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Product Review - Velodyne F-1500R Subwoofer - January, 1995 (Now Called the FSR-15 in April, 1998) - DF-661 Mini-Monitors

By John E. Johnson, Jr.


Servo feedback 15" driver, 250 watt amplifier with remote control included, size 20" x 20" x 20", black oak grained vinyl, weight 90 pounds, connections for left and right speaker inputs with filtered output plus RCA inputs from preamplifier, fixed 85 Hz passive high pass filter - first order, 40 - 100 Hz active variable low pass filter- first/fifth order. $1,595. Velodyne Acoustics, 1070 Commercial Street, Suite 101, San Jose, California 95112, (408) 436-7270.

Velodyne DF-661 Mini-Monitor. Three-way book shelf speaker (mini-monitor), one 6" woofer, one 6" mid-range driver, one 1" tweeter, size 18 7/8" x 8.5" x 10.5", available in black oak grained vinyl ($1,799 per pair) or rosewood ($2,399 per pair), nominal impedance 6 Ohms, weight 40 pounds. Velodyne Acoustics, 1070 Commercial Street, Suite 101, San Jose, California 95112, (408) 436-7270.

Velodyne Acoustics has been manufacturing subwoofers utilizing the servo-feedback principle for about 10 years now. Their ULD series, which is downward firing, remains popular, and with the addition of the F series which has variable low pass crossover filters and the amplifier built into the enclosure, as well as being forward firing, users have a wide selection to choose from. The servo-feedback type of mechanism employed by Velodyne consists of a 2.5 gram accelerometer which is attached to the voice coil of the speaker driver. When a signal is fed to the amplifier, which then is used to drive the speaker, the accelerometer senses the motion of the speaker cone, and this movement is converted to an electrical signal which is sent back to the built-in amplifier. The incoming signal from the main amplifier or preamplifier is then compared with the information supplied by the accelerometer. The difference, which consists mainly of harmonic distortion (even and odd order) produced by the speaker cone itself and effects on the cone by resonances inside the enclosure is electrically inverted and fed back to the speaker. Feedback occurs 3500 times per second, and the distorted movements of the cone are thus canceled out. This process works very well, and Velodyne has made a name for themselves, setting a standard in subwoofer design by which most others are compared.

How do the Velodyne subwoofers sound? We have compared a number of subwoofer designs, including servo- feedback, push-pull, as well as passive radiators. The servo- feedback subwoofers, in our experience, produce the purest low frequency sounds of all the designs. All musical instruments generate natural harmonics, mostly even order (second, fourth, sixth, etc.). It is the harmonics that are added by speakers that can render the sound unnatural (called harmonic distortion). Usually, a speaker will produce a strong second harmonic (e.g., 10%) and a less intense third harmonic (e.g., 5%), particularly at low frequencies and at high intensity. Most amplifiers have less than 0.5% Total Harmonic Distortion (THD), which is not audible. Therefore, it seems appropriate to design speakers with this level of performance. The Velodyne servo-feedback mechanism appears to reduce the harmonic distortion to about 1% at the frequencies covered by their units (18 - 100 Hz). The F series of subwoofers comes with a remote control powered by 2 AAA batteries. The control has buttons for volume, mute, and power on/off (actually a standby setting).

Listening to the F-1500 is quite an experience. Using a test CD which generates sine waves down to 13 Hz forces one to remove the grille cloth to watch the cone move, but nothing is audible (the ear is not very sensitive below 20 Hz). Somewhere between 15 - 20 Hz, many subwoofers start to rattle, producing a very irritating noise. Not the Velodyne! The cone continues to move smooth and silky at all frequencies, without a hint of breakup. And, of course, at about 18 Hz, one can begin to sense the sound waves being produced. It is difficult to say whether they are heard or felt; there is undoubtedly some of both. And this is one of the main reasons why a good subwoofer is so important to music or home theater: Intense low frequencies are indeed felt, even if they are not heard. Whether it is a low pedal note on a concert pipe organ, or engine room sounds from a star cruiser, the sense of being felt, along with being heard, adds a tremendous impact to the musical or audio - visual experience. It can literally raise the hair on the back of your neck, and make your arms break out in goosebumps.

