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Wednesday, June 13, 2012

Kenwood JL-545(B) Rebuild

I was at the cash/pawn shop, and listened to a pair of speakers,
A/B the JL-545B against some more recent but maybe also cheap 'digital-ready' pair of speakers (rather large, more than mere bookshelvers, but not quite full blown floor models).

Although I could hear that the 'digital ready' pair were cleaner and flatter, they simply didn't have the lively bass punch of the Kenwoods.

Also, the Kenwoods had a better, more real sounding midrange, with plenty of upper mid realism.

On the down side, the other speakers had a much clearer high end, while the Kenwoods sounded smeared or blurred, maybe a bit crunchy too. But the low SPL of the Kenwood tweeters masked most of this well, and made the Kenwoods sound musical.

Overall, the Kenwoods won out, with nice deep bass, almost boomy, and a live, warm upper mid in the vocal range. The other speakers sounded 'nasal' and so flat they were compressed horribly. Yet going the other way, the Kenwoods were clearly an effort, almost noticably fatiguing to listen to, while the other 'digital ready' speakers were effortless to listen to.

Against my better judgment (but in ine with my sense of another fun learning project), I walked off with the Kenwoods. I was convinced their larger boxes would make a better starting-point for a large livingroom floor-set.

The JL-545 (B=black version) are a large rectangular, somewhat flattened but tall cabinet, with a 3-way paper-cone speaker system.

A surprisingly generous 10" woofer, a light back-sealed mid, and paper tweeter,
are driven by a grossly simple "crossover",
consisting of two electrolytic caps, a 4.7 for the mid, and a 3.3 for the tweet.

The specs given were:
70 watts (music power?)
40 - 20kHz,
crossovers: 2000 Hz (woof to mid), 5000 Hz (mid to tweet)

Also disappointing, was the emptiness and flimsiness of the cabinet,
lacking any reinforcement and/or any matting or stuffing.

There is a small round port on the upper back panel, which contributes to the strong bass.

In fact, the whole secret of the rich bottom end seems to be the large cabinet-size and port,
which is well-proportioned to the 10" woofer.

Once I got over the shocking cheapness of manufacture,
I quickly warmed up to the project.
There were also some 'good' surprises here:

(1) The woofer had an extra ceramic ring glued onto the back, which effectively increased the size of the magnet. It seemed obvious that they had taken a basically cheap woofer (10 watt?) and strengthened the flux and power (SPL?) by this trick. Of course the coil would stay the same current rating, but no doubt this was a cost-effective way to make for a tighter more efficient bass response.

(claimed Sensitivity: (SPL?) : 91 db / W at 1 m

(2) It would be easy to drastically improve the flatness of this cabinet by judicious reinforcement/damping, and stuffing. I was ready to rock!

(1) First I added a cross-brace offset about halfway down across the back of the cab, effectively cutting the sides and back in half, vibration-wise.

(2) Next I glued in strips of triangular cross-section 2x1" up and down the sides, top and bottom, and upper back.

(3) Finally, I added a closed wooden box around the midrange from behind, making a 'cabinet inside a cabinet' to isolate the midrange (More on this later). This was braced against the back by a glued-in 1x2" strut pressing it against the inside front. I also lined this mini-cab with foam.

After this, I also stuffed the box with pillow-stuffing/blanket-liner, to kill standing waves.

The result was astounding:
The walls and sides, when tapped, went from 'pook' (like wooden drums) to 'tink', like a very stiff thick box.
I was confident the response would be very flat now in the bass and mid.

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