frequently asked about “hiss” or “hum” emanating from
speakers. Put your ear close to one of your speakers and you’ll be
able to sample the sounds we'll be talking about. Causes and potential remedies are
the subject of this article.
Two types of extraneous noises are common: hiss and
hum. Hiss is high frequency in nature, hum being lower
frequency, most commonly 60Hz.
we should understand that some amount of noise is normal. A
system that is totally silent is probably off! To
determine whether we are in the realm of normalcy, I’ll ask
the question “with volume set to a normal listening level,
how close to the speaker must you be to hear the noise.” If
the answer is a few feet (or less), the noise level is
likely normal. If you can hear it from the listening
position (or, especially if it intrudes upon the listening
experience), we have a problem that warrants further
Be advised that raising the
volume to maximum is not the way to check for noise
levels. Set the volume control to a normal listening
position to understand whether the amount of noise generated
will be noticeable while listening.
Where do these noises come
Self Generated Noise
The flow of electrons though a
conductor creates thermal noise. The active and passive parts
of a component create thermal noise in performing their normal
operations. The total amount of noise generated by a given
component depends on a wide number of factors including: the
type of parts and materials used, circuit topology, the
quantity of parts and the amount of gain in the circuit. The
power supply, in particular, is likely to create noise, some
of which leaks into nearby low-level circuits. The sum total
of the noise generated by a component we’ll refer to as
quiescent idling noise; the amount of noise when no signal
Simply put, signal to noise ratio is the amount of
noise present in a signal at a given level. The amount of
self-generated noise compared to a given signal level gives us
a signal to noise ratio (expressed in dB). As an example, a
component whose signal to noise ratio is measured at -90dB has
a noise floor ninety decibels below the signal. All things
being equal, the lower S/N ratio (higher number) the better,
but like any specific design criteria, tradeoffs exist.
Circuit designers carefully balance the pro and cons of any
potential topology, as choices made can impact sound quality.
Ultimately, concessions are made to optimize a
particular characteristic deemed important. In some cases S/N
ratio may be an acceptable tradeoff if performance gains can
be realized in other areas. Thus, it is important not to take
this specification out of context; overemphasizing it or any
other performance parameter. The bottom line is some
components are “quieter” than others, but that in and of
itself is not a measure of quality.
Further, the signal to noise ratio of a given
component does not necessarily tell us how much noise we’ll
hear though the speakers. That is determined by all of the
components in the system along with other influences.
What are some of the influences that affect system
Our audio/video systems are constantly bombarded by
a plethora of radiated signals. Radio, television and radar
are just a few examples of the cacophony of possible
disruptions ready willing and able to invade the relative
silence of our audio systems. Radio Frequency Interference
(RFI) is so pervasive that I doubt any system is unaffected by
it. RFI can manifest itself in the form of general
degradation, added noise or even direct translation of the
Electromagnetic Interference (EMI) can also induce
noise into the system. Radiated EMI fields are both common and
detrimental. In an audio system these fields are usually
induced by transformers and can manifest themselves as hum or
low frequency noise.
A high gain device is
more likely to be a bit noisier than a low gain design. All
things being equal, a preamp, let’s say, with more gain is
likely to have a bit more noise. So, if we are talking about
a very high gain application (e.g. phono), there will almost
certainly be a bit more noise, or his, apparent.
The single most
common "noise" problem we encounter, ground loops are a subject broad enough to demand an entire article,
we’ll skim the topic here as it relates to our theme at hand.
Manifested as a constant 60Hz tone that does not vary
with volume, ground loops are an especially common source of
noise and complaints. Checking for ground loops can be a bit
challenging. Following the suggestions mentioned below under Possible Remedies
would be a good place to start. Contact your dealer for more
specific help in solving ground loops.
Speaker sensitivity is
certainly the most important (and most often overlooked)
factor determining whether noise is an issue. Sensitivity
(expressed in Decibels) is the amount of loudness produced by
the speaker at a given distance (usually 1 meter), for a given
input signal (typically 1 watt). The higher the resulting
number, the higher the sensitivity. A speaker rated at 90dB
produces a louder volume than one rated at 85dB, with the same
input. Speakers with measured sensitivities below 85dB are
considered low sensitivity, moderately sensitive speakers
range from the mid 80’s to the low 90’s, and high sensitivity
speakers above that. There are very efficient speaker systems
(typically horns) rated at 100dB and above.
From this information, it
is not hard to see that the potential for noise becomes much
more of an issue as speaker sensitivity increases. A given set
of electronics mated with a low sensitivity speaker may
produce noise levels barely audible just a few feet away, but
substitute a very high sensitivity speaker and noise will be
easily heard at the listening position.
Ok, so now we have an understanding of the problem,
how can we reduce or eliminate the noise? The first step is to
determine the origin of the noise. It is a simple matter of
checking the components one by one to find the trouble maker.
Here's what to do.
Turn off all the components, and then disconnect the
inputs to the amplifier (or all the inputs from an integrated
amplifier or receiver). That will leave us with just the
speakers connected to the amplifier. Turn the amp on and
listen for noise. If all is quiet, we can assume
that the problem lay upstream. Onward.
Now turn the amplifier off
and connect the preamp (be sure all other components are
disconnected from the preamp). Turn on the preamp, then the
amplifier. If the noise returns, we can assume the preamp is
at least part of the problem, if things are silent, then the
problem is further upstream.
The next step is to begin connecting
your other sources,
one at a time, listening for when the noise returns. Since system noise is the sum total of the noise
generated by all the components in the chain, it may be that
you’ll find the noise level increases slightly with each
component you add to the mix. If this is the case, there is no simple way
of reducing noise levels, short of changing equipment.
If you find hum to be a problem, check for ground
loops and experiment with cable layout and locations. Be sure
to route interconnects to keep them away from AC cables. If
they must overlap, cross them at right angles to minimize hum
Grounds loops (constant 60Hz hum that
does not vary with volume setting) are the result of two or
more components being at different AC ground potentials. They
can occur in any system and often pop up when a new component
is added to the system. Many people feel there is "problem"
with the new component, or that it is a mismatch. Neither is
true and it is simply happenstance that the grounding scheme
of the new piece of gear is different from those currently in
play. Follow the step by step process outlined above. At some
point your 60Hz hum will appear. Floating the ground on that
component (and maybe others in addition) should cure the
problem. A common source of ground loops are cable and
satellite TV connections. There are specially designed in-line
isolation transformers designed specifically for this problem.
To sum up, remember that a certain amount of noise
from the system is to be expected and is quite normal. As long
as the noise does not intrude on the listening experience,
most people are satisfied. Do remember that the
quality of a system is not determined by the noise level it
generates but by the musical experience it imparts.