I had always thought that the rear speaker was a passive radiator. That plastic "thingy" over the rear driver is a sort of sound diffuser. The box has 2 ports - one each exiting from the front and back respectively. I had found that plugging the rear port by stuffing it with cloth tightened the bass considerably and since these speakers are placed with their back just 4" away from the wall, that's how they were played.
My Dad passed on early this year and the system was unused till a couple of months ago when I fired it up one day and discovered that the amp had stopped working. The CDP had given up the ghost a long time ago. I had a mini-amp
[I posted about this - I think it was last year] that I had ordered from e-bay and on connecting this to the JVC speakers with my iPod line-out as source, the sound was surprisingly very good and that's how the speakers were used ever since.
Last week, on a day I did not go to office, I decided to open up the speakers to see whether the rear driver was really a passive radiator. I was curious because if it were really a passive radiator I was wondering what the ports were doing there and I also wanted to see what the insides of the speaker looked like.
So:
Rear driver:
I prised out the diffuser which was just force embedded with plastic mounts on the perimeter of the driver cutouts. It came out easily. The driver had a shiny square frame typical of Jap. drivers of that period. Removed the driver mounting screws and found that the driver would not come out despite the frames not being attached to the baffle with glue or some such. The frame just came out a couple of millimeters but something was holding it back so I rotated the driver first anticlockwise and then clockwise. I found that on rotation, it moved out from the baffle a little. I kept at it till it was out fully. The driver had a bolt which was screwed on to it's back that extended to the front of the box. It was also not a passive radiator and it had its own enclosure - basically the box was divided into 2 parts vertically. One part housed the rear driver and its port exited from the front. The walls were bare - no damping material - naked!!! The crossover was 1st order - just an inductor in series. Impedance printed on it was 6 ?.
Front drivers:
For the mid-woofer - I prised out the circular decorative plastic frame on the perimeter of the driver cutout which was mounted on the baffle just like the rear diffuser. This driver came out easily along with the bolt mounted to its rear. So the front and rear drivers were attached together with the bolt to minimize vibration. Very impressive! This driver was identical to the driver on the rear. It was run full range and was connected in parallel to the rear driver. So, net Impedance for both drivers together = 3 ?. On the rear wall of its enclosure, just behind this driver there was just a pathetic strip of felt.
For the tweeters - After removing it, found that it was a sealed back tweeter which was crossed over 2nd order..
Cabinet Tweaks:
Front half of the enclosure: Since I was running the speakers with the rear port [for the front half] plugged, I densely stuffed this front part with some polyester stuffing that I had left over from a previous build.
Rear half of the enclosure: I lined the walls with convoluted foam that I had hoarded.
Both enclosures: There were cross pollinating leaks between the 2 parts of enclosures at the areas where the ports and the wires cut through the divider wall. I plugged these with carpenters wax that I purchased at the hardware store. I also cut new gaskets for the drivers from craft foam that I had lying around from a previous build.
I fixed the drivers back, fired the speakers up and:
The bass which was slightly flabby and which had an overhang had tightened up considerably. Sounds real good now.
The mid's which were good before now sound better. They are just "clearer".
The only bottleneck [compared to the other drivers] is the tweeter - it's an old fashioned paper cone tweeter and is not very articulate. But it's not harsh, is a bit too smooth but is something that I can live with. Not that I have a choice.
So there you are - for not much money and just a few hours of time - I managed to make a decent sounding speaker even better.
hyeah:
