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Specifying and Machining a Heatsink Extrusion for an Amplifier

August 4, 2026

On a power amplifier the heatsink is both the thermal design and half the visible product. That double role is why it deserves more thought than a catalogue lookup.

Fin geometry

For natural convection, which is what almost every hi-fi amplifier uses, the useful rules are:

• Fin height: taller is better up to a point. Beyond roughly 40 to 50 mm on a natural convection sink the tip of the fin is in air that has already been heated, so returns fall away.
• Fin thickness: at least 2 mm, typically 3 to 5 mm at the base. A thin fin cannot conduct heat to its tip, so the tip does nothing.
• Fin spacing: for natural convection, 8 to 12 mm between fins is the usual optimum. Closer spacing adds area but chokes the airflow between the fins, and below about 5 mm a natural convection sink starts losing performance as you add fins.
• Base thickness: thick enough to spread heat sideways from the device footprint. For a TO-247 or TO-3P on a large sink, 8 to 12 mm base is typical. Too thin and you get a hot spot under the device while the fins at the ends stay cool.

Forced convection changes all of this. With a fan, tighter spacing and taller fins both start to pay, which is why server heatsinks look nothing like amplifier heatsinks.

Reading a thermal resistance figure

Extruders publish a thermal resistance in degrees Celsius per watt, and it always comes with test conditions. The standard set is:

• A 75 mm (3 inch) long section
• Vertical mounting, fins vertical
• Free air, no enclosure
• 75 degrees Celsius rise above ambient

Two correction factors matter most:

1. Length. Doubling the length does not halve the resistance. The improvement flattens out, typically giving something like a 30 to 40 percent improvement for a doubling in the ranges used in audio. Use the extruder’s length correction curve.
2. Orientation. Fins must be vertical for the chimney effect to work. A sink mounted with horizontal fins loses a significant fraction of its capability. If your amplifier has side mounted heatsinks, the fins run vertically. If they run horizontally because it looks better, the thermal design has to account for it.

Then add the interface: device junction to case, case to sink through the insulator and the compound, sink to ambient. The insulator is often the biggest single term and the easiest to get wrong.

Stock profile or custom die

Stock profiles are held by extruders and by enclosure suppliers, often already sized for audio use with mounting slots and a flat device area. You get parts next month, no tooling cost and a proportion that is somebody else’s design.

A custom die gets you exactly the fin pitch, the depth and the face proportions your product needs, plus features extruded in: T-slots for chassis screws, a recessed logo band, a rebate for the top cover. Cost is typically a few thousand dollars and six to ten weeks for a first article, and the extruder will have minimum order quantities by weight or by length.

A useful middle path is a stock profile for the first production run, with a custom die ordered once the product has sold. The chassis dimensions usually shift slightly, but the electronics rarely do.

What the extruder cannot hold

Extrusion tolerances are generous compared with machining. Typical values for a medium profile:

• Cross section dimensions: a few tenths of a millimetre
• Straightness: often 1 mm per metre or looser
• Twist: measurable and visible over a long section
• Flatness of a wide face: not suitable for sealing or for mounting a power device

Which means secondary machining is not optional. Plan these operations:

• Cut to length with a square, clean cut. A saw cut end is visible on the finished product.
• Face the ends so the extrusion meets the front and back panels flat.
• Machine the device mounting area flat, typically a milled pad a few tenths deep across the transistor footprint.
• Drill and tap mounting holes, chassis screw holes and any through features.
• Chamfer the fin tips and the cut edges. Extruded fin tips are sharp and the ends are sharper. This is a safety and a finish issue.

Finishing an extruded heatsink

The fins make finishing harder than it looks.

• Bead blasting between fins requires a nozzle that reaches, or the sink is blasted from the ends and shows lighter roots. Ask how the shop does it.
• Brushing cannot reach between fins at all. A brushed heatsink means a brushed outer face with as machined or blasted fin flanks, which can look excellent if it is deliberate.
• Anodizing coats everything including between the fins. The coating is a thermal insulator, but at 10 to 20 microns its resistance is negligible compared with its benefit in emissivity and protection.
• Racking a heatsink is awkward and the contact point is usually on an end face. Specify which end.

Practical drawing notes

MATERIAL: ALUMINUM 6063-T5 EXTRUSION, PROFILE PER DRAWING 2201
CUT TO 300 mm, BOTH ENDS FACED SQUARE, 0.1 FLATNESS
MILL DEVICE PAD 120 x 40, FLATNESS 0.05, Ra 1.6 MAX
BREAK ALL FIN TIPS AND CUT EDGES 0.3 x 45
FINISH: BEAD BLAST ALL EXPOSED SURFACES INCLUDING BETWEEN FINS
ANODIZE TYPE II CLASS 2 BLACK 10-15 MICRONS
RACK POINT ON REAR END FACE ONLY

The device pad flatness and the fin tip break are the two lines shops most often miss, and they are the two that come back as a warranty problem and a cut finger.

Frequently asked questions

Keep reading

• What an Amplifier Chassis Actually Costs to Manufacture
• Billet, Extrusion or Folded Sheet: Choosing How to Build a Chassis
• Getting a 19 Inch Rack Panel Right the First Time

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