Restoring a Pair of Infinity Beta 50 Speakers

I was scanning Craig's List a while ago and found a listing for a pair of Infinity Beta 50 speakers that looked pretty good. I checked specs and reviews and found they were new around 2004 or so, making them 20 years old. They are floor standing towers that have two 8" bass drivers each, so they are capable of producing a satisfying amount of bass down to 30 Hz. 

Review #1

Review #2

Review #3

I dug up a manual on the speakers, some info on the CMMD (ceramic metal matrix diaphragm) drivers, and tech specs that include the crossover schematic. You can access all of that here.

The boxes are MDF covered in a black wood-grain vinyl that has seen better days- they were a bit dinged up especially at the bottom front edges, and one speaker had some "water"  damage where the joint between the side and bottom was splitting open. The seller hooked them up they speakers sounded OK, so I bought them with the intention of fixing up the cabinets. The seller didn't have the grill covers for them. I paid $125 for the pair.

Area affected by "water"- when MDF gets wet it swells up and then stays that way when it dries out. Whose idea was it to use this crap to make speakers?

An inside look at the water damaged area. It looks like a couple corner braces went missing, too. I replaced them with a couple wood blocks.

Damaged top-front edge. Both speakers were about the same. You can see the seam where the front panel was glued to the rest of the cabinet after the vinyl was applied to both.

Damaged bottom front edge- both speakers were about the same.

One bass driver had a dented dust cap, and a tiny dent near the edge of the cone. Not much can be done about that. Fortunately it doesn't affect performance. Grill covers would have been nice to hide this. Maybe I'll print some...

When I got them home I opened them up to check the interior and found a large handful of dog food inside one of them. I can think of a couple possibilities- either a rodent was stashing food inside the speaker, or maybe a little kid was playing with the dog food and dropped it in via the bass port on the back of the speaker. I didn't find any rodent poop, so I'll go with the latter explanation.

Some of the dog food can be seen here. Note- stuffing on only one side of each cabinet. Hmmm.

Recapping the crossovers

The crossovers had the usual cheapo parts- three nonpolarized electrolytic caps and four iron core inductors. I ordered replacements for the caps from Parts Express for a total of about $27 for both speakers. Considering the condition of the speakers, it's probably not worth upgrading the inductors to air core parts or the caps to film type. C4 and L5 that connect at the tweeter are film and air core parts.

It was nice of Infinity to provide such a helpful schematic! The numbers in the boxes on the right side are the sizes of the spade connectors on the ends of the wires (inches, of course!). They used different sizes so it would be impossible to connect the drivers with the wrong polarity.

One of the crossovers before I recapped it. The bass and mid/treble are set up for biamp/biwiring. There are three electrolytic caps (C1, C2, and C3) on the board.

When the parts arrived I opened up the speakers and got to work. The wires use spade type connectors at the drivers with the other end soldered to the crossover board. You have to remove and disconnect both bass drivers, the midrange driver, and the connector plate in order to get the crossover into position where you can work on it. The wires are color coded and the spade lugs on the + and - terminals of the drivers are different sizes, so it would be very hard to make a mistake when putting things back together.

Recapped crossover. Note the 30 uF cap (C2) is made by wiring two 15 uF caps in parallel and the 18.5 uF cap (C3) is made from a 17 uF and a 1.5 uF cap wired in parallel. The original caps were glued to the PCB, and you can't see it in this picture, but before positioning the new caps on the PCB I put a healthy beads of hot melt glue down at each cap location to minimize vibration that could cause microphonics.

While I had the crossovers out I measured the resistance of the four iron core coils, in case anyone wants to try rebuilding the crossovers with air core coils. Values are as follows:

L1:  0.81 Ohms

L2: 0.45 Ohms

L3: 0.58 Ohms

L4: 0.56 Ohms

If you replace the iron core coils with air core parts, you'll have trouble matching the exact resistances of the original coils. However, if you use coils with lower resistance, I don't think there's going to be any negative effect. I did some digging at Madisound and Parts Express and found most of the parts are cheapest at Parts Express. Among the parts I found, there are film caps that cost $10 and same-value film caps that cost $90. I wouldn't spend $90 for one cap. It can't possibly make that much difference in the performance of the speaker. If I were willing to spend so much that $90 caps wouldn't bother me, I'd probably just build an active crossover and bi- or tri-amp the speakers instead.

Here are the lowest cost air core inductors and film caps I found for the bass and midrange crossovers. The tweeter crossover already has an air core inductor and film cap, so no need to change anything there. Parts listed are for one speaker.

LF crossover, 600 Hz, 3rd order (18 dB/octave):

Jantzen 2.7 mH 18 gauge DCR=0.92 Ohms   $17.05 

Dayton 0.5 mH 18 gauge DCR=0.33 Ohms  $4.40

Solen 68 uF  film cap    $30.98

Total: $52.43

Mid/High crossover 3.3 kHz, 2nd order (12 dB/octave)

Dayton DPMC 30 uF film cap  $7.39

Dayton DPMC 18 uF film cap  $4.59

Jantzen 0.68 mH 15 gauge DCR=0.24 Ohms $17.44

Jantzen 0.75 mH 18 gauge DCR=0.42 Ohms $6.71

Total: $36.13

So the per speaker. the total would be $88.56, not including cost of a new PCB if needed to accommodate these much larger parts. 