The Velodyne series of speakers, using servo-feedback, excel at raising goosebumps with all types of sounds, whether it is the 1812 Overture, or Return of the Jedi. The F-1500 would be the best choice for home theater applications and large scale music, not only because of the size of the driver (15 inches), but the power of the included amplifier (250 watts). The F-1000 (10 inch driver - 80 watt amplifier) is best suited for music listening, while the F-1200 (12 inch driver - 100 watt amplifier) could be used for music listening and for home theater as long as you don't try to shatter the windows with car crash sounds.

The crossover network of the Velodyne subwoofer is unique. There are several components in all crossover networks: capacitors, which absorb low frequencies and pass high frequencies, inductors, which absorb high frequencies and pass low frequencies, and resistors. The various sets of frequencies are then fed to the appropriate driver (tweeter, midrange, or woofer). One combination of a capacitor, inductor and resistor will produce what is called a "first order" crossover, which means that the "rolloff" or decrease in frequencies being passed above or below the crossover point, will occur at the rate of 6 dB per octave. A second order crossover network would have two sets of the components and would rolloff at 12 dB per octave; a third order crossover network would have three sets of components and rolloff at 18 dB per octave, and so on. With each octave, the frequency doubles. So, one octave above 40 Hz is 80 Hz. An octave above 80 Hz is 160 Hz. The Velodyne subwoofers use a passive first order crossover fixed at 85 Hz connected to the speaker in/out and the line in/out. This means that if you connect the speaker outputs from your main amplifier, or the output from your preamplifier, to the Velodyne speaker in or line in, respectively, and feed the speaker out or line out from the Velodyne to your main speakers (called satellites when used in this manner) or to your main power amplifier, respectively, only frequencies above 85 Hz will be fed to them, and the rolloff below 85 Hz will be at 6 dB per octave. (This is called "high pass".) The signal fed to the Velodyne built-in amplifier from the speaker in or line in connectors, goes through an active (electrically powered) crossover which is adjustable to pass low frequencies below a setting from 40 to 100 Hz. Above the selected frequency, the rolloff is first order (6 dB per octave) followed by fifth order (30 dB per octave) once the signal reaches one octave above the selected frequency. (This is called "low pass".) So, if you set the adjustable frequency at 60 Hz, the rolloff would be 6 dB at 120 Hz (one octave) and 30 dB at 240 Hz. A signal of 85 Hz and above, rolled off at 6 dB per octave below 85 Hz, would continue to be fed back to your main speakers or power amplifier, regardless of the adjustable setting which feeds the Velodyne amplifier and speaker driver.

The Velodyne F series and ULD series have sealed enclosures. There is no port or vent. With any specific driver, ported woofer enclosures can produce deeper bass than sealed enclosures. This is because the air inside a sealed box acts like a spring to dampen the movements of the driver. However, this results in a tighter bass in the sealed enclosure than in a ported one. Thus, both types are readily available on the market. Velodyne solves the problem of obtaining deep bass in a sealed enclosure by using a driver, which they build at their factory, that has a very low free air resonant frequency (called "fs") of 4 Hz. Thus, even when placed in a sealed enclosure, the driver can produce very deep bass. We compared two Velodyne F-1500's to our reference system, which has fourteen 12" drivers (seven per channel) to reproduce sounds below 120 Hz, connected to amplifiers rated at almost 1000 watts RMS per channel. The Velodynes outperformed the reference system both in loudness, and the lowest frequency that could be reproduced (the reference system could not go below 20 Hz without rattling). It must be said that the reference system is dipolar, that is, the sound exits the front and rear of the enclosure. So, some cancellation of sound occurs. Still, to have the two Velodynes outperform fourteen bass drivers, whatever the configuration, is testimony to the outstanding design of these excellent subwoofers.

