Installing a quality sound system while retaining an authentic feel to our cars is challenging. Here are plans for a system that has reasonable fidelity, iPod capability, and a hidden subwoofer. (Please note that all photos referenced in this article are posted seperately on this web page as the Triumph Owners web site does not allow embedded photographs.)

Introduction:

Sound systems present many challenges in our beloved TR6s, including limited space, esthetics, poor speaker placement, and dismal performance of the original dealer installed electronics. In this article, I detail my solution to some of the challenges encountered in delivering quality TR6 sound at a reasonable cost, with volume levels sufficient for top down cruising and in a package that preserves the vintage interior appearance.

Choosing Your Signal Source:

The OEM sound system in the TR6 consisted of a dealer installed Bendix radio or Audiovox Cassette/8-Track tape deck. Power output was limited to approximately three watts RMS per channel delivered to a wheezy pair of eight ohm speakers mounted near the center console. While in its day it was on a par with sound systems being offered by other manufacturers, compared to today's iPod, satellite and CD capable multi-channel sound systems, the sound is embarrassingly primitive. To add insult to injury, the OEM sound system is simply too weak for top down listening, being about as loud as an asthmatic butterfly sneeze.
Installing a more up to date signal source frequently involves modification of the center console to permit installation of a DIN sized chassis, and demands punching large holes in interior panels to permit installation of adequately sized speakers. This is not only arduous; it results in loss of collector value and period esthetics.
Fortunately there are alternatives. Retro-style electronics are available that offer up to date design, period correct styling, and are sized to fit our cars without modifications. For my TR6, I chose the RetroSound Classic Radio (Figure 1). This radio offers modern features such as auto seek tuning, iPod support, AM/FM tuning for US and European frequencies, a clock, and four channel sound with 20 watts RMS of power per channel. Support for flash thumb drives and SD memory cards is optional.


Figure 1: RetroSound Classic radio

There are several other retro style radios available, but most are too large, especially for those cars that are blessed with especially narrow center consoles. Most TR6s allow radios of up to eight inches wide to be installed, however some fraction of TR6s came with a slightly narrower console that is 7-5/8 inches wide when measured from the rear. This rules out many contemporary radios. Even in the cars not so encumbered, installing an eight inch wide chassis can involve modifying the so called kidney panels to clear the chassis. One of the beauties of the RetroSound models is that they are a mere 5-1/2 wide, providing ample clearance and obviating the need for interior modifications. There are a variety of faceplates available, as well as several all metal knob styles. Only the so called Euro type face plates fit in our cars without modification. If choosing a knob set from RetroSound, be aware that different styles also differ in size. I recommend you consider only the more proportionate 68mm knobs if you go with a RetroSound product.

Acoustic considerations:

Once you have chosen your signal source, you now face several other important considerations relating to speaker selection and the resulting sound quality. The first is that the speaker openings in the kidney panels are a mere 4" in diameter. Those TR6s not originally equipped with a radio may not even have speaker openings. Unless you are willing to begin cutting up your interior (and I was not willing) you are stuck with the wimpy OEM speaker size. For these locations, I selected a Pioneer two way speaker with a co-axial tweeter, a space saving built in crossover, and included speaker grills. Whatever your choice it must boast excellent sensitivity, which means it produces good sound levels with modest power, usually expressed as sound pressure level at one watt. A rating of 90dB @ 1 watt would be a typical value, and higher values mean you will get more sound for a given amount of power. This is important as top down motoring means that sound levels must be high to be heard over the siren song of the TR6 exhaust note. If you are running a separate high powered amplifier then this is less of a consideration, but if you are not you should avoid sensitivities of less than 90dB.
The location of the OEM speakers is far from ideal, since high frequencies tend to beam from tweeters. Beaming is the phenomenon that results in the best audio being available only directly in front of a tweeter. Moving in any direction from the sweet spot dramatically reduces the sound level. In the TR6 there is no location that one hears clean high frequencies, a real problem. To overcome this, I briefly considered placing a pair of remote mounted tweeters on top of the dash pad, which can be oriented to face the driver. This would vastly improve stereo imaging but required drilling holes, routing wires, and mounting separate crossover networks for the tweeters. I chose to simply live with the poor imaging, and when my audio angst becomes insufferable I turn the stereo off and listen to that sweet exhaust note.
All of this brings me to the main impetus of this article, bass response. No matter which 4" speakers you choose, they will simply be incapable of delivering the bass response we have come to expect from modern auto sound systems. An obvious solution is a separate woofer of some sort, but that leaves you with two more problems; where do you put the darned thing, and how do you power it? The TR6 does not exactly have massive electrical reserves to support separate woofer amplifiers, and space is at a premium.
A possible option is a woofer with integrated amplifier, but most off the shelf designs simply will not fit anywhere that is unobtrusive. Some owners opt for a custom speaker/woofer packaging scheme residing on the parcel shelf, but this can interfere with soft top storage and worse, in my opinion, alters the appearance of the interior. Punching holes in the rear interior panel allows woofers or other large speakers to be mounted cleanly, but then covers them when the soft top is down and (here it comes again) also alters the appearance. Thus the solution offered in this article; A hidden center channel woofer that utilizes your signal source's rear channel amplifiers for power, and is small enough to be placed out of sight with no interior modifications. It can also be removed without tools in a few seconds, should the need arise.

