The Fosi Audio M04 is a single-channel Class D amplifier rated up to 100W @ 3Ω, with a SUB/PBTL switch for subwoofer or full-range mono use and a 19V/4.74A power adapter in the box. It suits adding a passive subwoofer, or powering one passive speaker, from an RCA line source.
Who the Fosi Audio M04 is for
Passive subwoofer owners: In SUB mode the M04 drives a passive sub with up to 100W @ 3Ω, and the SUB FREQ knob tunes it within 20–200Hz.
A single passive speaker: Flip the switch to PBTL and it runs one passive speaker as a full-range mono amplifier.
Line-level sources: A stereo RCA line input takes a phone, laptop, TV or other line source, per Fosi's connection diagram.
Simple controls: The power switch is built into the volume knob, next to the SUB FREQ knob and the SUB/PBTL switch on the front panel.
Skip it if: You need stereo or Bluetooth; the M04 is a single-channel amp with one RCA line input and one pair of speaker terminals.
Fosi Audio M04 key specs
Channels: 1 (mono), Class D
Max output power: 100W @ 3Ω (Fosi max rating)
Modes: SUB (subwoofer, with SUB FREQ control) or PBTL (full-range mono), set by a front switch
Input: RCA line in (L/R), 600mV input sensitivity
Output: Speaker terminals for a passive subwoofer or a passive speaker
Load impedance: 2–8Ω
Bass frequency range: 20Hz–200Hz (SUB mode)
THD / SNR: ≤0.02% (1kHz, 1W) / ≥102dB
Power: 19V/4.74A adapter included; accepts DC 12–24V
In the box: M04 amplifier, 19V power adapter with AC cord, user manual
Genuine Fosi Audio stock. Free shipping on every order, 45-day returns.
Fosi Audio M04 vs similar options
vs Fosi Audio TP-02: Adds up to 220W into 4Ω and an RCA SUB OUT for a powered sub, with a 24V/4.5A adapter; pick the M04 for its PBTL mode that also runs a passive speaker full-range.
vs Fosi Audio M03: Adds a 300W rating and an included 32V/5A supply for passive subwoofers; pick the M04 if 100W @ 3Ω is enough for your sub.
Yes. Every unit we sell is genuine and brand-new, shipped in its original packaging, and backed by our 1-year TonePier warranty and 45-day returns.
Where does my order ship from, and how long does it take?
Every order ships from our warehouse in China, with full tracking, and most orders ship within 3 business days. Shipping is free on every order; most US orders arrive within 7–15 business days and most orders to other countries within 7–20 business days — see our shipping policy for details.
What if it isn't for me?
You get 45 days to decide. Keep the packaging, keep it in like-new condition, and send it back for a refund. Ear tips unsealed for hygiene reasons are fine — we expect you to actually listen.
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