The Velodyne DF-661 mini-monitor is a relatively new product, and in keeping with Velodyne's philosophy of designing speakers with low distortion, the DF-661 is very innovative. If you remove the grille cloth, you will see two 6 inch aluminum cones with a tweeter between them. Actually, the woofer and mid-range drivers are not cone shaped, even though they appear that way. The tooling process, in which an aluminum sheet is depressed, first by computer driven lathe, and finally by hand, makes the light reflect in such a way that they appear to come to a point in the center. However, they are, in fact, cup shaped, and special techniques had to be developed just to handle them.

The voice coil is attached to the rear of the cone, and in both of the 6" drivers, a "counter coil" is also attached. The counter coil serves to cancel out harmonic distortion, which is more prevalent in low frequency drivers. In a way, this is a bit like the push pull effect used in amplifiers and some speakers, to diminish harmonic distortion. However, this alone is not enough to lower the distortion to levels where it is not audible (less than 0.5%). Velodyne's proprietary technology accomplishes this in the DF- 661, and distortion is very low. The cabinet is solidly constructed, well damped, and has heavy duty gold plated five way binding posts, capable of bi-wiring or bi-amping.

When initially listening to the DF-661, there is a sense of something missing. There is, indeed, since the distortion is gone. It reminds me of my first experience with a high end amplifier, a sensation of thin sound, which, again, turns out just to be the absence of distortion that I had been used to in previous equipment. However, once accustomed to the sound, you realize that individual instruments can be distinguished, that the extra fullness in other speakers was actually just harmonic distortion which never really belonged there in the first place, and you can never be satisfied with anything else.

As a percussionist, I have listened to hundreds of cymbals during the past 30 years when deciding on a purchase. Cymbals are unique in that they have many overtones, and each cymbal is different, even in the same brand, same batch, same size, and weight. This is due to the manufacturing process where the cymbal is lathed and then hammered. Since the hammering is not identical on any two cymbals, they all sound different, and most percussionists are very particular when selecting them. In testing the DF-661's, I noticed, for perhaps the first time, that cymbals sounded real. In some other speakers, because there are so many overtones, cymbals sound mushy, as the harmonic distortion crowds the actual overtones of the cymbal. With the DF-661, I could hear the distinctive natural cymbal overtones. If you are a percussionist, you will understand exactly what I mean by this. For the rest, I am sure you have a favorite instrument whose sound you are very familiar with. Go and listen to one of your best recordings on the DF-661, and discover the astounding clarity of these speakers for yourselves.

Every speaker has its own personality. Some are very bright in the high frequencies, others have a strong mid-range, and still others have intense bass. The personality of the DF-661 appears to be one of emphasized mid-range, which places most instruments forward on the sound stage. The woofer crosses over to the mid-range at 750 Hz, and the mid-range to the tweeter at 5 kHz.

The sensitivity of the DF-661 is stated at 88 dB/1 watt/1 meter. This implies that the speaker is not very sensitive (a speaker with a sensitivity rating of 90 or more dB/ 1 watt/ 1 meter would be considered sensitive, while a rating of less than 90 would be considered not so sensitive). The sensitivity rating is used as a reference to how much amplification power is needed to drive the speakers. Sensitive speakers need less power to produce a certain loudness than speakers which are not very sensitive. The instruction manual for the DF-661 states that you should have an amplifier of at least 75 watts per channel. We found that 1 watt per channel was good for background listening to Chopin in the afternoon with a nice cup of tea, 5 watts per channel were sufficient to get our attention, and 10 - 50 watts put us in the concert hall. A single ended Class A triode tube amplifier, with a maximum output of less than 10 watts per channel, drove the DF-661's to moderate listening levels. The sound quality with this combination was dazzling, since one of the virtues of a single ended amp is that it has primarily even order harmonic distortion (consonant - non irritating), and the DF-661 does not add any audible odd order harmonic distortion (dissonant - very irritating) throughout most of its spectrum.