The Hidden Woofer:

I decided early in this project that a single, center channel style sub woofer was the way to go, as it cut the space and electrical power requirements in half. I wanted an integrated woofer/amplifier both to simplify installation and (did I already mention this?) to save space. I searched the web extensively, but could not find one that was to my liking.
I finally decided that any woofer system should fit in the top of the passenger foot well which was the only unobtrusive, unutilized space I could identify. Some quick measurements showed that the woofer with enclosure could be no more than 9" x 9" and no taller than 6". This meant the largest woofer that would fit was an eight inch type. Alas, a Google search came up with nothing that met these size restrictions. A good car audio shop can fabricate a custom enclosure for you, but if I felt this was the way to go I wouldn't be writing this article, would I?
I then hit upon the idea of using PVC pipe as the enclosure for an eight inch woofer. PVC is easy to work with and is available in a many sizes. I used a five inch length of schedule 40 PVC pipe as my starting point, and acquired a suitable 8" woofer for the project (Figures 2 and 3). Depending on your selection of woofer and its space requirements, you may be able to shorten the pipe a bit to make it even more compact. Woofer depth is important! My woofer measured 3-3/4" from the mounting surface to the back of the magnet. The greater this distance, the more the woofer will intrude into the foot well, so keep this dimension as small as is practical. If the speaker voice coil is of the ventilated type (mine was) be sure to leave sufficient clearance between the speaker magnet and the floor of the enclosure to permit air cooling.


Figure 2: 5" long x 8" diameter PVC pipe
Figure 3: 8" diameter Woofer

The next consideration was sealing the end of the pipe in order to place a suitable acoustic load on the woofer. This extends the response lower into the frequency range and improves efficiency. Unfortunately, the laws of physics work against compact enclosures, so an enclosure this size represents an acoustic compromise, but one that is necessary. Did I mention how little space is available in a TR6?
A saber saw was used to cut a 5/8" laminated shelving board to provide a round plug that was then glued and screwed into one end of the pipe (Figure 4). Scrap pieces of PVC pipe were glued on the inside edge of the open end to facilitate mounting screws for the woofer basket.

Figure 4: 8" x 5/8" pipe plug
Figure 5: Crossover network

The crossover (Figure 5) is used to prevent unwanted high frequencies from reaching the woofer and thus degrading performance. The point at which the high frequency response is attenuated depends on a number of factors, including design philosophy. I chose 300 Hz as my cutoff point and in the interests of simplicity opted for a less complex -6 dB per octave design. The schematic for my crossover is shown in Figure 6, which shows the component values I used. The design was fabricated on a 2" x 2" electronic hobbyist perfboard, and the inductor was secured to the board using RTV. The crossover itself was secured to the interior wall of the pipe using JB Weld epoxy, carefully placed to avoid mechanical interference with the woofer. Wires were dressed using zip ties. Push on automotive style wire connectors were used to facilitate easy connection of both the woofer and the amplifier outputs.

Some noteworthy points on constructing the crossover:
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o OBSERVE POLARITIES on the woofer, the capacitors and the amplifier connections when assembling your crossover.
o The crossover design shown would ordinarily require non-polar electrolytic capacitors. Because these are difficult to source, polarized electrolytics were substituted and connected such that they function as non-polar capacitors. If you too roll your own capacitors, neglecting to observe polarities will quickly result in spectacular failure. The voltage ratings required depend on the power output of your amplifier, but in no case should be less than 16 volts. I used 35 volt capacitors.
o Because this design functions as a center channel woofer, only a single output connection from each rear channel is used. This approach sums the left and right channel signals and divides the result by two so that a single woofer will provide the bass from both channels. Because the human ear cannot identify the direction of origin for low frequencies, there is no loss of audio imaging.
o This design will ONLY function with a modern, double ended automotive audio amplifier. Double ended amplifiers drive both amplifier output terminals on each channel. Older designs, including the Triumph sourced radios and tape decks, are single ended designs and only one output terminal is driven while the other terminal is held at zero volts (i.e. chassis ground).
o Dual voice coil woofers are available. With a correct design, such a speaker would allow doubling the power to the center channel. If you really want to deliver head thumping bass but do not want to mount or wire a separate amp, this is a possibility. Such a design would require two crossovers, one for each voice coil.

