Design of the Esquire

  Overview

The Esquire is a two-way loudspeaker system incorporating the Vifa P13WH-00-08 woofer/midrange and the Vifa D27TG-45-06 one inch fabric dome tweeter. The impetus for the project was a need for a pair of extension loudspeakers of high quality but low cost. Because I did not need bass performance below 80 Hz., I decided to build a pair small of "mini-monitors" to fit on a bookshelf. I set a budget of $ 140.00 per pair. The basic design presented allows room for individual tweaking and upgrading, and depending on how many carats your golden ears are, and how deep your pockets are, you may pursue them. I did not pursue them, citing the law of diminishing returns.

Driver Selection

I believe the first and most important step in designing any loudspeaker system is selecting good drivers. If a designer successfully chooses good drivers, designing the loudspeaker system becomes much easier than trying to correct deficiencies intrinsic to the drivers themselves with crossover manipulation. After considering many drivers I chose the Vifa P13WH-00-08 5.25 inch midrange/woofer. My ears are convinced that it is one of the best 5.25 inch midrange/woofers in existence, especially considering its price to performance ratio. It comes highly recommended by many designers, and has found a home in some very high-end, high dollar loudspeakers. I chose as its mate, the Vifa D27TG-45-06 silk dome tweeter. The D27TG's specifications showed it to be an excellent match for the P13WH woofer. It incorporates all the best features of the more expensive silk domes currently available, such as aperiodic resonance damping, smooth and extended frequency response, treated and doped silk dome, etc., but at a fraction of their cost. I have seen what looks to be the D27TG in at least one high end loudspeaker costing $ 1500.00 dollars per pair.

Bass Loading

The enclosure volume and port dimensions are based on the Thiele/Small parameters for my individual drivers, optimized using Perfect Box and CALSOD. The drivers you receive will probably have different parameters than Vifa's specified parameters and my measured parameters. Therefore, the volume and tuning frequency of the enclosure I describe here may not provide the best performance for the woofers you purchase. Although my parameters differed from the manufacture's, they varied proportionally, and the enclosure volume and port frequency I calculated were similar to those recommended in Vifa's literature. Coincidence, no doubt. Be sure to add extra enclosure volume (10 % over volume is often used) to compensate for the space that will be occupied by the drivers, bracing, vent, and crossover. Also consider the effect of series resistance on the woofers Qtc (Qtc'). I calculated the initial vent length using a formula, but the final vent tuning was determined empirically using test equipment.

Enclosure Construction

The final enclosure is rectangular and made from 0.625 inch particle board I purchased from a Hechinger's scrap bin for one dollar. Initially I designed a complicated enclosure of PVC tube inside a rectangular enclosure with the annular space filled with sand or lead. But the final enclosure became too large to be considered a "minimonitor". Fortunately, the length of the panels are short compared to their thickness, and the final enclosure is very rigid. An internal diagonal brace has been placed to breakup standing waves due to the parallel walls of the enclosure, and to raise panel resonance. Corner braces are used extensively. The enclosure easily passes the "knuckle rap" test with flying colors, and a high pitched "ping" is heard if you rap on any of the panels. A tweak would certainly be to use 0.75 inch MDF or the construct the aforementioned PVC/lead enclosure, but I am not sure how significant the audible benefit would be. The enclosure is filled with virgin polyester batting to attenuate the P13WH's rear cone output and optimize the subjective bass quality.

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

Crossover Design

The crossover described here is not a cookbook design whose values were determined by plugging a nominal driver impedance value into a formula. All of the component values were optimized using the CALSOD computer program and later "tweaked" after listening tests. I choose these two drivers specifically for their smooth amplitude response, rolloff characteristics (especially the P13WH which is ruler flat with an almost perfect 2nd order rolloff), resonance control, off-axis dispersion, etc. to ease in crossover design; the effort paid off handsomely. Although the final crossover is complicated, I feel it is justified, and exploits the best features of the drivers in this system. I designed and experimented with many other types (Bessel, Chebychev, Butterworth) of filters of different orders (1st, 2nd, and 3rd) at many frequencies. But in the end, I found that a fourth order Linkwitz-Riley network at 3.0 kHz. looked the best on paper, and more importantly, sounded the best. Although the tweeter's excellent damping and low resonance suggest a lower crossover frequency could be used with a shallower rolloff, I found that such an approach often unbalanced the driver's bottom to top frequency response and resulted in audible distortion. The choice of a Linkwitz-Riley fourth order filter, with a crossover frequency located more than 2 octaves above the tweeter's resonance may seem a bit conservative. But listening tests indicate that this is a solution that allows the tweeter operate at its best. Fortunately, the relatively high crossover frequency poses no problem for the P13WH, because its off-axis performance is down only -1.0 dB 30 degrees off-axis below 3.0 kHz.

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.

Sound

I will not begin with the usual cliche " this loudspeaker sounds as good as loudspeakers costing five to ten times as much". I never know exactly what that means, other than too many loudspeakers are overpriced. However, I seriously doubt you could find a commercial loudspeaker this good for $70.00.

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.

Conclusions

Each loudspeaker costs approximately $ 70.00 to build. The Vifa drivers and their shipping make-up about 90 percent of that cost. Surplus items you have on hand will lower cost even more, while tweaks and "goldplating" will certainly add to the cost. I must say however, the basic loudspeaker described here yields unsurpassed "sonic" value when compared to available commercial models in its price class. Remember, this is a price class where one cheap electrolytic capacitor passes for a crossover, cheap cone tweeters are the status quo, and 1/8 or 1/4 inch unbraced undamped plastic or wood boxes pass for an enclosure.

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
Email: scott@smtp.dda.state.de.us