On Thu, 16 Mar 2006 23:23:34 -0000, "Serge Auckland"
wrote:


"Bill Taylor" wrote in message
...
On Thu, 16 Mar 2006 19:08:27 -0000, "Serge Auckland"
wrote:


Similarly, frequency response plots of moving magnets and moving coils
don't
show any particular benefit to the MC, nor does stereo separation or
harmonic and intermodulation distortion. So why *are* MC cartridges
throught
to be better?

If anyone knows of any good engineering reasons why this should be so, I
would be most interested to hear.

The only vaguely convincing explanation that I've heard is that MCs
can use less damping of the stylus assembly. Theoretically this
should allow better trackability at mid frequencies. I also have vague
recollections that the cantilever can more easily be made shorter,
which would help in keeping the effective tip mass down, which should
give better HF trackability. (Compliance only needs to be more than
about 10-12c.u. to track all records)

Not being a member of the MC owners club, I don't know if these
theories have any validity.

Bill

Interesting theories. Can you tell me a bit more as to why a compliance of
10-12cu is sufficient for all records? Interesting that in vinyl's heyday,
some cartridges were providing 30-40 cu. It could be a marketing exercise
rather than having a sound engineering reason for it, but it would be useful
to know why such high compliances are not necessary.


According to J Walton in "Pickups - The Key to Hi Fi" (published 1968,
but the physics haven't changed): the maximum excursion on an LP is
about .005cm, "if compliance were the only factor involved a
compliance of 2 c.u. is quite sufficient to track the largest stereo
amplitude of .005 cm at 3 gm tracking weight". It's possible that a
small number of modern LPs have slightly higher excursions, and
tracking weights are a bit lower, so a slightly higher compliance is
needed, but not that much higher.

The very high compliances of the 70s were very much a marketing
excercise. The other important factor with compliance is the LF
resonance with the cartridge/arm mass. The ideal resonant frequency is
about 10-15 Hz, any lower than that and the cartridge system is liable
to become unstable and skip grooves with the slightest disturbance,
and the oscillations caused by these disturbances produce very audible
wow and tracking force variations. Some of the high compliance
cartridges needed a negative mass arm in order to get the LF resonance
up to a reasonable frequency.

I recently changed my turntable. The only reasonably priced one that I
felt that I could trust was the Technics DJ turntable (basically a
1970s HiFi turntable with a speed control). The supplied arm has
quite a high effective mass and with the Shure V15-V that I had to buy
as well the LF resonance is plainly much to low, this cartridge has a
more reasonable compliance of about 23c.u., but it is still too high.
Forunately the Shure damper more or less controls the resonance.

There is another advantage of MCs that was certainly true in the 60s
and 70s. It is much easier to create a large magnetic field with a big
static magnet in the body of the cartridge than it is with a small
magnet on the end of the cantilever, so in the days of less effective
magnets it would probably be possible to keep the effective tip mass
lower with MC rather than MM cartridges.

Bill