TECH TALK: THE TRACKING DOWN CONVERTER AMPLIFIER
The tracking down converter amplifier is a variant of the class-h amplifier topology, and to understand how class-h works, you need to understand that in amplifiers that use conventional power supplies, the designer uses a power supply with a voltage high enough to deliver the maximum power the amplifier is capable of delivering. So, for example, if the amplifier were to be rated at 100-watts per channel into 4Ω, the power supply would need to deliver 20 volts (P=v2/R so 50 = 400/4).The problem is that when the amplifier is not operating at its maximum output, and instead is delivering, say, only 1-watt, which requires only 2 volts, the power supply will still be delivering 20 volts, so the remaining 18-volts is wasted (it’s dissipated as heat).This isn’t a problem with low-powered amplifiers, but as power output increases, the cost of building power supplies for them becomes prohibitively high, and it also becomes difficult and expensive to remove the increased heat that results from the increase in power (no matter whether you remove this heat using heat sinks, cooling fans or a combination of both).
in class-h designs the voltage of the power supply is varied depending on the power output required at any given moment, so that if you’re playing at 1-watt, it would supply only 2-volts, since this is all that’s required.Then, when you turned the volume up to 4-watts, the power supply voltage increases to 4-volts. At an output of 50-watts, the power supply would deliver the full 20-volts.This means there’s no waste, very little heat (so no need for heatsinks and fans) and importantly that the power supply can be made relatively cheaply.
The difficulty in designing class-h designs lies in the designer ensuring that the power supply always stays one step ahead of the output stage, otherwise it won’t be able to deliver the voltage required at the precise instant it’s required. carver’s tracking down-converter was a innovative way of doing this inexpensively and with very few components… indeed it was sufficiently innovative that he was awarded a patent for it—US Patent #4,484,150. (carver’s patent application cites more than thirty previous patents outlining previous similar approaches to the problem, so he didn’t ‘invent’ class-h, but he did invent a unique and very practical implementation of it: one that’s been so successful that it’s still being used four decades later, often in preference to class-D.) #