Does it have a sound? Most hear it as transparent; others hear it as dry and clinical, or as smoothed-over

A closer listen.

7 topics · 5 divided
Left: criticalRight: positiveTighter lines: more agreement

Power

Leans positiveDivided opinions
Reviewer sentiment14 supporting sources
CriticalNeutralPositive

Band: opinion spread · Dot: weighted average

Sources split, and the split tracks which output you use. Audio Science Review measured nearly 4 W into 50 ohms from the balanced jack, and reviewers and owners driving it balanced powered everything from an HD 650 to an HE6SEv2. Owners using the single-ended jacks, or feeding it from an RCA source, often found it no louder than cheaper single-ended amps like the Liquid Spark or JDS Labs EL Amp, and one HE6se owner heard bass without much impact.

Measured

Audio Science Review measured nearly 4 W per channel into 50 ohms balanced, and power neck and neck with the 789 into 33 and 300 ohms. Monoprice rates it at 6 W into 32 ohms and 700 mW into 300 ohms balanced, but 1.6 W and 185 mW single-ended.

vs. listeners — Both camps match the published figures: the single-ended jacks get roughly a quarter of the balanced power, and Monoprice lists single-ended gains of −10, 0 and +10 dB against −4, +6 and +16 dB balanced. Someone feeding it from RCA and listening on the 6.35 mm jack is using the weakest path through the amp.

Where it splits
drives almost anything from the balanced output64%

“And with nearly 4 watts of power on tap, you should be able to drive just about any headphone.”

amirm · Audio Science Review
single-ended output is modest — weaker than cheaper amps36%

“whereas on the 88 seven I have to turn it up to about the one o'clock position”

RandomStrata (video, single-ended with Fostex T60RP, vs the JDS Labs EL Amp at ten o'clock)
Link to this topic

The hardware

Specifications.

Checked

Design

Form factor
Desktop
Amplifier
Solid-state · THX AAA 887

Output

Output power
Single-ended: 3,200 mW @ 16 Ω · 1,600 mW @ 32 Ω · 185 mW @ 300 Ω · 100 mW @ 600 ΩBalanced: 8,800 mW @ 16 Ω · 6,000 mW @ 32 Ω · 700 mW @ 300 Ω · 350 mW @ 600 Ω
Output impedance
< 0.05 Ω SE · < 0.1 Ω bal.
Gain settings
3
Volume
Analog pot
Signal-to-noise
138 dB

Connection

Headphone outputs
6.35 mm, 3.5 mm, 4-pin XLR
Line outputs
RCA
Preamp output
No
Power
External power supply

Physical

Dimensions
221 × 198 × 51 mm

Manufacturer-published figures, not measurements · from monoprice.com (archived).

Follow the evidence

Sources 45

1–6 of 45 sources

Read with context

Limitations & method.

As of

What to keep in mind 7

  1. A read of where the reviews land as of October 2026. Nearly all of the reviews date from the 887's 2019–20 launch, and the field of clean, powerful desktop amps has grown a lot since.

  2. Only one independent test bench measured this amp, so the measured picture rests on a single, very thorough review plus Monoprice's own figures.