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Transformer Mounting: Weight, Hum and the Metalwork

September 22, 2026

The transformer is the heaviest object in most audio products, it is usually mounted to a plate that is also a cosmetic part, and it is the main source of both magnetic and mechanical noise. Those three facts pull the metalwork in different directions.

Weight is a structural problem

A 5 kg toroid bolted through a 2 mm aluminum plate is not a mounting, it is a hinge. The plate will dish around the bolt, and the dish will show on the other side of a cosmetic panel.

What to do:

• Increase local thickness rather than global thickness. A machined boss or a backing plate under the transformer costs far less than making the whole top plate 5 mm.
• Use a large diameter washer or a spreader plate. For a toroid, the supplied metal dish already does this. For an EI, the mounting frame does.
• Consider shock load, not static load. A unit dropped 100 mm onto a corner puts many times the static weight through that bolt. This is why transit damage on heavy audio gear usually appears as a bowed top plate.
• Keep the transformer close to a supported edge or a fold where the panel is naturally stiffer.

Toroidal mounting and the shorted turn

A toroid mounts with one bolt through the middle, a rubber dish below, a metal dish above and a nut.

The failure mode specific to this arrangement: if the bolt, the top dish, the chassis and the bolt again form a closed conductive loop that passes through the centre of the toroid, that loop is a single shorted turn linked to the core. It heats up and the transformer buzzes.

The rules that avoid it:

1. Insulate the top dish from the chassis, or insulate the bolt, so there is no complete conductive circuit through the core. Manufacturers supply an insulating washer for exactly this.
2. Never run a metal strap over the top of a toroid and bolt it down at both ends. That is a shorted turn by construction.
3. Never mount a toroid in a closed metal well or clamp ring that forms a loop around it.
4. Give the centre bolt a clearance hole that leaves room for the insulating washer, and put the washer on the drawing as a part number rather than a note.

EI transformers

A laminated EI transformer mounts on its frame with four bolts, or comes in a bell with two.

Manufacturing points:

• Four holes, one datum. The frame is not always as square as the drawing suggests. Give the mounting holes a slightly generous clearance, typically 0.5 to 1 mm over the bolt, so the frame is not forced.
• The frame is a spring. Bolting one corner down hard against a plate that is not flat twists the frame and can increase mechanical buzz.
• Laminations buzz. The noise is mechanical, at twice mains frequency and its harmonics. A thin gasket, a compliant washer or a bead of flexible adhesive under the frame usually reduces it more than anything done to the chassis.
• The external field is much higher than a toroid’s. Orientation matters. Rotating an EI transformer 90 degrees can change hum at the input stage by a large margin, and it costs nothing at the drawing stage.

Distance and orientation beat shielding

If there is one thing to take from this into a layout, it is that moving the transformer is free and shielding it is not.

In rough order of effectiveness per unit of cost:

1. Distance from the input stage and from signal transformers
2. Orientation, rotating the transformer so its field axis is not pointing at sensitive circuitry
3. A toroid instead of an EI, if the budget allows
4. A local steel or mu-metal can around the transformer
5. A steel partition between power and audio sections
6. Shielding the whole chassis, which does almost nothing for low frequency magnetic fields anyway

Aluminum chassis material provides essentially no low frequency magnetic shielding. If the design depends on magnetic shielding, the shielding part has to be ferrous and it has to be near the source. See the guide on shielding an audio chassis for the rest of that argument.

Thermal

Transformers run warm continuously, and in a sealed enclosure that heat has to go somewhere.

• A transformer bolted to a metal plate uses that plate as a heatsink, which is useful and also means the plate gets warm. On a cosmetic top plate this is usually fine and occasionally a surprise.
• Ventilation above the transformer helps. Ventilation only below it does not.
• Keep electrolytic capacitors away from it. Capacitor life halves for roughly every 10 degrees of temperature rise, and this is a layout decision with a metalwork consequence.

Drawing checklist

• Mounting hole positions from one datum, with clearance appropriate to the frame tolerance
• Local plate thickness or backing plate under the transformer
• Flatness callout on the mounting area
• The insulating washer for a toroid, called out as a part number
• A note prohibiting any conductive loop through the centre of a toroid
• Earth connection point, with a masked bare landing if the chassis is anodized
• Transformer weight noted on the assembly drawing, so the shop understands why the callouts are there
• Orientation, if the layout depends on it
• Clearance to mains terminals, per the applicable safety standard

Frequently asked questions

Keep reading

• What an Amplifier Chassis Actually Costs to Manufacture
• Billet, Extrusion or Folded Sheet: Choosing How to Build a Chassis
• Specifying and Machining a Heatsink Extrusion for an Amplifier

This guide is part of Audio Chassis Manufacturing: The Whole Picture, one of four sections in the guide library.