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"Veritas" Floorstanding "TMWW" LoudSpeakers Project, Text and Images courtesy of Wayne J. of Speakerbuilder.net After completing the Dayton IV "Lyra" design, I received many requests for a similar design employing dual 10" woofers.  I also received many questions asking about modifying the crossover design for various tweeters.  The most commonly requested tweeter modification was for the inexpensive variants of the HiVi RT-series isodynamic planar tweeters. When I began work on the dual-woofer version of the Lyra, it became apparent that the Dayton 275-070 tweeter used in the prior design did not have the necessary sensitivity that would be required to maintain the tonal balance of the speaker.  Accordingly, I researched higher-efficiency tweeters until I found candidates that would meet my requirement for open, detailed sound reproduction and smooth response (without breaking the piggy bank).

Introduction

Since this design originated from my desire to recreate the fantastic sound and style of various 1970's classics like the Infinity RS-6b's that used a ribbon tweeter, a polydome midrange driver, and either one or two 8" polymer cone woofers, it seemed only natural to consider ribbon-type tweeters.  Ultimately, I chose the HiVi RT2C isodynamic planar tweeter because of its detailed sound, extended frequency response, and relatively good price performance.  These types of drivers, however, are not inexpensive.  At just under $100 for the RT2C, this project is not cheap, but should compare very favorably with projects costing considerably more and retail loudspeakers selling for many times what it will cost you to build the Veritas.  Before anyone writes to me asking why I didn't use one of the $30'ish versions of the HiVi tweeter being sold by some vendors, it's because they are not the same as the retail-version of the RT2C.  Based on careful frequency response tests and critical listening, these "cheap" tweeters do not have a similar response or sound reproduction capability as the retail-version of the RT2C.  Accordingly, please do not request modifications to this project to allow the use of the discount ribbons -- I have already evaluated and rejected their use in this project because they provide inferior performance.

Measurements

Prior to beginning to design the speaker, driver measurements were made.  Using both the frequency response of each driver on a "pseudo" infinite baffle and impedance in free air, the speaker can be modeled using various software packages.  During the measurement and modeling process, such aspects of crossover frequency, relative sensitivity of the drivers, baffle geometry, room gain effects, off-axis horizontal and vertical dispersion, power response, and, of course, enclosure size are determined.

Once these aspects of the design are considered, the decision was made to use the HiVi RT2C ribbon, the Dayton 285-010 midrange driver, and a pair of Dayton 295-315 10" woofers.  Since the first and foremost objective in my designs is to get the best possible performance for the least possible cost, the Dayton 2" Dome midrange is the clear choice for most 3-way and higher designs.  The choice of woofer was quite simple, as well.  The Dayton 10" woofers are quite inexpensive and provide excellent sound quality in a sealed enclosure.  The 70's-era designs I am envision when I think of this process all used sealed alignments, for excellent  tonal balance and well-resolved bass reproduction.  The Dayton woofer exhibits superior performance in a sealed enclosure and the Veritas has excellent bass resolution with more than acceptable low-end extension.  To assure this during the design process, various low-frequency alignments were modeled with and without room gain effects (the low frequency "boost" the floors, walls, and corners provide).  With the Veritas placed approximately 24" from the wall, low-end extension is excellent into the lower 30 hz region.

The Crossover

Based on the off-axis performance of the woofer and midrange drivers as well as the impedance resonance of the midrange driver at about 350hz, crossover frequencies of 750hz and 4500hz were selected.  After extensive computer modeling various crossover designs were considered for their impedance profile, phase relationship between the drivers, and overall response characteristics.   The final crossover design takes advantage of the natural coincidence of the midrange driver's and tweeter's acoustic centers when flush mounted and uses low-order acoustic slopes to achieve near time and phase coherence along the listening axis starting at about 1500 hz.   Coupled with flat frequency response across the spectrum from about 45 hz to 20+ khz, the final crossover results in a deep, wide soundstage with precise imaging.

