Velodyne DF-661 Speaker System Review

For many years Velodyne has been known for its line of powered subwoofers, currently comprising at least eight models. The company has now taken the logical next step with the introduction of full-range speakers that can be used on their own or together with its subwoofers.

The DF-661’s model number reflects both its driver complement and its design goal. The DF stands for “distortion-free,” while 661 designates a 6-inch woofer, a 6-inch midrange driver, and a 1-inch tweeter.

The DF-661 is obviously a three-way system, though quite unlike any other we have seen. It is compact, but extraordinarily heavy for its size, and handsomely finished. The entire front is covered by an acoustically transparent black cloth grille.

Removing the grille gives you the second clue to this speaker’s unusual character (the first comes when you try to pick it up!): The woofer and midrange cones are made of spun aluminum, with no sign of the usual voice-coil dust cap. The voice coils and magnet structures are completely behind the cones, which merely come to a rounded point at their apexes.

Although the woofer and midrange cones are the same size (and have a superficial external resemblance to each other), the drivers are actually quite different. The woofer cone’s compliant surround allows the excursion needed to generate low frequencies at a useful level, whereas the midrange cone (which operates between 750 Hz and 5 kHz) has a relatively rigid mounting and does not visibly move. The tweeter, located between the two cone drivers, is a conventional 1-inch aluminum dome protected by a metal screen.

Each cone driver occupies a separate sub-enclosure. The 700-cubic-inch woofer volume is vented through a port in the back of the cabinet. The midrange driver operates in a 260-cu-bic-inch sealed volume.

Recessed into the rear panel are two pairs of gold-plated binding posts compatible with dual banana plugs as well as wires and lugs. The posts are normally paralleled by removable jumper straps. Removing the jumpers enables the DF-661 to be bi-wired or bi-amplified.

Velodyne stresses the importance of low nonlinear distortion (both harmonic and intermodulation) in a loudspeaker. The well-done, informative owner’s manual for the DF-661 devotes a couple of pages to nonlinear distortion and its effects, including a couple of spectrum-analyzer plots for the DF-661 and an unnamed “comparable system.” The plots indicate that the DF-661’s harmonic spectrum is appreciably lower in level than that of the other speaker.

We installed the speakers with the tweeters and midrange drivers roughly at ear level, several feet from any room walls, both for listening and for our room-response measurements. Initial listening tests demonstrated that the DF-661 was a smooth, clean-sounding system with no obvious colorations and surprisingly good bass considering its rated response, +2 dB from 60 Hz to 20 kHz.

The room curve (above approximately 300 Hz) sloped down gently to a plateau between 3 and 20 kHz. The close-miked woofer/midrange response showed a similar downward sloping characteristic, although its upper portion could not be measured accurately by this method. Response o measurements below 100 Hz were difficult to interpret, since most of the § output in that range came from the s port, which does not necessarily contribute to the overall sound quality in the same way as the output of the woofer cone. In any case, the cone’s response was flat within ±3 dB from 85 to 800 Hz but dropped steeply at lower frequencies (where the port output predominated).

The composite response curve sloped down (above the woofer range) by about 5 dB from 2 to 4 kHz and remained very smooth and flat (within +1 dB) from 4 to 11 kHz. There was a slight 3-dB peak (probably the tweeter resonance) at 13 kHz, and response returned to the average high-frequency level at 20 kHz. Quasi-anechoic MLS ground-plane on-axis response measurements showed a variation of less than ±3 dB from 300 Hz to 20 kHz.

Although the DF-661’s rated impedance is 6 ohms, we measured a minimum of 3.2 ohms at 230 Hz, and the impedance remained below 6 ohms from 120 Hz to 7 kHz. A 4-ohm rating would be more realistic. The impedance rose steadily at frequencies below 200 Hz, reaching 20 to 24 ohms in the octave from 20 to 40 Hz. That characteristic suggests that the speaker’s response was deliberately rolled off at low frequencies, perhaps to limit woofer excursion, and thus distortion, or to make it more compatible with subwoofers.

The tweeter’s horizontal dispersion was typical of 1-inch domes, with the output 45 degrees off-axis diverging from the on-axis response above about 7 kHz. The off-axis response was down (relative to the on-axis response) less than 4 dB at 10 kHz, 7 dB at 15 kHz, and 13 dB at 20 kHz.

Sensitivity was 88 dB sound-pressure level (SPL) at 1 meter, as rated, with a 2.83-volt input of random noise. With an input of 3.56 volts (corresponding to our standard reference output level of 90 dB SPL), the woofer distortion was lower than we have measured from other speakers with a similar driver complement (as well as from many far larger speakers). From a maximum of 10 percent between 20 and 30 Hz, distortion fell smoothly to 1.5 percent at 60 Hz, 1 percent at 70 Hz, and less than 0.5 percent between 90 Hz and 8.3 kHz. The lowest distortion reading, 0.23 percent, was at about 300 Hz.

We also measured the woofer distortion using the FFT spectrum-analysis feature of our Audio Precision System One. With 2.83 volts input at 100 Hz, the major distortion component was a third harmonic at -50 dB (0.3 percent). The second harmonic was at -60 dB (0.1 percent), and all others were at lower levels. Those are impressively low readings; for example, a roughly comparable speaker tested in the same way yielded a third-harmonic reading of about 1.1 percent and a fourth-harmonic level of 0.15 percent. A similar measurement of the DF-661’s midrange distortion revealed only 0.1 percent third harmonic and 0.05 percent second harmonic.

Finally, pulse power tests with single-cycle sine waves at 100 Hz, 1 kHz, and 10 kHz clipped the output of the amplifier before the speaker’s output waveform became seriously (audibly) distorted. This test subjected the speaker to momentary inputs of about 700 watts at the higher frequencies and 1,200 watts at 100 Hz!

We listened to the DF-661 speakers both alone and with subwoofers (not Velodynes, as it happened). Their sound was well balanced and unstrained, and clearly covered essentially the full audible frequency range. Used alone, they certainly had no lack of bass, but adding the subwoofers enabled the DF-661 system to create a seamless spectrum of sound from the deepest bass to above most people’s hearing range.

Regarding the manufacturer’s views on the importance of low speaker distortion, we have some minor reservations. Of course, low distortion is always desirable, but with most speakers we have heard and tested, we doubt that reducing their nonlinear distortions by a factor of several times (corresponding to the differences we measured between the DF-661 and some other good speakers) would make a significant difference in their sound. Nonetheless, the DF-661 certainly substantiated Velodyne’s claims of low distortion (allowing for differences in test conditions), and it proved itself, whether for that reason or others, to be a fine, highly listenable system, with or without the assistance of a subwoofer or two.

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