"I could never afford to purchase a Schroeder,
 and The Woody is every bit as good.
Every parameter you could
want is easily adjustable; and it's so damn beautiful,
and so musical too."
- Ted Danowski

Determining Tonearm Mounting Dimensions

Setting up an armboard or plinth to get a pivoting tonearm mounted in the right location can be a challenge until one figures it out. 

Pivoting tonearms can not keep the phono cartridge cantilver aligned with the record groove across the entire record.  In fact, theory shows that, in the swing of the arm over the record, the cantilever is aligned with the groove at only two points, called the inner and outer null points. 

Different alignment families, such as the Baerwald family, the Stevenson family, and the Lofgren A and Lofgren B families make different compromises in the effort to minimize tracing error (deviation of the cantilver axis from alignment with the groove). We personally have settled on the Baerwald family, but arguments can be made for each of the families.

Here is the deal:

The parameters that have to be balanced are 1) the tonearm mounting distance, 2) the overhang, and 3) the tonearm effective length.  These are shown in the image below.

The mounting distance is the horizontal distance between the vertical axis of the tonearm pivot, and the axis of the turntable spindle.  Ideally this distance will be adjustable to some degree by moving the armboard, but sometimes, particularly in old-time plinths, this distance has not been made adjustable.

The overhang is the horizontal distance the stylus point overhangs the center of the spindle, when the tonearm is swung to place the stylus point on an extension of the line joining the tonearm pivot vertical axis and the spindle axis.

The tonearm effective length is the sum of the mounting distance and the overhang.

For each alignment family, with accepted nominal values for outer record groove radius and inner record groove radius, 1) given a mounting distance we can calculate the correct effective length and overhang, or 2) given an effective length we can calculate the correct mounting distance and overhang.

To calculate the parameters for tonearm mounting, go to . Select Data from the choices at the top of the page.  On the page that comes up, select Tonearm Database,  then select Alignment Calculator.  Input either the effective length or mounting distance into the calculator and press the enter key.  Everything you want to know about tonearm parameters will be revealed.

Most tonearms are designed to allow the phono cartidge to be adjusted forward or rearward in the headshell.  For these tonearms it is generally best practice to attempt mounting the tonearm at the mounting distance specified by the tonearm manufacturer.  When the arm is mounted, measure the actual mounting distance achieved, and calculate the desired overhang using the actual measured mounting distance.

Some tonearms (such as the SME arms) are designed with the phono cartridge position fixed  in the headshell. These arms have a built in capability of adjustable mounting distance.  Because cartridge mounting geometry varies, the actual effective length of a tonearm with a fixed cartridge depends on the cartridge mounted.  For these arms the effective length specified by the manufacturer is only a rough guide.  For these arms the user can mount the cartridge, measure the effective length, and calculate the desired overhang using the measured effective length.