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Old 19th January 2012, 02:54 PM   #1
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Default Experiment with FF85k

This is a prototype to see if I can build a speaker with a sonic presentation similar to the RTR 280DR which I bought back in 1974 and enjoyed until we moved into a home which is too small for them. You see, the 280DR use six tweeters and four 10Ē woofers; on the front and each side are placed a woofer with tweeters above and below; the fourth woofer is downward firing, driving through slots around the base. If I had known anything about speaker design back in 1974, I might have had serious doubts about a TWT layout--it sounds like a really poor arrangement! However, it works extremely well, with response down to 20 Hz. Sounds great at a whisper or at ear-bleeding SPLs. When you think about it, with all those drivers, theyíre pretty much loafing unless you input enough power to shatter the windows in the room.

This may be a fools errand, but how about FAST configuration with multiple full range drivers? Dipping into my cesspool of audio knowledge and searching for othersí design features I can rip-off, I speculate that a modest bass unit (FAST-like covering 30 to perhaps 200 Hz) coupled with drivers on front and sides (like the old 280DR) might just give me what I want.

So here is the first test: a small, sealed box with four FF85k. I really only wanted drivers on the front and sides, BUT, I donít know how to make three drivers work together with even power distribution unless I employ a series resistor. And I donít want a driver on the back (like the four driver Micro-Tower), so there is a fourth driver on the top (a la the two driver Micro-Tower).

I happen to have four FF85k available: two in Micro-Fonkens and two in MJK passive OBs. The first picture shows the front/sides/top glue-up. The second shows the ďunfinishedĒ product: internal dimensions 7Ēx7Ēx6Ē. Filled with AcoustaStuff. Wired series/parallel for 8 ohms. Predicted F3 is 179 Hz.

I usually veneer the exterior and apply polyurethane inside and out. Since this is only a prototype, made from scraps, and built very quickly, I didnít bother with any finish (or pay attention to appearance). My sole concern is how it sounds.

I put an old sub with plate amp under it and adjusted as best I could to match the bass with the top end. Bottom line: sounds fine, as one would expect of the FF85k. Certainly can go louder than a single driver, but it just isnít the presentation Iím looking for. I suspect that itís the top driver that I donít like.

Guess the next step is to pull the top driver, add wood blocks to reduce the volume, and re-try two different ways: 1) put a resistor in place of the missing driver for balanced output from the 3 drivers; 2) just let the front driver run by itself in parallel with the side drivers in series--which should give half the acoustic power to the front and half split between the sides.

Not sure how soon I will get to that as I have other projects coming up, including building a Dynakit ST-35 tube amp.

Cheers, Jim
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File Type: jpg Exp FF85k-2.jpg (44.5 KB, 175 views)
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Old 19th January 2012, 05:23 PM   #2
von Ah is offline von Ah  United States
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Have some fun with this thread:
Stereophonic Sound from a Single Loudspeaker

Yours looks like a fun project!
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Old 19th January 2012, 06:35 PM   #3
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von,

Not sure what that thread has to do with my project, but it is interesting.

You understand that I only built one channel because I don't have 8 matched drivers available. And I wouldn't want to waste time building a pair anyway; not until I have some proof of concept.

Cheers, Jim
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Old 19th January 2012, 08:17 PM   #4
von Ah is offline von Ah  United States
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Your box just reminded me of that thread, and I thought you might be able to use something of what's been discussed there in terms of the relative outputs of all the identical drivers.
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