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ARSCLIST  January 2010

ARSCLIST January 2010

Subject:

Re: Styli for playback of old Edison Diamond Discs

From:

George Brock-Nannestad <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Tue, 12 Jan 2010 01:24:55 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (99 lines)

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 
modulation.

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 
think.

Kind regards,


George

-------------------------------------------------

Jerry Fabris wrote:

> I've located two historic measured drawings for the Edison Diamond Disc 
> phonograph diamond stylus.  Scanned photocopies are here:  
> http://www.box.net/shared/km1ycvl2i7
> 
> 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 
> up.
> 
> 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

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