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.
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.
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.
Download
high resolution drawings (2.7mb)
