too much humm...why?

[Farmdog]

Ok, here's the deal. I'm trying to record from my turntable to my pc. I have
a Newmark TT200 turntable and a good stylus. I'm using a Gemini PMX 01 mixer
as
my preamp. I have my turntable grounded to the Mixer as required. However, I
seem to be getting a lot of hum as if its not grounded well. last week I
grounded the turntable and the mixer separately to ground wires in the wall
and the hum is still there. Pretty hard to record with all that hum in the
background. my question is where is the hum coming from? My guess is that
its from crappy wiring in the Mixer. Am I doing something wrong in the
grounds? Someone once told my that to avoid electrical hum, use a batters
as an energy source instead of a wall plug..any ideas??

George

 

[SQLit]

Ok, here's the deal. I'm trying to record from my turntable to my pc. I have
a Newmark TT200 turntable and a good stylus. I'm using a Gemini PMX 01 mixer
as
my preamp. I have my turntable grounded to the Mixer as required. However, I
seem to be getting a lot of hum as if its not grounded well. last week I
grounded the turntable and the mixer separately to ground wires in the wall
and the hum is still there. Pretty hard to record with all that hum in the
background. my question is where is the hum coming from? My guess is that
its from crappy wiring in the Mixer. Am I doing something wrong in the
grounds? Someone once told my that to avoid electrical hum, use a batters
as an energy source instead of a wall plug..any ideas??

George

I had the same problem when first connected my stuff up.
I found that turntable had a green wire that when attached to my preamp
solved the problem. Separate from the power cord. Also I kept the turn table
and preamp plugged into the same outlet. Does your mixer have a separate
ground wire?

 

[Steve Cothran]

Might be worth checking the power supplies in the mixer and pnono
preamp. I'd hate to wear myself out over a little electrolytic cap.

Second, I would take a vom and see if there is any AC between the
computer case (ground?) and the unplugged shield contact of the mixer
output. It would only take a few mv to wipe you out.

 

[Bud]

The supplies do not get balanced. The high impedance inputs to
audio amps do, however, need to be balanced with reference to the
audio amp's chassis/zero reference, or 60 cycle hum gets introduced.

All it takes is about 20mV of difference to cause the problem, and
all the AC line conditioning in the world won't stop it. It has to do
with the line level outputs of the small signal sources compared to
the line level inputs of the amplifier, and what both of them call
"zero" volts. They have to be tied together at the chassis level, not
the patch cord.

The only talk of connectors is this thread was RCA connectors (which is
likely what is used). RCA connectors use a signal conductor and a
shield/ground - not ballanced. XLR (Cannon) connectors have 2
conductors and a shield/ground with the signal between the conductors -
balanced.

A grouund loop results from 2 devices that 'want' their chassis to be at
different voltages producing a current through the signal shield
producing a voltage which is added to the signal.

If the chassis of 2 devices are bonded (particularly if there is a
ground loop), the bond wire should have a much larger corss-section area
than the cross-seciton of the shield conductors. #14 would seem to be a
reasonable minumum.

The devices would best be powered from the same outlet. If they have
grounded power plugs this is necessary.

If 2 chassis are bonded and the shield is disconnected from one RCA
plug, the resistance between the 2 chassis is lowered (originally bond
in parallel with shield) producing more hum which is added to the
signal. If the chassis are not bonded, the voltage between them becomes
what the devices 'want' which will be far higher than the original hum.

Steel chassis vs 20 ga wire - the copper wire has a higher conductivity
but the chassis has a much larger cross-section and lower resistance.

Bud--