Car Stereos: Subwoofer Compatibility for Amplifier, dual voice coils, dual voice coil

Question
Hi Brian

I need some help in checking the compatibility or alternately choosing a subwoofer for the Harman Kardon CA470 amplifier whose specs are as follows
Type :    4-Channel
Amplification :    Class AB
RMS Power @ 4Ohms :    4 x 85W
RMS Power @ 2Ohms :    4 x 135W

The amplifier will be powering the cars rear speakers infinity Kappa 693.9i 3 way with the following specs
Power Handling, RMS - 110 Watts
Sensitivity - 96dB
Frequency Response - 35Hz - 30kHz
Impedance - 2 Ohms

Now I need to choose the subwoofer  and am confused due to the dual voice coils as to what would be the appropriate subwoofer because of the impedances. The specs for a single subwoofer are the following for the amp mentioned above

IDQ12V2.D4 - Image Dynamics 12" IDQ Dual 4 Ohm Subwoofer
Voice Coils: Dual 4 Ohm
RMS Power Handling: 350 Watts

or

would this subwoofer be appropriate
IDQ12V2.D2 - Image Dynamics 12" IDQ Dual 2 Ohm Subwoofer
Voice Coils: Dual 2 Ohm
RMS Power Handling: 350 Watts

Any help in choosing  the right amp  for the above configuration would be highly appreciated.

Thanks a lot in advance

Michael  

Answer
Hi Michael,

When you're using a subwoofer with a multi-channel amplifier like yours, you're going to be bridging the amp.  This means you'll be combining two amplifier channels into a mono channel with a higher output.  The rules change a bit for a bridged amplifier.  While this amp works well with 2-ohm speakers in stereo mode, it needs a speaker with a minimum impedance of 4 ohms in bridged mode.

Usually, that means you'd need to pick a 4-ohm sub to match; but the use of dual voice coil subs makes the issue a bit more complex.  The wiring and impedance for a DVC sub is similar to that of a pair of single voice coil subs.  You can't combine two 4-ohm subs and get a 4-ohm load; you have to choose a final impedance of 2 ohms or 8 ohms.  Similarly, you can't use a 4-ohm DVC sub and end up with a 4-ohm load.  However, you can use a 2-ohm DVC and get an impedance of 4 ohms, by wiring the voice coils in series.

For this reason, the best subwoofer choice for your amplifier would be the dual 2-ohm version.  The owner's manual should include a wiring diagram for a series wiring configuration, and the amplifier owner's manual will have instructions for bridged wiring.   The specs for the amp don't include the bridged power output, but based on the multi-channel specs, I'd expect a bridged RMS output of around 250 watts at 4 ohms.  That should make it a good match for the sub you've chosen.

Hope this helps!

Brian