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Ignorant speaker cap questions.

Posted by DavidLD (A) on December 28, 2003 at 07:52:53

This question was stimulated by Airtime sending me the drivers from a pair of Lafayette L-2 speakers, a 39.95(!!!) 3-way (!!!!) design dating from 1976, featuring an 8-inch woofer, cone mid-range, and cone tweeter.

I got to thinking about speaker building today with the emphasis on 6 1/2 inch woofer (or smaller) 2-way designs and how the catalog stores were able in the 70s to build 3-way designs and sell them at very modest prices. Ask a DIYer today about building a 3-way and you get a long story about how incredibly difficult this is to do and no one but an utter fool would try to develop one from scratch. But in the 70s, inexpensive and in many cases quite nice sounding three-ways were everywhere. The notes I have in the Lafayette catalog on the L-2 indicate that it had a "capacitor crossover" at 6000 and 9000 Hz. The 6000 Hz figure seems incredibly high for an 8-inch woofer design, and what they call the super-tweeter basically covers only an octave. Its a very tiny cone in a larger sealed metal frame with a quite big magnet on it. The "mid", also sealed, has a much tinier magnet but a slightly larger cone. I can't believe that in a $39 speaker they woulda used more than a couple of caps which would have made this a first order design with no real band-pass as such on the "mid".

The other thing I'm not sure about is the type of capacitor. I gather the common capacitor used in these kinds of designs were inexpensive Non-polarized capacitors that for reasonable values cost between $0.30 and $1 in the PE catalog. Was this what was normally used at this point in time. Again, my friends in would cringe at such parts, but the interesting thing is in the 70s they seemed to work pretty well. Can I substitute the inexpensive Mylar caps I picked up surplus at Madisound for these?

For a woofer that operated up to 6000 Hz, the inductor, if present at all, would be very tiny, maybe 0.2 mH, and cheap. The woofers are actually quite nice, they have good-sized magnets and they have what appear to be textile surrounds, but are different from the other cloth surrounds I have seen in that they arent crimped and look like an inverted foam surround.

My current thought is to build a single sealed partitioned box double the size of the original boxes for the woofers and aim the woofers in opposite directions at either end of the box. The original boxes were sealed. This is going to be my faux "acoustimass" module. Then build the mid and tweet into small satellite boxes. MY hunch is that these mid and tweet drivers are at least the equal of what Bose uses in their satellites. If there is some directionality with the bass, so what--thats true of the Bose as well....LOL

David

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