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A $1.00 Noise Reducing Speaker

Kill the crackle on the cheap.
Author, L.D. Blake, VE3VDC 

 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.

Easy stuff!

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


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.


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:

Two easy parts

Finally, reassemble the speaker.


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.

All done and working

So there you have it... less than a dollar for a noise reducing speaker that will turn your station into a headache free zone!




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