Thursday, July 16, 2009

Don’t Forget the Heatsink

Despite of its relatively low power output does the AN214 IC still deserve the tender loving care of adequate heatsinking to achieve better sound quality?

By: Vanessa Uy

During the AN214 IC-based amplifier’s heyday in the early 1980s, heatsinking is usually of little concern due to the device’s relatively low power output that barely reaches 5 watts. Hobbyists back then – especially those that lack adequate knowledge on how semiconductor ICs work – usually bolt the AN214’s metallic tab / backside of it’s 9-pin single inline package to a piece of scrap metal to serve as an ad hoc heatsink. Or maybe to the aluminum chassis of the amplifier’s intended housing, without further concerns on how hot it might get during the AN214 IC’s intended lifetime.

Ideally, when operated under the right conditions, solid state devices tend to last for hundreds of thousands of years. Some of these inadequately heatsinked amps just manage to last up to 8 months at most. As most electronic engineering textbooks – then and now – often states that low-powered applications require minimum thermal mass, a few centimeters of sheet metal, to transfer the small amount of heat generated by a low-power semiconductor device to the ambient air. But is common-sense engineering good enough for the perfectionist audiophile-grade applications for the AN214?

The raison d’être of heatsinking IC-based power amplifiers – even low-powered ones – is to obtain the IC amplifier’s maximum output voltage swing as specified by the manufacturers’ spec sheets. This can easily show up under triangular-wave or sine-wave tests when the inadequately heatsinked IC-based amplifier can’t even output one-tenth of it’s intended maximum output voltage swing - on the cathode ray oscilloscope, the sine and triangular wave peaks would be melting or clipped like crazy.

Non-linear reactive loads like loudspeakers – or the inter-stage transformer of the MJ2955 PNP transistor-based booster amplifier – tend to have some unwanted effects on inter-stage amplifiers (the AN214). The resulting back-EMF of the load may attempt to swing beyond the power supply voltages applied to the amplifier. Thus in order for the IC’s output protection circuitry – assuming it has one – to work, then a heatsink is always a necessity in “controlling” the inductively-generated electromotive force of whatever inductive load that happens to be connected to the AN214 IC-based power amplifier.

To obtain the best theoretically possible sound quality from your AN214 IC-based amplifier, an infinite heatsink – or a realistic equivalent – is a must. To make your intended finned aluminum alloy heatsink approach the heat-dissipating performance of an infinite heatsink, its main body – the point where you bolt-on the IC / semiconductor package – should be thicker than 1/16 of an inch. And this part should be polished to maximize heat transfer. You can also add silicone grease with metal oxide to further boost heat transfer. After bolting-on the IC to the heatsink, you can paint the aluminum alloy heatsink’s surface with a mat black-colored oil paint – preferably thinned a bit with linseed oil – to further boost the heatsinks’ emissivity rating. If done properly, your AN214 IC-based amplifier now has the potential to perform its very best.

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