In article
,
D.M. Procida wrote:
Johny B Good wrote:


There is one thing I don't understand about bias though.

It seems to me that since the spiral groove moves the needle inwards, it
must be the face of the grove that's towards the outer edge of the
record that's exerting a force on it.

In other words, if the groove were a wide U-shaped-channel rather than a
V, the needle would be pushed inwards by the wall of the channel on the
outside.

So I'd expect there to be higher pressure from the needle on that face
of the channel.

Yet bias adjustments exert a force moving the tonearm outwards, in the
opposite direction, which seems to me will further increase the pressure
on that wall, whereas to eliminate it, the bias should encourage the arm
to swing towards the centre.

As JBG has explained in detail. The (unwanted) radial force comes from the
geometry of the way the arm holds the stylus wrt the record. This is why I
referred to 'overhang'. The stylus is located at a point that is further
from the pivot than the place along the arm that is perpendicular to a
radial line. i.e. overhung. And angled.

The rotating groove 'drags' the stylus. Some of this force tries to pull
the arm. But the drag is inclined inward at an angle to the arm. So it also
tries to rotate the arm and move the stylus inward, radially. In the
absence of any other correcting forces being applied this (normally) means
the stylus pushes against the 'inner' wall (i.e. the one closer to the
center of the LP) than the 'outer' wall. So an "anti skating" or "bias"
force is added to try and counter this effect by trying to pull the arm
outward by a balancing amount.

This isn't just a matter of keeping the sylus in the groove. It also
affects the level of distortion and mistracking of louder sounds. So has an
audible effect on the music.

IIRC the 'bias' idea was actually devised by John Crabbe (editor for many
years of HFN) who (I think) came up with the 'thread and weight' method.

The problem is that the actual drag varies. Depends on the details of the
LP and the stylus. So varies as you play a normal LP, and from one LP to
another. Hence any adjustments made using a test LP tend to be just getting
you into the right kind of setup. Better than not bothering, but not a
complete solution.

Slainte,

Jim

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