1983 Porsche 911 Cabriolet owned by Rik Johnson, sound system designed/installed by Rik Johnson


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 IASCA’s 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 termination’s were done with aircraft quality heat shrink termination’s. 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 XLR’s, Canare RCA’s 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-2240’s (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 Alpine’s 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 Dynaudio’s did. (the front door locations were used, remember....no permanent modifications) So that the Dynaudio’s 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 901’s, 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 sub’s, 1 ¾ rings were added to the cabinets recessed mounting points to allow clearance for the magnets. To have a flushed grill over the sub’s, 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