Moving Iron Adventures


I have been a user of the London (Decca) Reference for the last eleven years. It has been good enough that I have had no desire to replace it—none at all! It seems a bit unusual for an audiophile to acquire some piece of kit and then say "That’s it!" for eleven years! Perhaps some of my complacency was due to having nice Quad tube amps and electrostatic speakers. But then things changed for me...

Everything got upset with moving house. My phono stage refused to work. My electrostatic speaker started to click and arc. And amongst it all, as I switched in other components to keep the music flowing, I listened more carefully to everything, including my turntable. I became aware my beloved Reference might no longer deserve that title as it was not now what it had been. Off it went to the UK, just in time as John Wright retires very shortly, for a full rebuild.

So in the meantime, I started to reluctantly expend monies on alternatives. I may have been a physician, but I had to stop working when I discovered that leukemia was to define my remaining life. Now I have to live upon my savings, and must balance audio delight against other necessities and a probable shortened lifespan. Go ask your financial advisor how to do that—I don’t have one and never have liked to think about money.

First job was to educate myself about other moving iron options, and several were available. None looked anything like a Decca cartridge: they have cantilevers! So, I learned that there were quislings who would make an MM cartridge where the magnets were not on the cantilever, but nearby both the fixed coils and the ferromagnetic mass on the end of the cantilever.They called the result moving iron. This was rubbish to a Decca user, but I realised it could only make any sense—compared to a moving magnet—if the moving ferromagnetic mass could be a whole lot smaller than the magnetic mass of an MM cartridge in the same place. Rare earth magnets have come along and make MM cartridges far better than they could be with iron magnets. Now what if the strong rare earth magnet sits still inside the cartridge body, and a small, lightweight sliver of ferromagnetic material sits on the end of the cantilever, waving around in the fixed magnetic field of the rare earth magnets, and induces a current in the fixed coils also in the cartridge body as it does so? Welcome to Grado, Soundsmith and Nagaoka. They do work, and surprisingly quite well. But don’t get excited, as they don’t work as well as the old Decca design.

Wisely or not, I bought an example from each company. A Grado Statement 3, a Soundsmith Sussurro MkII, and a Nagaoka MP-500. I didn’t tell you above that the London Reference was not my first Decca: I had bought a London Jubilee, and used it for about two weeks before I told myself that it was so good I simply had to spend twice as much to see what its big brother—the Reference—could do. I was still working then and could do that kind of thing. So now I could compare the new purchases to the Jubilee, knowing the Reference outclassed the Jubilee very handily.

All comparisons were on an SME Model 10 turntable, with a Series V tonearm. I was, and still am, surprised by the results. The MP-500 took about 15 hours to run in and after that it changed little. It was very close to the sound of the Deccas, so much so that I felt I had to take rather dramatic steps. I found another SME 10 table, used, and with the M10 arm (akin to the 309) that SME sold it with originally. I intended to sit the MP-500 on that that tonearm and table and that would delay the inevitable end of my two London Decca cartridges when they were no longer repairable after John Wright’s retirement. It is lively, dynamic and full of toe-tapping goodness.

I’ll sidetrack myself, and try to say what I’m trying to achieve. After 11 years of the London Reference, what is it that makes me want more of the same? I can’t speak to soundstage and imaging: I only have one somewhat damaged ear and I have no directional sense of hearing. Quality of sound is it—the whole thing—for me. Sure, I like some bass, and I don’t remember what my last audiogram said about high notes. But I did spend twenty five years flying to Toronto three times a year to attend two operas on each trip. I know what live orchestral music sounds like to my wretched ear, and I want more of it at home. I heard the difference when the COC moved from the O’Keefe/Hummingbird Centre to their new purpose built home. I was there for the first Ring Cycle and I mourn Richard Bradshaw’s early demise as much as anyone. In my earlier life, I had attended the Festival Hall in London for concerts (though with much cheaper seats!) As for modern live music, I have less experience. Two Supertramp concerts in Halifax, NS, and I was involved in supplying most of the pot for the Watchfield Concert in Wiltshire in 1975, where Hawkwind failed to turn up as booked. Attendees needed a lot of consolation.... So what am I looking for, aurally? Without clever terminology, it is the unconscious desire to tap my feet to the music. The sense that it is live, rather than a recording. I can’t identify anything in terms of frequency or timbre, and it probably comes down to responsiveness and timing to convince my ear that I’m hearing it live. Even SACDs through an Ayre C5-xe don’t quite match that, even if, analytically, I can’t say why they fail. Like Supreme Court Justice Potter Stewart on obscenity, "I know it when I [hear] it."

But to get back to important things, when the other cartridges came along...

The Grado Statement 3 proved to be accurate, honest and utterly boring. Very good at not reproducing surface noise. Maybe another tonearm would let it show its worth. I have gone back to it a couple of times, but I still get bored without being able to put a finger on a particular fault. Even after using an SPL meter to make sure I was comparing it fairly, it couldn’t compete against the (now returned and rebuilt) Reference.

