1-150 WATT PRO CLASS
By Rik Johnson
I am not new to car audio, I have been in the industry for over ten
years and have competed in C.A.N. (Car Audio Nationals) and N..A.C.A (National Auto Sound
Challenge Association) as well as I.A.S.C.A. (International Auto sound Challenge
Association). While being a sanctioned IASCA judging official and building award winning
auto sound vehicles for associates, its fortunate that I had enough time to assemble this
vehicle on-time for the 1996 competition season. Since auto sound is not my primary source
of revenue, I get to pick and choose which vehicles I desire to work on, so for the last
11 months I have dedicated my time to finishing the sound system in my Porsche 911
Cabriolet
The sound system in this vehicle was designed for competing in one of IASCAs
lowest power categories. Most systems in these power categories utilize passive x-over
components in their design. It was my intent to design a fully active sound system (for
the Pro 1-150 watt class) and not make any permanent modifications to the vehicle
(everything can be removed and returned to stock. No holes were cut or drilled, all
factory bolts, holes, and brackets were used).
THE TRUNK
We will start at the front of the car and systematically work our way to the rear. The
factory battery was replaced with a Optima 800. All the factory power/ground wire was
replaced with Phoenix Gold 4 awg power cable and then covered with a flex loom material.
All terminations were done with aircraft quality heat shrink terminations. The
main fuse is a wafer style from Phoenix Gold and the battery terminals are from Phoenix
Gold. The battery cover is actually one of the trunk trim panels, which is easily
removable for servicing of the battery.
Since this vehicle is not used for daily transportation, the entire trunk space was
available for use. The space saver spare tire had to go as well as the factory tool kit
and jack.
First off, the trunk floor (which is nowhere near to being a level surface) had to be
raised and constructed so as to secure to four factory bolts located in the trunk. All the
processors and amplifiers are secured to this panel. The trim panels (all of them) for the
trunk are pressure fit and covered with a four way stretch vinyl (a custom "hot
box" was designed to aid in the covering of the more curved panels). The amp rack and
associated trim pieces were hand fit one at a time. There are eight different trim panels
in the trunk. These panels were fabricated from quarter inch medite, PVC tubing,
fiberglass and body filler.
To stay with the design of the Porsche, all the trim panels exhibit multiple curved
surfaces and angles that match the contours of the fenders. The right side trim panel
covers the factory battery, capacitor and factory fuse block assembly. To access the
factory fuse block, a hinged door was added that follows the contours of the panel. The
interior of the hinged door was finished in black felt and well as the surrounding area of
the factory fuse block. The panel on the left side of the trunk follows the contours of
the fenders and if you notice, it has a raised notch for clearance of the factory shock.
The panel that covers the processors was shaped with fiberglass and PVC pipe to achieve a
seamless transition for the curved surface. The plexi glass that covers the processors was
curved in a specially designed oven to prevent warping and shrinking.
The most difficult of all the panels is the one in the far right corner that covers the
master cylinder. This panel not only had to clear the master cylinder but it also had to
allow access to the cylinder for fluid replacements. You will notice the small door on top
of the panel, this hinges upwards for access. The recessed compartment (finished in red
laminate) houses the temperature gauges for the amplifiers as well as the volume controls
for the rear satellites. The plexi cover over this panel rests in an aluminum
"U" shaped channel that allows the plexi to slide rearward for access to the
volume controls (I did not like the idea of a removable plexi cover. It is to easily
damaged). The amplifiers (all of which are protected from damage by plexi covers) were
laid out in a symmetrical fashion with the fourth location housing another compartment
that holds the main power fuse block and two additional fuse block assemblies for the
processors, amplifiers, fans and head unit.
The amplifier compartment is cooled via two small computer fans that are temperature
controlled. The fans initialize themselves when the amplifiers reach a temperature of 100
degrees (done via a custom fabricated circuit). The signal cable used throughout the
vehicle is Belden twisted pair computer grade cable. The speaker wire is Phoenix Gold Zero
Point reference. Additional power/ground wire is Phoenix Gold 4 gauge and 8 gauge. Because
of the connection nature of the Rane equipment and the Phoenix Gold equipment, all signal
cables were custom fabricated using Switchcraft XLRs, Canare RCAs and Belden
cable. All wiring is routed through factory locations when possible and wire tied every
four inches.
The electronics were all color matched to the exterior of the vehicle and re-silk
screened. A Phoenix Gold 1.2-farad capacitor is mounted next to the battery. (Hidden by
trim panel). The amplifiers are Phoenix Gold MPS-2240s (These amplifiers were chosen
because of their reputed reliability and sound reproduction characteristics).
Two Rane MQ-30 equalizers (These equalizers utilize a unique Interpolating constant-Q
design, This means that the bandwidth of each individual filter is guaranteed to be narrow
enough to prevent unwarranted interaction between filters, yet wide enough to produce
exactly the type of correction curve necessary in a automobile environment). The Rane
AC-23B provides three way stereo separation at 24db per octave. This crossover has
variable frequency adjustments along with an adjustable time delay circuit. These
processors are mounted in their own "quick release" cabinet arranged in a stair
configuration for easy viewing from the front of the trunk. The Rane processors were
modified from their original 120volt ac to 12-volt dc (using 18-volt dc power supplies).
