Fostex FX120 where did I go wrong?

[ODougbo]

This is the closest I came to a full range speaker project. I had 4 Fostex FX120 drivers (which are not inexpensive) that needed a home. The plan for the boxes came with the drivers; there’s a small ported box inside, which directs sound into a larger chamber (above speaker in photo).

No complaints about the bass, it’s pretty good, especially with all 4, more than you would expect. The midrange was too predominate, hence the mock up crossover and tweeter.

The question here, what was I doing wrong, even my rudimentary crossover & tweeter sounded much better than the speaker by themselves.

Where did I go wrong? The wrong application?

*For the 12db crossover, I found a site where you plug in the speaker values and hit enter; coils, caps and diagram is shown. I did try/experiment a few times.

[chrisb]

Originally Posted by ODougbo

This is the closest I came to a full range speaker project...

a few thoughts:

I wouldn't be a bit surprised if the enclosure plans accompanying the drivers did (can) not deliver balanced and extended low frequency performance, thereby "exaggerating" the mid-range, which many folks find a bit forward on all Fostex Full-rangers to start with. You might well be more sensitive than other folks to areas in which the wide metal dome "dust cap" could be at its worst.



Driver break-in - several hundred hours has come to be considered a basic requirement with full-rangers - Fostex in particular - before assessing tonal balance and dynamics.

Listening distance / room modes?

I have heard the FX120 in an enclosure quite different from the (assumed DBR) "factory" design, and while a single per enclosure of most 4.5" drivers can't be expected to lift heavy below 50Hz, they weren't particularly thin or misbalanced.



Currently priced at $125 ea, I wouldn't consider them inexpensive either, and you definitely want to find the right application.

[chris661]

BSC?
Will certainly help bring the mids in line. Though I'd also use a Zobel with that.

[ODougbo]

Yes very pricey. The cones are super light and they do seem to react faster then other speakers that I have.

The box is some kind of dual chamber design; the bass is enhanced in the larger chamber. It does work to a degree, all 4 together sounds like an 8” DJ speaker.

I though they may be for an infinite baffle speaker or designed for an information kiosk (and why I bought them in the first place).

I was curious if they would be more balanced a “huge” box?

Maybe a tube amp? (Which I don’t have)

[chazdrumzalot]

I have a suggestion. Why don't you turn one pair of drivers around to face the back, to simulate a bipole. This just might balance out the sound, but just be sure you have enough space behind the drivers for them to breathe. Secondly, having two full range drivers side by side like that is probably resulting in comb filtering at the high end, prompting the addition of the tweeter. If that's true then you might not need the tweeter at all in this configuration.

edit: you have the speakers against the wall right now, so I understand that bipole is probably not going to be a permanent solution for you. But it should approximate the balance of the sound if you incorporate a BSC filter, or roll off one driver to make a 1.5 way.

[ultrakaz]

If your listening space permits, I would stack the speakers; driver ends closest to each other. Then listen. There might be favorable cancellation that might tone down the treble. If not, then I would stick a 75mf cap in parallel with the bottom drivers and see how that sounds.

Otherwise, I would turn the speaker into a two-way.

[chrisb]

Originally Posted by ODougbo

[SIZE=3Yes very pricey. The cones are super light and they do seem to react faster then other speakers that I have.

The box is some kind of dual chamber design; the bass is enhanced in the larger chamber. It does work to a degree, all 4 together sounds like an 8” DJ speaker.

I though they may be for an infinite baffle speaker or designed for an information kiosk (and why I bought them in the first place).

I was curious if they would be more balanced a “huge” box?

I gotta resist a cheap answer to the question of relative size of enclosure / driver - suffice to say that aside from OB (in which these would be quite effective, provided XO'd high enough to mitigate restricted excursion capabilities), the enclosure's loading / tuning can get you closer to your desired results.

Maybe a[/FONT][/COLOR] tube amp? (Which I don’t have)

Quite possibly part of the equation. [/FONT][/SIZE]I should have included a couple of details in my prior post - the particular enclosure in which I've heard the FX120 and F120A are based on this one:




I have heard at least 10 different driver makes and models in variations on this design over the years - with appropriate tuning of internal volume and port dimensions, it's proved to be very successful.

Generally, I've found the Fostex FE series to mate very nicely with tube amps, ranging from 2A3 SET & EL84 SE in triode mode (obviously the "flea-watt" power levels restricts the attainable SPLs, particularly in larger venues), to EL34 & KT88 P/P pentode or UL. We all have our favorite combos, some of which remain constant over the years.

[ODougbo]

Very nice, thanks! It looks like the front baffle and blue sides – slide out of the outer box ( I guess they can be trimed for tuning.

Looks like somthing I'd like to build.

I thought there may be some type of notch filter for the middle range, just don’t know where to begin.

Originally Posted by chrisb

[FONT=Times New Roman][COLOR=black]

I gotta resist a cheap answer to the question of relative size of enclosure / driver - suffice to say that aside from OB (in which these would be quite effective, provided XO'd high enough to mitigate restricted excursion capabilities), the enclosure's loading / tuning can get you closer to your desired results.


Quite possibly part of the equation. I should have included a couple of details in my prior post - the particular enclosure in which I've heard the FX120 and F120A are based on this one:




I have heard at least 10 different driver makes and models in variations on this design over the years - with appropriate tuning of internal volume and port dimensions, it's proved to be very successful.

Generally, I've found the Fostex FE series to mate very nicely with tube amps, ranging from 2A3 SET & EL84 SE in triode mode (obviously the "flea-watt" power levels restricts the attainable SPLs, particularly in larger venues), to EL34 & KT88 P/P pentode or UL. We all have our favorite combos, some of which remain constant over the years.

[chrisb]

Originally Posted by ODougbo

Very nice, thanks! It looks like the front baffle and blue sides – slide out of the outer box ( I guess they can be trimed for tuning.)

nope, those colours are just to highlight the different layers of material and delineate the extent of slot ports - dimensions of slots / spacers are calculated for specific driver's T/S parameters - everything is very tightly glued up

Looks like something I'd like to build.

Well, as many builders of this design other than myself will attest, these are not a particularly simple build - definitely not as easy as any of the several Fostex factory designs I've completed.

They can sound quite wonderful though.

I thought there may be some type of notch filter for the middle range, just don’t know where to begin.

If you can run the front end of the system on a computer, it should be relatively easy to find a simple EQ plug-in - absent the wherewithal or interest to make detailed in-room measures, you could even tune by ear.

[ODougbo]

That box doesn’t look too completed, similar to a slot port, but there are two - on the sides.

The cab shown were super quick mock ups with scrap wood, only took 1 ½ - 2 hours (per pair). With some experimenting, I ended up using one thick pieces of square foam; with a large hole, right behind the speaker.

I built the one pair and there wasn’t much to like, with the second pair, at least I was getting some bass. I’m sure they are high performance woofers, but need something to soften the middle, or increse the low end.

Originally Posted by chrisb

nope, those colours are just to highlight the different layers of material and delineate the extent of slot ports - dimensions of slots / spacers are calculated for specific driver's T/S parameters - everything is very tightly glued up


Well, as many builders of this design other than myself will attest, these are not a particularly simple build - definitely not as easy as any of the several Fostex factory designs I've completed.

They can sound quite wonderful though.

If you can run the front end of the system on a computer, it should be relatively easy to find a simple EQ plug-in - absent the wherewithal or interest to make detailed in-room measures, you could even tune by ear.