Hi Jim,


Interesting that tone arm position had no effect but moving the entire unit
did. Did the move require changing the mains socket used?


No, this was from a socket shared with the PC it was tried horizontally
and vertically within the radius of the 1.5M mains lead. Changing
sockets resulted in somewhat worse hum.

Here I can measure changes in hum+noise level as the arm is moved, but I
can't compare that with what you get as I don't know the details of your
'test track'. The hum here is inaudible, though, regardless of where I play
the recordings or if I use speakers or headphones. cf below.


Test track was from an acoustic guitar album from the 1980's to get a
ballpark volume setting.


not audible to me at normal listening levels.

A complication is that how audible 'hum' may be will depend on more than
its simple measured level at the amp.


I appreciate that.

A) Depends on the room acoustics, speakers, etc. One room setup may make it
higher in level at the listener's ears than another.


Agreed, all of the above were different. I'm just trying to determine
how big a problem it is for me and how much time/effort I should spend
on chasing it. All ultimately subjective :-)


B) Is the hum pure 50Hz? Or does it have a lot of harmonics? That can make
a big difference to audibility.


It is harmonic rich although the main component is 50Hz.


Check the spectrum of the 'hum' and the above.

Picture = 1000 words, see he

http://www.nu-ware.com/Misc/Screenshots/Dual505A.png

Notes
1) The spectrum was obtained before I normalised the audio track to
examine the waveform.

2) Hum is with mains on at tt, motor off, tonearm in resting position.

3) As explained, I can reduce the Audacity measured RMS hum level to
around -55dB in this room by repositioning...moving it around while
looking at the levels.



Otherwise there might be a
risk that you make recordings with the hum level 'sounding low' but then
hear it when you play the results back elsewhere.


I'm in no rush...I'll be making test recordings and trying them on other
systems before I spend too much effort on the 'real thing'.