Tracking Down Problems in
Your Stereo System
The new amplifier seemed an
incredible bargain, considering the claims made by its manufacturer, so I gathered up the
few pennies I had scraped together as an impecunious student and bought it. I soon learned
that there really is no free lunch: one evening my system suddenly made a noise like a
dinosaur in distress and then quit with a sigh. As I approached the amp to find out what
had happened I detected a stench that only frying electrical things can make, and noticed
some cheery flames licking out of the ventilation holes.
Few audio breakdowns are that dramatic, of course. Often,
you turn on your equipment and something has simply stopped functioning. Or even more
insidious is the gradual sort of problem that eventually becomes obvious without your
quite realizing when it began. In either case, the first reaction is usually to dig out
the instruction manual and see if it has any help to offer. Most do include a section
devoted to troubleshooting, and these can sometimes direct you to a solution, at least if
the fault is confined to one component. Unfortunately, few such guides are very useful;
often the hints are confined to things like "No sound? Check that the unit is plugged
in."
Anyway, many problems (and virtually all the ones you can
cure yourself) occur between the individual parts of the audio chain. Components do fail
now and then, to be sure, but that's relatively rare; cables and connectors are a much
more vulnerable part of a system, followed at some distance by anything that includes
moving parts that can become misaligned or befouled. Such matters may be covered to some
extent in the manual of one or more components, but most just cover what can happen within
that piece of gear; for interconnection problems, you're on your own.
Humming along
When something does go amiss, the first task is to
determine the nature of the problem. Basically, this boils down to one of two things:
extraneous sounds or no sound at all. In the first category are hum problems,
radio-frequency interference (RFI), and various crackles and forms of distortion that can
creep into a signal when you're not looking.
Hum is probably the most common thing you will encounter,
and it is a tough one to correct because it has many possible causes. If, for example, it
simply appears all of a sudden, chances are that a cable coming loose has caused it. The
standard audio cable consists of a "hot" central lead and a braided outer
portion that acts both to shield the signal lead from stray electromagnetic signals and to
connect the chassis of one component to that of the next.
To minimize the possibility of hum entering the system,
there must be a good connection between the braided shield and the outer cuff of the RCA
plug at each end, and both cuffs must make tight contact with the jacks they are connected
to. The lower the signal level being carried by the cables, the greater the effect of hum
if it is picked up.
These days, very few of us wire our own interconnects, and
the factory-assembled ones rarely have problems except if they have been used in
applications where they have had to flex a great deal, so cable-related hum difficulties
are almost always related to the connection between plug and jack. The cause might be
something as simple as oxide buildup on the contacts, which is more or less inevitable
over time if the metal used is aluminum; the cure is often simply to undo and redo the
connection a few times. If the cuff has just become loose, crimping it slightly with a
pair of pliers will usually restore good contact; better still, replace the cable -- wire
is pretty cheap compared to the rest of the system.
Hum may also be a result of placing a signal lead too close
to an AC power cord. Distance is the remedy in such cases; and crossing leads at right
angles will minimize hum pickup as well.
A particularly knotty hum problem is one caused by a
"ground loop." Theoretically, the chassis of all the components in a system are
at ground potential, but in reality that is rarely the case. Slight differences do exist,
which means that small voltages can develop in the ground wires that connect one chassis
to another, and these can be picked up by the signal leads as hum. The problem may be
worsened if two or more components are independently connected to "house
ground," but this might also be okay -- the problem with ground loops is that they
are unpredictable. They can sometimes occur in places that have nothing to do with the
components that are causing them: it is not uncommon to have a ground loop develop just by
connecting a new component to a system, without even turning it on!
In principle, the way to avoid ground loops is to make sure
your system is connected to true ground only at one end, usually at the power amplifier.
If you can duplicate the audio chain by cascading the power cords of the components in the
same order (turntable to preamp, preamp to amplifier, for instance), so much the better,
but if this is impossible, and an intermediate component has a U-ground power plug, you
may have to insert a cheater cord that breaks the ground connection.
In extreme cases, the two shields of the stereo signal
leads can result in a ground loop and one of the connections must be broken. This will
involve a bit of cable surgery: strip the plastic sleeve off one lead only and carefully
cut the metal braid all the way around to break the contact (only at one end; the braid
must be attached at the other end to continue to provide shielding). You may have to do
this at several points in the system, particularly if the problem is caused by a component
that is connected at several spots, such as a tape deck.
Ground loops are not easy to identify. If the hum suddenly
appeared when you altered the wiring of the system or added new equipment, or if you find
it very hard to locate the problem and it yields to none of the other remedies, it may be
a ground loop. If so, be prepared to spend a lot of time experimenting to get rid of it.
It is possible that hum is caused by the internal failure
of a component. If you suspect this, isolating the component by disconnecting it from the
system entirely and listening to it through headphones should provide an answer. Faults of
this sort are very rare but they do occur; professional service is the only remedy.
We’ll look at some more gremlins next time.
...Ian G. Masters
ian@mastersonaudio.com
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