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 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/
I have a pair of M70's that my dad bought in about 1979. One of the midrange drivers has failed, and I want to replace it. Can you help me find a driver or at least the specs on this one?
ReplyDeleteDo you mean the 2" dome midrange? I bought one on eBay for about $60. Mfr: ITT Part LPKM 130/50/120T
ReplyDeleteI happen to have one that works, but it has a broken and chipped flange that I glued sort of back together. I had intended to keep it as a spare. . If you can't find one in good condition, contact me, and I may be persuaded to help rescue your set.
Hi, I just picked up a pair of M70's and I am glad to find your blog and all the information you have put into it.
ReplyDeleteSo far they sound just like I remember when they first came out when I was a service tech at a stereo store. I am very interested in the crossover information as I might need to check all the caps.
I look forward to any more information you will post.
Jim.
Hey, Jim, thanks for the shout out. I had charge of a B&O service department for a while in the 70's, too. These were definitely not like other components, were they?
DeleteB&O are one of a kind. While I was at the shop I picked up a 4002 turntable that I still am using almost every day.
ReplyDeleteNothing compares to it.
Hi Wizer,
ReplyDeleteI'm hoping you're still behind this blog where i'm coming back often as a ressource. I'm currently restoring a pair of M70. i'm having issues with the recapped crossover but only with the filler speaker signal curve bumps at approximately at 70hz then going down and up again around 100hz. Please reach me at charlesjean@videotron.ca. We can also discuss some kind help fee. Thanks in advance! Charles
Hey Charles, Yes, I'm still here but haven't advanced my project in many months (got much busier at work and at home for now). Still, very happy to help if I can. Are you measuring the filler frequency response near-field with a microphone?
ReplyDeleteGlad you answered so quickly. Let me say to start with that I left one speaker crossover untouched in order to benchmark the reworked one and I'm glad I did it. I used a tone generator plugged into a dummy amp and set the output VAC at 100mV at 20hz. I took all the measurements from 20hz to 20Kkz, from the original speaker, taking measurements from all drivers. I was able to draw the curves for each driver with consistent results and also finding the crossover points that match B&O spec sheet. I did the same for the reworked crossover and I'm getting pretty close results from the original to the reworked crossover for all drivers except the filler. Like I said previously, the filler driver follows its normal curve except a bump around 70hz. When I'm replacing the filler by another 4ohm speaker (el cheapo). The filler curve goes back to normal without a bump. Realizing that, I took the filler driver from the original speaker and fitted it with the reworked crossover but I’m still obtaining the same bump around 70hz I can live with a minor bump but it is almost as high as the higher curve point and I’m afraid to eventually damage the filler with such low frequency. I checked all the new capacitors and resistors. I used NP electrolytic for the large values and MKP for the small values. As a side note, only the filler crossover I used a mix for electrolytic and MKP, all other drivers are either MKP or Electrolytic. I checked and re-rechecked the circuit and can’t find anything wrong and reason seeking for help! ☺ Thanks in advance!
DeleteCharles, I'm still a little confused. Can you share the data? If I'm visualizing it correctly, there is a leak somewhere in the system. Gotta love a good mystery.
DeleteHi, Sure I can share the my data but how? Can't find anywhere an email address or a way to attach files? Thanks!
DeleteHi! Just wondering how your project worked out. Did you finish the M70's?
ReplyDeleteHi Jim,
ReplyDeleteThanks for asking. I just finished rewiring them, but haven't powered them up yet. The good news is I have recently acquired a BG4002 table, and got it working, and a BM2400. The wife is spending the winter in Florida, so maybe I'll get some serious work done this winter, and have it all put together. How's your project going?
Well my 4002 has been retiped and recapped and sounds wonderful. But I just found a bad tweeter on my M70 so on it goes.
ReplyDeleteFixed the M70!! Found the fine speaker wires were touching the magnet on both sides shorting the speaker!!
ReplyDeleteHello and thank you so much for documenting the work you have done in this blog.
ReplyDeleteYou'll be unsurprised to find that I've found a pair of M70s but what I intend to do with them might interest you.
I have spent many years as a sound engineer, working with some very fine speakers in my time, including ATC SCM 45A and ATC SCM100A monitors, which use a large, soft dome midrange driver, the ATC SM75-150 is remarkably similar to the ITT midrange unit in these speakers and from memory they sound quite similar. My intention is to build a pair of lower powered studio monitors, based on the combination of midrange and tweeter and taking advantage of all the good work already done to match these drivers nicely. Quite by chance, I found a pair of M70 on Facebook in a very poor state, with the boxes unusable, but all the drivers working, except for one of the tweeters, for which I've found an original, used replacement locally. Total cost so far £30 and a 120 mile round trip to collect them.
The cabinets were rotten and damp, but the drivers hadn't really suffered beyond the bass driver connections perishing and coming loose, which was a quick and easy repair.
Since I have carefully retrieved the front and rear baffles, I have an opportunity. The original M70 was incredibly good in terms of definition, but light on the bass. This isn't really surprising, since the SEAS 10" driver is confronted with a 27.7 litre, sealed encloser to work against, but the specifications of the driver show that it is suitable for a much larger cabinet, which could be either ported or sealed. Since the exercise will extend the bass, but won't change the efficiency of the system. I should be able to build new cabinets quite easily, with a greater capacity and extended bass. I've chosen a 65 Litre enclosure, which is within sensible boundaries for a sealed enclosure, but also ideal for a ported enclosure. The new enclosures, taking into account mounting the front baffle slightly differently, has internal dimensions of W33.8, D 30, H 63.8 for a 64L enclosure, taking into account the displacement of internal components. It can be conveniently ported with 50mm pipe (47mm bore), two ports at 7.7cm length. I can simply plug the port to choose between the infinite baffle and ported alignments. I'd like to upgrade the crossovers using components chosen for quality, rather than economy of manufacture, but I think that it makes better sense to upgrade one of the X-overs with like-for-like components at low cost, make sure it is working optimally, then upgrade the other X-over, checking for differences and doing my best to make adjustments as I go.
Your blog has proven an invaluable resource, even at this early stage. This is an exciting project, considering how I remember the sound of these drivers. Soft dome midrange units don't decouple in the same way fixed comes do. I think that's why they sound so smooth and dynamic. I think I stand a very good chance of achieving the sound of the M70 with the punch of the ATC monitors at a fraction of the cost!
Amazing! I've just found a used DATS V2. This opens up all sorts of possibilities, including perhaps using some different and perhaps more appropriate bass drivers!
ReplyDeleteNeckbone, thanks for reading and responding to my project blog. As you can see, it's been a while since I've published an update. I have a handful of reasons but will try to tell the rest of the story later this year. In the meantime, I'll read with interest your accounts, and thanks for sharing. I can't wait to learn how your bigger box project works out.
ReplyDelete