Aug 31, 2011 Ham radio
Many a good QSO has been wrecked by that high pitched crackling noise that is common on weak
signals. Sometimes it comes in louder than the voice and makes it impossible for you to hear your
partner. It’s called “Bacon Frying” or “Spike Noise” and it’s the enemy of every ham.
Spike noise is a high frequency pulse that rides on top of the voice on weak signals. It is usually the
highest pitched sound coming from your speaker and sometimes it’s the loudest. Traditionally this
has been handled by add on filtering systems using either analog or digital signal processing
methods. Only high end (i.e. expensive) rigs have had these devices built in. Whether analog or
digital, many low end and mobile radios don’t include them.
Fear not! There is a perfectly workable solution to the problem using nothing more than a couple of
The circuit, pictured below, is a simple low pass filter that can be added to any speaker and uses
only a pair of 50 cent parts.
Despite the unusual electronic symbol, C1 is a common non-polarized electrolytic capacitor used
mostly in speaker crossover networks. It has to be non-polarized because the speaker is working on
alternating current. You cannot use a polarized electrolytic capacitor here because under reverse
voltage it acts like a short and would cause considerable distortion.
The resistor, R1, is also a common part you can get at any electronics supply house. There is some
volume loss due to the resistor being in series with one of the speaker leads but it is barely
noticeable and given the benefits, it’s a good trade off.
Human speech doesn’t require full fidelity audio. Most voice energy is concentrated in the range of
400 to 3000 hz. Most of the really annoying noise is in the range of 2500 to 10,000 hz. Since these
two ranges don’t overlap very much we can effectively reduce the audible noise on a signal using a
low pass filter to remove the portion of the audio spectrum above 3,000hz. This effectively takes off
the noise and leaves the voice alone.
The combination of series resistor and shunt capacitor forms our low pass filter. At low frequencies
the capacitor appears as an open circuit and all the audio goes to the speaker. At higher frequencies
it acts as a short across the speaker, causing this energy to be dissipated across the resistor where
we don’t hear it.
The part values for noise canceling with minimal impact on voice quality are on the right. The
resistor should be 2 watts or better. The capacitor should be a non-polarized electrolytic type rated
for at least 16 volts.
There is no harm in experimenting with different capacitor values to get a tonality you like.
Increasing the capacitor’s value will increase the noise reduction but will also make voices sound
more bassy. Go too far and everything will sound muffled. To avoid excessive losses you should
always match the resistor with the impedance of your speaker.
Wire wound resistors work better in that situation than carbon ones… the inductance of the wire coil helps the curve of the filter.
Ok, a $1.00 circuit isn’t going to replace a $100.00 DSP unit but it can bring about considerable
relief from that nasty crackling noise and it might save you a couple of headaches when trying to
work that rare signal. So there you have it… less than a dollar for a noise reducing speaker that will
turn your station into a headache free zone!
An article by L.D. Blake – VE3VDC
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