Integrated Amplifiers

Almost all valve amps employ a transformer to step-down the high voltage/high impedance output of their power tubes – 6550s in this instance – to better match the lower impedance and higher current demand of the attached loudspeaker. Copland offers 8ohm and 4ohm output taps conceived to 'match' nominal 8 and 4ohm loudspeaker loads, respectively, even though almost no speakers provide such a 'flat' impedance. Instead the load swings up and down with frequency as the various reactive components of crossover and drivers come into play – the system response rising with increasing speaker impedance and falling with decreasing impedance.

I mention ARC's recently reviewed I/50 integrated in my Lab Report [see p59] as it shares the same tube and 50W specification as the CTA407, but differences in the output impedance of their '8ohm' taps are helpful in illustrating why two ostensibly similar tube amps can sound different with your (reviewer's) choice of loudspeaker. Through bass and midrange, the 8ohm taps of both the CTA407 and I/50 show a fairly flat output impedance of 0.75ohm and 0.9ohm, respectively. As a result both amplifiers react comparably to different loudspeaker loads up to about 8kHz. At higher frequencies, however, the I/50's output impedance starts to rise more swiftly than in the CTA407 – 2.6ohm vs. 0.8ohm/10kHz and 6.2ohm vs. 1.0ohm/20kHz, for example. The Perlisten S5t floorstanders mentioned by Mark in this review dip from 7ohm at 6kHz to 3ohm at 15-20kHz so the I/50's high treble roll-off would be more marked at –6dB/20kHz relative to the Copland/Perlisten pairing. The result? Copland's CTA407 will likely deliver a more consistent sound with different partnering loudspeakers. PM