First, in the credit where credit is due department, I want to introduce you all to my co-instigator and long time friend Jeff Swauger in LaGrange GA. We share common backgrounds, including a history with B&O products, speaker projects, and a dozen other loose alliances. Already, Jeff has pointed me towards other fine folks who have made the restoration of B&O speakers a passion, and has uncovered a number of helpful documents for the quest, including owner's manuals and schematics.
I benefit greatly from the help and support of the Southeastern Michigan Woofer and Tweeter Marching Society, whose members keep me well grounded in audio history and the large truths therein.
Bob Klacza's first thought when he saw the crossover was: "Hmmm. ...Mutual Inductance". He's right of course. Modern crossover design generally calls for placing the axes of inductor coils perpendicular to each other, so that flux lines do not induce parasitic currents. An experiment will necessarily ensue, to see the possible impact of mutual inductance on the crossovers.
Jim Snyder wanted to know: "What's up with the downward tweeter angle?" A good question, and we will discuss that at some length in the context of signal arrival times in a later post. For now, it's appropriate to note that the stand (aka "trumpet stand") that originally came with this model was designed to tilt the enclosure back slightly so that the tweeter was on axis to the listener, and the woofer slightly forward. I didn't happen to score a pair of the trumpet stands with this set, but might be shamed into getting them some day. I expect to use the speakers on bookshelfs at first.
Dave Clark counseled that projects of this sort invariably disappoint, in that the 40 year old speaker does not normally sound as good as one's memory of it. That may prove true, I even expect it, but there's no harm in trying; and Dave himself will have as much fun as any of us proving this axiom yet another time.
He also "reminds" me that one can not "hear phase", so this whole nonsense of aligning signals and sources so that a square wave could be reproduced on an oscilloscope is misguided and of little value. My goal is not to prove one way or another whether phase is audible; however, if this project is successful, it may just yield some additional insights into the answer to that question. I believe if there is any magic in these speakers, it is reflected in the crossover and the use of a narrow band midwoofer. We will spend a lot of time on this aspect, both in analysis and in later investigations.
Finally, I want to nominate for sainthood my wife Linda who, as a long suffering audio widow, must now wrap her mind around this: That I am about to spend countless hours and an undisclosed amount of money on a project that will result in nothing more than a pair of used, 40 year old loudspeakers. I reassured her that my hobbies are cheap in comparison. Classic cars, for example. Rock Bands. . Legos. This is way cheaper.
Spoke Too Soon Department
In our last episode, I mentioned a woofer arriving damaged in system #1. That's a minor dragon to slay, I thought to myself, as I set aside the first enclosure,, and examined the other. Turns out the other side had three driver problems hidden within its walls. There was no visible damage to the box, but somewhere before they were packed, there was a physical "event". In less formal terms, this unit must have been slammed off the shelf. I hope the cat is okay.
1): The dome midrange had broken away (apparently, quite forcefully) from the driver baffle, and was rattling around inside the enclosure.
For this break, both the baffle and the midrange driver sustained damage. The midrange flange was broken in four pieces. The dome, while intact, had collected a layer of cotton fiber from the material used in internal damping.
The inside surface of the baffle sustained a couple of divots on the left side when the other side of the midrange flange pulled loose.
The piece of baffle broken away from the midrange mount does show something I found interesting, though. Apparently the baffle is molded as a composite, with a millimeter thick substrate layer. My guess is this was a multi-stage process. This was one of the first speakers I ever encountered that had used plastics this extensively. In the 70's a full injection molded tool would have been very expensive, and I'd be interested in learning more about the process B&O used.
This next shot shows the screw posts extending back from the front baffle about 7-1/2 inches to mate up with the back baffle. Fairly common construction in later years, but once again, 40 years ago this was innovative stuff.
Problem #2 with this side became evident when I turned the enclosure over to remove the midwoofer (aka "filler" driver). A heavy portion of the driver sunk to the bottom of the subenclosure; as did my heart when I realized what I was about to find. The magnet had come off.
And since there was nowhere for the magnet to go but to hammer repeatedly at the coil, I'm afraid this driver has become a total loss.
3): Finally, the woofer in this box had the same symptom as woofer #1, previously reported in the blogosphere and here. The terminal had come loose from the basket, and was flapping along with the tinsel wire. This woofer is operational despite of that issue.
Of these various problems, it is believed that one, perhaps both woofers can be salvaged, while the two tweeters are fine. One of the filler drivers is bad, and one of the midrange drivers probably can be salvaged acoustically but not cosmetically. So, I've opted to replace one filler and one midrange.
I have ordered the new drivers (thanks eBay). These are expected to arrive from Colorado on Tuesday. In the meantime, I took a first look at repairing the woofer #1. If you recall, this woofer had terminal and coil lead problems. I elected to carefully pull the dustcap off, lifting it with the Xacto scalpel. I peeled back also the customary top layer of woofer cone, but hopefully the strength of the cone is not compromised.
The good news is that the coil wire break is above the neck joint, and should be easily fixable.
With the driver situation well on its way to resolution, I decided to turn my attention to the crossover networks. As I've said, I think there is some magic here, so I want to fully appreciate what we have to work with.
Crossover Teardown and analysis.
You may wonder why I tore down a perfectly good crossover. Yes, I did create quite a mess here.
First, I wanted to see what kind of deterioration I would find in an old crossover, and to do that I needed to separate the components and measure them. I did find some interesting results. Here is the schematic as reproduced from the owner's manual:
The next table shows the values and results of my measurement. I had expected I would have to replace any capacitors that had deteriorated out of spec, and there were some, though not a lot. My results and my notes are as follows:
Definitely some crossover deterioration, but perhaps not as much as might be expected. The curious one is the high (11-17%) dissipation reflected in the 12 uF capacitors. These were pretty substantial caps in size, and were used largely as conjugate (zobel) network elements, where a high series resistance isn't going to have the same significant negative effect as they would if they were in series with a speaker driver. This system probably would have performed within acceptable limits if not for the other issues.
After some reflection, I have elected to replace all the capacitors, and start with a clean slate. These were ordered from Martin Olsen in Denmark, and should be here in a week or two.
In part 3 I will have some color commentary on crossover designs and a discussion of the Baekgaard phase link technique, along with any other updates on this project. As you can see this has now become a bit of a rescue mission.