The filters and op-amps in these processors were replaced with more accurate ones as well
as eliminating the sub-sonic filter and ultra-sonic filter sections of the Rane AC-23B
crossover (cleaning up the analog signal path). Since compression drivers are notorious
for having problems in the 20k range, another modification was done to the crossover to
give an additional 5db of boost in that frequency range.
THE INTERIOR
The interior starts off with a Alpine 7909 (of course) CD which was chosen because of
its high output voltage and low source impedance. The Alpines out dated D/A
converters were replaced with Burr Brown 20 bit converters and a digital coax output was
added. Both sets of RCA out puts (each measured at 4.45 volts) are combined together using
a prototype Navone engineering balance line driver (thus giving me a maximum usable output
signal right around 9 volts with a source impedance of less then 300 ohms) This balanced
signal is sent directly into the trunk (about 3.5 feet) into the balanced inputs of the
Rane MQ-30 equalizers. (a balanced line yields upwards of 70db of common mode
rejection)
The front stage was designed to yield a true phantom center image high upon the dash. I
wanted to eliminate any chances of left/right bias (Haas effect) so to minimize path
length differences I chose to use Image Dynamics CD-2 horns and TAD 2001 compression
drivers. These horns are mounted equal distances from both doors and horizontal to the
plane of the passengers. The grill for the horns follows the contours of the dash and it
runs the full length of the dash as well. This grill assembly was constructed from quarter
inch medite and fiberglass. Perforated metal mesh was counter sunk so as to not ruin the
smooth surface once the grill cloth was applied.
I wanted as large a mid bass driver up front as possible so the 5.25" Dynaudio
15W75 mid-bass drivers were chosen. These drivers are very detailed, fast and also play
very low. I conducted research on a plethora of 5.25" drivers but nothing exhibited
the characteristics that the Dynaudios did. (the front door locations were used,
remember....no permanent modifications) So that the Dynaudios would not be playing
"free air" into the door cavities, special fiberglass pods were fabricated to
mount in the existing doors and to the existing screw holes. Since there was no way to get
the necessary volume from these pods I devised a ingenious plan. I purchased some flexible
(but airtight) wet/dry vac tubing, PVC elbows and PVC caps. The wet/dry vac tube is
approx. 3.5 feet long. One end is capped off, the other end has the PVC elbow mounted in
it. The elbow end was surgically placed in a hole cut into the bottom of the door pod and
then sealed into place. After loosely stuffing the wet/dry vac tubing with long hair wool,
the volume was estimated at .25 cubic foot. Perfect for a 5.25" mid-bass driver.
Another advantage of this strategy was that any type of door skin vibrations and
coloration has been completely eliminated.
Rear fill is supplied via a set of Bose 4" (cannibalized from a set of 901s,
remember...no permanent modifications, factory locations) mid-bass drivers mounted in the
factory rear side panel locations utilizing a custom built 30 ms time delay.
The JL Audio 10W6 subs were chosen because of their ability to function well in a small
volume 2nd order alignment. Both drivers are mounted side-by-side in a custom Zbox
enclosure that fits under the rear seats. The enclosure interior was coated with a
borasilicate ceramic and lightly packed with longhair wool to increase cabinet volume and
minimize cabinet coloration. This cabinet is constructed out of fiberglass and covered
with matching Porsche wool carpet. To get the necessary mounting depth for the JL Audio
10W6 subs, 1 ¾ rings were added to the cabinets recessed mounting points to allow
clearance for the magnets. To have a flushed grill over the subs, a new top panel
was constructed from ¾ inch medite and the grills were recessed (via a router) and then
covered with the carpet
SOUND
So how does it sound? I have spent about 120 hours fine-tuning the system for sound
quality. The RTA score runs a 37 most of the time and the sound quality and imaging scores
run very high. The vehicle is undefeated in zone 7 in the pro 1-150 watt class for 1996.
Special Thanks
Scott Whitehead/Brian Ellis-Innovative Audio, David Navone-Autosound 2000, Keith
Peterson/Larry Fredricks- Phoenix Gold, Todd Meek, Tjin Sin Tjo, Dave Friesen
Vitals
Vehicle - 1983 Porsche 911 Cabriolet
Owner - Rik Johnson, Kirkland Washington
Shop - AudioDesign, Kirkland Washington
Installer's - Rik Johnson, Todd Meek, Scott Whitehead (Innovative
Audio)
System Design - Rik Johnson
System Tuning - Rik Johnson
Head Unit - Alpine 7909
Processors - Rane
Electronic mods - Tjin Sin Tjo
Amplifiers - Phoenix Gold MPS-2240
Front Stage - Image Dynamic/TAD horns & compression drivers
Rear Fill - Bose
Subs - JL Audio
Security - Phantom 1100
Special Color matched processors, custom trunk panels
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