The Metronome

[Jim Shearer]

Steve,
Too bad you're all the way across the pond. I'd be happy to demo them for you! And the FE108eS Mets, so you could get a direct comparison to what you have.


Martin,
I'm looking forward to having you check out the F120a Mets. At the rate I'm going, they should be up & running in only another 2 months or so. And after they are completed, I have a pair of FF85K waiting for a micro-Fonken build. I am thinking that those might pay you a visit to see how they sound w/ really good upstream gear and a little help on the bottom end from, say, an 18" Goldwood.

Bluegti,
I tried a saw board--didn't like it. As for biscuits, I don't know their advantages over dowel pins, but a dry fit up works just fine w/ dowel pins. And the clamp/jig cost $50, compared w/ about twice that for the cheap biscuit cutter. Anyway, this is a big step up for me over using screws.

Cheers,
Jim

[Jim Shearer]

Finished drilling holes for dowel pins and began applying finish to exterior surfaces. I really enjoy applying that first coat of finish, watching the wood come to life.

As some doubt has been cast about the ability to do a dry fit-up when using dowel pins, see photo below. I don’t think I would want to crack open the glue without a dry fit-up to be sure everything is right. Despite some small errors, it’s starting to look good. You can see the sides have a coat of polyurethane.

Red oak veneer has been ordered and should be here tomorrow. I haven’t decided yet how much to veneer. Certainly the front baffles need to be covered, so that they have a more similar pattern than the plywood provides. The backs can probably be left as is. Sides remain open to discussion.

[Jim Shearer]

Progress report:

Finished final glue-up on the second of the F120a Mets today!

[Jim Shearer]

I made a couple of changes from the FE108eSigma Mets. First, I did claim at the end of the 108 Met build that I would make the next Mets with a removable port baffle. That after a total fiasco with lining ALL sides with foam, then having to cut open the port baffles to strip out foam from three of the sides. The F120a Mets use a 3 inch diameter port, which is a large enough hole to enable access. All I had to do was make the port press fit, which is easy to do thanks to the Parts Express Router Buddy. I will, however, make certain that the ports stay put and are well sealed by using a thin bead of PE speaker gasket material (soft, sticky polymer that doesn’t harden.)

As to the lining: having learned the hard way, I put 0.5 inch PE acoustic barrier foam on only the back panel from just above the driver down to the bottom. In the peak, I put a modest amount of Accousta-Stuff fiber fill. That can easily be modified via the driver hole.

The next change is that I used the router with a round-over bit to trim the vertical edges of the front baffles. Then I veneered the baffles with red oak. This may have sonic benefits by reducing diffraction, but there are other intangible benefits: The original Mets had a high WAF. These are even better!

Finally, I decided to put the binding posts on the port baffle rather than on the back panel. Time will tell if this was a good choice.

[planet10]

Originally posted by Jim Shearer
Finally, I decided to put the binding posts on the port baffle rather than on the back panel. Time will tell if this was a good choice.

We like doing that on our floorstanders... works well as long as you aren't switching them in and out a lot or using firehose cables.

dave

[Jim Shearer]

Originally posted by planet10


We like doing that on our floorstanders... works well as long as you aren't switching them in and out a lot or using firehose cables.

dave

I had to laugh at the thought of someone running 12 ga. cables to these speakers.

With a max rating of 10 watts rms, you can get along just fine with the fine (tiny gauge) wire!

[greenmark59]

Has anyone thought of using a Markaudio CHR-70 driver for a Metronome? And if so, what would the dimensions be for the "box"?

[Jim Shearer]

They’re finished!

Photo below shows one of the F120a Mets next to one of the FE108eSigma Mets (which were my first SFRD build two years ago.) You can see that I have learned a bit about veneering and improving appearance. And, yes, they are significantly larger than the 108 Mets. These look almost as good as they sound. Next post has a close-up of the driver, so you can see the round-over on the vertical edges where the veneer wraps around.

When I installed the drivers, I put washers between the driver and the baffle (a la Bob Brines.) This keeps the gasket from compressing too much; and with a stamped steel basket, it prevents distortion of the frame by the mounting screws. I have previously used steel washers and Nylon washers, both worked well. This time I thought I would try neoprene washers. Long story short: bad idea! With neoprene washers there was a muffling in the mid-range. Lara Fabian, Diana Krall, and Natalie Maines all sounded like they had cotton in their mouths. Swapped out neoprene for steel and, Voila! all is now good.

I tried them first without BSC: highs were too hot. The I put in the BSC I use with the 108 Mets; 1 mH inductor and 4.5 ohms: highs were too dull. I looked back at the BSC calculator: 1 mH and 4.5 ohms would be about 4 dB of correction; I think I want about 2 dB. So I tried 0.7 mH and 2 ohms: highs were just right.

I have about 150 hours on the drivers now, and they’re sounding very good. I haven’t measured yet, but I think the bass is reaching down into the high 30’s. For me, I want a system to get down to 30 Hz, so I may still use a sub with these Mets.

These are very revealing drivers, but not hyper-detailed. On classical CDs, edit points that are evident on other speakers are rather jarring on the F120a’s--perhaps because they are so involving that the disruption in the flow of the music is heightened. Moving on to jazz (where SFRDs really shine,) Jacintha’s performance of “And the Angels Sing” (from Autumn Leaves, the songs of Johnny Mercer) was fabulous. The opening bass line is well reproduced and doesn’t seem to need any help from a sub. I had intended to get a quick listen to “Lilly Was Here” from Candy Dulfer’s album SAXuality, but I couldn’t stop at just one track--I couldn’t turn it off. When you get drawn into the music like that, you know you have a winner.

It may be a bit early to make final judgements, but I think I like the new F120a Mets about as well as the FE108eSigmas in the mids; the F120a may be a bit better in the highs, and the F120a’s are a clear winner on the low end. If you add in a sub to fill out the low end, then I see it as a close contest. Not that they sound exactly the same--they are certainly different. As I listen more (and perhaps the drivers break in more,) I find myself leaning toward the F120a’s. As always: YMMV.

[Jim Shearer]

Close up of the driver area showing the round over and the veneer wrap around. Finish is semi-gloss polyurethane. Seems to have very high WAF.

[jamikl]

Was it ever agreed as to which of MJK's spreadsheets would give a close approximation to this build. I would like to work some designs for some bigger units. The link to the original plan does not seem to work now. Is it still available?
jamikl