Integrated Amplifiers

Faced with the daunting scale of the InPoL Legacy, and the stylistically disguised nature of the volume control, many enthusiasts will see it and think ‘power amp’. Instead, the InPoL Legacy is quite possibly the hi-fi world’s (physically) largest integrated amp so before we look under the cover, a word or two about that 99-step attenuator. This rotary governs a switched resistor network that covers a tight 1dB range over its top six positions (99 to 93), and 3dB over the top ten volume positions (99 to 89). This relaxes to a 7.8dB range from 99 to 79, 18.9dB from 99 to 59, 28.9dB from 99 down to 39, and a 51.4dB range from 99 down to 9. In practice, the maximum useable volume range is 70dB (3160:1) with only the top ~50dB being covered in usefully fine steps.

This volume control acts on the Legacy’s triode tube stage. This sets the amplifier’s voltage gain with the unity-gain InPoL output stage providing the supporting current. In practice, the InPoL (INseguitore a POmpa Lineare, or Linear Pump Follower) circuit is a single-ended Class A FET stage in a common source configuration but with an inductor, rather than series resistor, between the FET source and ground. This inductor – the frame transformer(s) seen near the front of the amp – stores energy, proportional to the inductance and the square of the current, that’s released by the FET as the current falls. This almost doubles the efficiency of the Class A stage to 50% but also defines the available quiescent current [see PM's Lab Report] and explains the high heatsink temperature. Two InPoL stages are operated here in a balanced/floating mode.

This ‘double InPoL’ configuration offers good PSU noise and ripple rejection but Pathos has still gone to town with a pair of massive, resin-filled toroidal power transformers (the 18kg black metal boxes at the rear, p50) with choke regulation (the coils covered by a red circular metal plate) and three large (also red!) reservoir/smoothing electrolytics amounting to 66,000µF per side. PM