Loudspeakers

As the Callisto 6 C has an ADC input for line-level analogue signals, obviating the need to use the Sound Hub, test signals were injected directly into the review pair for these measurements. Because the 6 C uses Dali's familiar twin-tweeter arrangement, the forward frequency responses were made with the microphone height set between that of the two HF drivers, with the results for the two review speakers shown overlaid [see Graph 1, below]. Response errors of ±5.7dB and ±4.8dB respectively (re. 200Hz to 20kHz), are high because an initial decline in output through the midrange to the first crossover is followed by shelved-up treble output, which continues to rise gently up to 20kHz.

Some of this treble excess will be countered by using the 6 C as Dali traditionally recommends – firing straight down the room rather than toed-in towards the listening seat, so that the direct sound is off-axis – but the hole at 2kHz will remain. As Dali could easily have equalised this using an analogue filter ahead of the internal power amplifier, we have to assume that it's a desired feature although it threatens to soften the sound and recess the stereo image a little. The deep, narrow dip in output above the audible range at about 30kHz is probably caused by interference between the two tweeters, in which case it will vary according to seated ear height.

Our diffraction-corrected nearfield measurement of the 6 C's bass response was frustrated, as with many modern speakers, by the use of highly flared ports which add some uncertainty to the relative weighting of driver and port outputs. In the event, our results show the 6 C's bass response being virtually flat down to 70Hz (–6dB re. 200Hz), below which the output falls off with a second-order (12dB per octave) slope before steepening below 30Hz. The cumulative spectral decay waterfall [Graph 2, above] reveals only a few low-level treble resonances above 3kHz. KH