My Metronome Experience

[Jim Shearer]

OK, when I registered & introduced myself, there was a request to report on my results. So as to not contaminate the genuine Metronome thread I will begin another.

Background:
When I was a teenager I built an amp and a couple pairs of speakers. The incentive to build vs. buy was strictly economic. I could buy a really cheap system or build one myself and have something significantly better. Then came a time when I could buy stuff that was way better than anything I could make myself. Among the gear I bought was a pair of RTR280DR speakers that I still love. However…the house we are in now is just not accommodating to these hefty beasts (which each contain four 10" woofers and six tweeters.) SO, with some regret, I put the RTRs on indefinite loan to my daughter and son-in-law (who have a nice, large home in desperate need of sonic goodness) and picked up a pair of Polk Rti6's and a PSW10 sub. I lived with them for a while, but felt audibly deprived (as compared to the RTRs.)
A two year search for something as sonically pleasing as the RTRs, but of a smaller dimension lead to the conclusion that I would like a pair of Totem Hawks or Forests. I did actually prefer the sound of Magnapan MG 1.6's, but they wouldn't fit in any better than the RTRs. So, I was ready to go for the Totems. BUT…we needed a new roof on the house, then there was a series of dental bills. Sigh… No expensive speakers for a while.
As I continued to browse speaker reviews on the web, I noticed single, full range driver systems getting positive comments (mostly.) And many of those comments mention qualities which I am looking for, like the open, boxless sound that I heard in the maggies. And then I saw a review of the Merril Zigmahornets on Six Moons Audio. Buy the drivers and plans; build them yourself. And I said to myself (like Bob the Builder that my grandsons watch,) "Can I build it? YES I can!"
Thus I revisit my teen years, with the difference that I can now build (with plans and advice) speakers of (hopefully) superior sonic character.
I will describe the process and hope that you experts will comment on how I (and other newbies) can do better on future projects.

[Jim Shearer]

Moving Forward:

I emailed for info on the Merril drivers and plans, but got no response. Further searching on the internet turned up some interesting information, including the DIYaudio.com site. Here are plans for suitable enclosures for specific, highly regarded drivers. I read M.J. King's papers on speaker theory, but don't have MathCAD anymore & don't really want to go there. I want plans for a fully defined system; one known to work and work well. I considered various designs and drivers, and decided that the Metronome with the Fostex FE108 driver was the within the range of my (limited) abilities, will fit into our home, offers a reasonable chance of success, and my wife is happy about the project. Her comments about the Metronome include "cute" and "pretty". There is also a measure of relief that I'm not asking to spend $3K.

Next I downloaded the drawings for the Metronome. (A big Thank You to Steve for creating the design and to Dave for making the drawings!) Then I made full size drawings to work out all the details. I'm a slightly obsessive detail person & need to "get everything straight in my head" before I can begin a project.
I ordered the drivers from Madisound and sundry items from Parts Express. Then off to the lumberyard for plywood, glue, stain, etc.

Preparing to Cut Wood:

I'm reasonably handy, but woodworking isn't a hobby of mine. I have a table saw, circular saw, jig saw, drills, Dremmel tool, and various hand tools. I took some scrap plywood and began making a 'test piece' before cutting into the nice (pricey) birch ply. I couldn't see how to do the odd angle cuts on the table saw. So I figured I'll use a pair of clamps and a good, straight piece 1"x4" x6' board as a 'fence' and run the circular saw against the fence. Oh, dear…the blade of the circular saw isn't parallel with it's 'foot'.

OK, it's the jig saw. Working very slowly and carefully, I can get a straight cut. But not good enough for me to do mitered corners. It's got to be butt joints. It should look OK if I veneer the exposed edges of the plywood--and I just happen to have some birch veneer left over from our kitchen remodeling. For those who may not be aware of it: flexible veneers with self adhesive backing are now available; sort of like contact paper, only real wood. I haven't seen anything like carbide tipped jig saw blades (and the steel ones get dull quickly,) so besides buying the narrow jig saw blades for cutting the arches, driver holes, and port holes, I picked up lots of 10 TPI blades for the long, straight cuts

[Jim Shearer]

On Cutting Wood:

I began by drawing cut lines on the plywood. Then I put foam plugs in my ears; I take great pains to preserve my hearing when working with tools or in noisy environments! The jig saw goes very slowly, so I cut a piece, then rest. The cutting process took about four days. (Did I mention that I work exceeding slowly?)

Since I used butt joints, I covered the edges of the front and back pieces with birch veneer so the 'ply' wouldn't show.

