Hi,
I have had some Forex FE207E drivers sitting on the shelf for 4 years
What is the current best proven design for this, is it still dimensions on the frugal-horn page below?:
The Metronome
http://p10hifi.net/FH/images/metronome-207m-v1.gif
What is that vertical plate behind the speaker?
What is the best wood to use, and should I only felt the back?
Sorry if this has all been answered somewhere in the 100's of pages in this thread.
Thanks
Yep, the dimensions on the Metronome page are still good.
The vertical plate behind the driver is a brace, that connects the front and rear panels. It cradles the rear of the driver magnet and is also full of holes so that it makes a negligible impact on the cabinet volume, yet is still strong enough to brace the driver magnet and the front to rear cab walls.
The brace also must be offset from a longitudinal centre line, drawn down the inside face of the rear panel. Easiest way is to just align one edge of the brace with your centre line and then fix in place.
The best wood is 18mm birch ply, which can be a pig to veneer, but a little care and attention to proper gluing, when veneering usually overcomes any problems.
My own speakers are lined with wool felt, but only on the inside rear face. The top third of the cabinet can be lightly (and I MEAN lightly) stuffed with Dacron fibre; overdo it and you will suck all the life out of the music - guaranteed!
Position the speakers next to walls, as that is where they are designed to be placed. If you bring them out into the room, they need a BSC circuit to compensate. Details of starting points for baffle step compensation circuits can be found in the Metronome section of the Frugal Horns website.
The Ella amp should sound great with Mets.
Hope that helps.
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Thanks Steve & Dave,
Luckily in this country, I can get hold of some Marine grade Birch Ply ... at a price.
Just to clarify, regarding the BSC. I only need it if the speaker is not against the wall? ... does that include the Zobel Network part of the circuit? I take it that is still needed with the 207's ..
What impedance should I be driving it with 4..8 ohms?
Does anyone have dimensions of the Arch vents at the bottom ... I could just measure them to scale from the drawing I guess ..
Thanks again ... great thread ...
Luckily in this country, I can get hold of some Marine grade Birch Ply ... at a price.
Just to clarify, regarding the BSC. I only need it if the speaker is not against the wall? ... does that include the Zobel Network part of the circuit? I take it that is still needed with the 207's ..
What impedance should I be driving it with 4..8 ohms?
Does anyone have dimensions of the Arch vents at the bottom ... I could just measure them to scale from the drawing I guess ..
Thanks again ... great thread ...
The Fostex is an 8Ohm driver.
The BSC will not be needed if the speaker is placed next to a wall.
If you have them out in the room, then BSC is needed. Jim Shearer has built quite a few Mets and as far as I know he has not used the Zobel network with his BSC experiments. You could PM him for his opinion.
If it helps, my own Mets use Fostex FF225WK drivers with a helper ribbon tweeter. They were built into the cabinet designed for the FE207E, the only change was the port length. They do not use BSC and sound great next to my wall.
Try them without the BSC and if you find the speakers too in-your-face then try out the BSC from the starting points given in the tables.
The arch vents are the diameter of the port tube at their highest point ie 3"
The BSC will not be needed if the speaker is placed next to a wall.
If you have them out in the room, then BSC is needed. Jim Shearer has built quite a few Mets and as far as I know he has not used the Zobel network with his BSC experiments. You could PM him for his opinion.
If it helps, my own Mets use Fostex FF225WK drivers with a helper ribbon tweeter. They were built into the cabinet designed for the FE207E, the only change was the port length. They do not use BSC and sound great next to my wall.
Try them without the BSC and if you find the speakers too in-your-face then try out the BSC from the starting points given in the tables.
The arch vents are the diameter of the port tube at their highest point ie 3"
Yep, I realise that. But with an 8 Ohm resistor and 6uF cap Zobel across it, won't it's impedance be more like 4 Ohm? .... (at most frequencies ....)
But like you say, I will try it without the Zobel first ..
Thanks.
No.
The Zobel is there to try and keep the impedance down to 8 ohms in the HF region where it rises.
Attached is an impedance measure for the FE207 (red) vrs the FE206 (blue).
The copper cap on the FE206 flattens the HF impedance, on the FE207 if you need flat impedance you need a zobel. The zobel is there more for the amp than the speaker.
dave
Attachments
Yep the brace touches the rear of the driver magnet. What I did was leave a 1mm or so gap between the back of the driver magnet and the brace. I then put a strip of blu-tak on the brace and offered up the driver. Tightening the driver bolts evenly causes the driver to bed down into the blu-tak, giving good coupling of the driver magnet to the brace.
I suspect that plywood will also dampen some frequencies so whether the magnet is hard up against the brace or has an intervening layer of blu-tak ought not to make a great deal of difference to the final outcome.
My reasons for leaving the slight gap, are purely practical. I would rather have some margin at the interface of the magnet and brace, than risk deformation of the driver frame.
