Users of mains-powered USB audio equipment can sometimes encounter a persistent hum when using Switch Mode Power Supplies (SMPS). This can be impossible to fix by any other means — this article explains why it happens and how to fix it with a pair of wire cutters.
The Y Capacitor
Most SMPS include a small capacitor that bridges the mains (primary) side of the circuit to the low-voltage (secondary) side. This is called a Y capacitor, rated to fail open-circuit in a fault condition. It is included by manufacturers to meet conducted EMI regulations — it’s a compliance component, not a protective safety feature.
The Y capacitor creates a leakage path from the mains supply into your equipment. At mains frequency (50Hz in the UK), a small current flows through this capacitor, you may have felt this as a faint tingle from a laptop chassis when using it on a wooden table. That is leakage current from the Y capacitor looking for an earth return through you.
When this leakage current finds its way into sensitive circuits such as a microphone amplifier, it causes audible hum at 50Hz or 100Hz — exactly mains frequency or twice mains frequency. No amount of shielding or grounding on the audio side will cure it, because the current is being injected directly into the circuit by the PSU itself.
Removing the leakage path
A simple modification to a regular SMPS resolves the interference — remove the Y capacitor. Without it, the leakage path is broken and the hum disappears completely.
I show below how I did this with a pair of wire cutters. The process takes a few minutes once you gain access to the SMPS internals.
WARNING: This involves opening and modifying a mains-connected device. Do not attempt this unless you are confident working with mains equipment. Never plug the SMPS into the mains while it is open or components are exposed.
On a well-designed SMPS a clear isolation gap separates the primary (mains) side from the secondary (low voltage) side of the PCB. I have marked this gap with a red line in the photo below.
Capacitor removed, time to reassemble the SMPS.
I put the top back on before plugging in the SMPS. The hum loop is resolved.
Does this affect EMI compliance?
Technically yes — removing the Y capacitor means the PSU no longer meets its original conducted EMI specification. In practice, for home use with short cable runs this is unlikely to cause any issue. If you are concerned, a quality USB power bank has no mains connection at all and therefore no Y capacitor leakage — an equally effective and entirely non-invasive solution.
Modifying a generic low-cost (Temu) USB supply
Here are photos where I made this mod to a £3 Temu USB supply.
Glue back together before plugging in!