Integrated Amplifiers

Copland suggests that 'a pair of 6550s in push-pull can deliver more than 100W of audio power' which is exactly what VAC achieves in its Sigma i200 [HFN Aug '21], even though most hi-fi applications will settle on a 50W specification for a pair of 6550s or KT88s in ultralinear configuration. That's the case here just as it is for the 6550s used in Audio Research's competing I/50 integrated tube amp [HFN Oct '22]. Power output meets Copland's 50W rating at 2x53W and 2x54W into 8 and 4ohm loads via the 8 and 4ohm taps, respectively, and there's a hint of headroom under dynamic conditions where 62W, 62W, 79W and 65W is achievable into 8ohm (8ohm tap) and 4, 2 and 1ohm [all 4ohm tap; see Graph 1]. With a maximum current delivery of 8.06A (10msec/1ohm at <1% THD) the CTA407 is capable, but clearly best partnered with sensitive speakers.

While the amplifier's power profile (and power consumption) is not dissimilar to that measured with the I/50, the CTA407's linearity versus power and frequency is arguably better controlled, resulting in a flatter response (–0.25dB/20kHz vs. –2.9dB/20kHz) and lower HF THD (0.7% vs. 3.2%/20kHz, all re. 10W). Like most tube amps, the CTA407's general distortion trend is higher at low bass frequencies (1.5%/20Hz, re. 10W) despite the novel toroidal output transformers [see Graph 2, below] while also increasing gently with level from 0.12%/1W, 0.3%/10W up to 0.4% at the rated 50W (all 1kHz into 8ohm). Overall gain is high at +41dB – a minimum of +20dB would be required to raise the rated 50W output with a modern 2V line-level source like a CD player or DAC – but noise is low and the A-wtd S/N ratio a very generous 90.5dB (re. 0dBW). PM