Ken Betts explains what possessed him to convert a stereo cartridge into a mono one and the steps he took in the process. One to try at home, if you have a steady hand and a keen eye.
Building a moving coil cartridge is not the first thing that comes to mind when I wake up in the morning, at least not normally. But I had found these funny looking cartridges a few years ago at the Tonbridge Audio Jumble and wondered what to do with them, I knew that they were a bit suspect when I bought them, price says everything. On this particular weekend I had been playing records using an Ortofon 2M Blue MC cartridge and compared to a moving magnet the results were pretty good, that is until I came to play some of my mono records. They did not sound brilliant and I was puzzled as to why this should be, could they be duff recordings? Could I have some sort of connection problem in the phono preamp? Could it be the preamp creating problems? A quick check of the last two reduced the probabilities to the first one; suspect recordings. But these are first and second pressings of top dollar black stuff so what the snuff is going on? I spent the whole evening scratching my head over this and by the following morning I had decided to make a mono MC cartridge. Yes, that’s right, no knowledge, no experience, in fact like 99% of sane people I had only ever bought cartridges and that was the sum total of my dealings with the beasts.
Fast and furious
I had to read lots concerning how a stereo cartridge worked to produce two independent channels, not concentrating on construction but how the stylus and cantilever moved, and in what direction whilst it was being ‘dragged along’ the groove. Now I realised that the cantilever and hence the MC was being waggled not up and down or side to side but in fact diagonally and in a combination of both planes simultaneously. I spent an hour or two studying the construction of the Sumiko Blue Point Special to see if I could work out how to dismantle it, and just as important how to reassemble it after any work. I had read that the Sumiko was not a bringer of much in the way of bass notes and seeing the size of the four coils, two left channel and two right channel, it occurred to me that the stylus/cantilever had its work cut out trying to shift that lot; fast and furious was not really on the agenda. So I began to understand why some MCs are good and others are brilliant, its down to kinetic energy and the suspension including the damping properties of same.
The first thing that I had to sort out was the difference in the way the stylus tracked the groove in a mono record, it goes from side to side and this means that if a stereo cartridge is used then the recovered audio voltage will contain any information added to the left and right walls of groove, perhaps dirt or scratches, and also because of its design, any dirt or scratches that move the stylus vertically. Next was to figure out a way of getting the cartridge to produce output that is only composed of voltage generated by horizontal movement of the generator coils within the cartridge. That in itself is quite straightforward, just have a cantilever fitted with a coil and only allow it to move horizontally right? Not so, what about tracking weight for a start, it must have some form of flexible suspension to set the stylus at the correct angle to the surface of the record, if there is no suspension then it chews up the record. Another major problem that raises it head when trying to convert stereo to mono is the mounting of the stylus with reference to the coils. 45 degrees is how it’s done in stereo cartridges and possibly 90 degrees in mono. Looking at the back end of a stereo cantilever with the stylus vertical the coil armature simplistically forms a letter X with diagonal opposing arms carrying one of the two series connected coils for the left channel and the other diagonally opposing arms carrying coils for the right.
First I removed the cantilever complete with the coils and stylus, then the suspension and damping pad had to come out, preferably without damaging the cantilever and stylus. Using a microscope was the only way and some special tools, mostly of a cutting nature, had to be fashioned. As clock and watch repair has been a hobby for many years, this was not as difficult as it seems, the use of a fine oilstone mean that sharpening watch oilers into mini scalpels was surprisingly simple, and as they are made of hardened steel they took a fine edge. These scalpels were then used to remove the coil windings from the armature, at this point I should point out that not all armatures are of this construction and some may pose greater problems. In reality the cross shape armature is probably a better form as it is lighter than the square armature and suits this project well. The next step was the worst one for me as it meant passing the point of no return, get this wrong and I've blown it. I cut the aluminium tube cantilever off, Blu-tack is invaluble for this sort of job, leaving a small amount behind, probably 1.5mm long. This left me with the X armature and suspension and the cantilever as two pieces ready to refit later.
The suspension spring was removed from the armature in order to wind 20 turns of 0.03mm pure silver wire onto two arms of the armature. The plan here was to hold the armature in a small pin vice and twirl it round counting as I went, then reverse the armature and do the other side. Hah! Forgot that that side would be full of wire, back to drawing board, also nearly forgot that the silver wire is not insulated.
