On Thu, 05 Mar 2015 00:55:31 +0000, Sumatriptan
wrote:

What do you mean by the 'deck'?

I meant how is the deck powered, by mains AC, or low-voltage DC from a
wall-wart or inlne PSU block, but you've answered that below.

The interface between tt and PC is a
single unit containing pre-amp and RIAA eq. and a ADC followed by a USB
interface. It is powered via the USB from the PC.

With tt input disconnected from the pre-amp front end, hum/noise is too
low to be measurable using Audacity. S/N ratio is given in the specs as
89 dB @1 kHz

So, that would seem to make it the actual deck then - nothing else
seems to fit the facts.

1) Is the deck powered by a wall-wart or an inline unit and
low-voltage DC actually coming into the deck, or is it mains coming in
and its motor works directly off it? If the former, I'd suspect the
PSU.


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

Do you mean actually pulling the plug out of the wall, or just
switching off, which might leave the deck still attached to neutral?
(I can't remember for sure whether power-points have double-pole
switches, but I think not.)

That also suggests that the hum may have two, or more, sources, and by
doing that you are fixing a less important one.

2) What sort of PC is it? A desktop, or a laptop powered by a
wall-wart or inline unit as described for the deck?

Desktop powered by mains.

Wouldn't have expected problems with that, though ISTR that Jim thinks
PC PSUs are noisy, electrically-speaking. Nevertheless I've digitised
stuff via a SB Live soundcard on a standard, homebuilt PC without any
difficulty. However, thinking about it, that may mostly have been
using an optical input, as described in the previously linked post, or
another of around that time, and that of course wouldn't hum!

Java Jive: ROT: PC Audio Recording & Playback (was: Holst and a
Headache), 6/12/11
https://groups.google.com/d/msg/uk.t...k/a7R64KmusM8J


I just had a quick skim and your mention of laptops reminded me that I
do have a laptop available that I could try on battery power.

Certainly worth a try.

I am aware of the headshell contact issues with this turntable so will
be looking at that when I get more time to play.

Good luck.
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In article , Java Jive
wrote:
Wouldn't have expected problems with that, though ISTR that Jim thinks
PC PSUs are noisy, electrically-speaking. Nevertheless I've digitised
stuff via a SB Live soundcard on a standard, homebuilt PC without any
difficulty. However, thinking about it, that may mostly have been using
an optical input, as described in the previously linked post, or another
of around that time, and that of course wouldn't hum!

The problem is that its a matter of chance what kind of PSU you get in a
computer. Some may be OK, others may be a disaster.

I just got a new machine to use as an 'AV' box with my new HDTV. The
supplier who put it together for me tried the standard PSU that comes with
the main box. Found it produced so much crap that it stopped wired ethernet
working! Changed to a different external brick, no problems.

FWIW I routinely now use a computer to record from LP, etc, and it works
fine. I could probably get the noise and hum down, but in practice it rises
as soon as the stylus hits a real LP anyway! So no point in doing so. pun
alert!

I do use an external USB ADC, though.

Jim

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Audio Misc http://www.audiomisc.co.uk/index.html

So, that would seem to make it the actual deck then - nothing else
seems to fit the facts.


Could be, see below regarding laptop test.



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

Do you mean actually pulling the plug out of the wall,


Yes, unplugging.



That also suggests that the hum may have two, or more, sources, and by
doing that you are fixing a less important one.



A reduction of 5 dB is worth looking for. Subjectively, it reduces hum
from perceptible to only just perceptible on quiet passages under my
listening conditions here.

I would imagine that there are often multiple hum sources and the object
is to reduce/eliminate enough of them so that what remains is not a
problem to the listener (me).


I just had a quick skim and your mention of laptops reminded me that I
do have a laptop available that I could try on battery power.

Certainly worth a try.


I tried it with my laptop. With laptop PSU plugged in Audacity gave
exactly the same hum levels as with the desktop.

With laptop PSU unplugged the hum drops by about 6 dB. This is almost
identical to the drop in level when I unplug the turntable mains supply
when connected to desktop. At this lower level the turntable sounds very
quiet. (In this room, with these speakers...etc etc.)

(BTW, I've been calling this 'hum' but with components up to and beyond
3 kHz I know hum/noise is a better description)

Above makes me think the problem is earth loop related but I can't see
how, yet. Would be interesting to isolate the tt motor supply with a 1:1
transformer to see if that 'breaks the loop'. I may have one around
somewhere, would be good if it had an isolating screen...


I am aware of the headshell contact issues with this turntable so will
be looking at that when I get more time to play.

Good luck.


I would have expected a headshell contact problem to result in erratic
and rapid signal level changes or discrepancies in hum between channels
etc. Still, something to check if all else fails.

Thanks for the ideas.