Prebuild tips for Metronome F167 Build

[AdamThorne]

So I built my Cyburgs Needles, and they're good. Good enough that one of my friends is going to buy 'em, and free me up for another project. Sweet!

The needles have been running off a modified sonic impact t-amp. It's been enough to rock the room, but not quite the whole house. So I'm thinking of something with a little more efficiency, a little more low-end authority, but not a whole lot more footprint. And I'm thinking shielded. My room is maybe 10ft by 20ft, with a dining room L maybe 13 x 16. The configuration of the house leads to a listening area across the short dimension of the room - built in 1909 the layout makes no consideration for an entertainment center. I listen to electronic music, latin jazz, rock, and various random other things.

So I've come to the Fostex 167, in a Metrinome cabinet. frugal-horn.com has an example box. (PVC pipe, or purpose built port?) Also baffle step and zobel are suggested. I figured I'd go with these. After trying to make mdf + paint look nice on the needles I'd thought I'd try the birch ply with the Metronomes. One of you metrinome guys posted finishing tips:

"On birch I like a lighter look than tung oil. The air will darken birch nicely after a year or so regardless. Tung oil is very durable and fine for darker woods like gunstocks etc., but birch goes blotchy a bit.
A better goo is polyurethane (oil based, Sherwin Williams or any oil based varnish gloss) mixed 60% poly - 40% mineral spirits (maybe stiffer weather permitting) and applied like a tung oil. Brush on and wipe off about 3 - 8 coats (24 hrs between coats min) sanding with 320 grit on a padded block. Then use a nice beeswax diluted into min. spirits (shave the beeswax with a chisel a few days ahead) add lilac or juniper oil -maybe some cheap perfume. Apply wax with #0000 steel wool and buff with cotton. And you have a faux French polish that will wear better than shellac. Has a stronger film against abrasion and moisture than tung. Smells good too. Basically this is Simon Watts and Jason La Trobe Bateman's finish, cabinetmakers to the Rothchilds, drawers and furniture etc. Some of the better Italian finishers inspect the cotton seed fields the varnish is squished from, Varnish-o-philes. TC"

I'm down with this, but I'm not sure how diluting beeswax works, or just what is meant by 'shave the beeswax with a chisel a few days ahead'.

I've also read about the fostex drivers being friendlier with low damping factor amps, and if your amp is higher df you can approximate this with some series resistance. I believe my little t-amp has been sorted into the 'low df' pile, but uh the box it came in doesn't say one way or the other...

Can someone out there sanity check this? I don't have any screwed up bad ideas in here, do I? Are there any building tips? Are the plan and circuit on p10's website still the current state-of-the-art? What are people doing for stuffing? The design (http://homepage.mac.com/tlinespeaker...me-207m-v1.gif) looks like it has a holey-brace in it? I guess I could bust out some trigonometry to figure out dimensions for it, but if someone else has them handy...

And I'm gonna use this project as an excuse to buy a router.... new tools!

So I'll probably order parts later this weekend. Thought I'd bounce this off y'all first though.

Adam

[Scottmoose]

Those finishing tips were written by our friend, the late, and much missed Terry Cain, who passed away early in December last year. TC was a fine-furniture maker turned speaker-designer. To give you an idea of what a woodworking God the man was, see his IM-BENs below in, IIRC, an acid-etched cherry finish. Feel inadequate yet? Yep, join the club. We all do when we see his work.

AFAIK, it's simply a case of using a chisel or similar to shave pieces from a block of pure-beeswax, and add mineral spirits to dissolve the wax. You can add more wax to make it thicker, or more spirits to thin the mix.

The T-amp is fairly low DF, though the FE167E, having a mid-q, isn't too worried either way; it'll work cheerfully enough with low and high DF amps. The plans on Frugal-horn are current. You can add the brace or do without it -it's optional. Adding it will brace the driver & cut down on cabinet noise, but the SPLs it generates aren't high, so it's not as critical as with a larger 8in unit.

Best
Scott

[AdamThorne]

thanks, Scott!

[vitalstates]

just stumbled on this thread.....have to add my two penneth of caution..

