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Wednesday, June 13, 2012

Kenwood JL-545(B) Rebuild (2)

Posted by chris661  "The extra magnet is a bucking magnet, ...It's attached with opposite polarity to the main magnet, so that the fields cancel. The result is that you can put it near a CRT television without interference."

 I have low expectations for all Hi-Fi speakers,
having repeatedly tried them for musical instrument use with disastrous results.
It will be hard to shake off the 40 year impression that all HiFi speakers are shiite,
and never good for much more than 10 watts.

actually, the circuit-board seen lying on top of the box
in the first picture is unrelated to this project.
Its a board for a tube power-amp built in a rack-mount format.

The real (old) crossover discussed in this thread is below:

The above 'xover' got replaced with this:

On the upper left is the 3.3 uF cap for tweeter, bypassed with a poly.
This is my crossover so far.
Its a 3rd order two-way crossing over at about 1200 Hz,
with an added 3.3 uF cap for the tweeter.

Okay here are some pics showing the mini "box within a box" inner cabinet for the midrange:

It was actually very easy to make the midrange box:
I simply got these little knik knack boxes second-hand from a thrift-store.
They are meant to hang on the wall in some quaint kitchen and store ceramic dolls or some crap.

I commandeered them and glued/screwed a 3/8" plywood back to them, and then glued and propped them in the cab.

The box was lined with foam to cut reflections.

The whole reason for this extra effort was because I was replacing the midrange driver.

The new driver is pictured below.
My son is holding for camera,
hence the discrepancy between left and right hands in photos...

As reflecting my previously documented attitude toward 'stock' HiFi speakers,
this is at least rated at "200 watts" (system total), and comes with an SPL response curve.
The original looked like the kind of speaker you find in a cheap transistor radio from the 60s.

The only problem is it is an open-back midrange,
and hence the added mini-box cab within a cab.

You'll notice that this midrange claims to go down to 150 Hz!
Actually, they recommend rolling off around 800 Hz,
so I then selected 1000-1200 Hz as the real rolloff,
assuming the bass speaker could handle everything below 1000 Hz reasonably well.
I may regret this choice of xover point.

At the top end, I intend eventually to replace the crap tweeter with a good one,
but for now I just subbed a 3.3 uF poly-cap to reconnect the tweeter.
The mid has been left in reverse-phase, as the original was,
(for testing against the original tweeter/xover),
but since 3rd orders are only 90 degree phase-shift,
I am assuming the system will work equally well with mids in or out of phase with the woofer.

When I get a new tweeter pair,
I will probably want to xover mid/tweet at around 2000 Hz - 3000 Hz,
because I don't like the look of the peak/dip on the graph.
I hope I can find a tweeter than can behave down to 2000 Hz, so I don't have to worry.

The Midrange will end up only doing between 1000-2000 or 1000-3000 Hz.

At that point, I may want to lower the xover between woof/mid to something sensible like 200-800 Hz, and see how this midrange driver holds up.
That might free the woofer to handle low bass better.

Just some Ideas so far.

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