Hitachi Foam Fix and upgrade

[nazaroo]

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.

 

[nazaroo]

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':

 

[nazaroo]

A first listen was encouraging at low volumes,
but when I put the screens back on and listened louder,
it became harsh or perhaps 'transistor-like' in sound.
Also the bass was as perhaps expected, somewhat lacking.

I re-examined the screens,
and to my disappointment, noticed that as well as cloth,
there was a backing support screen of plastic (molded)
with tiny round holes through it.
It looks as though this backing screen blocks about half the output of the mid and tweeters.
I decided to cut out the backing screen, leaving only the cloth, for the tweeter and mid sections.

 

[nazaroo]

Here's some pics of the operation:

The grill, with the backing plastic grating seen from back:

Turns out that the cloth is glued to the grating.
But the grating is so flimsy it can be easily cracked and broken out,
and peeled off from the back inside, leaving the front cloth and grill mostly intact.

You have to take care to leave enough of the backing grate behind around the edges,
so that the cloth is still supported and retains its basic shape stretched across the gap:

From here, I had to re-glue the remaining framework holding the cloth,
back to the hardshell frame.
There was some problem with the superglue leaking through the cloth,
but other more experienced DIYers might do better on this than I did.
In fact, in subdued evening lighting, the grill doesn't look much different/worse,
than it did before.

But my goal was achieved - the removal of the blocking support grillwork,
leaving the light, cloth cover, so that the tweeters/woofers could project sound cleanly,
unhindered by a diffusing grill or reflecting surface.


I will give the speakers another listen at moderate/high volumes,
and decide whether to keep them as is, (and do the crossover),
or save the tweeters for the Kenwoods, and junk these babies.
If I do the crossover, I may still replace the woofers with something better.
I do like the cabinets with their non-rectangular shape.
The bottom 2" stands are plastic, and not exciting.

 

[nazaroo]

Well, I thought it through again,
and decided that the best thing about these was the mid and tweeter,
and the only way to increase bass response was to find big bad 8" woofers, and abandon the ones I re-surrounded.
Maybe those will work in a larger case, so I won't throw them away.

But I've decided to cannibalize these for the tweeter and midrange,
and I've inserted them into the Kenwoods (see other thread).

The box here is a good design, but simply too small,
and aren't giving bass any better than a paradigm or mission 717 or probably any good near-fields.

It would be nice to copy it in a larger format, but thats a faraway project.

For now I'm sticking the Kenwood tweeter/midrange in this box,
and sticking the higher power/better tweeter/mid in the Kenwoods.

This is a much better match than what I was doing over there.
It has improved the sound of the Kenwoods immensely already, so for $30 I got 2 good tweets and 2 good mids.
I think I'm ahead.

I don't know what to do about a crossover for this cab,
I have just left the two caps in for now.
With such cheap speakers it will have to be a low priority.


Below: Hitachi versus Kenwood Midranges. One magnet is is triple-size,
and the semi-closed back is also a much larger volume.