Spica TC-50 loudspeaker 1988 Measurements
Anechoic data were available for the review, providing a foundation on which to base the technical analysis. The LF system resonance was noted at 65Hz, with a -6dB free-field response at 48Hz, indicating a desirably slow, well-damped rolloff. In-room, well sited within the boundaries, the response extended down to 35Hz, provided that excessively loud demonstration levels were not demanded.
Although the speaker is rated at 4 ohms, the impedance curve (fig.1) related to a 6 ohm nominal rating with minima at 4 ohms and above, while the peaks remained within 12 ohms. This is a relatively kind load, and should not cause matching problems for modern amplifiers.
Fig.1 Spica TC-50, modulus of impedance in ohms (logarithmic vertical scale).
The voltage sensitivity checked out at a below-average 86dB for 2.83V, consistent with the manufacturer's spec. (The 4 ohm rating means that more than 1W will be drawn from the amplifier to maintain the 2.83V level.) The power handling was not exceptional; I advise caution in driving up to 75W peak program, and also advise against high-level driving with continuous synthesizer tones, especially in the bass. On the basis of this power handling, a minimum of 20Wpc will be required, while the peak input will result in satisfactory in room sound levels of 99dBA. This is not particularly loud, but more should not be expected; remember, the TC-50 is a miniature at heart, regardless of its generous musical performance.
Measured on-axis at 1m, the left- and right-hand examples did not provide particularly good pair-matching; both responses have been plotted for comparison (fig.2). The upper midrange was 1dB softer in one example, while with the same enclosure, the treble was mildly elevated, this making for a "thinner" tonal balance on the dashed-line example (footnote 2). We had not picked the most favorable vertical axis for this particular measurement, which resulted in the 1.5-3kHz depression evident in the crossover range.
Fig.2 Spica TC-50, on-axis response and pair match (5dB/vertical div.).
Moving on to the set of forward characteristic responses averaged by a 1/3-octave window taken at a more representative 2m microphone distance, these produced a more favorable axial response (fig.3). Here, the on-axis response held within close ±2dB limits from 65Hz to 19kHz. At 10 degrees above-axis (shown dotted), the presence-range dip occurred; so the need for a sufficiently high stand, or one angled back a bit, was confirmed—when all has been squared up, the ear should be at mid-driver level. Off-axis in the lateral plane, the indication was of some lack of phase integration in the crossover, as the response fell off more quickly than expected in the crossover range. This notch deepened significantly, by the time the microphone was 45 degrees off-axis (dash-dot-dash).
Fig.3 Spica TC-50, forward response family (5dB/vertical div.).
The midrange was mildly lifted relative to the fundamental bass range by typically 1dB, this pattern continuing in the main treble register. Given the good diffraction properties, such a small enclosure might have been designed to have the alternative of a slightly falling response to impart a more natural balance in a typical listening room.
This opinion was largely confirmed by the room-averaged response, fig.4. In general terms, this was impressively even, and confirmed the ability of a small stand-mounted speaker to interface well with a room. ±3dB limits held virtually for a 25Hz to 12kHz range. By comparison with the classic reference speakers, the upper treble was considered to be a little too healthy, denoted by the horizontal marker level. The lift amounted to 2-2.5dB and was responsible for the "light" tonal quality noted in the subjective appraisal.
Fig.4 Spica TC-50, spatially averaged, 1/3-octave response in MC listening room (5dB/vertical div.).
Distortion measurements for second and third harmonics at a 96dB spl at 1m (fig.5) showed the Spica was working fairly hard, with second harmonics up to the 1% level over much of the range, and peaking at a modest 2.5% by 60Hz. Third harmonics peaked at 1% in the midband, but otherwise averaged a very good 0.2%. When the sound level was reduced to a more moderate 86dB spl (fig.6), third harmonic remained much the same, but the second-harmonic levels were greatly reduced to a typical 0.25%, with maximum at 0.35%; a good result.—Martin Colloms
Fig.5 Spica TC-50, harmonic distortion vs frequency at 96dB spl.
Fig.6 Spica TC-50, harmonic distortion vs frequency at 86dB spl.
Footnote 2: Spica takes pride in the closeness of their pair-matching. Designer John Bau postulates that the review pair had somehow been mixed up with another pair to produce two odd pairs as this would account for the exact 1dB difference.—John Atkinson