Amplifier Cooling Shelves

Abstract

This Class-A amplifier has adequate heat sinking for passive cooling most days of the year, but if the ambient temperature exceeded 35°C then the 60°C thermal cut-off switches would trigger.  Mild forced air cooling with PC case fans installed horizontally in a perforated shelf upon which the amplifier sits was the answer.  The idea is best for amplifiers having external heat sinks, but those with internal heatsinks and ventilation apertures would also benefit.

Aside

Some people suggest that water cooling is a good idea for Class-A amplifiers, but as a mechanical engineer (who actually did quite well in thermodynamics) I can confidently say that such a regime could only transfer the heat of the transistors to some other location where it would need a radiator and a fan anyway! Here, perfectly useable heatsinks are right there asking for a breeze.

OK

Since the amplifier’s heat sink fins are external, there wasn’t much point looking at internal fans.  The amp was sitting between solid shelves in a rack, so I had a couple of perforated steel cooling shelves fabricated and powder coated as replacements:

These sit on tabs in the frame:

Placed above and below the amp, natural convection through the perforations took about 3 or 4° off the heat sinks directly behind the output transistors (measured with an IR thermometer).  That was not really much, so I decided to install a pair of low-speed 120mm computer fans to the bottom shelf to create a mild updraught past the heat sinks.  The fans were bolted under the shelf to align with the heatsinks and also direct air upwardly through the amplifier via the chassis floor slots to force-cool anything inside such as the driver transistor heat sinks and the power supply inductors – then out through the lid slots.

Fans under shelf

There are variable speed temperature sensing fan controller projects out there, but with extremely quiet modern fans now available, I saw little point in a controller – just turn them on.  It seems the demand for quiet media serving PCs has resulted in many choices.  Anyway I grabbed a couple of 120mm “Gentle Typhoon” case fans.  These are available in many speeds, so I chose the second slowest version which runs at approx. 800rpm on 12Vdc to move 0.8m³/min of air while producing just 9dB (measured who-knows-how).  Hardly a typhoon but gentle for sure.  These particular fans have blades which hug the cowling very closely indeed.  They therefore provide good pressure (as against mere unobstructed flow rate) to provide a decent flow through the shelf perforations.

To power them I dug out an old 300mA/9V linear plug pack (I have a box-full and that’s good because you can’t get them any more).  It measured at 11.5V with no load.  The fans each draw only 23mA so that’s good enough.  I cut off the plug and soldered in an electrolytic smoothing capacitor (under the heat shrink in the next photo) and split it off to two Molex plugs which came with the fans:

Old plug pack etc.

Nothing sits on the perforated shelf above the amp.  This allows the warm air to escape freely without heating any other component.

The fans are completely inaudible from anywhere more than a foot away and produce a mildly turbulent upward breeze – sufficient to displace hot air from between the heat sink fins.  Now about 8° is removed from the heat sinks directly behind the output transistors.  Also the amplifier chassis is generally cooler so it’ll be great in winter to supplement the central heating.  😀

Athena

 

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