Above is one of the first ‘coils’ made to see if the whole idea would work before committing. Rethinking the generator coil winding lead me to wind the coils separately and then transfer them to the armature. The pin vice now held a small brass taper clock pin coated with candle wax, under the microscope I was able to wind the 20 turns in three layers, using cyanoacrylate (super glue) to hold the layers in place each time. I also insulated the silver wire with water based polyurethane varnish. As the brass pin I used was tapered and waxed the coil was now removable when the glue set, in fact I think I only had to make three to get two with almost exactly the same resistance. Resistance measurement proved insulation, no shorted turns, and also confirmed the number of turns, which affect the output voltage balance. The coils were put aside until almost the end of the project as they are vulnerable and easily fitted over the two required arms of the armature at the last moment.
Turning attention to the cantilever and stylus I decided that I preferred the stylus of an Ortofon MM that I had as it has a fineline profile and I believe that it’s more suitable for the groove in mono records than the elliptical stylus fitted to the BPS. I had to fit a cantilever and stylus onto the stub left behind on the BPS armature, so I removed the same length plus 1.5mm of cantilever, including stylus, from the donor Ortofon MM. This had an aluminium tube cantilever that is almost the correct size to fit inside the BPS stub, however a small amount of resizing by inserting a fine steel pin into the stub and the rubbing the aluminium tube increases the internal diameter sufficiently to enable a snug fit of the ‘new’ cantilever. This is fixed permanently using cyanoacrylate glue which does a good job as it’s thin enough to get well into the joint via capillary action.
The original BPS armature is now fitted with a cantilever and stylus but the stylus is in line with one pair of ‘arms’ and at right angles to the other two, this is what we need to enable two coils to be mounted horizontally for maximum voltage generation in that plane but minimum in the vertical plane. Because the generator coils are positioned near to the cantilever’s fulcrum point the coils will tend to rotate slightly with vertical motion but move in a clear arc with horizontal motion. It did occur to me to remove the two unused ‘arms’ but I realised they would act as a vertical support when pressed up to the damping pad, with little or no effect on the horizontal motion. Subsequent tests with stylus gauge showed this to be true.
The assembly was now offered up to the body of the BPS and the suspension pulled-up just enough to hold every thing in place against the damping foam. Next came the trickiest part of the job, fitting the coils over the ‘arms’ of the armature and soldering the wire ends to the connecting pins. It was at this point I realised that the coils generated voltage in opposite polarities whilst moving horizontally within the magnetic field and came to the conclusion that if I fitted the coils so that they were in fact wound in opposition I could generate the same signal phase and polarity in both coils. This should result in the same phase/polarity audio signal in both left and right channels of the amplifier chain, I considered this to be much better than the summing arrangements normally used so went ahead with assembly in that manner, again fingers crossed that I was not theorizing incorrectly. The construction of MC assemblies does not really allow many rethinks so it had to be right. Now it was time to ‘bolt it all together’ and try things out.
The suspension was set by using a down-force gauge to make the stylus effectively sit perpendicular to the record with the tracking weight at about 2.5gms, this meant that when installed in a tone arm with tracking weight set to 2.5gms the stylus should be at the correct angle to the record surface. With a fibre paper dust cover fitted over coils listening tests proved that my idea does work and that because of the small coils and hence small inertia requirement it really is extremely quick and the stylus does pick out fine details. Happily my theory about surface noise again proved correct as the mono playback from good but old records is very quiet with scratches being minimised.
Above is a picture of the second MCI built using copper coils, this time I fitted a sapphire cantilever with an FG stylus and mounted the motor of the other BPS I had into a body carved from a piece of well seasoned oak, which takes ages if you are not clever with wood carving tools. But it seems to have worked out and even sounds good.
Now I have good mono as well as stereo cartridges I suppose I should try to figure out away of using them without the hassle of changing cartridge when moving from a stereo to mono record. The best way is to have a turntable fitted with two arms and all the necessary switching and connections to suit. I’m not sure about that, it means modifying and perhaps a bit of butchery to some bit of kit I would have to buy. I'll sleep on it.