Infinity SM82 (or SM80) 2-Way Loudspeaker Rebuild

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Infinity SM82 (or SM80) 2-Way Loudspeaker Rebuild

Postby Shannon Parks » Wed Apr 11, 2012 6:40 am

I re-did the foam surrounds on my woofers last year and wasn't pleased with the results. So I've been meaning to re-use the cabs and maybe some of the crossover parts. Ultimately, I chose this woofer and tweeter:
Dayton DS215-8
Dayton DC28F-8
Port Tube rec'd by Parts Express

The woofer and tweeter were picked for their cabinet suitability and matching together. The woofer in particular looks perfect for this cab. Also, I *think* I won't need to do any wood cuts for them. The port tube will require the old port tube hole to be enlarged by a 1/4", so I'll use a jig saw for that. Using WinISD everything confirms the Parts Express specs, with the port tube length at 4". The original crossovers were a 2nd order LR. I will switch to a 1st order Butterworth. I have my 10uF Sprague 730P caps and some 600uH inductors to use for that. I'll probably adjust those for 2500Hz.

Parts Express had the parts to me in record time. I ordered on Sunday afternoon and got them at 9am Tuesday morning - love it! I will do the upgrade this coming weekend. Meanwhile I am experimenting with speaker sweeps with my RS digital SPL meter and my Boonton. I'm getting some useful data with a simple setup, only getting too many resonances from the room. Hopefully the wind will die down and I can sweep these in my yard facing up from the grass. I think I should be able to get decent tests that way for comparison before and after the upgrade.

Shannon
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Postby TomMcNally » Wed Apr 11, 2012 7:34 am

I hear the dogs howling now when you get up past 10k !

I reconed my 1980 vintage Cerwin-Vega speakers about 10 years
ago with a kit from Parts Express. It was tedious, but went well
and they still sound good.
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Postby Shannon Parks » Wed Apr 11, 2012 9:21 am

I hope my neighbors don't get mad at me. But loudspeaker testing is serious business. LOL!

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Postby TomMcNally » Wed Apr 11, 2012 9:25 am

I worked at a radio station in Philadelphia back in the 80's ...
the Chief Engineer said "watch this" and connected his Leader
audio oscillator into the inputs of a new Phase Linear 400 amp,
connected to a big pair of speakers, and fed low frequencies,
like 10 to 30 Hz ... the building kind of shook like an earthquake.
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Postby Ty_Bower » Wed Apr 11, 2012 2:54 pm

That looks like a nice woofer.

I picked up a set of the MCM 55-2421 a while back to use as subs. I stuck them inside some old 36 liter sealed boxes that had outlived their previous cheapo drivers. Honestly, the alignment wasn't tuned right (boxes way too big) but they still sounded half decent after a little EQ. I bet they would have worked well with a good tweeter.

Image Image

I'll be keen to see how your project turns out. Keep us posted!
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Stock Sweep

Postby Shannon Parks » Thu Apr 12, 2012 5:33 am

Pretty happy with my test setup, sans $250K anechoic chamber. I ended up just opening my garage door and facing the speaker down the driveway, just placed outside the garage door opening. This meant I didn't have to drag everything out to my yard, and the results were 100% better than indoor testing which was basically useless, other than seeing how colored the environment is. The data:

Image

The low frequency dips at 120Hz, 350Hz and 700Hz are resonances due to the test setup, so those can be ignored. They appear in my Fostex speaker, too. You can see that the low frequency -3dB cut off is around 70Hz, and that is indeed the spec for this speaker. Above 4kHz, we have a rising output due to a mismatch between the tweeter and woofer. The stock crossover has a 10W 1.5 ohm resistor that could've been larger. It's not horrible, but I think this explains why this speaker's tweeter is referred to as harsh by many listeners - it just slightly overpowers everything else. Finally, note that I'm not using a calibrated, lab grade mic, but a RS SPL Digital meter (with inverse C weighted transfer function). But I think my sweep is pretty close to reality and makes sense to me. I'll post my Fostex speaker for comparison:

Image

The two low freq dips are exactly the same as the Infinity, so I am confident it is my imperfect test setup. The flatter response between 4kHz and 10kHz seems correct to me, and leads me to believe that the Infinity sweep isn't faulty in the same region.

I was pretty happy with this sweep. My -3dB cutoff is around 80Hz, and I have fairly flat response for this design - I expected much worse resonances. I think it is obvious I need to add a tweeter to this speaker - just a simple cap coupled tweet and no other crossover components.

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Initial Speaker Placement Checkout

Postby Shannon Parks » Thu Apr 12, 2012 7:04 am

The Dayton speakers will fit A-OK. I needed to gently bend the tweeter hookup tabs a little to shimmy it in, but I don't *need* to make any cuts. The magnet on the tweeter just fits inside the hole. The new 8" woofer is twice as heavy as the old one. The only cut with my jig saw will be for the port tube.