You could lower the cost a bit by winding the coils yourself. You'd have to buy the wire and make or buy forms to wind it on, and then have some means to test to make sure you've got the right inductance. There's a calculator here that will tell you the amount of wire required for a specified inductance and form size, and calculates its resistance. Copper magnet wire runs about $20-25 per lb on ebay. For reference, the 2.7 mH inductor uses about 1/2 lb of 18 gauge wire, and more copper when using larger diameter wire to get lower resistance.

If I were going to be restoring speakers and rebuilding/improving crossovers on a regular basis, I'd probably design and build (3D print) a coil winding machine powered by my electric drill and buy wire in bulk on 5-10 lb spools.

Tweeters

When I listened to these speakers before I bought them, all the drivers were producing clean sound. After listening to them at home for a while, I decided that the tweeter output wasn't what I would have expected. I did some research and found out they used ferrofluid, and that it gets gummy over time and affects the performance of the driver. I looked up the part number and emailed Harmon International about the possibility of ordering a pair of them. They emailed back asking for my address and promptly sent me a pair of the tweeters for free! I installed them and listened to the speakers again and now the high frequency output was what I expected it to be.

Cabinet Rework

I pulled all the drivers, crossovers and stuffing out of them, then peeled the vinyl off to assess the damage and make cabinet repairs. I used a razor knife to cut the vinyl into 2-4" wide strips before attempting to peel it off the speaker boxes.  One speaker had some "water" damage, so I chiseled and ground off some of the MDF and used bondo wood filler to replace it, did the same for the damage at the front edges and any other dings, and the seam between the front panel and the rest of the cabinet, then sanded everything smooth.

I attacked the worst one first. This one has the "water" damage at the bottom of the cabinet. It's the one I found dog kibble inside. It's not hard to guess what kind of "water" damaged the speaker.

Close up of the "water" damage. Surprise! MDF, aka cardboard, doesn't like to get wet.

Here it is after removing the worst of the damaged stuff with a chisel and coarse sand paper.

Bondo Wood Filler applied. I filled in the sockets for the grilles because I don't have them and don't plan on making any. I also filled in the seam between the front panel and the rest of the cabinet.

After the first round of sanding. I added a bit more Bondo to fill in the voids and sanded again before  applying the vinyl.

I could have refinished them with more wood-grain vinyl, but there are a lot of other, more interesting options available in the vinyl that's used to wrap cars. I ordered a 5' x 10' roll of 3M 2080 satin flip psychedelic vinyl film from rvinyl for $170. Yes, that's more than I paid for the speakers.

This is the new vinyl.

I wiped the cabinets down with a tack cloth to make sure there was no residue or sanding dust on them and then applied vinyl to the tops and bottoms of the cabinets to get a feel for working with the stuff. It went on easily with minimal effort. I started by peeling the backing from one edge of the vinyl, sticking it to the speaker box, and slowly pulled the backing off the vinyl as I went along. I used a vinyl squeegee to chase out air and apply pressure to the film and followed that with a 2" wide brayer to really press it down. 

The top of one of the speakers after installing the vinyl. I used the flash on the phone to light it up- otherwise it just looks gray.

I left the original black woodgrain vinyl on the backs of the cabinets, but had to wrap the film around the edges at the back and cut the corners at 45 degrees. I used a framing square to get the 45 degree cuts exact.

This is how I got the corner cuts just right.

The back edge of the top of the speaker.

I wrapped a single large piece of vinyl over the sides and front of the speaker. This took a little more effort than the top and bottom, but I had help from my hot girlfriend. Thanks Carol! I started with the speaker box laying on one side, then stuck the film on the other side of the cabinet, then rotated the box onto it's back and covered the front surface, and finally rotated the box again to finish the other side. I used a heat gun in some areas to allow stretching the vinyl over curved parts of the front panel. Once the film was stuck down I used a razor knife to trim the edges and cut the openings for the drivers. I also used the framing square to cut the back corners at 45 degrees as I did on the tops and bottoms of the cabinets.

Here's one of them completely covered with the new vinyl. As you can see, there are many imperfections, but the overall effect is pretty nice. A lot of the rough surface you can see on the front of the speaker was the result of the old vinyl pulling off some of the MDF surface. This is the one that didn't have so much doggy damage.

Finished speakers set up in my bedroom. I'm happy with the results!

More impressive in sunlight:

The speakers weigh about 60 lbs each, so it's very easy to damage the edges if you aren't super careful. I made a CAD model of them because I wanted to print TPU skirts to protect the bottom edge from getting dinged up again by getting bumped by vacuum cleaner, or from moving them around. The skirts will print in two parts for each speaker and go all around the bottom edges of the speakers. I haven't decided if I am going to print and install them...

CAD render of the skirt I designed for the speakers and printed in TPU. It's designed to be stapled to the bottom of the speaker to protect the delicate edges from bumps.