Sircom’s First Law of Amplification states that the thermal output of any amplifier under test is directly proportional to the ambient temperature of the environment. In other words, when it’s freezing cold outside, all you get to review is Class D iceboxes, and the all-valve Audio Research 75 SE’s time in the sun was around Wimbledon fortnight. It’s a hot-running amplifier at the best of times, but with the sunshine outside the house pushing body temperature, and no air-con inside the house, those hours spent running in the Reference 75 SE were hours spent in a sauna.
There is little to distinguish the original Reference 75 from its new SE model from the outside, apart from a tiny badge on the top. It’s still the same classic Audio Research finish in silver (or black) with now-matching grab handles on the front and two big blue glowing meters on the front panel. It still uses the same hybrid solid-state/valve circuit with each channel featuring a JFET input stage feeding a 6H30 double triode line driver and it is still an all balanced amplifier, with not even the implication of a single-ended RCA input. It still needs a 20A IEC socket. The similarities sort of end there.
Something happened at Audio Research a couple of years ago, between the launch of the Ref 75 and Ref 75 SE. What happened was the GS range, and, in particular, the GS150 stereo power amplifier. Audio Research shares with Naim Audio a very conservative customer base, and any change to the exterior design is met with gasps of disapproval – even the move from black handles on a silver front to matching handles on ARC amps a few years ago was met with the sort of opprobrium normally reserved for impeached presidents and profligate hedge-fund managers. But after the inevitable feeding frenzy over the GS new look, when people began actually listening to these amplifiers, they seriously liked what they heard. A touch of ‘trickle down’ was required.
The Reference 75 stereo, the Reference 150 mono (the original design platform for the Ref 75), and the Reference 250 mono amps underwent some significant improvements, with changes to several active and passive components in the circuitry, and a move from two matched pairs of the 6550-derived KT120 to the two pairs of all new KT150 power tube. Despite the tube swap, power output remains unchanged at 75W per channel.