Fostex FF125WK First Build

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For my first speaker build I've decided on a single speaker system using the FF125_wk speaker and suggested cabinet design by Fostex, which can be found here.

The speaker enclosures are simple 9 litre cabinets ported and tuned to 57Hz. However I'm a little unsure on a few points and would like some clarification.

According to BassBox Pro the F3 is at 58Hz (for this cabinet design) as shown in the graph below. Would it be a good idea to add a low frequency High Pass Filter at just below this frequency, say at 55Hz to protect the speaker, because the amp produces frequencies from 20Hz? Or should I be introducing the cut off at a higher frequency because the the cut of is not sudden and absolute.

Also, do I really need to add an impedance EQ network to flatten the 2 peaks caused by the vented box at such low frequencies? Or am I totally misunderstanding all of this?

Basically, I want to keep the speaker as component free as possible as not to distort the signal from my Audiolab 8000a amplifier, but at the same time I don't want to totally mess this build up.

Your thoughts and suggestion are most welcome.


J Smith.


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Most music has very little content below 40 Hz unless it is HT soundtrack from movies. Most folks using this as a full range run it straight without high pass filter. Those peaks are impedance peaks not freq response - you don't hear it but your amp feels a change in resistance of the load through those frequencies. The only filter you may need is a high freq attenuator (baffle step compensation or bsc) of about 6 dB from 500 Hz and above if your speaker is in a narrow box. If flat wall mounted or in a back loaded horn with lots of bass gain, no need for BSC. The BSC can be done at line level before the amp or at the driver with a resistor and inductor in parallel. Or it can be done be EQ'ing. Good luck with your build.
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9L/57Hz looks good to me. I was considering this driver for a time and 10L/55Hz was what I kept coming to.

As for implementing BSC, I think passive line-level is you best bet, adjustable to boot:

Baffle Step Compensation

Nothing wrong with speaker-level though:

Loudspeaker Diffraction Loss and Baffle Step Compensation Circuits

Some drivers may require a Zobel to have other types of network function properly, but I'm pretty sure a low-inductance fullrange driver voice-coil should not. Here is a link to that for reference in any case:

Speaker Zobel / Impedance Equalization Network Circuit Calculator

my only observations would be

Fostex drivers are notorious / beloved for their long break-in periods (up to several hundred hours depending on program content and playing levels), so you could expect extension and dynamics of lowest 2-3 octaves to improves substantially, as well as overall open-ness,

With the exception of some of the BLH for much larger drivers ( i.e. FE206, 208ESr, etc), you could probably do much better than almost any of the "recommended" enclosures that generally accompany Fostex drivers. Indeed, there are countless dozens to be found by "amateur/ DIYers" herein, as well as at commercial sites.

I've heard the FF125Wk in several designs from small stand mounters to a Pensil-like (floorlength) MLTL, and they work very well indeed.

As xrk971 notes, enclosure design and placement can go a long way to mitigate the need for BSC, and the impedance peaks are generally far more benign than they appear in print or pixels on your screen. Also note that measured or modeled FR & impedance plots are with very simple frequency sweeps - far less chaotic and dynamic than almost any musical program material.

Frankly, I'd give them a couple of weeks of break-in before dialing in any EQ / BSC , and note that your thread title includes "first build" - I'd predict more to follow, during which you might want to explore the capabilities of this driver in other designs.;)
Thanks for the advice lads, it always reassuring to hear from such experienced people.

I was somewhat concerned about the low frequencies and those 2 impedance peaks but you've put my mind to rest. The BSC and Zobel references made for interesting reading.

So the plan is this... Bang the 125's in to the 9L cabinets that we cut over the weekend, hook them up to the good old trusted Audiolab 8000a amp and Bobs your uncle!

Well actually, with a little bit more care then that. I'm going to take your advice there Chrisb and run them in without any EQ/BSC for a week or 2, and while I'm doing that I'm going to make up a few circuits on breadboards and see how they sound with them.

And yes, even before we started cutting the cabinets I was in Sketchup, These 125's will see much wood over the coming years, lucky them :)

Thanks again.

J Smith.
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