Figure 6: Center channel, 300 Hz crossover network.

I painted my completed woofer satin black to make it difficult to notice when installed, and should you also chose to paint your woofer, test the paint for adherence to the laminated bottom plug and the PVC pipe. If the paint does not adhere well on the bottom plug, the mounting system described hereafter will not work. I also recommend the use of a speaker grill, to prevent some wayward passenger from accidently putting a foot through the speaker cone. I used 3M weather-strip adhesive to fasten the grill, and used SEM brand vinyl dash coating to paint the enclosure after lightly sanding the bottom plug. I tried several alternate paints including conventional enamel, Krylon Fusion paint for plastics, automotive primer, and high temperature paint, all of which when dry easily rubbed off at least one of the two materials.
The completed unit cost about $90 to build inclusive of shipping, and weighs eight pounds (Figure 7).

Figure 7: Completed center channel woofer
Figure 8: Suggested paint

When installing the woofer assembly in the passenger foot well, it is possible to do so without altering the car in any way. I used 6 thin magnets, which I sourced surplus from the supplier listed at the end of this article. The magnets hold nearly four pounds apiece, which is adequate to account for routine shocks from potholes or running over the occasional MG Midget. If you find you need a stronger mount, simply add a few more magnets to the bottom of the enclosure and reinstall. Do not be tempted to use the peel and stick strip magnets you find at the hardware store; They will not support enough of a load and your woofer will end up dropping on your passengers toes. Of course, if you do not get along with your passenger this can be a good thing. By the way, Velcro will not work either, as it has very poor vibration resistance.
If you are a sacrilegious TR6 owner, you may of course drill some holes in the top of the foot well from the engine compartment side, and then insert at least three suitably sized lag screws into the back of the woofer enclosure. Just don't tell me you did it, as you will then see a grown man cry. Be careful that your screws do not overly penetrate the enclosure, or you could damage the woofer or crossover network.

Figure 9: Installed woofer

Concluding Remarks:

You will note in Figure 7 that there are four speaker connections provided on the woofer. Only two of the terminals are connected internally. This was done to provide a convenient way to park the unused wire from each channel when wiring the woofer to the rear channel amplifier outputs. For my installation the two rear channels were otherwise unused, and this design allowed me to power the woofer without the need for a separate amplifier and to use the fader control to adjust the volume of the woofer.
Placing the woofer in the foot well extends bass response, offsetting the weaker performance inherent in small woofer enclosures. The result is bass response that in my opinion exceeds that of any parcel shelf mounted woofer enclosure compatible with normal soft top stowage. Because this design does not use a separate woofer amplifier, you will not be able to deliver the window popping bass so favored by rice boy types, but on the plus side you will be able to enjoy good fidelity when cruising. If you wish to use your stereo to suck the breath out of innocent bystanders while stopped at a traffic light, try this instead; Wear your baseball cap backwards, cover your car with Honda vTEC stickers, and offer an occasional gang sign to pedestrians. In this way, you will at least be able to intimidate the Jaguar and Austin Healy owners of the world.
In my opinion, the best part of this project is impressing your friends when you show them your equipment - by which I mean your spiffy new audio equipment.

List of suppliers Used by the Author:

Item . . . . . . . . . . . . . Supplier . . . . . . . . . . . . . . Web Site
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PVC pipe . . . . . . . . . . Swampgas . . . . . . . . . . . . eBay
Fasteners, epoxy, etc . Ace Hardware . . . . . . . . . . http://www.acehardware.com/
Woofer . . . . . . . . . . . Audio Maven . . . . . . . . . . . http://stores.ebay.com/Mavin-The-Store/
Capacitors, etc . . . . . Surplus Gizmos . . . . . . . . . http://www.surplusgizmos.com
SEM vinyl coating . . . Baxter Auto Parts . . . . . . . . http://www.baxterautoparts.com/
Classic Radio . . . . . . . Retro Sound USA . . . . . . . . http://www.retrosoundusa.com/
Pioneer speakers . . . . Best Buy . . . . . . . . . . . . . . http://www.bestbuy.com/
Speaker Grill . . . . . . . SoundExtreme1902 . . . . . . eBay