Jamo CBR 1303 Speaker System Review

The Danish-made Jamo loudspeakers feature a novel “CBR” (Center Bass Reflex) woofer design. The basically conventional woofer is mounted at the end of a rigid tube of the same diameter as its basket rim. The entire structure is suspended from the front panel of the cabinet by four soft rubber blocks so as to minimize transmission of vibrations from the woofer to the cabinet. A slightly larger-diameter outer tube, firmly attached to the front panel, surrounds the driver and its inner tube.

The CBR woofer operates as a reflex (ported) duct-loaded system. The space between the inner and outer tubes is the duct, which opens to the outside around the rim of the speaker. Although Jamo claims that this construction, by providing symmetrical loading of the rear of the bass diaphragm, reduces distortion, this seems improbable in view of the wavelengths involved. In any event, it appears to be a convenient way to design a speaker with vibration isolation from the front panel as well as reducing the number of large openings in the panel and thus increasing its rigidity.

The CBR 1303 is the next-to-largest unit in the Jamo line. Its walnut-veneer-finished cabinet is 28 inches high, 14 inches wide, and 13 inches deep, and it weighs 48 pounds. The drivers are mounted on a slightly curved panel designed to provide a phase-coherent wavefront at the listening position. The top of the panel (containing the two higher-frequency drivers) is angled slightly downward when the speaker is shelf mounted. For floor mounting, the speakers should be placed on the supplied heavy-duty steel stands, which tilt them slightly upward and raise their tops 35 inches above the floor.

The Jamo CBR 1303 is a three-way speaker system with a nominal impedance rating of 4 to 8 ohms. It is rated to handle up to 130 watts input (200 watts of music program), and it has a relatively high rated sensitivity of 96 dB sound-pressure level at 1 meter with an input of 1.8 watts (corresponding to about 93.5 dB SPL with our usual reference input of 1 watt). The 11-inch woofer crosses over at 750 Hz to a 4-1/2-inch-cone midrange driver. The second crossover, to a 1-inch dome radiator, is at 3,500 Hz. The two higher-frequency drivers are mounted in a vertical line slightly to the left of the woofer’s center line. Most of the curved front panel of the speaker is covered with a grey flocked finish, but the section surrounding the midrange and high-frequency drivers is made of a resilient foam material, presumably to damp unwanted resonance. The two higher-frequency drivers are also mounted slightly behind the panel, coupled to the room through short tapered conical sections.

The front of the speaker is covered by a removable curved black cloth grille retained by plastic snap fasteners. At the top front of the cabinet are two level-balancing knobs for the midrange and high-frequency drivers. They are normally flush with the exterior of the cabinet, but pressing and releasing a knob causes it to protrude slightly from the surrounding surface for easy adjustment. A second push relocks the knob in its retracted position. Adjacent to each knob is a small red LED that flashes when excessive drive levels are applied to its driver. Insulated spring clips for the speaker connections are recessed into the rear of the cabinet. Price: $439.95.

Laboratory Measurements

All our measurements of the Jamo CBR 1303 were made with its midrange and tweeter level controls set to their maximum (0-dB) positions, which also provided the best listening balance in our room.

The room-averaged frequency response of the CBR 1303 speakers was quite uniform from 400 to 20,000 Hz, with most of the variations resulting from room reflections and standing waves that could not be eliminated completely with the frequency “warble” on our sweeping sine-wave test signal. Some of the more distinct departures from flatness correlated well with response curves supplied to us by Jamo, which were made under both anechoic and averaged-power-response conditions, although their magnitudes tended to be smaller in our measurements.

However, a broad 5-dB response peak from 10,000 to 15,000 Hz was apparent in the Jamo anechoic-response curve in our IQS FFT response measurement and (somewhat modified) even in our room-response measurement. We were, therefore, not surprised to find that the speakers’ sound had a tendency toward crispness (or even “edginess” if the program had an emphasized top end).

The close-miked woofer response was combined with the response measured at the annular port opening (after correction for their relative areas) to produce an anechoic bass response that was flat within ± 2 dB from 53 to 500 Hz. It had a broad rise of + 2 to +3 dB from about 60 to 150 Hz and fell off at 18 dB per octave below 50 Hz. Splicing this to the room curve yielded a composite frequency response flat within ±2.5 dB from 50 to 18,000 Hz.