The Sussurro MkII initially disappointed me. Eventually I RTFM and connected its 0.47mV output as an MC cartridge and set the loading to 800Ω as recommended. Now it is close to being as good as the Nagaoka, but doesn’t quite match it. But, along the way...

Like many, I have some older cartridges stored away. The Benz Micro Ruby 3 turned out to be a competitor, but the Ortofon Kontrapunkt C actually came the closest to the Decca sound I was trying to simulate. This surprised me a great deal, as I had used it some 12 years ago before the Deccas, where I thought it was an overly detailed and rather etched sounding pickup. I’m probably a bit more experienced at setting the VTA now, and my phono stage is certainly a lot more capable than the one I was using then.

So what to do now? I have spent good money and can’t honestly sell my purchases on as "you might like it but I didn’t" items. I have decided to keep my options open, with two tonearm pods custom machined to allow two extra tonearms to play on the two SME 10 tables. I was going to use my collection of SME 309 headshells, but the price of the cheapest SME tonearm still sold without a table underneath it is prohibitive. ($4.2kCDN each for the M2-9, which is more than I paid for a Series V arm!) So they will have to have Rega RB330 arms, and I can sort out the HTA and VTA by moving the pods around, and up and down). Pretty obviously pride of place on the Series V will be given to the London Reference, with the Jubilee as backup when it wears out, and if no one can be found to re-tip it at that time. The M10 arm will have the Ortofon Kontrapunkt C on it, and while I had thought I might replace when needed with a Cadenza Black, I’m now thinking I’d do better to re-tip it. The two extra tonearms will house the lovely MP-500 (I already bought two spare styli as it is that good!) and the Soundsmith Sussurro MkII. My phono stage (Musical Fidelity NuVista Vinyl) lets me attach up to five cartridges and remembers their settings as MM, MC, capacitance loading, resistance loading, ±6dB amplification, rumble filter on/off. The Grado Statement 3 goes in the cupboard with my old Benz Micro Wood H2 and Ruby 3. Should I get lucky and my recent bone marrow transplant lasts longer than expected, one day the MP-500 will replace the worn out Reference and Jubilee on the Series V arm. I don’t think that will happen in my time, but these ideas might provide guidance to others!

Right now, I allow myself one LP a day via the Reference. Everything else is played on the second table, so that provides the majority of my music now. In a couple of months the tonearm pods will be here, and I shall have three other options, populated as described, to prolong the lifespan of the Reference.

 

dogberry

Markus,

This is what I wrote about it:

I spent three hours today looking over the shoulder of a skillful repairman as he dissected a Quad 2905. It takes about 20 minutes to get the speaker stripped down to the point where you can see and test the panels. There are six separate panels, two bass at the bottom, two mid/high range in the middle, and two more bass panels at the top. For those who don't know, each panel is a sandwich, with a plastic grid of 2cm squares on the outside front and back. Inside this is a copper panel perforated by a grid of small circular holes, resting against a phenolic board like a PCB (again, front and rear). Between the phenolic boards is a Mylar membrane, glued at the edge to the rear plastic grid's outer edge. There is a 2-3mm gap between the Mylar and the PCB in front of, and behind it. The Mylar has a graphite coating all over except for the very edges. High voltage is fed into the copper stator on each side of the Mylar, and the graphite responds by by being drawn to or repelled from the stators. The stators are solid single pieces of copper for the bass panels as low sounds are non-directional. The mid range/treble panels have the classic Quad arrangements of concentric circles of the copper mesh, each part circle connected separately through delay lines so that the two treble panels act like one large concave/convex panel, simulating a point source.

All the delay lines must be unsoldered to remove panels, and reconnected afterwards.

My faulty panel, this time, was the lowest bass panel, and the phenolic board at the front of the sandwich with the copper stator on it was detached from the plastic frame at one side. Originally, it was glued in place. Careful probing with an insulated tool makes the loose part of the stator arc noisily as it is pushed towards the Mylar. So that panel was removed and replaced with a new one ordered from Mo-Fi some weeks ago.

However, the four original panels in that speaker (two now being replaced with new ones) all have the same problem developing, and all on the same side of the front face. They have come loose from the frame, but are not yet loose enough to cause arcing. Comparing old and new panels one can see some differences. The glue on the originals is slopped around a bit, but is extremely sparse or absent where the phenolic board is loose. The new panels have a visible bead of glue connecting the stator to the plastic grid.

