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Background
The Bose Acoustic Wave system came in some models of third-generation
RX-7s (1993-1995) and serves as a high-tech subwoofer. It consists
of a curvy, plastic tube that sits in
the back of the trunk, and pokes up through the cargo cover just
behind the seats. It contains an amplifier and at least one speaker.
[ I will soon open my Wave up and see what exactly is in there. I
suspect I will find a low pass filter and two speakers. ]
Some complain about the sound of the Wave, and most complain about
the space it takes up. Being a bass player myself, I expect more
detail in the low end, and the Wave cannot provide the sound
quality.
The Problem
When I first got my car, I posted some questions to the RX-7 mailing
list about the premium Bose stereo package, and how I could make
some simple modifications to it. I had an old Rockford Fosgate Punch
75 amplifier from my last car that was still in good shape, and I
was eager to make use of it again. It doesn't have the best signal
to noise ratio, but it did wonders for the sound quality in my old
Honda (especially the bass). The answers I found were that all the
speakers with the premium RX-7 audio package had their own
amplifiers, and the speakers were most likely
of the 1 ohm variety, which are very difficult to use with a third
party amplifier.
After finally getting my shop manual, I studied the wiring diagram
for the audio system. I noticed that power and left and right
channels were being sent to the Wave in one wiring harness. I
surmised that the audio signals were line level.
Removing The Wave
The owner's manual describes how to take out half of the Wave
system. Looking from the rear of the car towards the front, there
is a clip at the right side that flips up, and a wing nut at the
front of the right tube. This is part of the procedure to get
access to the spare tire. On the left side, there are three 10mm
nuts to remove. At this point, you can free the Wave assembly from
the clamp on the right side and lift it up slightly to reveal the
wiring connection. The Wave has an accordian section in the middle
(towards the left side) which allows it to bend. This
makes it easier to remove it, and I'm sure it "adds" to the
wonderful sound quality.
The wiring connection is on the left, underneath. Find it with your
hand, squeeze the connector, and wiggle it free. Do not pull on
the wires themselves, only the connector.
How does it sound without the Wave?
The front speakers are full range, and do an adequate job on their
own. What I noticed after taking the Wave out is a lack of overall
power, particularly in the lower end. At one point, while on the
highway, I found I had turned the volume all the way up. On the other
hand, the low end did seem to tighten up a little bit. I listened to a
tape of a song with a very prominent 5 string electric bass (lots of
sustained, open low B string notes), and everything around and below
the low E (40 Hz) was quite audible. I definitely got a sense that I
was hearing more harmonics than fundamental at that level, giving only
the impression of the pitch. This is not a major problem, and it's a
nice feature of the human ear to still allow us to perceive a pitch,
even if we can't hear the actual frequency.
Replacing the Wave
To make a long story short, I rigged up a temporary connection of my
Punch 75 to the wiring connector. Here are the color codes of the
six connections, as they are arranged on the connector:
Blue/Yellow stripe
Right -
| Yellow/Blue stripe
Left -
| Black/Red stripe
Power +
|
White/Blue stripe
Right +
| Yellow/Green stripe
Left +
| Black/Orange stripe
Ground
|
Connections
The power is 12 volts, perfect for hooking up an amplifier. Since the
power is only turned on at this lead when the stereo is on, it makes an
adequate turn-on connection for the amp. The ideal situation would be
a seperate power connection directly to the battery (with in-line fuse
of course), but for test purposes I connected the power and turn-on
connections to the power connection of the harness. I connected the
ground wire of the amplifier to one of the bolts previously used to
hold down the Wave tube. The speaker connections at this point should
be self-explanatory.
I recommend getting some quick-disconnect connectors with 1/4" tabs
and crimping them on the ends of all the wires, as they can just be
slid into the harness connection. I bought a pack of 10 for $1.49 at Radio Shack, Cat No. 64-3038A, which had 2 insulated and 4 uninsulated 1/4" connectors. A harness connector (like
the one on the Wave) is not really necessary, although it would add the
protection of locking the connections together. If you don't intend to
use or sell the Wave system, you could cut the female harness connector
off and use that instead of the quick-disconnects.
How does it sound?
Experiment #1
I had an old 8-inch woofer that I used to use in my old Honda before it
was baked at 375 degrees in the hot San Jose summer sun for a week
until golden brown. The rubber surround was totally missing! Anyway,
I did not want to test this setup with a good speaker just yet, and I
knew this old speaker would at least make sound.
It worked! It rattled like hell, but there was plenty of volume.
Standing at the rear of the car with the trunk open, the one 8" speaker
drowned out the front speakers. This is without adjusting the gain on
my amp, which I suspect had not been touched since I bought it and had
it installed in my Honda. As a reference, the gain on the Punch 75 is
"...factory-preset to a level which matches most 500 millivolt to 1
volt rated preamp-output source units."
Experiment #2
Time for some more beef. I opened up an old Fisher speaker that had
a 15" woofer. I never realized how cheap these speakers were until I
did this. In any case, it worked, and it sounded much better than
the 8-incher. With the speaker just laying in the trunk of my car,
the bass extension was much deeper, although due to the cheapness of
the driver there was a lot of ringing from the frame. The woofer
also could not handle much volume, although at this point I had still
not adjusted the input volume on the Rockford. It is worthwhile to
note that this was an 8 ohm speaker.
Experiment #3
OK, time to get a little weird. I have a small guitar/keyboard amp
that someone I knew found in a trash can and gave to me. It has a
single 12" speaker, the details of which are a mystery (it is probably
4 ohms). I disconnected the guitar amp leads from the speaker and
plugged it into the Rockford amp. The guitar amp has an open back, not
ideal for a subwoofer installation. When I first plugged it in, the
sound was a bit harsh, and I got a lot of high end content. Rather
than wire in a choke, I turned the guitar amp face down on the trunk
carpet, and placed a block of wood across its open back. I also
adjusted the gain on the Rockford all the way down (14 dBV), and
eventually bridged the amp to mono. I was extraordinarily impressed
with the results.
The bass was strong and tight when compared to the Bose system.
Even with the loose block of wood on the back of the guitar amp, the
balance was surprising.
Other findings
I had expected to hear only deep bass through the speaker, but it
was clearly a full range signal. That means the head unit does no
filtering, and the low pass filter is fully contained in the Wave unit.
This means you are free to add whatever amp, crossover and speakers
you desire!
What am I planning next?
For those that are interested in my own personal endeavors... Well,
I'm leaving out the Wave for now, and I'm riding around with the guitar
amp setup in my trunk (it just barely fits :-). My plan is to get a
10" or 12" subwoofer, possibly from Infinity, although I will be doing
some listening tests before I make my decision. I'll build a small box
to contain the sub and attach the amp to it in such a way that it can
be removed at a moments notice. This will be temporary until I can
afford to really soup this system up and build a nice enclosure for the
sub(s) and extra amps. But until then, the combination of a single sub
with the Punch 75 ought to put some serious bass in my trunk.
Fine print:
I have no affiliation with Rockford Fosgate, Infinity, Mazda, Bose,
Radio Shack. I take no responsibility for any modifications made
suggested in these pages. Use this information at your own risk.
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