Dahlquist DQ-20 Speaker System Review

The introduction of Dahlquist’s original Phased Array speaker system, the DQ-10, more than fourteen years ago aroused considerable interest in the audiophile community. Despite a superficial external resemblance to the contemporary Quad electrostatic speaker, the DQ-10 was a four-way dynamic system that, unlike most other dynamic speakers of the time, was designed to deliver a phase-coherent acoustic output waveform. Still a part of the Dahlquist speaker line, the DQ-10 has now been joined by the DQ-20, a larger and more refined system embodying similar principles.

Dahlquist’s Phased Array design was developed to deal with two basic areas of loudspeaker inaccuracy: time-delay distortion and diffraction effects. In general, the direct sounds from the different drivers of a multiway system arrive at the listener’s position at slightly different times because of differences in the inertia of their moving systems and unequal distances from their voice coils to their front mounting planes and from the mounting planes to the listener. The combined result of these effects can be degraded spatial definition.

Diffraction, which can create similar aberrations in the listening field, occurs when the sound wave leaving a speaker driver encounters an abrupt discontinuity in its transmission path, usually at the juncture of the speaker cone and the mounting board, at the edge of a cabinet, or at the grille frame. The discontinuity bends the wave front and scatters the sound so that parts of it reach the listener over slightly different paths. Whenever the same signal arrives at a point in space over more than one path (almost always of different lengths), the resulting interference produces irregularities in the frequency response and impairs the spatial accuracy of the stereo imaging.

The DQ-20 is a three-way dynamic speaker system with vertically aligned drivers whose 10-inch acoustic-suspension woofer crosses over to a 5-inch midrange cone at 400 Hz. The second crossover, at 3,500 Hz, is to a 3/4-inch soft-dome tweeter. The midrange and high-frequency drivers are on a separate mounting board extending from the top of the woofer cabinet. The board is tapered to minimize diffraction of the drivers’ radiation patterns, and it is also stepped back slightly from the front of the woofer cabinet so as to locate all the drivers in the same acoustic plane. The drivers themselves are further aligned to compensate for their different inertias.

The midrange cone driver is back-loaded by a cylindrical duct with a small opening in its rear surface. The duct provides aperiodic (nonresonant) resistive loading of the cone and eliminates rear radiation, which would disturb the intended polar pattern of the system. The DQ-20’s crossover network, which is completely hand-wired, uses air-core inductors and high-quality capacitors. The nominal system impedance is 6 ohms (4 ohms minimum), and its rated frequency response is 50 to 20,000 Hz ±2.5 dB, or 1,000 to 20,000 Hz ± 1 dB. It is recommended for use with amplifiers rated to deliver between 50 and 250 watts.

The woofer enclosure and the mid/high-frequency driver board are finished in flat black, and they are surrounded by a frame of light oak with an open, black metal-mesh grille covering its front and rear. The entire system is mounted on an integral base about 3 inches high, and overall it measures 41 inches high, 20-3/4 inches wide, and 11-3/4 inches deep; it weighs 60 pounds.

On the back of the speaker box are the insulated multiway binding posts and protective fuses for the tweeter and woofer. The DQ-20 is furnished with Black Diamond spiked feet for making a firm contact with the floor, which is said to improve its imaging qualities. Price: $1,800 a pair.

Lab Tests

The averaged room response we measured from our pair of Dahlquist DQ-20 speakers was an exceptionally uniform 20 to 20,000 Hz ±4.5 dB. The treble range, in particular, was flat within ±3 dB from 1,500 to 20,000 Hz. When the close-miked woofer response was spliced to the room curve, the composite frequency response was within ±4 dB from 33 to 20,000 Hz.

The system impedance was 5 ohms in the 100-Hz range and reached a minimum of 4 ohms at 1,100 Hz and a maximum of 16 ohms at 43 and 2,400 Hz. Its average value over the audio range was approximately 9 ohms. With the standard input of 2.83 volts of pink noise, the sound-pressure level (SPL) at 1 meter measured 86 dB. The bass distortion with 4.5 volts input (equivalent to a 90-dB SPL) was less than 0.7 percent from 100 to 70 Hz, 2.3 percent at 50 Hz, and 5.3 percent at 40 Hz.

In our pulse power tests, using a 1-cycle burst followed by 128 cycles of silence, the woofer’s 100-Hz output waveform began to flatten at an input of 195 watts into its 5-ohm impedance. Unlike many woofers, however, it overloaded gradually, with a smoothly increasing level of second-harmonic distortion before the cone and voice coil reached their physical limits and emitted the harsh rasping sounds that for many speakers are the first indication of overload. At 1,000 Hz the midrange driver absorbed the amplifier’s maximum output of 1,640 watts into its 4.2-ohm impedance without showing signs of overload, and at 10.000 Hz the amplifier clipped, at 575 watts into the tweeter’s 12-ohm impedance, before the speaker distorted.

Our quasi-anechoic FFT frequency-response measurements at 1 meter on the axis of the mid/high drivers were generally similar to the room measurements. The response at 45 degrees off-axis was close to the axial response, with an average difference of no more than 6 dB between the two at frequencies up to 15.000 Hz. The excellent phase linearity of the DQ-20 system was confirmed by its group delay, which was within ±0.2 millisecond from about 2,500 to 28,000 Hz.

Comments

Our measurements indicated that the Dahlquist DQ-20 was a very smooth, wide-range speaker with a highly linear phase response— essentially the qualities claimed for it. Its sensitivity was moderate, but it had no difficulty in handling very large peak-power inputs without distortion or overload. Its impedance was high enough over the full audio range to be compatible with any good amplifier.

The installation instructions advise placing the speakers well away from the wall behind them and not too far apart. We located them 2-1/2 to 3 feet from the wall and about 5-1/2 feet apart, with excellent results. It was apparent that exact placement was not at all critical.

The frequency balance of the DQ-20 was remarkably close to that of our KEF 105.2 reference speakers, which is not surprising since both have very flat, smooth frequency-response characteristics. The KEF speaker, which has a 12-inch woofer in a larger enclosure, had slightly better low-bass response, but the DQ-20 had an airier sound and a clearly extended top end.

The grilles of the DQ-20 appear to be as transparent visually as they are acoustically. The wall behind the speakers was clearly visible through the grilles, which might or might not be a desirable effect. Since the speaker is not a dipole (although its appearance would be consistent with bidirectional radiation), it is not obvious why the rear grille is also transparent.

Overall, the Dahlquist DQ-20 is a fine speaker, ranking with some of the best we have tested, including far more expensive units. It offers a combination of smooth, extended response, wide dispersion, and low bass distortion that is especially noteworthy in a speaker of its moderate size and weight, along with clean, attractive styling. And in these times, its price seems very reasonable for what it provides.

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