Also, thanks to the wonderful properties of the HiVi ribbon tweeter, the high-end sound is simply spectacular. Those of you who have never heard a good ribbon driver will be amazed at how, when properly used, these tweeters can produce exceptionally smooth, detailed, open sound without harshness or sibilance.  Further, the Veritas highpass consists of only a single, series capacitor in front of the tweeter.  This is something that would not be possible with the less-expensive versions of this driver being sold at various places -- those drivers do not exhibit the exceptionally smooth response of the retail version of the RT2C.  However, with this tweeter being controlled only by a single components, the openness and purity of the sound is worth the added cost.  Additionally, having a only a single component made this a prime opportunity to evaluate different capacitors.

For the Veritas, I strongly recommend using the AudioCap Theta capacitors.  These capacitors were tested and compared to Solen, Dayton, and other brands.  Despite their high cost, the Thetas were, by far, superior at reproducing detail, ambience, and presence within the music.  Probably because of the high-frequency extension of the RT2C, the high-quality capacitor is able to elicit performance that is otherwise lost by other brands.  The real question is whether it's worth an extra $20 per speaker for the Audiocaps.  To that question, I answer a firm "yes!" because you would justify spending an extra $20 for a tweeter that sounds this much better, why not spend that money on the capacitor that feeds it?

The midrange filter for the Dayton 285-010 driver employs a 2nd order highpass and 3rd order lowpass in a cascade bandpass topology, although not necessarily to achieve particular acoustic rolloff rates as much as to achieve the desired rates with specific phasing and impedance properties.  By controlling the driver phase using the second series-capacitor in the lowpass section of the bandpass filter, the coincidence of the acoustic centers of the tweeter and midrange driver are able to perform with a high degree of time and phase coherence, allowing the Veritas to produce incredibly smooth midrange and high-frequency performance with an exceptional soundstage and imaging.

Finally, the lowpass crossover for the 10" woofers was chosen for the combination of simplicity and to achieve the desired acoustic rolloff rates.  As well, due to the geometry of the Veritas speaker, the design axis and crossover design summation distance (i.e., the distance at which you're predicting the response of the speaker) had to be carefully considered.  Two major issues influenced this aspect of the design -- the relatively poor vertical dispersion of planar drivers and a response peak in the 3-4khz region of the Dayton 10" woofer.  If measurements are made at a summation point too close to the speaker along the design axis, the results are skewed by the fact that you are evaluating the woofer considerably off-axis where the peak in the woofer's response will not be adequately considered.  For the Veritas, the design axis is the midpoint between the tweeter and midrange driver and the summation distance for the design was 2.5m, which reflects a typical listening distance.

Part Number	Description	Quantity	$/Unit	Sub-total
027-406	.47uF bypass capacitor	2	$0.83	$1.66
027-410	1.0 uF capacitor	2	$1.15	$2.30
027-429	7.5 uF cap	2	$2.40	$4.80
027-427	6.2 uF cap	2	$2.15	$4.30
027-440	30 uF cap	2	$7.88	$15.76
027-412	1.5 uF cap	2	$1.35	$2.70
027-428	10 uF cap	2	$3.15	$6.30
027-352	47 uF nonpolar electrolytic	2	$0.90	$1.80
027-722	1.0 uF Audiocap Theta	2	$10.91	$21.82
027-718	0.47 uF Audiocap Theta	2	$10.45	$20.90
005-2	2 ohm Mills resistor	2	$1.50	$3.00
005-3	3 ohm Mills resistor	2	$1.50	$3.00
266-904	2 mH Erse inductor	2	$10.90	$21.80
266-658	0.22 mH foil inductor, 16 awg	2	$6.30	$12.60
266-674	1.0 mH foil inductor, 16 awg	2	$11.15	$22.30
295-315	Dayton 10" woofer	4	$20.15	$80.60
285-010	Dayton 2" dome midrange	2	$29.90	$59.80
297-406	HiVi RT2C tweeter	2	$99.80	$199.60
			Total	$485.04

