Stereo Amplifier

Review: Spica TC-50 Speakers

Wedge shaped loudspeakers do not have instant eye-appeal, especially when they are placed with the thin edge of the wedge pointing upwards. Even after a couple of weeks of living with these Spica TC50s I find myself looking at them sideways and wondering if they really need to be that odd shape.

However, the point is that I am still living with them and enjoying the sounds they make. There is an open spaciousness about the sound which makes an immediate appeal, and if their odd shape draws attention to them visually, the neutral unobtrusive sound quality almost makes them aurally transparent.

I understand that the paramount objective of the designer John Bau was to go for phase coherence. This obliged him to time-align the two drive units, by which we mean pushing back the tweeter, or bringing forward the woofer, to position their voice-coils (the effective source of the radiation) at precisely the same distance from the listener's ears.

The Spica's sloping front panel achieves the time alignment without protuberances and, as an extra move against diffraction interference effects in the overlapping frequency bands of the two drivers, the panel is covered by a 15mm thick pad of absorbent material. This smooths out diffraction kinks and helps the dispersal 'windows' of the drivers to merge in a more seamless manner. The voice-coil alignment is designed to work best with the listener's ears opposite the centre of the vertical back panel. This means putting the loudspeakers on rather high stands unless you are sitting on a low armchair or sofa. Some slight tilting of the loudspeakers is suggested in the owner's manual as a means of fine tuning this listening angle.

For the design objective of good phase linearity to be preserved throughout the system, special care was needed in putting together the crossover network. Computer-aided design has speeded up this optimization process and the final result is described as "fourth order Bessel" for the woofer and "approximately first order" for the tweeter. Input impedance is nominally 4 Ohms, which is no problem for the vast majority of amplifiers and may give a subjectively more powerful feeling of efficiency in comparison with standard 8-Ohm loudspeakers than the modest 84dB for 1 watt at 1 metre quoted.

Cabinet construction and finish are excellent, all-black in the case of my review samples, though walnut and oak are also available at the same price. The grille cloths are virtually transparent to sound and stand just clear of the absorbent pad on the baffle. The enclosure is a closed-box type with a system resonance at 56Hz. The output terminals are set at an angle into a recessed panel. Presence Audio also offer two upgraded versions of this loudspeaker. The TC-50SE has additional internal cabinet bracing and a split crossover network wired to two pairs of terminals to permit bi-amp and bi-wire drive. The TC-50S EX removes the crossover network from the cabinet and supplies it mounted in a plastic box (one for each loudspeaker) to be placed near the power amplifier. The drive units comprise a 25mm soft domed (impregnated cloth) tweeter and a 165mm treated paper cone woofer.

How they performed

The design-talk about linear phase and computer derived crossovers would certainly appear in this case to be something more than mere talk. Having done enough listening to persuade myself that this was a musically very satisfying loudspeaker, I tried to be coldly objective and do some measuring. The impedance/frequency check produced an acceptably easy load characteristic, never falling below about 3.5 Ohms. Sensitivity was as per the specification. More interestingly, the frequency response tested in-room with warble tones produced an unusually smooth and consistent level plot, steady within about + 1.5dB all the way from 80Hz to 16kHz. Here was a definite indicator that the subjective smoothness mentioned earlier was no conjuring trick: clearly the designer had put together two drive units which in themselves seem nothing out of the ordinary, and had combined them electronically and physically to produce a near-ideal response. After noting the smooth roll-off at each end of the 80Hz to 16kHz 'flat' bandwidth, I came across a coloured leaflet for the TC-50 containing the maker's own response graph. It matched my result almost exactly, even agreeing to a 10dB fall by 20kHz.

You will gather from this that the TC-50s are not so sharp-toned as some high-quality loudspeakers, but this degree of treble roll-off is of little consequence for lots of musical source material (and, dare I say it, for lots of ears). Provided it is balanced by subtle bass limitation, it will always score well in subjective listening tests-as producing very 'musical' or easy-on-the-ear results.

A major benefit from the triangular enclosure used here is the considerable suppression of internal standing-wave resonances. The only parallel surfaces are the two sides and standing waves between them are broken up by the presence of the drive units. A tight elimination of box resonances results and this helps to provide real-sounding, quick-acting bass, although inevitably lacking in ultimate depth. Bass extension can virtually be written off for compact loudspeakers and the wise designer will look for a musical balance rather than an aggressive treble. The Spica TC-50 succeeds in the sense of sounding pleasingly musical at all times. If its dimensions and price are attractive go along to your nearest dealer and ask to hear it.