Random factoid: I once worked at a speaker factory! I worked for Panasonic in Knoxville, TN for three or four months in 1994. It was a neat experience to see a Japanese factory first hand. A lot of the Japanese managers used Westernized names like Hank and Tex. The line I worked on usually made 6x9s for the various car manufacturers. I worked many different positions on the line, but I forget a lot of it. Our line did about 3 to 4 thousand speakers a day. I do remember one station where I had to glue the cone to the frame, and I hated it. I had to glue about 4 units a minute for 8 hours with a partner. No wonder I didn't enjoy the refoam job.

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SNAFU

Postby Shannon Parks » Sat Apr 14, 2012 6:35 am

1st SNAFU: the plastic housing for the tweeter has a lip that is 2.9" and my hole is 2.85". I hadn't noticed that it didn't sit flush the other morning when I initially placed it. So I taped up the area with some shipping tape and jig sawed the hole a little larger. Ugly, but not bad and now my terminals have some more breathing room, too. The speakers are mounted and the woofer is connected. I will sweep it from 500Hz to 5kHz, and then sweep it a second time with my 600uH inductor.

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Postby Shannon Parks » Mon Apr 16, 2012 6:14 am

I learned some things this weekend while testing (and gardening and doing taxes). First off, let me point to the links I heavily frequent:
Rod Elliott's Design of Passive Crossovers
Crossover Calculator

I wanted to do a KISS two-way 1st order crossover, but it wasn't in the cards. The woofer's impedance characteristic is like a moving target for the inductor crossover frequency, so the 6dB roll-off was more like a 4dB roll-off. So a zobel network was needed for the woofer. I used the calcs on Rod's page, and it was 37.5uF and 5.8 ohms. I tested with that, and then with a simplified 10uF and 10 ohm. The simplified zobel looked better with an impedance sweep (the calc'd zobel had a more slopey shoulder, instead of a crisp shoulder). I was then near my 6dB roll-off, but I then decided I needed more attenuation due to a SPL response peak at close to 3kHz. So I had my zobel, and it looked like a 12dB 2nd order L-R was the obvious route.

Impedance sweeping the woofer with the 10-10 zobel and a crossover of 1.16mH and 5uF gave great results. I then did the tweeter, and then adjusted it to 5uF and 600mH - very nice with the lower inductor. This gave the tweeter less of a shoulder , so it might match with the woofer characteristic better. I can add more wraps to it if needed.

I plan to go ahead and mount this on a plexiglass board - maybe 5" by 6" - and mount it in the bottom. I can then do some SPL tests. Anyone have tips on cutting plexiglass?

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Postby snitch56 » Mon Apr 16, 2012 9:23 am

Shannon,
I dug up some old documentation on those Dayton Tweeters. You are in the ballpark of their "recommended X-over".
http://www.diytube.com/phpBB2/album_pic.php?pic_id=158

I tried using them in some A25's where they were almost a direct fit other than the three vs. four mounting holes. Some work needed to be done to lower the X-over frequency. In the end I ended up fixing the original Seas driver (one out of two was bad) with some conductive epoxy. The wire between the terminal and coil had fatigued.

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Postby Shannon Parks » Mon Apr 16, 2012 10:21 am

snitch56 wrote:Shannon,
I dug up some old documentation on those Dayton Tweeters. You are in the ballpark of their "recommended X-over".
http://www.diytube.com/phpBB2/album_pic.php?pic_id=158


Thanks, Brad!

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Postby nyazzip » Tue Apr 17, 2012 12:12 am

Anyone have tips on cutting plexiglass?

....tablesaw, with a plywood(finer/more numerous teeth) blade. some people recommend spinning the blade backwards, so it abrades rather than cuts, but i have not tried that. i have had very good results with a bottom-of-the-line plywood blade; the brand and size eludes me at the moment.
that, or, a hacksaw, then refine the cut using sandpaper on a hard flat surface. polycarbonate is more brittle and less forgiving of flexing than plexi, but the same principles apply
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Postby Shannon Parks » Tue Apr 17, 2012 2:55 am

We have a shearing tool at work that we use on PCBs and brass sheets that I plan to give a try today. It does seem like pretty brittle material (at least with a jigsaw hammering it).

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Postby snitch56 » Tue Apr 17, 2012 6:01 am

For cast acrylic, I typically use a panel saw and sandwich the piece between two pieces of thin (1/4" - 3/8") plywood. The plywood protects the acrylic from edge cracking. For thin pieces a laser cutter works well and for thick pieces use a water jet if available.

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Postby Shannon Parks » Thu Apr 19, 2012 6:35 am

I tried a CD jewel case for the cross-over for the heck of it - cheap and ubiquitous. We'll see if it rattles, but I can dampen it if needed. Borrowed the glue gun from the missus. The inductors are glommed from the SM82 crossovers and my original (defunct) SM80s. The 8 ohm, 10W resistors were part of the headphone jack on the Sansui I parted out - Ed Brown has the iron. The wire to the speaker is 18AWG teflon silver stranded surplus (nice stuff!) and the wire to the speaker jack is 16AWG. DCR from one end to the other through the woofer inductors is 420 milliohms with a 4-wire measurement.

Shannon

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