Quad The Quad II Classic

The Quad II-Classic is a valve power amplifier that faithfully follows the design of the original amplifier launched in 1953 by Peter Walker. This reissue reproduces the architecture and characteristics of one of the most studied amplifiers in the history of audio, retaining the sonic spirit of the original version, which remained in production until 1970.

Its output stage relies on a cathode-coupled arrangement that reproduces the behaviour of triode operation while retaining the size and efficiency of a pentode arrangement. It is this particular feature that set the original Quad II apart from its contemporaries and explains the longevity of its design. The Quad II-Classic uses valves equivalent to those of the historic model: two EF86, two KT66 and one GZ34 rectifier, for Class A operation delivering approximately 15 watts per channel.

The measured values reflect a carefully considered design: an input sensitivity of 775 mV RMS, harmonic distortion of 0.8% at 12 W (measured at 700 Hz), residual noise better than -90 dB, a frequency response of 10 Hz to 20 kHz (± 0.2 dB) and a damping factor of 11.75 at 700 Hz, referenced to 8 ohms. The output impedance is 8 ohms.

This type of amplifier is aimed at enthusiasts of valve sound seeking a smooth and detailed presentation, to be paired with speakers of suitable sensitivity given the modest power delivered in Class A. The Quad II-Classic will suit attentive listening, in a system where sonic refinement takes precedence over raw power.

When buying such a valve amplifier, it is worth checking the condition of the valves fitted, in particular the GZ34 rectifier and the KT66 output valves, parts that wear with hours of operation and whose replacement must respect the original values. It is also useful to check the condition of the output transformers, a central element of Peter Walker's design, as well as the connectors and internal passive components, which can degrade over time on a unit of this architecture.

• Power: 15 W RMS per channel
• Class A operation
• Valves: 2 x EF86, 2 x KT66, 1 x GZ34
• Output impedance: 8 ohms