by Scott C. Blaier Written December 1995 11-APR-96
|Vifa's Enclosure:||Volume: 5 liters, Vent: 1 inch diameter 1.5 inches long (74 Hz.), F3: 80 Hz, Alignment: QB3.|
|Esquire:||Volume: 5.3 liters, Vent: 1.25 inch diameter 1.625 inches long (75 Hz.), F3: 75 Hz., Alignment: SC4 (better transient stability than QB3).|
The front baffle is sloped approximately 5.0 degrees (the rim of the drivers separated by 3/8 inches) to better align the drivers acoustic centers. This technique is an admittedly poor solution to align the drivers' acoustic centers since one can not easily determine a driver's acoustic center by visual inspection. But I had to at least improve the drivers' alignment to some extent because CALSOD modeling showed a slight trough in response around crossover frequency and increased phase disturbance if the acoustic centers were significantly out of alignment. I flushed mounted the drivers on the baffle by routing, but did not find it necessary to round the baffle's edges or use a "felt tweeter ring" to further suppress edge diffraction.
I chose to veneer my enclosures with Pau Farro (Brazilian Rosewood), an absolutely beautiful wood. The cost was $10.00 per enclosure, but if properly applied and finished, the Pau Farro will give you a gorgeous cabinet on par with the most expensive loudspeakers available. The choice of finish for the enclosure is a personal choice, and less exotic, politically correct, domestic wood veneers such as walnut, oak, and cherry are available and less expensive. A photograph of the loudspeakers is included with the article. Please note that the wood veneer has not been finished at this stage.
Enclosure Dimensions: Width 7.50 inches Height 11.25 inches Depth 5.875 inches top to produce the required 6.875 inches bottom 5.0 degree front baffle slope.Conversion: 1.0 inch = 2.54 centimeters
The graph show a near theoretically perfect fourth order Linkwitz-Riley response centered around 3.0 kHz. The power response dips - 3.0 dB around the crossover frequency, as it should, and total amplitude response is flat through the crossover region. Also note the near perfect symmetry and smoothness of the each driver's rolloff curve around the crossover. The electrical filters' response exactly compliment each driver's acoustic rolloff to strictly maintain a 4th order Linkwitz-Riley transfer function well above and below the crossover point. Please note that the drivers are operating in phase, and reversing the polarity of the tweeter will produce a severe response null of - 25.0 dB centered around the crossover frequency of 3.0 kHz.
Both drivers have outstanding off axis response, and coupled with Linkwitz-Riley's absence of frequency dependent polar tilt, superior damping, and minimum driver interaction, provide excellent stereo imaging.
The cost of the complicated 4th order Linkwitz-Riley network is somewhat compromised phase and transient response. These design compromises are unacceptable to some listeners; however, my ears are less sensitive to these shortcomings then to the many problems solved by using a high order crossover. Loudspeaker design is an exercise in comprise, and it is these compromises that determine the overall "sound" of any loudspeaker system. The nice part about building your own loudspeaker is that you can choose what to compromise on, based on personal listening preferences. I did not use the crossover to compensate for the response step because these loudspeakers are to be used on a bookshelf against the wall, which will reinforce bass response.
A diagram of the crossover schematic is included with the article. I used quality Mylar capacitors in the tweeter circuit and quality electrolytics in the woofer circuit. Both circuits use quality air core inductors whose parasitic d.c. resistance is considered throughout the design. Certainly tweaks such as buying expensive polypropylene capacitors and exotic inductors are possible. But these tweaks can double the overall project budget for perhaps a modest overall increase in performance per dollar spent. I purchased a surplus of Mylar capacitors and precision matched each capacitor to 1% of the theoretical CALSOD value. Finally, if you wish to improve inductor quality, please note that it will effect each driver's Qts by changing its Qes, and overall crossover performance. Replacing the inductors in series with the woofer with lower d.c. resistance "exotic" inductors may improve overall sound. But the d.c. resistance in the tweeter circuit inductors contribute to each legs total damping resistance, and should not be changed; if you insist on using low resistance "exotic" inductors, you must make up the lost resistance by using a larger value series resistors.
In my opinion, the greatest strength of the system is its neutrality. It is exceptionally clean, resonance and peak free, with no emphasis on any frequency. It can play surprisingly loud for its size, without losing any its sonic composure. It likes all types of music. Cloth domes are back! The Vifa tweeters are silky and airy, and the fourth order crossover allows the tweeter to perform without "harshness" or "raspiness". The crossover transition between the drivers is seamless and smooth, while dispersion off-axis response is excellent, providing a very stable and credible stereo image. The P13WH exceeded all of my expectations reproducing midrange, but as you would expect, a 5.25 inch woofer can not reproduce the bottom octaves with authority. However, perceived bass is robust and taught, and quite satisfying. A quality subwoofer and crossover would remove bass from the Vifa woofer and further improve midrange performance, as well as provide the missing bass extension. I have already began design work on a large three-way floor monitor loudspeaker system for a friend using these two drivers and crossover teamed with the widely acclaimed NHT 1259 12 inch woofer/subwoofer. Stay tuned.
In addition to high fidelity, you will have a aesthetically pleasing loudspeaker, in contrast to the many wood- grained "vinyl siding" clad commercial loudspeaker in this price class.
The cabinet and crossover are easy to construct for any competent hobbyist, and I found the whole project quite enjoyable. In conclusion, if you need a set of very high quality loudspeakers, but your on a budget, these loudspeakers are an excellent choice. In addition, the basic design certainly leaves room for you to give it a personal touch by upgrading items such as the crossover components, to further refine overall performance. A word of caution though, the Vifa drivers and crossover design are fatally linked in this design, and they can not be changed or altered without penalty. Good luck.
Scott C. Blaier