Subwoofer powers up but makes no sound: supply board or amplifier board?

The LED lights, the relay clicks, and there is silence. Measuring two voltages tells you which board to buy — and prevents the single most expensive misdiagnosis in subwoofer repair.

5 min read · Replacement Boards

Why this fault gets misdiagnosed

When a subwoofer is completely dead, everyone looks at the power supply. When it lights up and stays silent, too many people still look at the power supply — because the supply is the board they already understand. The result is a replacement supply board fitted to a unit whose supply was never faulty.

The two measurements that settle it

Take the unit apart far enough to reach the amplifier board input. With the unit powered and a signal present, measure the DC rails feeding the amplifier stage.

• Correct rails present (±35 V on most HKTS-platform units) → the supply is healthy. The fault is on the amplifier board.
• No rails, or one rail missing → the supply has failed. The amplifier board is probably innocent.
• Rails present but sagging badly under load → supply is failing under load. This is the one ambiguous case; look for a hot component on the supply board.

That is the whole test. It takes three minutes and it replaces a guessing game that regularly costs technicians the price of the wrong board.

Reading the symptom more precisely

Before you even open the cabinet, the symptom narrows things down.

• Powers up, LED normal, silent on all sources → amplifier board, or the signal path into it
• Powers up, LED normal, faint crackle or hum but no bass → amplifier board output stage
• Clicks repeatedly, relay cycling → supply protection tripping, treat as a supply fault first
• Dead, no LED → supply board, see the other guide
• Works cold, cuts out when warm → thermal fault, usually the amplifier stage or its heatsink interface

The thermal failure that looks intermittent

A subwoofer that works for twenty minutes and then goes quiet has a thermal fault, and in most cases it is the amplifier board. The output devices heat up, the thermal interface between them and the heatsink has degraded, and the protection circuit does exactly what it is supposed to do.

Technicians often replace the supply board in these cases because the failure is intermittent and the supply is the board they can test most easily. It rarely helps. If the unit fails reproducibly after a warm-up period and recovers after cooling, suspect the amplifier stage and its thermal path.

What to do next

Once you know which board is at fault, the repair is straightforward: both are available as drop-in replacements with the original footprint and connectors. If you are still unsure, photograph the board and the model label and send them to us — we will tell you which one you need before you order anything, including from us.