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Suggestions For Cap Size With Monsoon Tweeter w/ Fulton FMI80

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Posted on March 7, 2017 at 03:25:39
tubav
Audiophile

Posts: 325
Joined: March 21, 2009
I picked up a pair of Fulton FMI80 on Craigslist. Sadly the tweeters were not working. I had a pair of Monsoon ribbon tweeters laying around I'd like to try since finding 4 Peerless tweeters won't happen. The Fulton crossover is a 6uf cap on the tweeters with the woofer running full range. Would that be a good place to start with a series resistor to suit my taste? Thanks.

 

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RE: Suggestions For Cap Size With Monsoon Tweeter w/ Fulton FMI80, posted on March 7, 2017 at 04:01:35
Salectric
Audiophile

Posts: 998
Location: East Coast
Joined: February 23, 2003
Contributor
  Since:
November 13, 2015
I don't know anything about your ribbon tweeters but I have had a pair of FMI-80s since the 1970s so I do know something about them. The "crossover" is as you described---just a 6uf cap in series with the Peerless tweeters. The 8" woofer runs full range. The woofer's natural rolloff in the highs allows it to blend nicely with the tweeters. This may seem like a haphazard or economy crossover, but it is actually one of the keys to the 80's high quality sound especially its natural dynamics. At one point I disconnected the Peerless tweeters and just used my RTR ESR-6 electrostatic tweeters with a 1.5k crossover. I figured since the RTRs were rated down to the same frequency used in the 80's crossover, this should give me the benefit of the electrostats down to a lower frequency and be an overall improvement. Wrong! The sound had an obvious hole or suckout in the crossover region and what was left was an incoherent mess. That taught me that the Peerless tweeters were important for a smooth transition from the paper cone woofer to the super-fast electrostats. I suspect you will find something similar with your ribbons. However no harm in trying provided your crossover cap provides enough protection for the delicate ribbons. The proper value of the cap will depend on the impedance characteristics of the ribbon in the crossover region. You will probably also find that the ribbons are not the same sensitivity as the woofer so the ribbons may be too loud or not not loud enough. If I were you I would just keep an eye out for some Peerless cone tweeters. I suspect they come up on EBay pretty regularly although I have never checked.

 

It's possible you could have corrected the suckout in the crossover region..., posted on March 7, 2017 at 07:50:38
Mr. Dick Hertz
Audiophile

Posts: 19261
Location: So. Calif.
Joined: May 25, 2006
...by simply reversing the polarity of the tweeters.




When you can't make them see the light, make them feel the heat.- Ronald Reagan

 

RE: It's possible you could have corrected the suckout in the crossover region..., posted on March 7, 2017 at 09:19:30
Salectric
Audiophile

Posts: 998
Location: East Coast
Joined: February 23, 2003
Contributor
  Since:
November 13, 2015
I tried reversing the polarity. As I recall there wasn't much difference. The 6 electrostatic panels were angled for better dispersion and they were also open on the back. Perhaps those made the phasing less critical.

 

RE: Suggestions For Cap Size With Monsoon Tweeter w/ Fulton FMI80, posted on March 7, 2017 at 17:02:11
Duster
Audiophile

Posts: 12355
Location: Pacific Northwest
Joined: August 25, 2002
It may depend on the fs resonant frequency of the ribbon tweeter. 6uf is a rather large value 1st order capacitor if it is an 8 ohm ribbon tweeter.

 

On My Digi-Meter, posted on March 7, 2017 at 19:01:11
tubav
Audiophile

Posts: 325
Joined: March 21, 2009
It says the Monsoon tweeter is about 2.25 ohms. I won't hit it with a lot of power. I use Heathkit W3M tube monoblocks, about 18 wpc or Eico ST-40 which is about the same. What would you suggest? Thanks.

 

RE: On My Digi-Meter, posted on March 7, 2017 at 19:21:02
Duster
Audiophile

Posts: 12355
Location: Pacific Northwest
Joined: August 25, 2002
That reading seems pretty low. Are you sure you used the right setting? Take a look at this web page.

See link:

 

RE: On My Digi-Meter, posted on March 7, 2017 at 21:25:31
Tidycat1
Audiophile

Posts: 302
Location: Brecksville
Joined: June 18, 2013
You might need an L-pad to adjust the tweeter level.I suggest over estimating the wattage on the L-pad.Maybe 50 watt or above.
8mfd to l-pad,then to tweeter.
Tom:cat

 

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