APOGEE CALIPER SIGNATURE

Apogee's smaller Signature proves a worthy sibling for the superb Duetta Model
by Martin Colloms

Some months ago now, I reviewed the Apogee Duetta Signatures, and found them very worthy performers -truly high end products. It ended with me unwilling to let them leave the house! As a result, they were purchased and now form the basis for a second listening station. I utilised some skilled help to review the Duettas, but the technical data had to rely on measurements made in the listening room.
Fortune, however, smiled on the Caliper project, as a group of HFN/RR review speakers was planned for anechoic testing and despite all logistical difficulties the Calipers were included in this. For the first time in this magazine, an Apogee review will contain frequency responseand other measurements made in an anechoic chamber and, given the reputation and cost of these speakers, many technically-minded readers will be interested to see how well such speakers performed under conditions found in the laboratory.

The Caliper Signature is a new speaker derived from the existing Caliper model, in the way that the Duetta Signature is a derivative of the Duetta II. Although the appearance remains unaltered, improvements to the Caliper concern frequency linearity and tonal balance, and also an increase in sensitivity. When I last reviewed the standard Calipers I found much to commend but ultimately I felt dissatisfied with the lower bass which tended to thump excessively in my room. Some colouration in the upper mid and upper treble also disturbed the performance more than I would have liked considering the price level involved. Given these findings, the comparison with Apogee's own successful larger model might seem unlikely to show the Caliper Signature in its best light. But fortunately we were able to place the speaker in a group of other models, auditioned by a practised panel of six judges under unsighted conditions. The highly positive results of this test were later backed by some extended personal listening, something essential for a future review on a product of this calibre.

Priced at £3150 the Caliper Signature represents a serious investment, which will require solid backing by ancillary equipment of matching performance. Offering a considerable price saving over the Duetta Signature, the Caliper Signature also has the benefit (for some at least) of a reduced panel area, and thus a less obtrusive profile. It certainly looks more 'domesticated' in smaller rooms, but use in a room much less than 17 feet in length is not to be recommended since, as with all dipole panels (radiating from both front and rear planes), 2-3 feet (or more) will be required behind the panel to avoid the consequences of the worst rear wall reflections. The Caliper Signature is less efficient than the matching Duetta and, due to its smaller size, will not play as loudly or take as much power.

Considerable care is needed in fine tuning the speaker axes of radiation, which must take account of the listener's own height as well as that of the favourite Listening position. Fairly low-backed armchairs are recommended, upholstered in a light, absorbent material. High-backed leather furniture is the worst, leading to the creation of local reflections and thus disturbing one's perception of the sound stage. Stereo focus tends to improve with listener distance, with a sizable 22-24ft room length representing the ideal for this model in my view.

Fine tuning extends to the minor angle presented to the listener, the spacing from the side walls and the rear walls, while noting that such a dipole may be placed nearer the side walls than can an equivalent box design. The vertical axis is the most critical, however, and to this end Apogee supplies a plumb line whose bob may be used to define accurately and match the exact degree of backward tilt required for optimum sound quality and tonal balance at the listening position. One or two degrees is significant in this context, an aspect reminiscent of the first series Quad Electrostatic.

Rated at 100W per channel the speaker is quoted at 3Ohms. This means that the matching amplifier will have to work pretty hard for its living, and the use of one of the higher current '4ohm' rated solid-state models is clearly advised. At a relatively modest price level, a Krell KSA80 is an obvious and excellent choice, but depending on the sound quality of the non Class A Krells, these may also serve well. My Goldmund Mimesis, power amplifiers proved to be unsuitable for this 3 Ohm loading. Depending on the cable length, the quality and resistance of the cable run is also important, in terms of both power loss and fidelity, and must be accorded serious attention. Bell wire will most definitely not suffice for a Caliper! We tried bi-wiring, & and liked the result.

For the auditioning, sighted and unsighted assessment was undertaken, with a source system including Krell KMA160 monoblocks, customised Musical Fidelity MVX and a Pink Triangle PIP pre-amp, custom silver wiring, a Goldmund Studio/T4/Koetsu Red Signature, plus CD source generated by a Cambridge CD1 and a custom digital decoder.

