THIEAUDIO Monarch MKIV 2DD + 6BA + 2EST IEM

A tribrid IEM combines three different driver technologies in a single shell, each handling the frequency range it's best suited to. The Monarch MK4 uses 10 drivers per side in a 4-way configuration: two 8mm dynamic drivers arranged in an isobaric IMPACT² design for sub-bass, six Sonion balanced armature drivers for midrange and lower treble, and two Sonion electrostatic tweeters (E50) plus a Sonion 28UAP ultra-high frequency driver for the top end. The crossover network uses 9 passive components across 4 independent sound tubes to precisely hand off frequencies between driver types, extending the frequency response from 10Hz all the way to 44kHz.

The MK4 is the first Monarch to feature a physical tuning switch, switchable without tools directly on the shell. Standard mode delivers the Monarch's signature studio-neutral sound — a fast, clean sub-bass with a sharp rolloff at 150Hz, neutral midrange, and natural treble. It's the reference tuning, ideal for critical listening, acoustic music, and vocals. Rumble mode adds 3dB to the entire sub-bass shelf and boosts the 200–500Hz region by around 2dB, making the overall presentation thicker, warmer, and more powerful. It's well suited to electronic music, hip-hop, and anything that benefits from more low-end weight and body. Importantly, neither mode sacrifices the technical performance of the MK4 — it's not a case of one mode being "better," just different.

Electrostatic drivers work differently to dynamic and balanced armature drivers — instead of a moving coil or armature, they use an ultra-thin diaphragm suspended between two charged conductive plates that respond almost instantaneously to the audio signal. This gives them exceptional speed and resolution in the upper frequencies, with vanishingly low distortion. In the MK4, the Sonion EST drivers are specifically targeted at the 10–40kHz range — the extreme upper treble and beyond — rather than the broader treble region. This focused crossover design is a key improvement over earlier Monarch generations, resulting in a smoother handoff between the balanced armature tweeters and the ESTs, with less harshness and a more natural, airy treble character.

At 10.9Ω impedance and 100dB sensitivity, the Monarch MK4 is easy to drive and will reach comfortable listening volumes from a smartphone or basic dongle DAC. That said, it scales noticeably with better sources — a capable DAC/amp will improve dynamics, soundstage, and fine detail retrieval significantly. For the best result, a low output impedance source is important given the MK4's relatively low impedance; sources with high output impedance can alter the frequency response. Portable DACs and DAPs are ideal pairings, and the MK4 connects via standard 0.78mm 2-pin.

The MK4 ships with ThieAudio's Chocolate modular cable — an 8-core hybrid construction of ultra-pure oxygen-free copper (OFC) and silver-plated OFC wires. The modular plug system means both a 4.4mm balanced plug and a 3.5mm single-ended plug are included in the box, so you can use the MK4 with any source without needing to buy an adapter or a separate cable. Swapping plugs takes seconds. The IEM end uses a standard 0.78mm 2-pin connector, so the cable is replaceable or upgradeable with any compatible aftermarket option.