Writer's Note: This is an archive of the original article, published in the SMWTMS newsletter, the LC. Many thanks to Bob and the elders for kindly publishing my first attempt at writing for this hobby. I promise to do better next week.
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In my early days in the audio business, I was a service department manager at a high end audio saloon out in Lincoln Nebraska. The dealer (eventually "The Sound Environment") had a lot of good audio gear, and a nice cross section of the available technology at the time: Magneplanar, Celestion, Canton, Klipsch, Audio Research, Tandberg, and Revox to name a few. I had occasion to listen and compare a wide range of speakers costing much more than I could afford. But why take them home at all when you can sit there after work, and play around?
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In my early days in the audio business, I was a service department manager at a high end audio saloon out in Lincoln Nebraska. The dealer (eventually "The Sound Environment") had a lot of good audio gear, and a nice cross section of the available technology at the time: Magneplanar, Celestion, Canton, Klipsch, Audio Research, Tandberg, and Revox to name a few. I had occasion to listen and compare a wide range of speakers costing much more than I could afford. But why take them home at all when you can sit there after work, and play around?
The Sound Environment, was also the biggest Bang and Olufsen dealer in the world at the time, so it was only natural that we would celebrate the next product coming out of Denmark. We all remember the Beogram 4000/4002 turntables, the sleek Beomaster 3000 with it's cool black and rosewood components, and who could forget the first "Touch" component, the Beomaster 1900. For speakers they had the Beovox line, a very well regarded range of speakers from bookshelf to floor models. I was drawn to the Beovox M70, in part because I could see throwing 6 weeks of salary at them some day, and because there were some unique qualities of these speakers that I really liked. I thought they had a nice solid transient behavior, and an image quality that was hard to put into words. Pianos sounded right, for one thing, which is not that easy to do at any price. I would listen for many hours, using the Mobile Fidelity and Sheffield Labs demo material of the day, and all kinds of music really. I solidified my preference of this speaker over anything else in its size and price class. I enjoyed my time in the audio retail business, but you can't make a career out of listening to music, can you? I left Nebraska in 1978, without having earned enough to own a pair; and had not encountered the M70 again before now
Fast forward to 2015, and I was looking for an excuse to build something new in my mad scientist lair, a space I just acquired for the price of the condo built on top of it. But what kind of project should christen this space? I've got it; a loudspeaker project! I should build something I could live with for the long run. What I need is a bunch of good drivers, some crossover components, and solid cabinets, and about 14-26 weeks of time. I could do that, and maybe spend 6 weeks of salary and maybe have something to show for it. Then I happened upon an ad from AudioClassics.com where they had 300 different speakers and they needed to get rid of one.
So I happened down the list and saw an ad for the Beovox M70, and the wheels started turning. Rather than reinvent the wheel, I reasoned, why don't I revisit my roots? The price was $99.00. Had to be a typo. These speakers were $800 in 1977. That is about $3200 in 2015 federal reserve notes. How can these of all speakers be sitting out there for $99.00? Well, there's the one catch. One woofer doesn't work. Well, there is my project, then, I thought to myself. It will be nice to have the first project to grace the floors of Waterford Sound Works (A name I just made up as I was typing this) be a refurbished Beovox M70. If I can restore it, I will. If I have to improvise, so be it, but Im really interested in recapturing the magic of an old favorite loudspeaker.
This is a blog of sorts to amuse my friends and to invite them to come to my rescue as I struggle with the various issues of taking apart a 40 year old speaker and putting it back together good as new. There will be several questions along the way.
Do these sound as good as I remember? Why / Why Not? Can I even hope to recreate a 40 year old memory?
What was it about the speakers that captured my attention, and what aspect(s) have proved important?
Were the resin based front and rear baffles part of the magic? What else is going on inside this treasure chest?
What is with the 12 cm "Filler Driver" and was it a precursor of B&O Phase Link crossover topology?
Is sealed box still the right way to do loudspeaker systems?
Enough back story, it's time to start the investigation....
The speakers arrived from Audio Classics on Wednesday, in good shape. I unboxed the first of the two speakers, and have begun a preliminary assessment.
They arrived well protected in a box from another classic speaker, the KEF104/2. I have experience with this speaker from 1990, but only one nostalgia trip at a time, please...
First thing I noticed was that these were in great shape except for one scrape on the side. I don't have a strategy yet for fixing it, but first things first, need to finish the damage assessment.
The grille came off with some gentle prying. It was held in with a thin bead of caulk that probably dried solid 25 years ago. Still, no damage, and I consider myself fortunate that my screwdriver gambit was a success.
The baffle was lined with a soft open cell foam that will be difficult to match. It is an open cell, coarse foam, experiencing some age distress.
Removing the back panel, there were no obvious signs of distress in or around the cabinet. No obvious evidence that there was any service previously done on the box.
The inner workings were typical of a good speaker from those days. Bracing and reclaimed fiber where it could do some good. More fiber wrapped around the harness so it wouldn't buzz.
The crossover has about 20 parts, and in part two, I will document the crossover design, show the original schematic, and have some more to share about the crossover philosophy.
You can see some of the other elements in the box; a two liter midwoofer enclosure, and a midrange back load of about a half liter.
After inspection of the woofer, I confirmed that this indeed was the bad woofer. It was open circuit, and had a metallic clunk indicating a voice coil that strays too close to a magnetic pole.
Something else about the woofer that you will notice by inspection: The woofer terminal had come loose, and the ferrule had slidden down to the woofer cone. This may or may not be the root failure in the woofer, but it was a topic discussed in several posts on the B&O tech forum (e.g. here). I will do a teardown analysis next time and let you know.
It is a SEAS 25F-EWX in a four ohm10" driver which is no longer available in the wild. I will check with the B&O service world, but already feeling that I may need a fallback strategy for woofers.
So, that's it for now. The story is unfolding. Next time I will take out the good cabinet, and make a few more observations on it, and as promised talk some about crossover design.
I hope you'll come along for the ride, and interject anything you think will be important. Already, I have had $99 worth of fun. The rest is gravy.
