From: Patent Tactics, George Brock-Nannestad
Hello, on stylii for Diamond Disc
First of all, the groove in Diamond Discs has an elliptical, almost circular
profile; there are no "walls" as such. The reason the shape is elliptical in
principle is that it is cut by the edge of a cylindrical stylus, which is
held at an angle, and that geometrically creates an ellipse. The modulation
is very, very small, and in order to create a sufficient sound pressure by
mechanical replay, the lever ratio between the reproducing stylus movement
and the diaphragm movement has to be very large. This means that the pressure
on the groove is huge. This is the reason why a vertical recording is
essential here: the pressure is supported by the record material. A lateral
groove wall would never carry the same pressure.
The small modulation requires a very fine-grain and hard record material.
Now, as the groove is "part-circular" in cross section, what is available to
the reproducing stylus is an arc of a circle that varies from what it is at
maximum depth (as shown in the 1917 drawing; it should not go much deeper
than that, because it then cuts away the material from the neighbouring
grooves) to very little, when the stylus is near the surface of the cylinder.
The pressure on the material is largest in the latter situation, because the
area of contact is the smallest. And this is the reason why we need a good
fit of the stylus for mechanical reproduction: to maximise the area of
contact. Now, even if the modulation were deeper at its deepest and right at
the surface at it most shallow, the stylus will still track beautifully,
because there is a threaded spindle that carries the soundbox across. It does
not really matter if the neighbouring grooves are partly eaten by deep
The situation is remarkably different with a modern pickup that may not be
driven by a threaded spindle: the tracking has all to come from the tip of
the stylus. Here, the use of a pickup inside the shell of an old reproducer
on an original phonograph is not such a bad idea, if the rumble figures are
small enough. The stylus obviously ought to have the correct radius circle in
contact with the groove, but it is not nearly as essential as in the case of
mechanical replay, because the wear is less; there is virtually no pressure
when compared with mechanical replay.
When truncated elliptical stylii are used, it is actually the truncation that
is in contact with the groove, and that is not at all well controlled, and
you may find cases where they do not conform well. One way of making
truncated "elliptical" stylii is to have a conical stylus and polish off
facets fore and aft, creating a chisel-sharp edge, which you then flatten and
polish round. Sometimes you have a good shape here that fits well in 2-minute
cylinder grooves (DD grooves are however quite narrow). What you really want
is a line-contact diamond with a circular section for the line (a door knob,
in fact). A sphere is no good, because it does not track very well--in
principle!! Because, due to the small modulation and the long wavelengths at
80rpm, sometimes there is not much to be gained by going from sphere to
doorknob. When you go for line contact, it must be oriented correctly in
space, or it will not interface properly with the groove. This is very
difficult with a high-compliance pickup, because the angle varies
tremendously with the vertical stylus pressure, and dependent on the angle of
the cantilever, the point (or circle) of contact crawls back and forth.
Reproduction of mechanical recordings is not nearly as simple as you would
Jerry Fabris wrote:
> I've located two historic measured drawings for the Edison Diamond Disc
> phonograph diamond stylus. Scanned photocopies are here:
> The June 28, 1917 drawing shows a tolerance of 0.00375 inch (3.75 mil)
> maximum radius to 0.0035 inch (3.5 mil) minimum radius. The October 25,
> 1916 drawing shows a 0.0035 inch (3.5 mil) radius with a 60 degree angle
> Expert Stylus Co. offers a 0.0037 inch (3.7 mil) radius spherical stylus
> for Diamond Discs. The owner of the company told me that this size was
> based on measurements he took from an actual Edison Diamond Disc
> phonograph reproducer. I have tried this Expert 0.0037 inch radius
> Diamond Discs stylus, mounted on a Stanton 500 cartridge, playing
> electrically. It doesn't sound right to me. I hear unnecessary noise.
> For electrical playback of Diamond Discs, I've had the most consistent
> luck with a 0.0020 inch (2.0 mil) radius spherical size. This is based on
> what sounds best to my ears, not based on objective measurements.
> Results can vary from disc to disc, so try different sizes.
> In general, I'd say that it doesn't make sense to use truncated styli on
> vertical cut records, since the information is on the bottom. A good
> argument can be made for using elliptical shapes. Beyond Diamond Discs,
> when playing wax or other soft material records with non-standard
> sizes/shapes, one should always be extremely wary of causing wear damage.
> -Jerry Fabris, Museum Curator
> Thomas Edison National Historical Park
> West Orange, NJ