SPW-1, Woofers

Read more on the Acoustat SPW-1 in our home audio section

Brian Wallen2005-05-19 19:28

I can\'t seem to find any information on the SPW-1 woofer model that was added to the Spectra series. I\'ve never seen one in the flesh, but seem to remember that these had about 6-inch drivers.

I have one Xerox copy of a brochure about the Spectras with the following information:

\"Present day source material will tax the most capable of speakers, regardless of driving principle. Spectra 22 and particularly the 33 will handle all or most of the source material you care to listen to--as a full range electrostat. However, an electrostat\'s high intimacy of drive can occassionally lead to ionization of the air between its elements, on heavy, deep bass material, resulting in audible popping noices. The addition of Spectra Passive Woofer 1 (below 100 Hz) extends both panel performance and total system dynamics by 8db or more!

Most important of all, listening reveals the SPW-1 to be the perfect Spectra companion....\"

First, I assume that the drivers, or at least one of the drivers in the SPW-1 were electronically active, not just drones. Was the SPW-1 just a box with two drivers? Any crossover components, or are those built into the Spectra panels? Did this unit extend the range of the combination, in addition to increasing dynamic range?

Andy Szabo has said in other places that the woofers for use with Acoustats should be low efficiency models. Does the use of small diameter drivers in the SPW-1 suggest that small cones will be quicker and come closer to matching the quickness of the electrostatic panels? What can we learn from looking at the SPW-1 design that may be helpful in constructing passive woofer units to use in place of the scarce SPW-1?

I have a pair of KEF B-139 drivers--the oval ones with the flat diaphram--mounted in an approximately 70 litre sealed box. These are part of a KEF Constructor Series system that also includes a 110mm mid range and a dome tweeter. I built these so that the B-139s had their own dedicated box. KEF rates the characteristic sensitity level of these at 87dB spl at 1m on reference axis for pink noise input of 1W. I biamp these with an electronic crossover with a 100Hz crossover point, so there are no passive crossover components connected to the B-139s, although I used the standard KEF crossover for the MF/HF unit. Would the B-139s make suitable \"passive\" woofers for Spectra 22s?


Andy Szabo2005-09-11 19:33

The SPW-1 is a passive, vented system, consisting of four 6” drivers with high-level crossovers contained in the woofer box. It is a “stereo” woofer in that each channel drives a separate crossover and two of the side-firing woofers. Should there be any stereo information below 100 Hz, this arrangement will tend to preserve the image better than a single “mono-sum” woofer.

The woofer output on the Spectra series interface is a full-range output: essentially just a pass-through from the input. This allows the use of a variety of woofer systems.

Yes, the SPW-1 does extend the low frequency response somewhat, but it still does not provide extension to the lowest of notes, as is possible with more expensive systems.

I can’t advise on the suitability of your KEF drivers in the described enclosure. Certainly, the use of an active electronic crossover has the potential for steeper crossover slopes and better matching between high-pass and low-pass curves. Active crossovers usually have level controls as well, so the relative efficiency of the woofer system becomes a moot point.

The idea of “quickness” of small woofers vs. large is not really an issue below 100 Hz. However, due to the potential of better coupling between the voice coil and the smaller cone, small woofers can have lower distortion, if used well within their excursion limits. With so many other variables in woofer quality and system design, other factors may be more important. Larger woofers typically have a lower resonant point, and are therefore capable of better low frequency extension without electronic equalization.

I have a Spectra 1100, which is a hybrid system consisting of a single electrostatic panel coupled with an 8” woofer. It has quite credible low frequency response, but is naturally limited in dynamic output due to the 8” woofer. I have added a woofer system made by Dayton Loudspeakers, available from Parts Express (www.partsexpress.com). This is an active system, with a very robust 10” woofer, active crossover, and 250-watt amplifier. It is by no means a perfect system, but it is a very good value at only $350 in kit form. The kit was so easy to build I was disappointed it didn’t take longer – what can I say, I like building things! The Dayton system has been a positive addition to my system for about 2-1/2 years now, and has increased my enjoyment of music and video. They also make similar systems with 12” and 15” woofers.

Like any woofer system without a computer-based room-equalization system (such as the Velodyne), it took quite a while to ‘dial-in’ the room placement, phase, crossover point, and relative level. To roll-off the main Spectra speakers, I inserted a capacitor in series with the main amplifier (working against the input impedance of the amp), to roll-off at 6 dB per octave.

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