On 4/03/2015 3:57 AM, Sumatriptan wrote:
Thanks all for comments.

Trevor, as per one of your suggestions...The biggest source of hum was
right channel RCA plug with screen intermittent O/C. Fixed that and
replaced both with gold plated versions while I was at it.

Java, I *like* a challenge so am happy to to chase hum. This tt has a 2
core mains lead, as expected, with the earthing being provided by a
earth wire connected to the pre-amp. Also, a continuity check showed
that the tone arm *is* isolated from the tt body, although a temporary
connection to it made no difference to the hum level.

Mick..thnks for cartridge wiring details, actually 5 wires including the
black earth wire from the tt metalwork to the headshell via the tone arm.

The hum level is now slight but perceptible at normal listening levels
in this room. Here's some figures from Audacity with record input set to
max for this test and tt connected to the phono-usb unit.

TT mains off, hum/noise RMS -60 dB
TT mains on, hum/noise RMS around -55 dB but varies a lot, see below.
Test track used at -20 dB RMS -6 dB peaks (not worried about headroom
for testing)
Motor on/off made no change.
Tone arm position gives no change.

Moving tt position in this office/workshop gave large, up to 15 dB,
changes in hum/noise. Maybe due to lots of mains wiring underfloor as
well as trailing extension leads to PCs etc. So...

I moved the tt to an acoustically and electrically quieter room and
tried it with my Pioneer amp (I had forgotten all about it in my earlier
posts). Result...hum not audible to me at normal listening levels.

Looks like my efforts will be aimed at finding somewhere in my office
that is electrically quiet for recording to PC. Or perhaps the problem
is with my phono-USB unit. Or the tt earthing which is getting to it via
UPS/power conditioning/PC chassis.

Comments please?

**Brand and model of cartridge would be helpful.


--
Trevor Wilson
www.rageaudio.com.au

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