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Wednesday, June 13, 2012

Hitachi Foam Fix (1)

Well, I picked up what looked like a nice pair of small floor speakers by Hitachi, not sure the model number:

The guy sold them for $30. So I couldn't really say much about their fitness.

I figured I couldn't lose, since I couldn't even buy a pair of tweeters that cheap!

When I got em home, I noticed that the soft foam surrounds on the woofers were in fact ready to fall apart:
just a light touch would tear them.
Having someone else re-do the surrounds was probably not cost effective for him, so he sold them.

But the cone itself seemed to be a tough rubber/plastic material, and maybe the speakers were worth saving.

But luckily my friend at the Speaker Shop gave me a pair of cobbled rubber surrounds. These are obviously heavier, and I was warned they would reduce the SPL of the woofers a db or so. I didn't really care that much.

My buddy gave me a few tips:
Don't try to glue the inside and outside at the same time! Patience...
The rubbers he gave me were reverse-half-round, meant to be glued behind the cone.
I decided to glue them on top of the cone instead however, for looks.
The rubber seemed to accept this position, without putting too much backwards pressure on the cone.

I noticed that even the slightest off-center push would scrape these cones.
It would be very tricky to get them glued without rubbing.
I decided to skip the cardboard layer between the original foam and the metal cage.
I kept the outer foam gasket though, again for looks, since the speakers were actually mounted on the surface of the frontplate from the outside, and not from behind.

I used Gorilla superglue for both the cone and the rim, first gluing the cone.

I then sat the speaker on the table (as per photo) with sound hooked up (mostly bass).
A small amount of volume ensured that the speaker cone would find its natural center.
I then glued opposing sides (in four stretches between the screw-holes) to the rim.
The crazy glue takes a while to dry, especially with metal and rubber,
so there was plenty of time to slide the rubber a bit back and forth.

Mostly however, there was nothing to do, as even a small amount of play caused an audible scrape noise,
as music was being played.

It went quite well, and seemed to result in a non-scraping fix.
However, the speaker out of the box sounded pretty awful, and without significant base.
But what disturbed me was the amount of mid and high that came through into the woofer.
It was obnoxious in sound, and I thought to myself that this must be what was bleeding into the speaker in the background even when it was hooked up in the box.

A glance at the 'crossover' (simply two caps, as with Kenwoods, one for mid, one for tweet),
confirmed that there was nothing between the input and the woofer to block high end.

The cheap crossover was however just one step better than the Kenwood,
because for the top tweeter-cap they splurged on a poly (the mid was still an electrolytic).

I thought I would just leave things until I could plan and build a proper crossover.
I think that here again the sound could be significantly improved.

Several nice things about this rig should be noted:

(1) The front panel is angled back, to point the speakers more toward listener-level,
and this means that the front-back panels are not parallel, which significantly reduces standing waves and reflections in the cab.

(2) Unlike the Kenwoods, the cab is stuffed with heavier-style cotton/felt matting. This also reduces unwanted standing waves between sides, top and bottom.

Here's a look at the 'crossover' and stuffing:

And the tweeter looks like it has good potential;
it appears to be a soft-dome, and is marked as 'ferro-fluid cooled tweeter':

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