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
cheap parts...
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.
| Values For Minimal
Effect |
|
Speaker |
R1 |
C1 |
| 4
ohms |
5
ohms |
10
uf |
| 8
ohms |
10
ohms |
4.7
uf |
| 16
ohms |
15
ohms |
2.2
uf |
The part values for noise
cancelling 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 impedence of your speaker.
Construction
Adding this to your station
speakers is easy:
Take the back off of the speaker
you are going to modify.
Unsolder one wire from the speaker
connection point. It doesn't matter which wire.
Now solder the capacitor to the
speaker's two connections, one wire on each.
Solder one end of the resistor to
the speaker connection where you took off the wire.
Then solder the wire you took off
the speaker to the other end of the resistor.
Tuck the parts down out of the
way, making sure they don't touch the speaker itself as this would cause
a buzzing sound at louder volumes.
The finished project should look
something like the picture below:
Finally, reassemble the speaker.
Conclusions
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!
73,
L.D. BLAKE, VE3VDC
