Integrated Amplifiers

While the Virtus I2 may not quite achieve the rating hoped for by Canor, the brand is still clear that the amplifier remains operationally in 'Class A'. In practice a pair of KT88s/6550s will typically achieve 50W/8ohm in push-pull/UL guise [HFN Nov '23 & Oct '22] but can, if pushed hard, deliver closer to 100W [HFN Aug '21]. But Canor does not advocate 'tube rolling' – its use of moderate plate current with a proprietary auto-bias regime confirms this is a 'plug and play/sit back and listen' solution. Tweakers look elsewhere! Meanwhile, as the definition of 'Class A' in tube circuits is often contested, I'm tempted to refer to the RCA's (Radio Corporation of America) Receiving Tube Manual, published back in 1947. Buried in this literary treasure is a section that explains Class A operation as a configuration where 'the grid bias and alternating grid voltages are such that the plate current ... flows at all times'.

While single-ended tube stages [HFN Dec '23] are fundamentally Class A in operation, Canor can remain faithful to the RCA's definition for push-pull Class A by idling the tubes midway between cut-off (low) and full saturation (high). Here audio current still swings through both the 'negative' and 'positive' push-pull tubes, increasing in one tube above the idle bias up to the point of saturation while simultaneously decreasing in the other, but avoiding cut-off. The two phases are summed in the output transformer, also suppressing even-order distortions and noise. Strictly, a Class A amp's idle power consumption will be high and virtually unchanged regardless of amplifier output but, like most, the Virtus I2 idles 'hot' (220W, in this instance) but draws higher (250W) at full output. Class A or not, the I2 remains very inefficient as its KT88s continue to dissipate a high current even if there is no input signal. PM