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How to tame a cheap plastic Foster compression horn tweeter! (long)

So, last summer I got an old pair of Zenith Allegro 10" 2-way speakers at a yard sale for 5 bucks. At the time, I only wanted the cabinets and woofers to go under a pair of small sealed mini-monitors I built awhile back that have an f6 of 132Hz. I lowpassed the Allegro woofers with 18mH series, 240uF shunt, and a Zobel of 8 Ohms / 36uF, that yielded -6dB at 135 Hz and resulted in a pair of decent 3-ways.

I had of course removed the Foster compression horns from the Allegros, and they sat around in the parts drawer until a few weeks ago I started a "cheapskate" build with some semi-decent Goldwood 7" woofers. I decided to use the Fosters in this build, crossed in at 1900 Hz 1st order (I know, I know, too shallow, too low,) because that was where I could lowpass the Goldwoods with parts on hand. I figured the Fosters would be "hot" (like around 100dB/W/M sensitivity), so I installed rotary L-pads on the cabinet backs to be able to fine-tune the output level. I wound up matching the (measured) level of the woofers at -16dB attenuation.

Even padded down, these tweeters sounded hideous! To compare them to a buzz saw would be an insult to buzz saws. Think shrill, strident, metallic harshness with an edge on violins and soprano vocals like jagged, rusty, tetanus-infected razor blades. Severe listening fatigue set in after about 30 seconds. At best, it sounded like a miniature version of a bad PA system in a gymnasium.

At this point I decided to order some real tweeters from PE and re-do the highpass filter, blowing the budget but making ultimately for listenable speakers. The new Vifa dome tweets will be here in about a week.

Meanwhile, could the Fosters be rendered semi-listenable?

I figured a steeper crossover at the same frequency would reduce distortion at the bottom of their range. With on-hand caps and coils, I went 2nd order electrical. The improvement was infinitesimal.

The next thing to address was possible resonances in the horns. Wrapping the outside of the horns tightly in two layers of rope caulk deadened them nicely, and made for a small but audible improvement. I could now listen for up to 60 seconds before ear bleeding commenced.

Final step was to attenuate the upper-range nasties acoustically. After experimenting with various grille materials, I wound up with some 1/8" thick medium density felt taped over the horn mouths, augmented by a small cotton ball impaled on the "bullet" phase plug of each tweeter. This of course muffled the output over the entire passband of the tweeters. Reducing the L-pad attenuation to "only" -10dB got them back to the same level as the woofers, with a measured system response slowly shelving down to 12kHz, above which they roll off precipitously.

But that was OK. While I'm sacrificing a bit of upper-end air and sparkle (chimes and cymbals still have a nice "ting" but it decays early), gone too is the edgy nerve-grating harshness and distortion. These things now sound almost like music! Last night I made it through a whole CD of Mozart violin concerti, and a couple of discs of lady folk singers (Kate Wolf, Connie Dover), without once wincing or running for the volume control! I can now almost enjoy these speakers until the new dome tweeters arrive.

So, if you ever come across a good deal on "vintage" speakers with some nasty compression horn tweets, don't despair. They CAN be made to sound at least tolerable.






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Topic - How to tame a cheap plastic Foster compression horn tweeter! (long) - Brian H P 11:33:57 05/30/14 (2)

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