Spica Angelus loudspeaker 1988 Measurement
Measured nearfield, the Angelus' -6dB point referred to the level at 100Hz was a highish 41Hz, though the rate of rolloff was shallow, implying a somewhat overdamped alignment. The spatially averaged room-response also revealed the low frequencies to be a little lightweight, there being a lack of energy in my room—which normally reinforces the mid-to-upper bass—right up to 200Hz. The rate of roll-off was gentle, however, as there was still significant output, albeit 10dB down, at 31.5Hz (though the woofer was audibly working very hard at this frequency and level).
The response sounded a little resonant in the 200Hz 1/3-octave band, with a dip at 250Hz, but above that frequency, the in-room response was essentially flat (±0.8dB) from the lower midrange up to 5kHz or so (fig.1), and sounded very clean. The treble was also relatively flat, apart from a prominence in the 5kHz and 6.3kHz bands, due, I am sure, to the contribution to the reverberant soundfield in the room from the accentuated output in this region above the listener's head.
Fig.1 Spica Angelus, spatially averaged, 1/3-octave, freefield response in JA's Santa Fe listening room.
As with other designs featuring this relatively large-diaphragm tweeter, the final octave rolled off a little early, the in-room -6dB point being at 16kHz (though note that this is a relative measure only, the exact roll-off, dependent on room furnishings and distance from the speakers to the listening seat, being unique to each room). Pair-matching appeared excellent, while the horizontal dispersion seemed even and well-controlled.
The impedance plot showed that the Angelus is well-behaved, rarely dropping below 8 ohms apart from a drop to 5.2 ohms around 8kHz. The speaker should work well even with highish-output-impedance tube amplifiers. The LF resonance was of low Q, but positioned a little lower than spec at around 30Hz. Measured sensitivity at 1kHz was spot on at 87dB/W/m. Unless you have a small room, I recommend using 100W amplifiers with the Angelus.—John Atkinson