Design and technology

Briefly covering the principles behind this two way model: it comprises two vertically aligned, open backed elements; the smaller is a wide-frequency-range mid/treble ribbon which is effective down to at least 300Hz and freely suspended over its full height, aside from a single directivity-control clamp of low-mass foam. This bends the ribbon forward in the vertical plane, to optimise the perceived frequency response and directivity at the listener position. The mid/treble ribbon is of of classic geometry, with a massive array of magnets on each vertical side. The second radiating element, the bass unit, is a large surface area Kapton film, horizontally corrugated to minimise bending in the vertical direction, and quite flexibly suspended on all four edges.
A high temperature rating film is used for both elements. In the mid/treble three vertical conductors carry the same direction of current flow. For the bass element the conductor foil is presented as an aluminium sheet with slots cut horizontally to achieve a zigzag current path, these current sections lying over magnetic bars bonded to a perforated steel support plate lying behind the diaphragm element. While this technique is neither as linear nor as acoustically transparent as the free ribbon type, it clearly works well over the required frequency range.
Free from dominant resonances, both driver elements present an essentially resistive 3Ohm loading and the Linear phase design is completed by simple first order 6dB/octave crossover executed in top quality components, including all polypropylene capacitors.
As with the Duetta Signature, a three position switch and equaliser is provided to control treble level. Further investigation of the Duetta left me confused by this facility, since it appeared co change the frequency response as well as the mid/ treble level. for both systems I preferred the 'normal' setting.
Caliper Signature may be bi-wired and certainly benefits from this; it does not have the Duetta facility for bypassing the crossover for true active operation. As regards the generating area, the bass driver measures 93cm high and is tapered, with an average width of 31cm. An area of approximately 2,700cm is driven, equivalent, with an allowance for average diaphragm excursions, of a couple of 330mm, 12in bass drivers.
The mid treble clement is built like the Duetta model, running 34cm high in a 3cm wide aperture. Electrical connection is via high quality gold plated 4mm socket binding post. On delivery the only assembly required is the fitting of the rear bracket feet and the spikes, plus the adjustment of the spikes for levelling.

Sound Quality

It is not unknown for expensive speakers to perform indifferently under unsighted listening test conditions. Factors at work include a lack of panel familiarity with the room sound, the different room sound of a bi-planar model, problems of directivity, plus sound-source height and the difficulty of acoustically addressing six panelists with reasonable uniformity. Nevertheless, we found the Caliper achieved fine scores while unsighted.
Comparing the Caliper with the Duetta (this and all subsequent references here are to the Signature versions of course) reveals that the stage width is comparatively wide but the impression of height and scale is understandably reduced. Stereo focus is very similar, clearly less 'holographic' than with several precision miniatures, but the Caliper sound is so consistent in terms of amplitude and phase that one soon adapts to the altered presentation and finds an equally valid perceptual inter[relation of focus and stereo perspective. As regards transparency and detail, the CaIiper comes a close second to the Duetta except when driven hard the greater power handling capacity' of the bigger model allows it to draw away at high sound levels. Put in perspective, this means that on wide range programme the Caliper can be run substantially harder than the Quad, while the Duetta offers the same advantage over the Caliper.
In the bass the Caliper did not extend quite as deep as the Duetta, and like the Duetta, it did tend to sound at the bass extremes the degree dependent on the room size, and the speaker/listener placement. However the one note effect of the original Caliper has been successfully avoided in the signature. Interestingly, the Caliper now sounded a little faster in the Bass than the Duetta and in general the bass quality was considered to be a major strength like the Duetta, the Caliper had the ability to reproduce all the elements of a percussive bass note as one sound - the edge, the slam, the crack and the bang. Rhythm and drive were present in abundance the bass sounding 'open' till the way Into the midrange
Similar in frequency balance to the bigger model, and consistent with the optimum vertical imaging, the Caliper) was at times marginally preferred to the Duetta on treble sweetness, not a bad thing for a model which might have to partner a less costly amplifier.
Taken overall, the Caliper Signature did make the grade on all major aspects of sound quality including those areas difficult for panel speakers, namely tonal balance and neutrality. As regards musical detail and transparency, these were exceptionally good, and were allied to the lively musical dynamics. It is hard to place a value on such aspects, and this is where this speaker's particular technology demonstrates its unique performance, first seen in the very large and costly Apogee 'Apogee'. On the unsighted testing, the top scores attained were a clear recognition of this, speaker's merits - in particular, the clarity and dynamics. Much of the full musical force of the drive system was felt to have been expressed.

