On Sat, 07 Mar 2015 09:48:56 +0000 (GMT), Jim Lesurf
wrote:

In article , Johny B Good
wrote:

Echo the comments about choice of cable. But if changing this keep in
mind that the cable capacitance also matters for MM cartridges.


Surely, you must have meant Moving iron or variable reluctance
cartridges with a notional 47K ohms impedance where this does actually
matter rather than the very low impedance MM types, typically 10 to 30
ohm impedance, which even several hundred pF's worth won't disturb in
the slightest.

Erm... Its normal to refer to moving iron or variable reluctance as 'MM' or
moving magnet as a class to distinguish them from MC or moving coil.

As Phil has pointed out MMs tend to have coil resistances much higher than
the "10 to 30" you state. I've just been doing some stats on this for
reasons I mentioned a while ago. And again as he says, the load capacitance
affects MMs and the makers tend to recommend values. (I've only found one
or two MC with recommended load capacitances. These being above 1 *micro*
Farad because their coil inductance is so small.)

FWIW since I did go though old reviews to collate values I can give some
results he

http://jcgl.orpheusweb.co.uk/temp/trans.png

shows the sensitivity and coil resistance (not the recommended loading)
of some MM and MC examples. It also shows the effect of using an x10 or
x20 voltage step-up transformer with the MCs.


Perhaps you're confusing the recommended *loading* with the actual
cartridge impedances, and saying MM when you mean MC?

This last bit, conflating MM as MC (in my mind) as you may already
have seen in my previous reply to Phil's post[1], is the error I'm
totally guilty of. Once you translate my use of MM as MC, my post then
makes perfect sense (other than for the fact that it was a pointless
response to yours).

[1] In my embarassed response to the mistake so eloquently picked up
by Phil, I overlooked his remarks about transformer shielding.

Mu-metal shielding _is_ sometimes employed, not internally but as a
total shroud around the transformer to screen magnetically sensitive
components from the inevitable stray leakage flux of classic E&I (T&U)
cored designs.

In this case, the flux levels are only a tiny fraction of that which
links the windings so doesn't represent a saturation risk to the
Mu-metal[2]. Although it can slightly mag up the Mu-metal or soft
steel often used as a cheaper substitute, this permanent magnetism
isn't a problem to nearby pickup coils (tape heads and phono
cartridges) which, in any case, is confined to the screening material
used, whether it be Mu-metal or soft steel.

Toroidal transformers otoh, so effectively confine their magnetic
flux, they don't normally require any such screening, not even when
placed inside the confines of an IBM 14 inch CRT monitor originally
designed for 120v mains in order to convert it to 240v mains. A feat
not possible with a conventional transformer even when placed outside
of said monitor if not a good 60 cms or more away as I discovered two
decades back.

[2] Is Mu-metal any worse than soft iron for self magnetisation? If
so, it seems a strange choice for the pole pieces of magnetic
replay/recording heads, especially since it has such poor wear
properties.
--
J B Good