On Fri, 6 Mar 2015 20:02:38 -0800 (PST), Phil Allison
wrote:

Johny B Good wrote:


One possible workaround, using such an RIAA USB ADC, is to fit it
into the TT itself, along with a USB B socket and either locally
supply the 5v from an analogue PSU using a small toroidal mains
transformer or quality Mu-metal screened (with interwinding foil
shielding connected to the local ground return point of the amplifier)
or else a battery sourced 5v supply.


** There are no Mu-metal shielded mains transformers.
Normally, a copper strap around the outside suffices or a wrap made from transformer steel or simply a steel box.

Don't think Mu-metal can be used, as inrush surges and saturation of the core would likely cause the material to become magnetised.


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.

** MM = moving magnet, the most common type of magnetic PU cartridge.

Most have about 500 to 700 ohms DC resistance, 0.5H of inductance and a recommended load of 47kohms in parallel with 250pF.

Moving coil ( MC ) cartridges have low resistances, generally from about 1ohm to 30ohms.



Drat! I misread (Gawd knows why!) MM as MC. Yeah, Moving Magnet is in
the same class as Moving Iron / variable reluctance which allows for a
stronger magnetic flux without trying to move a relatively heavy
magnet around, along with the luxury of static windings with enough
turns of wire to generate higher voltages into more sane load
impedances, notwithstanding the necessity to aim for a preferred
capacitive loading to help hold up the frequency response at the high
end by making use of the resulting high inductance of the pickup coil
windings to provide a rather damped LC parallel resonance voltage
magnification effect.

Modern MM carts using today's wonder rare earth Neodium magnets would
probably compete well against variable reluctance designs in terms of
both output sensitivity and transient response.

Apologies to Jim for picking up on an a non-error.

Obviously I was referring to MC types with my low impedance remarks.
I just misread MM as MC and failed to realise the error even when
typing the response. :-(
--
J B Good