Measurement

We expected problems In the anechoic chamber, since proximity effects tend to dominate in the case of a speaker such as this, one with a large frontal aspect and bipolar directivity, when measured at such close range. The main graph for sine wave excitation (Fig 1) was taken at 2 metres, rather than the usual reference 1 metre. This 2 metre response was quite decent, contrasting strongly with the proximity-distorted, dashed trace shown below, this the 1 metre output taken under the same conditions. On the listening axis, and given an allowance for chamber error, the Caliper achieved 38Hz to 20kHz, ±2.5dB a remarkably fine result. Estimating the absolute sensitivity proved to be difficult owing to its ability to throw' sound energy forward to the listening position. On the basis that the Duetta Signature was rated as equivalent to 85-86dBW (8ohms) 1 metre, perceived at the listening seat, the Caliper would rate 3-4dB lower at say 82.5dB per 8ohms watt. Measured at 1 metre, the actual output, in error due to proximity, and was nearer 80dB.Third-octave equivalent averaging gives a good idea of the overall frequency response trends, while the registration of the off-axis outputs indicates much about the integration of the two drivers and also the forward energy characteristics. The 15" vertical above-axis response is worth examining for its 7dB level loss above 2kHz. The speaker is clearly very angle sensitive in the vertical plane, and the noted fine adjustment of vertical angle is essential. (The spiked screw feet provided do, in fact make this quite easy.)
In the preferred lateral directions, the Caliper Signature demonstrated very competent responses. This was equivalent to the speakers set with the tweeter ribbons inwards; the panels almost plane to the back wall. To verify this recommendation, the lateral responses were also plotted at 30" for the alternate, outside direction (Fig 3). Here the result was a distinctly unwelcome bumpy response.
Averaging 3.5Ohms, the speaker's load impedance fell within narrow 3.2 and 4.2Ohms limits, and although low, it did not present any significant reactance to upset amplifiers. Taking into account the 3.5Ohm value, the low 82dB/W sensitivity as well as the inherently unlimited dynamic range, the Caliper Signature ideally requires an amplifier with a solid 4Ohm performance and a normal 8ohm rated power of 80-100watts per channel.
The main bass resonance was most clearly shown on the 2 metre response and lay between 40 and 45Hz (varying a little with input amplitude) and interacted with the listening room to generate a strong lift. This will also vary with rooms, but in mine it lay at 30Hz. Take reasonable care with the speaker and the listener position and this can generally be controlled to give a satisfactory subjective result. In my listening room, the Caliper Signature essentially matched the fine result shown by the Duetta Signature In delivering a remarkably accurate frequency/power response to the listening area, rear wall reflections Included. Note that an unavoidable mic change to a 6mm from a 12mm capsule for this measurement has resulted in a brighter presentation in this room averaged response, one showing more energy at high frequencies. As regards distortion, the output at 96dB spl was interesting with the Caliper clearly having to work harder than the Duetta. The designs also show a distinct difference in operation between the bass and treble sections. There was not significant distortion above 1Kz, because this was where the free ribbon works in a linear, balanced magnetic field. The bass section, on the other hand, is not push- pull, and significant second harmonic distortion was to be expected. Above 6OHz, this averaged l.5% and peaked at 3%. Aside from a minor modal resonance at 130Hz, third harmonic was generally lower than second, the preferred result. When driven hard, some progressive bass doubling is to be expected, but its onset was gentle and the speaker was hard to 'crack'. At the lower 86dB level, the distortion had reduced significantly except for the bass resonance region where a 5-8% distortion level was still present.
In an effort to approach the limit of this speaker's residual distortion at average music levels, an analysis of just 80dB spl was made, enhanced to give a base line of 0.03%. Again at this lower level, the performance of the low frequency panel improved further, while the mid/treble ribbon continued its excellent performance, here remaining on the 0.03% measurement threshold for both second and third harmonic components up to 20kHz and beyond. Thus a major part of the working range of this speaker, the most critical range from a subjective viewpoint, offers very low distortion, comparable with the noted Quad Electrostatic in this respect. With a maximum input of 200W/40hms on speech and music programme, this Apogee should deliver peak sound levels of l05dB at the listener position using a stereo pair, which should be sufficient for most applications. For higher sound levels, the Duetta Signature Is the correct choice.

Conclusion

This somewhat smaller edition of the Duetta Signature, the Caliper Signature is a strong member of Apogee's latest range. Passing two ordeals-by-fire in this review, namely anechoic lab analysis and unsighted listening, these speakers offer lasting quality in terms of finish, build, design, neutrality, clarity, dynamics and stereo sound stage. By association, the technical performance of the Duetta Signature was confirmed. The minor reservations concerning the CaIiper Signature concern the fairly heavy amplifier loading, the low sensitivity, the critical vertical axis alignment and the potential for an excess in the low bass under certain conditions. However, these factors do not prevent the Caliper Signature from attaining genuine audiophile status and the design was easily capable of allowing the listener to distinguish between some of the worlds finest signal sources and amplifiers. The Caliper Signatures fully deserve a careful choice of system and location, and in return will reward their owner with a substantial slice of the excellent Duetta Signature performance, from a smaller, less costly package - in fact, just the job they were designed to do .