Crossover Construction

Building the crossovers for the Veritas is not terribly difficult.  The highpass, consisting of only a single capacitor, represents the ultimate in simplicity.  The lowpass uses only three components in what is called a "strange second order" topology.  To most, it will resemble a first order crossover with a zobel.   Regardless of what you call it, it's not hard to build.  The picture at the right shows what the components look like when laid out prior to assembly.  Notice that the 47 uF non-polar electrolytic capacitor is shown with the 0.47uF Dayton bypass capacitor connected in parallel with it. The two AudioCap Theta's are shown in the upper left corner of the photo connected in parallel to make the required 1.47 uF capacitor.

The next step is to prepare the remaining capacitors.  All of the capacitor values for this project are achieved by combining multiple capacitors in parallel.  While sometimes this is done to produce a capacitor with a lower ESR, in this case it was done merely to achieve the required component values as closely as possible.  To the left, you will see the that the 6.2 uF and 7.5uF capacitor are combined in parallel to make the needed 13.5 uF capacitor for the bandpass filter.  Also, a 30 uF capacitor is combined with a 1.0 uF capacitor to make the 31.0 uF value; and also a 10 uF capacitor joined with a 1.5 uF capacitor to arrive at 11.5 uF.  The schematic below shows where each is required in the crossover.

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Once the components are prepared, the parts should be arranged by the section of the crossover network they will be used in.  The photo to the right shows the components separated by section.  The parts are then mounted on a suitable surface that can be inserted into the enclosure.  I prefer pegboard, masonite, or a similar, rigid material.  The lowpass section is shown on the left with the components affixed to pegboard using hot-melt glue.

    On the right is a photo of the bandpass filter components arrange on pegboard prior to soldering.  This is just one possible arrangement for the components.  I later moved the components around in a more compact arrangement and mounted them on a smaller piece of pegboard.  It's more important, however, to make sure that the components are firmly attached to the mounting board and clean connections are made between them than it is to have a pretty looking board.

 

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The Enclosure

Most of the questions I receive about my designs relate to building the enclosures.  I have attempted to provide drawings that are sufficient to guide you through the construction process.  As well, I have made high-resolutions scans of the drawings and crossover network available for download, for those of you who need to view precise measurements when constructing the speakers.

The enclosures are approximately 3.8 ft³ sealed, providing an F₃ for the enclosure in the 40 hz region, when stuffed with approximately 1 lb/ft³ of R19 fiberglass or about .75 lb/ft³ Acoustastuf, or similar, spun-fiber dampening material.  While the actual F₃ frequency of about 49 hz may indicate that these speakers lack bass output, the opposite is true.  Due to the effects of floor coupling, the amount of baffle step compensation in the crossover, and the naturally shallow rolloff of a sealed alignment, the Veritas produce smooth, well resolved bass in a very natural manner.

All construction shown in the drawings is 3/4" medium-density fiberboard  some builders may elect to construct the front baffle from 1" or 1 1/2" thick material.  This is recommended, provided that the internal volume of the enclosure is maintained.  You'll also notice that these enclosure use three cross-braces to minimize cabinet resonance.  The placement of these braces also controls woofer loading and care should be taken to ensure that any modifications to the enclosures do not produce uneven woofer loading.

Venting these enclosures is something many people suggest as an almost "knee-jerk" reaction.  I do not recommend venting them, but some of you may choose to do so regardless.  In my listening evaluation and throughout the consideration of the many design choices made in this design, I have found that venting these enclosures significantly reduces overall tonal balance and low-frequency resolution, only to gain a little low-end extension.   The price for this low-end "boom" is very steep.  In addition to the factors I just mentioned, the enclosure volume should be increased to about 7.5 ft3 and at least a 5" diameter vent should be used to avoid excessive port noise.  Careful bracing should also be used to avoid resonance in the port tube.  I will not be providing any further guidance on venting these enclosures, but hope that those who choose to make this modification will share their results.

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Download high resolution drawings (2.7mb)