Integrated Amplifiers

While Copland offers two output taps from its speaker coupling transformers – 8ohm and 4ohm – its 75W power is specified, unusually, into 8ohm and 3ohm. Either way, the CTA408 readily bests this figure, delivering 2x95W/8ohm and 2x90W/4ohm via its 8ohm and 4ohm taps, respectively. Under dynamic conditions this increases to 110W and 100W into 8/4ohm loads (8/4ohm taps) up to 1% THD and 155W/160W into 2/1ohm loads (4ohm tap) with the distortion limit relaxed to 2% [see Graph 1, below]. Distortion scales with output level and frequency, and although the pairs of KT150 tubes are matched at Copland's factory, and the bias adapted accordingly, there was still a difference between L and R channels in our sample. (The fact that it's the protective shipping boxes and not the tubes themselves that are marked V103/V104, etc, is simply asking for trouble the moment a CTA408 is packed and moved from demo to demo...)

In this instance, distortion increases from 0.022%/1W to 0.2%/10W (left) and 0.07%/10W (right) before coalescing at 0.75%/75W – all at 1kHz/8ohm. The modest 15dB overall NF also means distortion increases with frequency – 0.7%/10W/20kHz (left) and 1.5%/10W/20kHz (right) – and at low bass frequencies through transformer core saturation [a feature of every such tube amp; see Graph 2, above]. Otherwise, the 0.9-1.1ohm output impedance is well matched between channels, as is the 83.5dB A-wtd S/N ratio (re. 0dBW), but is sufficient to cause variations in system response with swings in loudspeaker load impedance. Into a 'flat' 8ohm load the response is good for –0.2dB/20kHz and –3dB/60kHz (8ohm tap) and holds firm out to 30kHz right down to 1ohm loads (via 4ohm tap). PM