The Italian Job
Sydney Morning Herald
Monday June 18, 2001
PATHOS Acoustics is careful to warn owners of its Twin Towers amplifier that they should not operate it near a swimming pool, kitchen sink or stove. Hello? Is there anyone out there silly enough to put $10,000 worth of electronics by the pool?
As you've probably guessed, the Pathos is no ordinary amplifier. It's Italian. It's also a valve/solid-state hybrid. All input signals are routed though a pair of 12AX77 valves before heading off for final amplification by transistors. The output stage is also unconventional. Rather than use NPN/PNP pairs of transistors in a classic Class A/B output, Pathos uses only NPN transistors in pure Class-A mode.
In a Class A/B amplifier, audio signals are split into two, with the ``positive" half going through one transistor, and the ``negative" through another. The difficulty is that at some stage the two halves have to be rejoined, something that is hard to get right. As a result, most Class A/B amplifiers suffer from harsh distortion when the audio signal crosses over from one transistor to another. In a Class-A design, a single transistor handles the entire signal, so there's no crossover distortion.
The problem with Class-A is that it's difficult to achieve high-power output levels. The Pathos is rated at just 30 watts continuous per channel into 8-ohm loudspeakers, and the manufacturer specifically warns against using low-efficiency loudspeakers. Pathos also suggests that because of the particular Class-A circuit it uses, called INPOL, the Twin Towers should be used only with speakers with an impedance higher than 5 ohms, effectively ruling out the use of all modern high-efficiency 4-ohm loudspeakers.
Pathos is not very forthcoming about how its INPOL circuit operates. The name itself derives from the Italian description of the circuit: inseguitore a pompa lineare (literally, linear pump tracker). It appears that the basis of INPOL is that it achieves low distortion without the use of feedback. In a conventional amplifier, distortion is kept low because the distortion in the output is reversed in polarity then taken back to the input, where it's mixed with the audio signal. The idea is that the introduced distortion will cancel the inherent distortion, a technique called negative feedback. It's inherently flawed, however, because with rapidly changing music signals, the correction signal arrives too late to guarantee perfect cancellation. Electronics engineers liken it to trying to lift yourself up by your shoelaces.
Teamed with a pair of high-efficiency floor-standing loudspeakers, I was amazed at how loudly the Pathos could play, as well as by the delicate transparency of the reproduced sound. The tonal quality was different to anything I have heard, but whether this was caused by the use of valves, the use of Class-A or by not using feedback, I couldn't say. What I can say is that aurally and visually it's a gorgeous amplifier.
Greg Borrowman is editor of Australian HI-FI Magazine.
© 2001 Sydney Morning Herald