Active subwoofer is silent, hums or shuts off — how to find the cause

You're settling in for a film, the upper channels are playing, but there's no bass at all — or it came in and dropped out again once the sound got louder. The other common picture: the subwoofer sits in the corner and hums constantly even with the film paused. For an active (self-powered) subwoofer these are three completely different faults, and each has its own source.

The key thing to grasp straight away: an active subwoofer has two separate parts — the driver (the speaker itself, with its voice coil and cone) and the amplifier plate at the back (plate amp), together with the power supply and the auto-on circuit. Most faults come from the plate and the power supply, not from the driver itself. That's good news: the plate can be repaired, whereas a blown driver often means a new speaker. This article goes in exactly that order: power supply and amplifier first, then the driver, then cables and grounding last.

Before we go on — this is about hi-fi and home-cinema active subwoofers (SVS, REL, BK Electronics, Wharfedale, Yamaha, Klipsch). Repair of portable and Bluetooth speakers is not covered here; that is a different design with a battery and a wireless module.

No sound at all — power supply and amplifier plate

The subwoofer is completely silent. The first step is not to open the cabinet but to find out whether the plate is alive at all.

Look at the back panel: is the standby indicator lit? With a hand on the driver, do you feel even the faintest thump or click when switching on? Silence plus a dark indicator points to the power supply — the main fuse, the mains switch or the power transformer. A lit indicator but no sound is more likely the amplifier stage itself or the input path.

In older Riga buildings, add one more variable. Voltage dips and spikes in worn stairwell distribution boards kill power-supply components over the years. A blown mains fuse or a swollen capacitor is a typical find after a few years on that kind of supply.

How to separate the plate from the driver (on the bench). This is the key test, and we do it first. The driver can be checked on its own — disconnect it from the plate and measure the coil resistance with a multimeter. A healthy bass coil reads a few ohms (usually close to its nominal impedance); infinity means an open coil, zero means a short. If the driver is healthy, the fault is in the plate or the power supply — and that's the verdict in most cases.

The DIY boundary here is clear: you can check the cables, the input (LFE vs Line) and whether a signal is reaching the unit from the receiver at all. Opening the cabinet, measuring plate voltages and replacing capacitors is bench work. The power supply holds capacitors that retain a charge even after the mains is unplugged.

Won't turn on from signal (auto-on not working)

A very common complaint, and one that's often confused with complete silence. The subwoofer isn't dead — it simply isn't waking from standby when you start playing sound.

Almost every active subwoofer has a signal-sense (auto-on) circuit: when a low signal appears at the LFE input, it wakes the amplifier from standby, and after a few minutes of silence it sends it back to sleep. When this circuit fails, the driver and amplifier can be perfectly healthy — the fault is in the small sense part of the plate.

Before thinking about repair, run the DIY check — half the time it settles the matter:

• Switch the mode to "On" (not "Auto"). If sound now appears, the circuit is to blame, not the amplifier. This is also the best quick test.
• The receiver's bass level is too low. Auto-on needs a certain signal threshold. On quiet evening viewing the signal may not reach the threshold — raise the subwoofer channel level in the receiver.
• LFE vs stereo input. Make sure the cable is in the right input and the receiver's output is configured to "Subwoofer/LFE".

If there's sound in "On" mode but it never wakes in "Auto" mode, the sense circuit needs bench repair. It can be fixed — that's no reason to write the speaker off.

Hums or buzzes (ground loop vs a fault)

A constant low hum from the subwoofer is one of two different stories, and telling them apart is simple — it comes down only to when the hum appears.

Ground loop (most common, and this is NOT a subwoofer fault). If the hum appears only when the subwoofer is joined to the receiver or TV by a signal cable, but disappears the moment you unplug that cable, the fault is a ground loop — the system's units are grounded through two different points and a small current flows between them. That is a matter of cables and installation, not a component defect.

Power-supply capacitor (a real fault). If the hum is always there — even with the signal cable unplugged and the subwoofer only connected to the mains — then the low-frequency background almost always means the power-supply filter capacitor is worn out and no longer smooths the mains ripple. That is bench repair.

In Riga flats there's one more common source of hum — humidity and the salty Baltic air. Oxidised contacts and connectors behind the panel create a poorly defined ground that amplifies the loop effect. First check whether it hums with the signal cable unplugged; that one step separates the installation from a component fault.

The DIY boundary: you can localise a ground loop yourself (unplug the cable, change the socket, disconnect other units one by one). Replacing the filter capacitor is bench work.

Weak or distorted bass

The subwoofer plays, but the bass is feeble, boomy or rasps on the loudest scenes. Here the cause can be in the electronics, in the mechanics, or simply in the settings.

First rule out the settings: check the phase switch (0/180°). If the subwoofer is out of phase with the main speakers, the bass partially cancels and goes thin — turn the phase and listen for whether the lower notes become fuller. Also check the low-pass control and the crossover setting in the receiver.

Once the settings are ruled out, electronics and mechanics remain:

• Distortion that grows with volume — typical of an amplifier stage that's clipping (overloading) or of a power supply that "sags" under load. That's a bench case.
• A rasp or rubbing straight from the cone — a mechanical driver defect: a deformed or cracked surround rubber at the cone edge, or a voice coil scraping the magnet. The driver dies physically less often than the plate, but salty, damp air and the dry heat of the heating season make the cone edge brittle over the years.
• Evenly quiet bass with no distortion — more often settings or phase than a fault.

A practical check at home: press the cone gently and evenly with both hands (with the subwoofer switched off). A healthy driver's travel should be smooth and symmetrical; scraping or catching points to the voice coil and means an inspection on the bench.

Shuts off when you turn up the volume (protection/heat)

The subwoofer plays normally until a loud, bass-heavy scene arrives — then it suddenly goes quiet, sometimes with a click, and comes back to life a moment later. That's a classic protection or thermal-load pattern, not a random fault.

Most plate amps have a protection circuit that cuts the output when the amplifier stage overheats or the power supply can no longer hold its voltage under heavy load. The typical causes:

• Thermal protection. The amplifier or heatsink overheats when driving high power for a long time. If the subwoofer sits in a closed recess or TV cabinet with no airflow, it overheats faster. The first check — pull it away from the wall, give the air room, and see whether the shutdowns become less frequent.
• A collapsing power supply. If the power-supply capacitors are worn out, the voltage "sags" on bass peaks and protection trips. It's the same worn capacitor that causes the hum elsewhere — here it just shows up as a shutdown under load.
• An output-stage or thermal-paste problem. Dried thermal paste between the output transistors and the heatsink worsens heat transfer and speeds up the protection trip.

The DIY boundary: you can fix ventilation and placement yourself — that's the first thing to check. If the shutdowns continue even with free airflow, the fault is in the power supply or the output stage, and that's bench repair.

Is an active subwoofer worth repairing? (decision table)

The short answer: almost always yes — because most faults are in the plate and the power supply, which are repairable, while the cabinet and the driver stay healthy. The subwoofer's cabinet and driver are its most expensive, most massive part; the amplifier plate is what tends to wear out. Add the typical 4–6 year ownership window and the fact that a good subwoofer outlasts its first amplifier repair by a wide margin.

Rather than a blanket verdict on whether your subwoofer is worth repairing, we first open the plate and measure the driver — we identify the fault on-site, once it's clear whether the fault is in a small circuit or in the driver itself. For more on how we handle this kind of equipment, see the audio equipment repair in Riga page. If the symptom is closer to complete silence across the whole system, the articles on amplifier troubleshooting and on an AV receiver with no sound will help too.