Aiwa MA 3000 + Polk db840DVC Q. from newbie

Shackman

Hi one and all. John, a newbie in Australia writing. Firts post actually, so don't be too cruel!
I have just upgraded my car stereo from the OEM cassette/radio to a nice new Kenwood system. The head unit is a 22W RMS/ 50W peak times 4 (front and rear, L+R) CD/AM/FM job with provision for aux. in via a 3.5mm socket, (handy to plug your Zoom H4 in after recording a gig or band practice) and additionally a USB socket, so a USB stick full of mp3's works well and sounds great. Speakers are 6 1/2" 3 way in the doors and a couple of 6 by 9's on the parcel shelf, all Kenwood and all 4 ohm.

I feel however (as a bass player) that the bottom end with this set up is a bit lacking, and no amount of bass emphasis using the standard EQ controls is really adequate. So I've bought a Polk Audio subwoofer, model db840DVC. This is an 8 inch unit 4 ohm unit with a claimed capacity of 180W RMS and 360W peak. It utilises dual 4 ohm voice coils and consistent with that has 2 pairs of speaker terminals, one each opposite the other on the speaker basket. Claimed freq. range is 30hz to 200Khz and sensitivity is a claimed 85db at 1 watt (and I assume I meter- that's approx 39 " to you guys). The plan is to mount the subwoofer within the boot, or "trunk" as you guys would say, on to a conveniently placed circular depression at the rear of the parcel shelf in my '98 Mitsubishi Diamente, sold here as a "Magna". Whether I cut a hole of the appropriate diameter in that circular depression in the parcel shelf stamping for the sub to fire through, or just drill it full of holes, I haven't yet decided. It doesn't look to be stress bearing.

Powering the subwoofer will be a stand alone amp, one I have had lying around for years from a previous car stereo installation. It is an Awia MA - 3000, a model dating from the early 90's when it was considered high powered. Specs are 35+35W RMS stereo/65+65W peak. It claims on the side of the unit, "Matching Impedance 4-8 Ohms". I'm assuming that the quoted wattage figures relate to a 4 ohm load rather than 8 ohm. It is bridgeable with an external switch from stereo to mono usage and mono is obviously how I plan to use it, so in theory I should have 70W RMS/130W peak available. It is old but in good nick. Usage wise, I don't listen to "doof doof" music, as car based rap is often characterized, but enjoy rock, jazz, pop and even orchestral. I'm not going for deafening volume, but headroom and a bit of bona fide thump. After all, I turn 53 this year!
The plan is to connect both of the amps output terminal pairs (ie. L+R) to the subs pair of speaker terminals. As the amp will be bridged it should deliver 35W RMS (max), across 4 ohm, to both coils. THe amp has an external sensitivity adjustment, and the head unit provides for independent subwoofer level adjustment

So much for the background. Now to the questions. (BTW, I've searched the Polk site to attempt to answer these Q.'s- no specific hard answers for this kind of issue forthcoming, so have posted on your forum).

Can anyone see problems with this proposition?

Is this insufficient power for the speaker?

How does this wiring plan look to you?

Glad of any help,

Cheers,

John

MacLeod

I dont like using 2 channel amps for DVC subs. Most amps dont put out equally to both channels and so youve got 2 different signals pulling on the sub.

If youre able to, youd be better off returning that sub for a SVC model. Then you could hook up just about any 2 channel amp to it and be set. Its gonna be hard trying to find a mono amp that only makes 200 watts RMS at 2 ohms. However, if you can find one that does, thats would be a better choice for you.

Also, you might want to consider an amp for your speakers. Head units make horrible power sources regardless what their "ratings" claim.

Shackman

Thanks MacLeod, appreciate your assist. I have another Q. though.

I plan to make a subwoofer enclosure per the Polk specs for my db840DVC and this will go in the trunk of course. Here is my dumb question:

Does it matter in terms of sound dispersion or percieved volume which way the sub speaker box is pointed- ie. side, front facing or rearwards? Or for that matter, where in the trunk the sub is placed?

The 2 candidate positions in my car, a sedan, are:

(1) As far forward as poss. and thus close to the rear seat back, or
(2) At the rear of the trunk and to one side behind a wheel well

I'm asking this because, unlike hearing door mounted or rear deck speakers, with a sub in an enclosure sitting in the trunk there is of course a barrier to the subs sound entering the passenger compartment and thus the listeners ears, formed by the rear deck/parcel shelf and the back of the rear seat. And these barriers being formed by a thin sheet of steel plus some upholstery, surely some muffling of sound originating in the trunk must occur.

Over to you. Thanks, John

rubicrawler_lj

I would recommend removing your parcel shelf covering, adding some sound resonance material, opening the parcel shelf covering to match existing openings, covering it in speaker cloth and **** it down properly.
An alternative to speaker cloth is to use speaker grills for speakers that fit your particular auto.
I would then build a sealed enclosure (subwoofer mounted face up) that fits 4 in. (10.16 cm) below the parcel shelf for best sound transmission.

Rockford Fosgate has an online sealed enclosure design wizard that could help you out.
[URL=]http://www.rockfordfosgate.com/rftech/box_wizard.asp[/URL]

You could also build an enclosure that mounts the subwoofer mounted toward the rear seat, just try to maintain the 10.16 cm. distance to avoid muffling the sound.

Good luck!