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

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.

Yes. As a rule of thumb at these low frequencies I tend to expect it to
'differentiate' the signal, so leading to a rise in sensitivity
proportional to frequency. The currents on mains systems (and to a lesser
extent, the voltages) tend to pick up a wild set of harmonics due to the
way other PSUs around the place react. Hence the fairly hairy result that
looks nothing like a 50Hz sinewave! Alas, that makes the result much more
audible than a pure 50Hz tone. :-/


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.

IIRC you're using one of the UCA series - i.e. in the same series as the
one I tested and reported on at

http://www.audiomisc.co.uk/HFN/ADC/USBrecording.html

but I can't recall if you're using one that includes the RIAA.

If you're not already doing so, you may get better results by using a
dedicated RIAA preamp and having longer coax leads from its output to the
Behringer. This would keep the fields from the Behringer and computer
further away from the low level signals from cartridge to RIAA preamp.

A problem with using the RIAA in the Behringer is that you'd have to use
longer signal leads at low level to move the unit away, and so that may not
help - or make things worse! Better to keep low-level signal leads short,
the snag being if that also moves the source of interference closer! catch
22.

FWIW I did some more recordings here this morning so can give some values
for reference. Here I get about -72 to -74 dB hum+noise before the needle
drops onto the LP. This is with the 0dB RIAA reference level at about
-15dB. So the nominal level of hum+noise is in the ballpark of -60
dB(RIAA). However the hum is almost pure 50Hz and some of the value is
noise over a wide band as I'm using a 96k sample rate. In practice, the
noise goes up as soon as the needle finds the groove. Dominated by LF
rumbles for the best LPs as the system goes down to below 10Hz.

Here this is with the 'PC' next to the deck and the ADC just under it. But
the 'PC's power supply is a brick down on the floor.

FWIW The RIAA preamp I use does have a 10 Ohm lift resistor in the
grounding. I found this made a measureable improvement in hum in most
circumstances of normal use.

Ignore the hum values given in handbooks and reviews. They tend to use a
weighting filter and reference level that is, erm, 'kind' compared with a
straight measurement like the above.

FWIW2 I've just been doing some theory estimates of thermal noise for other
reasons. (Comparing MM with MC as a result of a daft assertion in a
magazine! Old campaigners can probably guess the source given that the
assertion showed 'innocence' of the physics of thermal noise. :-) ) So in
context this is an interesting comparison between ideal world and reality.

Jim

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