On 05/03/2015 09:40, Jim Lesurf wrote:
In article , Sumatriptan
wrote:


Most are higher order harmonics of 50Hz mains but some change in level
and frequency over periods of seconds and minutes.

Again, that seems like the cartridge and its wiring are acting like a
pickup for EM fields around them.

Is it possible for you to try something like a 10 Ohm resistor in series
with the deck's ground wire to the amp? And if you have a spare headshell,
try that with its wires open and then connected to make shorts?

No idea if that will help, but its the kind of experiment I'd try to get
some more info to think about.


Yes, what is suggestive of EM pickup is the preponderance of higher
order components. As you will know, EM coupling is dependent on frequency.

Haven't tried the 10 Ohm resistor yet but pursuing the wiring EMC idea
has given an interesting result. I wondered if it was movement of the
phono leads and USB cable rather than tt itself that causes the
hum/noise levels to change. Maybe they were acting as the pickup
antenna. The Behringer interface USB lead is almost 2M long so I tried
'shortening' it. Not physically! I just sort of scrumbled it up so it
formed a quite small EM antenna.

Result....the hum level dropped by 5 dB and is now very close to
satisfactory when listening. For me :-)

Removing the mains supply to the turntable gives a further 5 dB
reduction and there may be a second hum source. Doing this gives a RMS
hum/noise level below the -60 dB 'floor' of the Audacity level meters.
Not sure if I need to achieve this but will be fun trying.

I'll do some more tests later.



Motor is mains. If this mains supply is disconnected the hum level falls
by about 5 dB

By "disconnected" do you mean 'pull plug from wall socket' or 'using the
switch on the deck'? Do they give different results?


Unplugging and switching off give the sane result...a 5 dB reduction as
mentioned above.