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Old 5th October 2007, 07:52 AM   #1
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Default Fostex 3"-5" FR or ribbon?

It's a few years since my last speaker build, and are thinking of something like:
Two 12" or 15" woofers crossed over to either 4 fostex 3"- 5" FR drivers or a large ribbon transducer. I would prefer to cross over around 400-500Hz, and in that case I think the Fostex' are the best bet since few ribbons go that far down?
But will the Fostexs be as detailed as a ribbon? Would it be better to cross over around 2kHz and use a ribbon?
Which Fostex 3"-5" would any-yous recommend?
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Old 5th October 2007, 09:58 AM   #2
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Default Re: Fostex 3"-5" FR or ribbon?

Quote:
Originally posted by SemperFi
But will the Fostexs be as detailed as a ribbon?
More to the point, the outputs from the multiple FR drivers will comb with each other, causing lobing and progressively severe HF attenuation from ~4.5KHz upward (with the 3in drivers -about 2.75KHz for 5in), so you'll need to EQ them flat or add a tweeter. Close in you might get away with it with Jordan units, but those a) cost a packet, and b) I'm still unconvinced. Ted Jordan is one of the greatest driver designers in the world, but even he can't beat the laws of physics.
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Old 5th October 2007, 10:21 AM   #3
kepa1 is offline kepa1  France
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True, my 4" Fostex never sounded that good since I added a Beyma horn tweeter (AST 05). They go higher and have a better definition than the full rangers.
With woofers that large you will have to use a dedicated midrange or a full range, preferably not a 3", and a ribbon in the highs.
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Old 5th October 2007, 07:00 PM   #4
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Thanks for replies. I am trying to make high impedance speakers for my OTLs and thought of using several units in series. But if I get weird cancellation effects then perhaps it wont work.
How do line arrays work then? Do they use EQ?
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Old 5th October 2007, 07:43 PM   #5
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Line arrays using large numbers of FR drivers don't work, bluntly. They need huge Eq to lift the HF, and that does nothing about the lobing issues. As an illustration of what I mean, have a look at the plots presented here: http://www.partsexpress.com/projects.../Kuze3201.html And that's using 2in drivers. When centre to centre spacing between drivers is greater than 1 wavelength of the highest frequency to be reproduced, you get this lobing effect. Fact of life, & there's nothing that can be done about it.

Most arrays are 2 way setups. With small dome tweeters, you can just about get away with it, but ribbons are usually favoured because they'll behave as a column. Problem with those is that it almost invariably involves needing to place an XO slap-bang in the middle of the telephone band, where our hearing is most sensitive. So the XO needs to be good. Very good.

The other alternative is a focused array, where the front is curved into a concave shape. That helps to an extent, though it's of more use in combatting the delay issues involved with column speakers where the output from the ends has to travel a greater distance to reach the listener than that in the middle.

Having said all that, Roger Russel uses 25 3in FR units in his array, then EQs out the HF loss (& the LF come to that, as they're sealed boxes, using high Fs drivers, so not much natural LF action without electronic correction), and some people think it's the best thing since the pre-sliced bread days. Ted Jordan advocates using 4 of his smaller drivers in a tiny array (the beaming probably helps). And Fostex suggest using two of their FF85Ks in one BLH (ditto, and they have a rising response, which probably helps compensate). So, as ever, YMMV. All I can say is that it wouldn't be my first port of call.
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Old 6th October 2007, 02:20 AM   #6
el`Ol is offline el`Ol  Germany
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Maybe not in your price range, but here is a large ribbon:
http://www.sonus.de/php/p_produkte.p...1&produkt_id=1
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Old 6th October 2007, 04:16 AM   #7
OzMikeH is offline OzMikeH  Australia
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I suggest looking for a single high efficiency high impedance fullrange driver for your OTL amplifier.
Something better than 98dB/W/m and 16 ohms should not be too expensive. One of the smaller Lowthers perhaps..
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Old 6th October 2007, 05:37 AM   #8
el`Ol is offline el`Ol  Germany
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These look nice for OTL.
http://cgi.ebay.com/Vintage-Pioneer-...QQcmdZViewItem
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Old 7th October 2007, 03:59 AM   #9
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Quote:
Originally posted by el`Ol
These look nice for OTL.
Very nice period

dave
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Old 7th October 2007, 04:06 PM   #10
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Thanks for good info.
Since the lobing effect occurs at high frequencies, is it safe to use several bass drivers and cross over somewhere in the low-mids? Say 4x12" bass drivers crossed over to a single Fostex FR around 400Hz.
That should present a relatively high impedance load for my OTLs at the low end, which is where most power is sucked out.
So then remains the question of which FR driver to select?
Hmmm, sitting here thinking of the expence of 4 bass drivers per side, and the size of the speaker that'll fit all those drivers, I realize a horn loaded FR may be best after all...
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