Protection circuit for Fostex FE167E - diyAudio
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Old 10th January 2007, 12:30 AM   #1
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Default Protection circuit for Fostex FE167E

A friend of mine and I are going to be building a set of speakers using the Fostex FE167E driver in a bass reflex enclosure. I have a concern that since the Fostex has an Xmax of 0.6 mm that there could be a risk of overexcursion using this driver with his 100W/channel receiver. I am thinking of using a high pass filter in the speakers to cross them over to his subwoofer. However, I am unsure of a couple of things. First, is there an easy way to determine the excursion of the driver at a particular frequency/amplifier power given a certain size enclosure and tuning frequency? Second, I would like to use a first order RC high pass filter to protect from overexcursion, but I am unsure of what exact values of the resistor and capacitor should be. Obviously I can calculate the product of RC, but I am not sure if there is a certain size of resistor/capacitor that should be used.

Thanks for any help.

Cheers,
Geoff
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Old 10th January 2007, 03:30 PM   #2
MJK is offline MJK  United States
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Unless you are playing the 1812 Overture at very loud volumes, I would not worry too much about this issue. For normal listening, even at very loud volumes, I have not had a problem with my Fostex and Lowther systems running full range powered by a 200 watt/channel amp.

If you really are intent in providing some protection I would recommend a small closed box with the volume set to provide an acoustic crossover (12 dB/octave) at the lower frequency of your choice. Simple, saves parts, less expensive, and less likely to be screwed up and more likely to perform as designed.
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Old 10th January 2007, 04:44 PM   #3
chrisb is offline chrisb  Canada
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Quote:
Originally posted by MJK
Unless you are playing the 1812 Overture at very loud volumes, I would not worry too much about this issue. For normal listening, even at very loud volumes, I have not had a problem with my Fostex and Lowther systems running full range powered by a 200 watt/channel amp.

If you really are intent in providing some protection I would recommend a small closed box with the volume set to provide an acoustic crossover (12 dB/octave) at the lower frequency of your choice. Simple, saves parts, less expensive, and less likely to be screwed up and more likely to perform as designed.

DaveD (planet10) recently had a pair of quite small sealed boxes contructed for FE167s for exactly that purpose. They'll be serving as L&R mains for a complete surround HT system, with dual powered woofers up front (FE167 centre, and in-wall FE127 MLTLs for surrounds)

It's always nice to meet a loudspeaker's acoustic goals with mechanical rather than electronic methods.
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Old 11th January 2007, 05:24 PM   #4
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Thanks for the replies.

I guess I should note that these speakers are going to be dual use for music and home theatre. I've looked at a sealed enclosure, but I can't seem to get a f3 much below 125 Hz. I would like to get into the 60-80 Hz range with these drivers and that doesn't seem to be possible in a sealed enclosure.
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Old 17th January 2007, 02:25 PM   #5
navin is offline navin  India
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Quote:
Originally posted by chrisb



DaveD (planet10) recently had a pair of quite small sealed boxes contructed for FE167s for exactly that purpose.
It's always nice to meet a loudspeaker's acoustic goals with mechanical rather than electronic methods.
Is there a link for this?

How low does it go?
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Old 17th January 2007, 03:37 PM   #6
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Originally posted by navin
Is there a link for this?

How low does it go?
Drawing attached. They reach 106 Hz, They are XOed 1st order at that frequency (using an appropriate sized coupling cap in the amp) to give a 3rd order roll-off. Each is XOed to a push-push Extremis woofer 3rd order (active tubed XO using gyrators). Sats are driven with a Red Light District, woofers with a Bryston 4B.

dave
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Old 17th January 2007, 03:39 PM   #7
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a pic (along with the "matching" centre channel. Phase plugs are the same wood & finish as the speaker boxes.
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Old 17th January 2007, 03:54 PM   #8
navin is offline navin  India
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Quote:
Originally posted by planet10


Drawing attached. ...using an appropriate sized coupling cap in the amp) to give a 3rd order roll-off. Each is XOed to a push-push Extremis woofer 3rd order...dave

That system must sound sweet. Dave, that was to quote another canadian "just what the doctor ordered"! 100hz would just about make the grade.

Q1: any reason for the sloped rear baffle other than standing waves?

Q2; for the rear channels one idea could be to mount the driver on the sloped baffle and have the speaker "upside down" so that the speaker is mounted high on a wall and looking down. Do you see any merits/de-merits in this? After all this would only apply to the surrounds.

edit...

excuse me if these questions have been answered before in some FAQ?
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Old 17th January 2007, 06:42 PM   #9
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Originally posted by navin
Q1: any reason for the sloped rear baffle other than standing waves?
No. In such a small box, they are a real issue. You'll note that they are built into the centre and also into the 4.5 litre aperiodic CSS FR125S box posted elsewhere (althou in that the slant provides a support for the resistive damping.

dave
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