On 07/03/2015 01:58, 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. You might actually get away with
using the 5v from the USB host but, since you already have a mains
feed for the TT motor, it's best to avoid use of the host supplied 5v
feed.


Makes sense, but I don't think it is the 5v USB feed causing the
problem. I'm now convinced it is stray field pickup due to the poor
quality, long phono leads. I tried 'shortening' the leads by wrapping
them into a small volume and the hum level reduced considerably. That
workaround plus the isolating transformer in the motor 50Hz feed has
given this result:

http://www.nu-ware.com/Misc/Screenshots/Dual505C.png

Left spectrum is motor running, stylus raised. Right spectrum is stylus
lowered into the lead-in groove. Hum is now considerably lower than
vinyl surface noise.

The waveform shows the (normalised to -1 dB) signal before/after the
stylus contacts the lead-in and before any audio is played.

I consider this to be good enough for my needs since I can't hear any
hum with this workaround, even at higher than normal listening levels.


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.


See my reply to Phil.