Velodyne does not make specific recommendations about placement of the speakers on stands, although most mini-monitors are displayed on them. We found that putting the speakers on the floor (wooden) worked fine, although the bass was accentuated. Raising them above the floor reduced this phenomenon, and this is probably the best position for them, but since they do perform on the floor, you should try a variety of placements if you purchase these speakers (your dealer will demonstrate any placement you request, I am sure).

Compared to our reference system, which uses ribbon technology, the DF-661's were more precise, and more focused. The harmonic distortion present in the ribbons, which reproduce all sounds above 120 Hz, was apparent after listening to the DF-661's, and these ribbon speakers are no slouches. They came close, but still, the difference was noticeable. A conventional three-way bookshelf type speaker that we also have in the lab paled in comparison to the DF-661's.

Now comes the good part. We paired the DF-661's with two F-1500 subwoofers and listened to music as well as home theater sound. To say we were astonished by the combination of unprecedented clarity and power throughout the audio spectrum is an understatement. Although movie soundtracks do not always have terrific fidelity, they still benefit from speakers with low distortion.

Using a third DF-661 (either vertically or on its side) for the center channel provides perfect balance for the front section (left/right/center). However, the DF-661 is not magnetically shielded, so, although you could put the DF-661 on top of a large rear projection TV where the electronics are in the bottom of the cabinet (try this out in the dealer's showroom using a rear projection TV the same size as the one you have at home), you should not place one of these speakers on top of a direct view TV. Velodyne has a new center channel speaker, which is magnetically shielded, for this purpose (Velodyne Model LD-3C, $649). While the center speaker can be driven directly by the surround sound amplifier, the main front left/right configuration necessitates a decision. If you have a high quality surround sound amplifier, the preamplifier out can be disconnected from the main in (this is separate from the center channel or rear channel amplifier outputs and inputs). Using a "Y" connector, you can connect one of the Y arms to the pre out on the surround amplifier, a second arm to the line in on the subwoofer, and the third arm back to the main in on the surround amplifier (or to an additional power amplifier with higher wattage if you wish). In this situation, the subwoofer will reproduce only those frequencies below the setting on the adjustable crossover, while the main amplifier will reproduce all frequencies. This is the manner in which we set up the F-1500 subwoofers with the DF-661's, and since the DF-661's rolloff at about 60 Hz, we adjusted the F-1500 filter to between 60 and 75 Hz, and the F-1500 volume control to suit the music or movie surround sound track. Another option is to connect the pre out from the surround sound amplifier to the line in on the subwoofer, and then run a second cable from the line out on the subwoofer back to the main in on the surround amplifier. In this case, the subwoofer will handle the low frequencies, and your main front left/right speakers will handle frequencies above 85 Hz. You would then essentially have a four-way, bi-amplified stereo sound system. Of course, you can utilize just one subwoofer if you wish, since each unit has inputs for both channels. Some cancellation of frequencies that are out of phase will occur, but one subwoofer is better than none, and you can always add a second subwoofer later. Velodyne now manufactures two-way speakers (Model LD-2, magnetically shielded, $800/pair) which can be used for the rear surround channel, and these would complete the surround sound system. When the new digital full range surround sound systems become available, you can add small (or large!) subwoofers to the rear speakers for full range surround sound.

There is an old platitude in business that states there are three factors which determine a store's success: location, location, and location. It is becoming increasingly apparent that in audio, the three most important factors are distortion free, distortion free, and distortion free. Velodyne has proved themselves in this regard with their subwoofers, and now, with the DF-661's they extend their skills at achieving new levels of low distortion to the remainder of the audio spectrum. These are superb products and are highly recommended to audiophiles and home theater enthusiasts who insist on extreme high quality with low distortion.

John E. Johnson Jr.

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Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
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