Help needed for a Fostex FE 204 driver/BVR Double Mouth

Phats

Member
2007-06-26 7:17 pm
Hi all,

After many years of being 'loudspeaker-less' I was planning a new build in the near future.
My last pair of serious speakers were the Jericho/FE 208sigma and they performed well with a SET amp. I liked them very much in dynamics and micro resolution. Now I have a pair of FE 204's left in the cupboard and the urge to built me another pair of nice speakers. And years ago I tumbled upon Scottmoose's 'Bruce' double mouth BVR's and I am still in love with the looks of that design. And, big plus, even my wife thinks they are beautiful.
There are some more factors to consider, the dimensions are exactly fitting for our rather limited living space and, painted in high-gloss white we think they are not that much imposing.
But now problems do arise, from what I have understood in forum posts the Bruce was developed for the FE 207 with a Qts of 0,26 and speakers with a lower Qts did not perform that well in the Bruce.
So, my FE 204 has a Qts value of 0,2. Significantly smaller. A solution for too low a Qts was was given by Dave (of Planet10 fame) in reaction to a dissappointed builder in using a 4/5 ohm resistor in series, but that feels like a stopgap. So, a no-go for the 204 and ofcourse the Bruce has been discontinued. And I cannot find anything remotely as pretty and of acceptable dimensions as that Bruce.

What to do? My wife will never accept something as a Replikon horn or some other serious backloaded horn. Just too darn imposing and not even functional given our living room... And where should our dachshund live? In the hornmouth? Well, best not ask, I am sure a lot of folk would suggest just that!
Another solution is the purchase of a 207, however funds do not permit that and I like that 204. I have it and want to use it!

So. all in all, quite an introduction to a difficult question. Are there people in the know, (there are some, I have read a few posts of people who definitely know their stuff) who would be willing to dissect the functions and dimensions of this Bruce double mouth BVR and adapt it to the FE 204?
I really do hope Scottmoose will chime in! I am really sorry to say I am something of a nitwit concerning mathematics, my Mac probably can't run horn/bvr whatever designing software (I know I can't) but quite good with loudspeaker building... The Jericho's were a nice built as well!
I can post the FE 204's parameters and building plan of the Bruce when people are interested in this project!

With kind regards, Stef
 
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If you don't mind, I'd rather people did not post plans for my discontinued designs on the forum or anywhere else. It causes me no end of problems, and I like to keep tabs on what is going on, as far as possible.

Be that as it may. The design in question was intended for the FE207E, which is significantly different to the FE204. The latter was the predecessor to the FE206E. Lower Q driver, higher mass-corner frequency & designed to be used on the end of high output impedance [usually valve] amplification. This type of enclosure is not naturally suited to drivers with a high mass-corner, & is the reason why I did the long-path designs. On the end of a high output impedance amplifier ( preferably > 4ohm) it will come more into line. An alternative is to use some series R as was traditionally done. You could use a variable resistor; adjust until you get a balance you're happy with, and either leave in place, or measure & substitute fixed value resistors. Remember, traditionally drivers with very powerful motors / lots of damping were specifically intended to be used on the end of variable output impedance amplification, so they could be dialled in to suit individual requirements / preferences. Not many of those around any more. So there's no shame in using some series R -it's essentially the equivalent, the downside being you do burn up some power in the resistor as the overall efficiency is reduced.
 

Phats

Member
2007-06-26 7:17 pm
Hi Scott,

Big thanks for the reply as such... Of course I will refrain from posting your design if so wanted! And, hats off for your energies and efforts you put in those remarkable loudspeakerplans... I was really hoping that with some adjustments in chamber and throat dimensions the 204 could be put in good working order. In this regard I expose myself in not being able to understand the T/S parameters of LS units in perspective and correspondence of suitable cabinets.
I gather that in responding like you did it's not possible to adjust the cabinet in favor of the 204 and the only solution will be the series resistor.
Well, probably for the best the BVR is supposed to be run by a SET at 8 ohms. If I understand your explanation of high output impedance amplifiers correctly.... so a series R could be used.
But what to expect? A suck-out in midd-bass? Hollow bass response? That is what i remember from that disappointed builder of the old Bruce.
As a sidestep, in what enclosures could the 204 be used then?

Of course anyone is free to chime in, thanks again Scott and,

Kind regards, Stef.
 
Re the 204 in this enclosure, basically what you'd expect, viz. reduced gain throughout the LF end & lower midrange ( < 300Hz). A high output impedance amplifier or some series R effectively raises system Q, lowering the mass-corner & rebalancing the system, at the price of reduced system efficiency. About 4ohms should do the job quite nicely. Note that I'm not refering to the output taps when I'm talking about the amplifier output impedance, I'm talking about damping factor, or (preferably in this case) the lack thereof. 2.5ohms - 4ohms is fairly typical for most SET amps.

As far as the 204 in general is concerned, if it's to be run without LF support or any help from the amplifier / series R, then you need a cabinet that can provide broadband gain from its lower tuning up to it's acoustic low-pass Fh, which ideally should be around the mass corner frequency of the speaker based on system Q. Hence the long-path designs. An alternative might possibly be a Karlson, since these BP variations sacrifice some LF extension for gain (and a few other issues -not everybody likes them). Given that you like the Bruce enclosure, I wouldn't have much hesitation in getting a few ohms of series R on them, providing the loss of a few dB of headroom is acceptable.
 

Phats

Member
2007-06-26 7:17 pm
Thanks for the reply Scott, I think I could live well with a bit lowered efficiency. Given the smallish living room it would not be a problem. Hopefully the series R will balance the response enough. I'm thinking of ways to reasonably 'fix' most of the panels without going overboard in wall thickness, ie. clever bracing. And using a sensible variety of scraps of wood to calm the chamber. Even thinking of making those scraps of a softish material (used under carpets or wooden floors), I don't know what to call that in english. Thinking as well to route the cables in the panels and fix them with a damping material, such as (beeswax). Etc. Want to make it a fine build.... And I hope to enAble the Fostexes some day, intriguing concept.

So, thanks for your comments and,

Kind regards, Stef.
 

Phats

Member
2007-06-26 7:17 pm
Hi All,

In my last post I was boasting about 'smart bracing'. And I probably know what i am doing.... (for instance; making a brace in which the Fostexses magnet is locked in a brace, which as well fixates the sidepanels in the chamber) But! I was just wondering how it would affect the performance of the cabinet. I mean this, I would really like to secure the backpanel with a brace to the loudspeaker 'chamber', from the chamber to the top and to the bottom. In effect the 'horn/port' will be reduced in width. Will it suffice to leave the cabinet dimensions 'as is'? Or do I have to widen it? And, of course, Scott probably knows the answer....

Kind regards, Stef.
 
Bracing right to the back panel would reduce the size of the throat & introduce an unintended discontinuity into the expansion. While that's not likely to be heart-breaking, it's not ideal either. It should not be necessary with a reasonable build material (18mm BB ply or similar). Design is always about balancing some features off against others. In this case, I would say the balance is in favour of not running an internal base right to the back panel -I would keep it internal to the chamber. It will get plenty of support from the panels above / below.
 
I'm an advocate for such bracing, and you will see that in some of Scott's design they are drawn in. You do have to account for the volume of the braces and ensure that they do not affect the areas or volumes of the horn/BVR.

Bracing the driver against more of the enclosure material will help distribute its reactive energy more widely so that panel resonances are less likely to get excited.

Are we still talking about Bruce? Taking a brace outside of the air cavity chamber gets tricky. A holey brace inside that cavity that ends well before the throat could be volume compensated for with some small amount of stuffing.

dave