Refurbishing A Classic Loudpeaker: the Beovox M70, Part 3

Continuing the rebuild project for the Bang & Olufsen Beovox M70 speakers first described in the July and September issues of SMWTMS newsletter. If you missed it, the previous two entries are reproduced here and here.

New for this installment is the breaking in of a few new hand tools, a decent solder station and a nifty component tester from Parts Express: The DATS V2 speaker test dongle and app.

I'd like to talk first a little about crossovers. One "dirty little secret" in loudspeaker design is a common practice of inverting the phase of a tweeter to provide decent summation in the crossover frequency of a typical two way design. Sometimes the reactances of the speakers have signal content that works out substantially out of phase at the matched polarity. Usually it's easier to move the red dot than to explain to the user why his tweeter is out of phase. Good summation in the crossover frequency arguably fixes an output problem, but it's not without its own issues. What may work on axis, may not work so well off axis, and at all frequencies. Many very good loudspeakers use this technique. 

I'm not a polarity bigot as a rule, but I have never been comfortable with the idea that phase is not always known and dealt with a priori; that is, figure out the right thing to do and do that. As opposed to flipping things around until the response is less broken. Call me a purist, or at least superstitious. I've respected phase since my early encounters with schizophrenic tweeter polarity..

Attempts to bring order to this evil include some admirable solutions. A constant voltage crossover has the virtue of forcing a unity summation at the crossover frequency, and phase is forced to play nice too.

One principle of this type of the crossover is that it is possible through adjusting values to effect a high amplitude slope near the crossover point. This topology is sometimes called a "Quasi 2nd order" for that reason. It's been described by Small (1), Ashley& Kaminsky (2) , and others.

Another concept for dealing with phase at the crossover points is the B&O Phase Link topology. I came across this first in a 1977 paper by Erik Bækgaard (3). A recent re-cap by Geoff Martin is posted in his interesting blog (4) Not to take anything away from the 1970's, but it's clear B&O continues to do some very interesting loudspeaker work today.



Phase link describes a method for inserting an additional driver

The M70 Crossover has elements of both a series crossover and a phase link topology. The next figure shows both types of elements, after factoring out the zobel and pad resistor elements.

The filler driver shows a series resonant circuit established by L2 and C5||C3

The Tweeter and midrange crossovers are a little more complex, but note that there is a constant voltage section set up by C8 and L4

So, this should be an interesting piece to study further.
To help in breaking down the measurements for further analysis, I relaid the crossover to more closely emulate the schematic. (I would flip it top to bottom to line up the same element positions, but we'll let that be my nod to the OCD gods. Not superstitious about that at all).

Astute newsletter editors will notice that I took steps to eliminate the mutual inductance problem cited in part II:

The original layout had a characteristic of about 0.1% of mutual inductance. Certainly smaller than the working tolerances of the project, but no sentiment for carrying over the issue.

I have been taking a number of measurements using my DATS impedance tester. It's a remarkable little device. A USB speaker parameter tester and LRC wrapped in one app. I'll have more to report on the usefulness of that device next time.

It has given me a few new problems to worry about. Dammit. I believe in order for the narrow range of the filler driver to do what it must, it is desirable, possibly crucial, to have drivers that are matched pretty well in the filler range. Unfortunately, since one of my filler drivers was defunct, I had to get a replacement driver from another production lot.

Here is the difference in resonance between the two drivers:

The transfer impedance of the crossover network is as follows:

Clearly it is time to stop cutting bait, and put this thing together. Other projects are wanting their share of the lair.

More on the deconstruction and reconstruction of the Bang & Olufsen M70 speaker next time.


(1) Richard H. Small, Constant Voltage Crossover Network Design, JAES Volume 19 Issue 1 pp. 12-19; January 1971

(2) Robert J. Ashley, and Allan Kaminsky, Active and Passive Filters as Loudspeaker Crossover Network, JAES Volume 19 Issue 6 pp. 494-502; June 1971

(3) Erik Bækgaard, A Novel Approach to Linear Phase Loudspeakers Using Passive Crossover Networks, JAES Volume 25 Issue 5 pp. 284-294; May 1977

(4) Geoffrey Martin, Blog Post: B&O Tech: Uni-Phase Loudspeakers
http://www.tonmeister.ca/wordpress/2015/10/29/bo-tech-uni-phase-loudspeakers/

Refurbishing A Classic Loudpeaker: the Beovox M70, Part 2

This is part two in a series of posts about a classic Beovox M70 loudspeaker pair which I am rebuilding for fun and to satisfy a few open questions in my mind about the virtues of the design. If you want to read or revisit part one, I have posted it at http://waterfordsoundworks.blogspot.com/.

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.

Seeking to perform triage and get a full assessment of where we stood on the driver front, I did a rudimentary inspection of all the drivers. Four of the drivers had no apparent problem, and four were going to need work.

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.

Refurbishing A Classic Loudspeaker: the Beovox M70, Part 1

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?

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.