Owner`s manual

13
Hum can come from many sources both internal and external. In the previous section we
focused our attention to hum that can originate internally to the amplifier. In this section
we will focus our attention on external sources of hum.
One of the main causes of hum are ground loops – caused when a circuit has more than
one ground path for the power supply or signal currents to follow. The IR drop associated
with each path will be different and therefore result in a voltage differential within the
circuit. This of course will be amplified and is usually presented in the form of 60 Hz or
120Hz hum. It is also very common that there will be more than one source of hum - this
can lead to chasing one’s tail since the shotgun approach can introduce additional ground
loops. Therefore is very important that we follow the isolation and elimination
methodology.
The three prong line cords found on some equipment cause one of the most common
sources of ground loop hum. The ground loop is set up when the equipment grounds are
connected via the signal interconnect and then again connected via another path through
the AC line cords third prong ground lead. I have found that when a system is suffering
from hum, it is worth the time to insert the ground isolators (available from any hardware
store) on to each line cord with the three wire plug. In this way this particular ground
loop is eliminated. This is always a good first step as it eliminates this from the “tail
chasing” exercise.
The three prong problem is one of several external causes can be addressed so simply and
quickly and therefore it makes sense to first eliminate those.
• Install ground isolating adaptors (available at any hardware store) on any component
that has been equipped with three prong power plugs. Power up and note the effect. Has
it been eliminated (or reduced)? If there is an improvement keep the adaptors
permanently in place. (Personally, I always remove the ground connection from all of
my components since with high end equipment utilizing power transformers, there is
virtually no danger whatsoever and it eliminates a continual source of hum generating
problems)
• Turn off any nearby fluorescent or dimmer operated lighting fixture
• Replace the interconnects – some esoteric interconnects are virtually ineffective at
shielding. Try simple cheap interconnects from Radio Shack or similar - not a
permanent solution but it will help you isolate the source of the problem.
If the hum is eliminated, reconnect the preamplifier to the power amplifier. Remove all
inputs from the preamp but otherwise keep the connections in place from the preamp
output through to and including the loudspeakers. Insert a pair of RCA shorting plugs
into a preamp line level input (say the tuner input) and select that input. Turn on your
system and observe. Rotate the volume control from fully attenuated to full up. If the
hum remains the problem is in the preamp line stage (or interconnect but hopefully you
have eliminated that possibility earlier). If the hum is disappears, you can assume that the