On Wed, 04 Mar 2015 19:09:58 +0000, Sumatriptan
wrote:

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

Wow! That's impressively rich in HF harmonics which suggests the use
of an unscreened[1] transformer for powering the RIAA pre-amp[2]
allowing capacitive coupling of the higher frequency harmonics and
noise to dominate the mains hum interference.

The actual plot of the waveform bears absolutely no resemblence to
what I see via cheap 6v ac wallwart transformer used to supply a
nominal 1v level sampling of the mains via a simple resistive
attenuator for feeding into the line input of a laptop for comparitive
measurements between the real mains and a petrol generator I foolishly
thought would make a good IT kit standby supply a few years ago.

The resulting waveform in CoolEdit's window looked indistinguishable
from an oscilloscope's trace of its own mains derived 1v Pk2Pk
'calibration source' which quite clearly showed that the mains was far
from being a pure sinewave (essentially it looked like a sinewave with
the tops slightly but most definitely clipped with a slight downward
tilt to the 'flat tops' on the positive peaks and vice versa for the
negative peaks.

When I first observed this nearly two decades ago, I just assumed it
was an effect of the transformer. It was only much later when I
repeated the test with a SmartUPS2000 in line that I discovered this
was not so since the waveform of the UPS looked a perfect example of a
sinewave, except for some very low level sample switching artifacts
(around the 5KHz mark afaicr).


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'.


[1] I'm referring to the inter-winding screen not a whole transformer
shield which would typically be either mu-metal or soft iron unless a
toroidal transformer is being used to make such magnetic screening
totally redundent as I well know from experience.

[2] I've assumed you're using an RIAA pre-amp of some sort in the TT
itself (quite honestly _the_ only place to do the RIAA
pre-amplification).

You'll have enough trouble competing against the millivolt or so of
mains hum at line levels even when the equalised cartridge signal is
safely boosted into the volts range in the hum loop free environment
of the confnes of the TT itself before it has to bully its way past
the hum loop afflicted line level interconnection to the main amp's
auxilliary input.

Unless you take extreme measures to eliminate hum loop noise, feeding
the cartidge output directly to the phono input of the main amp via a
yard or so of high quality screened cable often leaves you on a hiding
to nothing.

Mention of "high quality screened cable" reminds me of another
possible reason for the predominence of HF harmonics and other noise
in your test signal. Some, so called screened cable can have very poor
screening, in some cases just 3 or 4 strands spiral wrapped around the
signal wire, which makes them very prone to pick up electric fields
radiated by nearby (and sometimes, not so nearby) mains wiring around
the room. I'd certainly be examining the 'screened' cabling very
closely if I were in your current predicament (not forgetting that
phono plugs can fail to make a proper earth return connection in the
socket which can also produce the same symptoms).
--
J B Good