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Phono Hum Troubleshooting
If you experience a hum when using a record player, the problem may be
caused by a lack of shielding (allowing noise to enter) or by a ground
loop (usually a redundant connection between the system's ground and
earth). Perform the following tests and you will likely solve the
problem. (The table in this example is a Linn Sondek LP12, but
this will likely apply to most any table.)
Disconnect the Turntable from the Hi-Fi system, and
from the mains (the power outlet on the wall) or outboard power supply.
You may now begin.
Check to make sure that the cartridge tags are
correctly connected to the cartridge and the headshell (see
"Continuity Test" below for the proper headshell
configuration) and that none of the cartridge wires' ("tags"
we call them) connectors are touching the headshell or the cartridge
body. Also make sure that none of the tags are caught between the
cartridge and the headshell, or beneath the head of a cartridge mounting
bolt.
Earth Tests: With an ohm meter on it's lowest scale
(200 ohms on most units), check to insure that all desired earth
continuities exist and are no more than about two ohms, as follows:
1) Unplug the cable from the arm and check that
the tonearm earth lead spade (which attaches to the preamp's earth
{"ground"} post) is continuous to the center pin of the
connector which plugs into the tonearm. If you have a problem here, the
ground lead may have broken internally due to stress. Cut about an inch
off of the lead and test it again. If it's still bad, the arm cable will
have to be replaced. If it's ok, plug it back in.
2) If you are dealing with an LP12, check from the
earth spade to the turntable as follows:
 | The stainless steel top plate (use a bolt head
so you don't scratch the plate). |
| The subchassis and main bearing (if
you have continuity to the record spindle then you also have
continuity to the subchassis). |
| The earth pin of the mains lead (if applicable;
Basik and Valhalla power supplies only). |
3) Check from the earth spade to the tonearm as
follows:
| The main pillar (try the bare threads of a
screw hole). |
| The arm tube (try the little screw which holds
the nylon "thingie" on to the bottom of the tube where the
counterweight threads on, or scrape a little spot of paint off of
the bottom of the tube at the rear). |
| The headshell (the cartridge bolts usually cut
through the paint and make contact with the headshell, but if not
just scrape a tiny bare spot on the bottom of the headshell). |
If you find a problem here, check the set screw
that holds the din connector into the arm and makes the earth connection
(it's the one which is 7/8" up from the bottom of the main pillar).
It should be very snug.
Continuity Test: Disconnect the cartridge tags from
the cartridge. With an ohm meter on it's lowest scale, check to insure
that all desired internal tonearm and cable continuities exist and are
no more than about two ohms, as follows:
| Left channel RCA plug positive (pin) is
continuous to the White cartridge tag (upper left headshell pin). |
| Left channel RCA plug negative (sleeve) is
continuous to the Blue cartridge tag (lower left headshell pin). |
| Right channel RCA plug positive (pin) is
continuous to the Red cartridge tag (upper right headshell pin). |
| Right channel RCA plug negative (sleeve) is
continuous to the Green cartridge tag (lower right headshell pin). |
Ground Loop Test: With an ohm meter on it's highest
scale (20 Meg-ohms on most units) and the cartridge tags still
disconnected from the cartridge, test that there is no continuity
(measurement is greater than 20 Meg-ohms) between the tonearm earth lead
spade and any of the arm (and cable) or cartridge contacts as follows:
| Left channel RCA plug positive (pin). |
| Left channel RCA plug negative (sleeve). |
| Right channel RCA plug positive (pin). |
| Right channel RCA plug negative (sleeve). |
| Left channel cartridge positive. |
| Left channel cartridge negative. |
| Right channel cartridge positive. * |
| Right channel cartridge negative. * |
Most moving magnet cartridges will be shielded and as
such one channel's negative pin will be connected to the body of the
cartridge. The body should not be continuous to the cartridge's mounting
ears (the tabs through which the mounting bolts pass). If it is, a
ground loop is created and a hum will result. In such a case the
cartridge must be replaced.
LP12 Motor and Supply Short Test: With an ohm meter
on it's highest scale, test to insure that no part of the turntable's
electrical system is continuous to the turntable's chassis. Check from
the tonearm earth lead spade to:
| The four motor wires (one blue, one red, and
two gray). |
| The two power conductors of the mains lead
(black or ribbed is Live = 120v, white or plain is Neutral). (In the
UK. and Europe, brown is Live and blue is Neutral = 0v.) |
| If the turntable has an early Basik supply on
which the components are connected via a two-row terminal strip,
check for any continuity to any terminal on the strip. |
If you have continuity to any of the above, do
not use the turntable until the source of this continuity has
been found and corrected!
Misc.: If all of the above test out all right, we
suggest that the trouble may be:
* The earthing arrangement of the associated
electronics. Experimentation here as to what is earthed and
where may help. (Do not alter the earthing arrangement of any Linn
equipment.)
* Video or antenna ground loop. See the
Ground Blocker section of
Hi-Fi Tips.
* Hum pickup from the power
transformer of the associated electronics: Temporarily turn off and
disconnect from the system any equipment unnecessary to the task of
listening to records. One at a time, move the preamplifier and power
amplifier (and turntable power supply if it's an outboard unit) to a
different position relative to the turntable. (At low volume settings
some equipment continues playing for a few seconds after the power is
switched off. This is very handy. If after switch-off the music remains
but the hum stops, the transformer in the component you just switched
off is most likely the cause of the hum. Warning: Some preamplifiers
emit a very large "pop" when switched off or on with the power
amplifier on. This could cause amplifier or loudspeaker damage.) |