I may be wrong but I think Terry was talking about hardwood solid...In my experience of french polishing over the years...faux or otherwise, its good to exercise a bit of caution with laminates like BB ply. I have found that trad french polish sealer has changed over the years and the more spirit there is in it(as in modern stuff) the more chance of reaction with the first layer of glue....upshot being a very much darker finish than one would expect....similar effects will be realised with sealing wax.

fwiw my advice would be to test thoroughly on off-cuts or go with propriatary product that has been field tested with laminates. Certainly in the UK there are some really good finishes from the likes of liberon and 'house of harbru'.

[AdamThorne]

vitalstates - thx for the tip, I'll test the finish on a bit of leftover.

probably a good call to try figure out what I'm doing with the finish before screwing up the speakers anyway.

[Scottmoose]

Good tip[s] Ed -thanks.

IIRC, Terry was talking about ply, but (big but) being in the position he was, he had very high quality stuff, and also had the facilities to make up his own too.

[AdamThorne]

I thought I saw somewhere that a bit of stuffing goes into the top of the metrinomes? I can't find that bit now. Did I really see it?

I've got my drivers and hardware now. The fostex has a vented dustcap! I couldn't tell from the previous pictures I'd seen of it. Does this driver still love a phase plug, even with a vented dustcap?

Picked up a router as well. The number of times I've seen woodworking tips around here that involve using a flush-cut bit has driven me to buy one. That plus rabbeting joints and chamfering driver cut-outs. All I need now is some quality wood. From what I've read here I'll avoid the Home Despot birch ply. So I'll have to go looking for someone to sell me some worthwhile stuff. Who knows, maybe someone will have ply veneered with something else unusual or interesting that doesn't cost a meeellion dollars.

[Scottmoose]

You can either stuff the top, above the driver, and add a piece of felt or similar to the rear directly behind it, or line the box. Personally I think lining it all will be overkill. The top, rear wall & one sidewall should be fine, although it does depend on what material you use.

[Steve Cresswell]

Hi Adam

Scott is right on the nose with the question of stuffing. Use as little as possible.
The spec for mine could do with changing on the drawing to remove the instruction to line the
cabinet throughout.

This was necessary during the very early days of my development of the Metronome enclosure
to tame the peaks and dips in the bass response, as at that time it had an open bottom.

As soon as I applied Scott's suggested mass loading them to them with the 2 inch port. The damping killed the sound stone dead and brought forward the most dreadful harsh and tinny sound I had ever heard. Quite frankly they were bloody awful!

Removing most of the damping except for that on the cabinet back had the desired effect and brought back all the lovely midrange I had had before, but I had gotten rid of the peaky bass just as Scott had said the mass loading would achieve.

I would love to have a go with my FE108EZ Metronomes in corners as I think they would sound great, but it would involve too much swapping of furniture around. One day I might try it whilst the wife is out if I can get over the inertia.
Steve.

[AdamThorne]

K. So I'm looking at the FE167e/207e box on frugal-horn. The sample box design indicates that the speaker is externally 64 and 3/8 tall. Is this the standing dimension of the speaker, or the vertical dimension of the front baffle?

Since the metronome table lists L=60 which agrees with the drawing, it leads me to believe that height, and not the actual dimension of the baffle are what's in the drawing. The baffle would be longer than the standing height of the speaker, since the speaker walls all slant back.

In this case the amount the baffle drops back from bottom to top is:
1/2 * (11 + 7/8) - 1/2 * (3 + 7/8) = 4

So our right triangle becomes:
(baffle length)^2 = (64 + 3/8)^2 + 4^2

So to cut the actual front baffle I want to make it 64.499 in. tall, rather than 64 + 3/8, right?

The side baffles drop back a little bit more, at 4.8125 in. This makes their actual length 64.555 in.

Not that I'm cutting wood accurate down to .056 inches, or even the 1/8 in difference between panel dimension and the standing speaker height, but I might as well at least shoot at the correct target, right? I remember reading someone else's post about the little tiny angles off of 90deg he needed to do his cuts at for the metronome...

This may have been covered in the Metrinome thread - if so I apologize for the duplication. I haven't managed to read the entire thread, it's fairly monster.

EXECUTIVE SUMMARY: the FE167e/207e/hemp fr8 example enclosure drawing on frugal-horn.com is labled with standing speaker dimensions, which are not the same as actual panel dimensions. Is this the case?

A