The top and bottom pieces I cut on the table saw. I set the blade angle to 86.5 degrees to get the taper on the edges for a good fit. On these little pieces, I cut, then checked for fit. When they weren't close enough to suit me, I cut them again or made small adjustments. (Another day's work.) Then I cut the arches on the bottoms of each side, holes for the drivers, and holes for the port tubes. The interior of the driver hole got a 30 degree bevel to allow the driver to "breathe".

[zacster]

Maybe it is too late, but some advice on the angled cuts. I'm not a woodworker either but I've been obsessing over how to build these things right and I've come to a few conclusions.

First, since you have a table saw, it is your best bet to get straight cuts. Build a jig with the angle set into it. If you are not going to use a table saw, build a jig with a guide edge built into it, with a cleat at the bottom. Then make one cut, and flip the wood to make the other cut. This way you get the exact same angle on both sides.

To get perfectly straight and square edges, use a router with a pattern or trim bit and a straight edge after you make the saw cut.

And here is the thing about the Metronome. The butt joints are not at 90 degree angles. If you cut everything perfectly square, they wouldn't join up flush. The difference is slight, but if you are NOT getting square edges, take advantage of them so that the angle is oriented the correct way. You would need laser cuts to get everything perfect. I have a spreadsheet with all the angles calculated out to 2 or 4 places.

[planet10]

Good to see another Metronome gestating

That jigsaw reminds me of one i got from my dad that i have in my antique electric tools collection ... is that the same one you used to build with in your teens... i have a couple oldies but goodies (unfortunately i just had to bury one of my favorite drills of all time

dave

[zacster]

Where upstate NY are you anyway? Are you close to the center of the universe, NYC? I'd be interested in hearing them when you're done.

[Steve Cresswell]

Hi Jim,

Great to see another Metronnome under construction.
You guys make my efforts look pretty lame.
I'm still using the prototypes I built nearly two years ago and really ought to build some "proper" ones
I used nothing but a handsaw, a plane and much cursing to build mine.

Dave: Maybe you ought to get Norm Abram to build a pair on his New Yankee Workshop and give us all a few pointers

Steve.

[Jim Shearer]

Dave,

Believe it or not, that is the same old Sears jig saw I used many years (decades) ago. One of the tools I picked up when we cleaned out my parents house when they were both gone. My defective Black & Decker circular saw is one I bought years later. Maybe I should have bargined w/ my brother for our Dad's old circular saw!

Cheers. Jim

[Jim Shearer]

Assembling:

I started by applying finish to the interior surfaces. I have read that speakers perform better if ALL surfaces are finished. I have no hard data one way or the other, but what the heck? Why not? I sealed the interior, but didn't stain it, figuring wood color shouldn't significantly affect timbre (sound color.)
I don't have those nice, big wood working clamps, so I had to tack the face to the sides with wire nails (like little finish nails) to hold while the glue set.

Next, I cut the port tubes (from Parts Express) to length and put them into the bottom plate. They fit quite tightly, but just to be sure they sealed well, I put a soft, rubbery, tacky material (Parts Express speaker gasket) around them on the inside. (See photo.) Then I glued the top and bottom pieces in place.
I installed the binding posts (Parts Express p/n 091-1245) on the back of the speakers. I originally wanted to put them underneath by the port, but there wasn't sufficient clearance to use banana plugs there. I took the advice of another person on this forum and used crimp on quick connects rather than soldering wires to the speaker terminals and binding posts. And while it seemed a bit of overkill to me, I did use 12 gauge wire from the binding posts to the speakers. (Don't want those electrons to feel constrained!)
I lined the interior with Parts Express Sonic Barrier 0.5 inch thick Acoustic Foam (p/n 260-520) which has an adhesive backing. No fiber stuffing.

I glued and nailed the backs on. I had not 'trimmed' the top of the front, back and sides to length yet. I waited until they were glued up, then laid the enclosure face down on the table saw. I carefully adjusted the cut line, and with the blade at an 86.5 degree angle, powered up the saw and SLOWLY raised the blade to get a nice, smooth backward sloping top cut. (See photo.) I suspect this is in violation of safety procedures for table saw use, but it seemed like a good way to get the top cut just right. I applied birch veneer to the top, just as I had with the edges of the front and back pieces.
I applied a light oak stain and polyurethane finish to the exterior and puttied the nail holes. Note that I painted the undersides black.

[planet10]

Originally posted by Steve Cresswell
Dave: Maybe you ought to get Norm Abram to build a pair on his New Yankee Workshop and give us all a few pointers

Who is Norm Abram?

dave