Mounting a Toroidal Transformer (and Fixing Hum)

The mounting kit

Most toroidal transformers are supplied with a standard kit: a centre bolt, a steel dished washer, and two rubber (neoprene) pads. The transformer sits on one pad, the second pad goes on top, and the dished washer spreads the clamping force evenly when the bolt is tightened.

• Tighten firmly enough that the transformer cannot rotate under a strong twist. Do not tighten so hard that the washer flattens completely or cuts into the winding.
• Never mount the transformer without the pads. They protect the winding insulation and absorb vibration.
• For vertical mounting or high vibration (vehicles, flight cases), consider a potted-centre toroid. Its centre hole is filled with resin and it mounts with a single screw.

The shorted-turn trap

The centre bolt passes through the middle of the transformer, so it acts like one turn of a winding. If the bolt's top touches the equipment's lid, a bracket or any other metal, and that metal connects back to the chassis at the bolt's bottom, you have made a closed conductive loop through the core: a shorted turn.

A shorted turn behaves like a secondary winding with a dead short across it. It can carry a very large current, heat the bolt and chassis, and damage the transformer or blow fuses.

Placement and orientation

• Keep it away from input circuitry. The stray field of a toroid is small but not zero. Place it as far as practical from input sockets, volume controls, phono stages and input wiring.
• Rotate to find the quiet spot. The leakage field is strongest where the leads leave the winding. With the amplifier running and no input connected, rotate the transformer slowly and listen through speakers or headphones. Fix it in the quietest position.
• Allow airflow. Transformers are designed for a maximum temperature rise. Do not bury one against a hot heatsink or in a sealed box.

Wiring the leads

• Follow the label on the transformer for lead colours. There is no universal standard, and colours vary between manufacturers.
• Twist the primary pair together, and twist each secondary pair together. Twisting reduces the radiated field from the wires themselves.
• Dual-primary transformers (2 × 115 V) are connected in series for 230 V. Getting the phasing wrong gives zero output and a very high primary current, so follow the diagram exactly.
• Earth the chassis properly. A transformer's electrostatic shield, if fitted, is usually connected to earth.

Troubleshooting hum and buzz

First work out whether the noise is mechanical (you can hear the transformer itself hum or buzz) or electrical (hum comes out of the speakers).

Mechanical buzz from the transformer

• DC on the mains. This is the most common cause with toroids. Appliances that draw current on only one half of the cycle (some hair dryers on low, heaters and chargers) put a small DC offset on the household supply. A toroid's gap-free core is very sensitive to this and will buzz even though nothing is wrong with it. Test by moving the equipment to another circuit or switching off other appliances. A mains DC blocker is the permanent fix.
• Loose mounting. Retighten the bolt and check the pads are in place.
• Overload. A transformer driven beyond its rating gets hot and noisy. Check the load current.
• Shorted turn. Loud buzz plus heat soon after switch-on means you should check the bolt, as described above.

Hum in the speakers

• Magnetic coupling. Rotate or move the transformer as described above, and route input wiring away from it.
• Ground loops and layout. Most amplifier hum comes from grounding and power-supply layout, not the transformer. Use a star ground, keep rectifier charging-current loops short, and do not run signal ground through the reservoir capacitor wiring.
• Insufficient filtering. A 100 Hz buzz that rises with volume points to too little reservoir capacitance or an undersized transformer.

Still stuck? The team at Vaani can advise on transformer-related noise problems.