Sure, you could keep trimming until the driver sat flush with the front of the cab, whilst touching the brace at the rear, but wood expands and contracts with changes in humidity, so you can never be sure that the magnet will touch the brace at all times. Neither can you be sure that the brace isn't putting undue pressure on the driver frame. The blu-tak interface solves this potential problem.
I've used my Fostex FF225WK Mets for the past five years with zero issues. They are fast, clean and neutral across the frequency range and sound wonderful.
My reasons for leaving the slight gap, are purely practical. I would rather have some margin at the interface of the magnet and brace, than risk deformation of the driver frame.
Sure, you could keep trimming until the driver sat flush with the front of the cab, whilst touching the brace at the rear, but wood expands and contracts with changes in humidity, so you can never be sure that the magnet will touch the brace at all times. Neither can you be sure that the brace isn't putting undue pressure on the driver frame. The blu-tak interface solves this potential problem.
I've used my Fostex FF225WK Mets for the past five years with zero issues. They are fast, clean and neutral across the frequency range and sound wonderful.
Steve,
+1 on the FF225WK ! I wish more people could get their ears on this driver. If they could, I think there would be many more builds using it. In the TLs I built, it has usable extension down to the mid 30's and a nice smooth response up to about 8KHz. It does need a super tweeter to supplement on top; I use an Eminence APT-80 located behind the cabinets (XOed w/ just a 1 micro-farad cap) & firing upward toward the ceiling.
+1 on the FF225WK ! I wish more people could get their ears on this driver. If they could, I think there would be many more builds using it. In the TLs I built, it has usable extension down to the mid 30's and a nice smooth response up to about 8KHz. It does need a super tweeter to supplement on top; I use an Eminence APT-80 located behind the cabinets (XOed w/ just a 1 micro-farad cap) & firing upward toward the ceiling.
Hi all, I am planning to start on a metronome build this weekend but thought I should run my driver choice by the experts first.
I bought a pair of Tang Band w4-1052sdf's to put into a Cyburgs stick but didn't like the sound from the test box I built, so onto other designs. Will this driver work in the Metronome that AmadeusMozart designed for the ff105wk in post 1000? The specs for the two drivers are almost identical, and I understand that mltl's are quite flexible and forgiving.
I have the wood,the plans and the saw already to go,the drivers sound great in another folded tl I built a while back, I would just like to know that I,m not going to be making a mistake with this setup.
Thanks
Graham
I bought a pair of Tang Band w4-1052sdf's to put into a Cyburgs stick but didn't like the sound from the test box I built, so onto other designs. Will this driver work in the Metronome that AmadeusMozart designed for the ff105wk in post 1000? The specs for the two drivers are almost identical, and I understand that mltl's are quite flexible and forgiving.
I have the wood,the plans and the saw already to go,the drivers sound great in another folded tl I built a while back, I would just like to know that I,m not going to be making a mistake with this setup.
Thanks
Graham
Thanks GM.
So the formula gives a result of 333 for the TB speaker (fs 70, qts .42) and 365 for the FF105 speaker(fs 75, qts .41) seems close to me but I don't know squat about these numbers. How close should it be before a redesign is recommended? The vas of the speakers is 5.2 and 4.8L respectively, does this play into these cabinets at all?
Graham
So the formula gives a result of 333 for the TB speaker (fs 70, qts .42) and 365 for the FF105 speaker(fs 75, qts .41) seems close to me but I don't know squat about these numbers. How close should it be before a redesign is recommended? The vas of the speakers is 5.2 and 4.8L respectively, does this play into these cabinets at all?
Graham
You're welcome!
Oops, forgot to say that factory specs tolerance limit has historically been +/-10% and while I personally would have preferred the TB having the higher of the two corner frequencies to allow more measured specs variance, it meets the <10%, so good enough for me in a 'swapping' drivers scenario with an existing cab.
Design wise, on paper [published specs], the TB has a lower Fs, so theoretically should be taller and with a higher Vas to boot, proportionally taller and since Qts dominates, it would theoretically need to be a bit 'fatter' too. In short, whatever the Fb, net Vb ratio differences are for them in a T/S max flat vented alignment, so too there should theoretically be at least this much difference in Fb, net Vb in a MLTL, etc..
GM
Oops, forgot to say that factory specs tolerance limit has historically been +/-10% and while I personally would have preferred the TB having the higher of the two corner frequencies to allow more measured specs variance, it meets the <10%, so good enough for me in a 'swapping' drivers scenario with an existing cab.
Design wise, on paper [published specs], the TB has a lower Fs, so theoretically should be taller and with a higher Vas to boot, proportionally taller and since Qts dominates, it would theoretically need to be a bit 'fatter' too. In short, whatever the Fb, net Vb ratio differences are for them in a T/S max flat vented alignment, so too there should theoretically be at least this much difference in Fb, net Vb in a MLTL, etc..
GM
Great, I am going to go ahead with this build. The 10% variances will probably be able to be handled with changing the stuffing and lining densities. I think I would rather build a cabinet that I know works with another driver and have to buy those later, than re-invent it for this driver that may work.
Thank you again for your help.
Graham
Thank you again for your help.
Graham