So we scratched our heads for a bit, and discussed various glues. We don't want water based glue, and we don't want it to penetrate through the phenolic board. We decided to place a blob of industrial hot glue at each cross of the grid, and then used a dental pick to pull the copper stator back up against the glue as it set. We used one of the old panels as a test first to ensure the Mylar would not be hurt by the heat of the glue - it wasn't. Then I remembered the roasting bags for poultry, which I think are made of Mylar, so it should resist heat OK. At the end, all the stators on the front are well-affixed to their frame. None of the stators on the back of the panels were loose at all. Looks like a poor manufacturing process led to insufficient glue, and I'm guessing Quad became aware of it since the new panels have lots more glue on them. Then the speaker was reassembled and we set off home.
It works just fine now. So what was the point of the hot glue? As the end of the stator starts to flap in the breeze it gradually detaches for more of its length, and at some point it can move enough to touch the Mylar and it arcs. After a short (pun intended) while there is a hole burned right through the Mylar. By fixing the stators firmly with glue I hope we have reset the clock on those panels, or at least at that end of them, as the other end had no looseness, and nor did the rear stator as I said above. Time will tell.

As for doing it myself if needed, I think I can. There are airtight dust screens that look like Saran wrap on the front and rear of the speaker, to keep dust out (!) - because an accumulation of dust would increase the risk of arcing. The PCB with the delay coils and circuitry on it is coated in wax, probably beeswax, as it is outside of the sealed dust screens, and again any dust gathering between the terminals that the delay lines are soldered onto will encourage the terminals to arc. So after re-soldering the delay lines we dripped a beeswax candle onto the terminals. Not as pretty as the factory finish, but it should work.

Now the other speaker hasn't had a problem at all, so far. I could take it apart now and go at it with the glue gun if needed, but I think I'll wait until it starts to cause trouble, replace the panel that goes, and apply glue to all the others if showing signs of loosening. Fingers crossed, it won't happen...

So if you are hearing a buzzing, you probably have a panel that has begun to detach from the plastic grid. Put enough energy into it when you turn up the volume and you make it vibrate, and it will gradually come loose over a longer section. Ultimately it gets so loose it comes close to the Mylar and it arcs. Very quickly a hole burns through the Mylar and the panel is toast. The hot glue serves to affix the stator panel back to the plastic frame, so that it doesn't work loose and get close to the Mylar. I wish I'd had the foresight to photograph everything as we did it. The anatomy inside each panel is hard to describe with words alone. There are some photos on this site. At least you have but four panels in each speaker to worry about: I have six!

@dogberry , Mylar will shrink when heated with a heat gun. After final assembly Acoustat panels were run under an infrared heater to tighten the Mylar. If the speakers were not crossed to a subwoofer, with age the diaphragms would loosen a bit, and they could rap against the stators with loud bass or a bad warp. A heat gun fixes the problem. Jim Strickland, the brain behind Acoustat, discovered that by using a certain type of insulation on that stators you could prevent dangerous arching altogether. Roger West of Sound Labs uses a similar construct but smaller individual panel sizes. They never get lose enough to rattle even if you require them to make bass.

In regards to cartridge design, it is the effective mass that matters, the distance of the mass from the fulcrum along with it's size. 

I have heard both the Strain Gauge and Hyperion. Peter spells his name with two Ns, Ledermann. The Strain Gauge immediately gets your attention with fast transients and a brightness that does not increase sibilance, so it is easily tolerated, and harder to identify. The Hyperion sounds rather boring on first listen. Turn up the volume to more realistic levels, and it becomes apparent that the Hyperion is the more accurate cartridge. It is also a fabulous tracker, the Strain Gauge is not. I could live with an Hyperion had I a phono stage that could mate with it. I did have The Voice for about 1 year. It is the most accurate high output cartridge I have ever heard, and a terrific value matching cartridges twice the price. The ART 1000 is a scary design. My instinct, right or wrong, tells me to steer clear.

 

Small point: Sound Lab panels are a single large sheet of Mylar that is sandwiched between a pair of egg crate grill-like hard plastic  dividers, resulting in small rectangular divisions of the Mylar that are variously sized to spread out their respective resonant peaks. SL like to bias their panels at very high voltages compared to most other commercial ESLs, and possibly as a result they had problems with arcing for many years. The combination of high voltage and inadequate stator insulation were culprits. SL used to endorse having customers “kill” any of those individual rectangles that were arcing, by injecting silicone caulk across the space between the Mylar and the static where arcing was occurring. The affected Mylar would also develop holes, eventually. The whole problem was a nightmare for owners like me, until eventually after killing so many rectangles, you sent the whole panel back to the factory for total rebuild. About 10 or more years ago, they seem to have solved the insulation problem. My 845PXs , purchased back then, are doing fine.

Autocorrect made when one of its usual blunders.  In the phrase "...the space between the Mylar and the static where arcing was occurring...", the word "static" should be replaced by the word "stator".  ESL enthusiasts would have figured that out.

 I wasn't really aware of moving iron cartridges until I bought used TT with one mounted. I have no complaints with the sound, but yes, they certainly are prone to induced hum. I still have the cartridge but have replaced it with a MM which is in the ballpark sonically.