The IQS analysis system showed the effective anechoic frequency response of the system (at a 1-meter distance) to be flat within ± 3 dB from a few hundred hertz to 17,000 Hz. The high-frequency level control affected frequencies above 3,000 Hz, reducing their level by up to 15 dB at its setting limit. The midrange control had a similar effect on frequencies between 800 and 4,000 Hz. Calibrations surrounding the knobs show their effect on the signal over a range of 0 to —13 dB. (Unfortunately, the minus signs are printed as division signs, which makes no sense and could lead to their being read carelessly as plus signs.) The response curves measured on the speaker axis and 45 degrees horizontally off axis were essentially identical up to at least 10.000 Hz, although they diverged considerably in the 15,000- to 20,000-Hz range. The phase shift of the system was highly linear with frequency, resulting in a group delay of only 0.2 to 0.4 milliseconds between 2,000 and 20,000 Hz.

The system impedance reached its minimum value of 5 ohms between 100 and 200 Hz, although it was at least 8 ohms over most of the audio range. The sensitivity was almost exactly as rated, with a sound-pressure level of 93 dB measured 1 meter from the speaker with an input of 2.83 volts of random noise in the octave centered at 1.000 Hz. The woofer distortion was measured with inputs of 1 and 10 watts (based on an 8-ohm impedance). Separate readings were made of the port and cone outputs, each in the frequency range where it was the major contributor to the total speaker output. The distortion was extremely low down to about 50 Hz, typically reading 0.3 to 0.4 per cent at 1 watt. The effective crossover to the port radiation was at about 40 Hz, and the distortion rose rapidly in the low bass to 2 per cent at 40 Hz and 15 per cent at 30 Hz. Increasing the drive to 10 watts resulted in moderate distortion readings of 0.5 to 2 per cent down to 70 Hz, rising to 9 per cent at 50 Hz and 20 per cent at 40 Hz.

Comment

As we noted earlier, some of the sound character of the Jamo CBR 1303 could be inferred from its response measurements, but, as always, this is an unreliable way to judge the sound of a speaker. True, as its high-frequency response peak would suggest, the CBR 1303 did sound brighter than some of the speakers we are accustomed to hearing. Still, its balance was good, with the lows holding their own against the extended high-frequency and smooth midrange outputs. We did not find any advantage in reducing the middle or high-frequency output with the speaker’s controls, although our listening room is rather absorbent and thus tends toward a “warm” sound.

Obviously, generating a powerful, room-shaking low-bass output is not the forte of the Jamo CBR 1303. On the other hand, unless your taste is strongly biased toward pipe-organ music at live levels, it could hardly be considered inadequate in that respect. The bass-distortion readings speak eloquently for the quality of the woofer; we have rarely seen distortion readings as low as those of the CBR 1303 from a dynamic speaker system. And remember that the 93-dB sensitivity of this speaker is about 6 dB higher than that of other, typical high-fidelity speakers. This means that our 1-watt measurement was the equivalent of a 4-watt measurement on most speakers and that our 10-watt readings should be compared with others made at 40 watts (which we do not make, since few speakers take kindly to such treatment).

We were curious to see what it would take to light the overload LED’s of the CBR 1303, since the instructions say nothing about this. We never found out, though we tried driving the speakers as loud as we could with Phase Linear Model 400 and with Perreaux PMF 2150B amplifiers playing typical FM-broadcast music. With the preamplifier volume at maximum, we enjoyed (?) ear-splitting but amazingly clean sound. This is hardly our preferred mode of listening, but there were no signs of distress from the speakers (or from the amplifiers).

Unfortunately (or perhaps fortunately), we also never saw a glimmer from the overload lights on either speaker. The manufacturer says that the speakers can handle 200 watts of program material, and we were certainly giving them all of that and more—the equivalent of driving a typical home hi-fi speaker with an 800-watt amplifier pushed to its limits.

Our conclusion is that the Jamo CBR 1303 is a handsome, very well constructed and finished speaker whose sound quality, high sensitivity, and ruggedness merit its serious consideraton by anyone contemplating buying speakers in its price class.

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