In this column, Kristen answers questions about all aspects of electronics, including
computer hardware, software, circuits, electronic theory, troubleshooting, and anything
else of interest to the hobbyist. Feel free to participate with your questions, comments, or
suggestions. Send all questions and comments to: Q&A@nutsvolts.com.
; WITH KRISTEN A. McINTYRE
Humming an Unpleasant Tune
QWhen I hook up my Amazon Dot to an external amplifier, I get a constant hum. I tried using an expensive audio connector (Monster), but the hum persists. I’ve heard that ground loops could
be the cause, but I don’t know how to verify there’s a
ground loop. Can you give me some pointers?
AHum like this can often be difficult to diagnose. It usually comes from the power line’s 50 or 60 Hz electromagnetic interference. This is compounded by the relatively high impedance
inputs to and high gain of most amplifiers.
If you know someone who plays the electric guitar
(like my son does), you can really hear this effect. It’s
accentuated because of the high gain that guitar players
like to use to provide “distortion.” The high gain actually
drives the amplifiers into a state called clipping (Figure 1).
When this happens, the input signal is large enough
that the amplifier cannot reproduce the higher voltage
parts of the waveform, so the voltage actually just stops
rising. It’s as if the top of the waveform is “clipped”
Clipped sine waves — the form of the voltage
variations on the AC mains — are almost like square
waves, particularly as the clipping occurs lower
and lower down the sine function. Square waves have
a lot of harmonic content, which we know from doing
something called a Fourier decomposition. This operation
decomposes a time-domain waveform into its frequency
components — a series of sine waves at a given phase,
amplitude, and frequency.
When you look at it that way, you see that there are
many odd harmonics — multiples of the base frequency
by an odd integer (Figure 2). These odd harmonics are
particularly objectionable to our ears (that’s a whole
other thing to discuss sometime), and that’s why we hear
this unpleasant hum. If it were just at 50 Hz or 60 Hz,
we wouldn’t hear it that well. Our ears don’t respond
sufficiently at that frequency.
All of this is well and good, but how do we get rid of
the hum you’re hearing? Let’s first look at the potential
ways we could be coupling that energy from the mains
into the audio amplifier.
One would be stray electromagnetic fields. In a
typical home environment, these fields are present and
significant. If you have an
oscilloscope handy, you can
observe this. Just turn the
• Humming an Unpleasant Tune
• Computer Mind Tricks
Q & A
; FIGURE 1. A clipped
; FIGURE 2. Harmonic content of a clipped sine
(Wikipedia CC BY-SA 4.0 by Binksternet).
8 September/October 2018