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Noise reduction

Sep 25th 2011, 16:31


Joined: Apr 4th 1998, 00:00
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Total Posts: 0
Several years ago I used to solder a capcitor on a speaker termal and then to ground . but dont remember what size or type capacitor or what speaker terminal to solder it to can any one help or am i just dreaming that i once did this thanks
Sep 26th 2011, 06:30


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
It sounds like you were just putting a capacitor across the speaker leads to act as a simple low pass filter. To find the size of the capacitor, you can use Xc=1/(6.28xFxC). I guess you may want to start by trying a Xc of 8 ohm at 3000 Hz. This is not a real practicial way of building a low pass filter as the capacitor is going to be large. I would place the capacitor at the volume control or some other place in the audio amplifier circuit. I do not know what type of receiver that you have, but with modern radios, you should not need any extra filtering. I assume that you are trying to filter out high frequency noise. Can you give us more detail in what you are trying to do?
73 Don KE8DO
Sep 28th 2011, 11:10


Joined: Jul 25th 2011, 14:25
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Total Posts: 0
Rarely would a capacitor across the speaker leads be needed. This will do nothing to eliminate noise being picked up the by receiver's antenna, for example. It could help with a piece of equipment suffering interference from RF radiated by the station, but, as noted, it usually would not be necessary.

One should also generally not install a filte across the speaker leads, or from one or both speaker leads to ground, on any solid-state equipment. This can cause the audio amplifier to become unstable and may actually cause equipment damage.

Back when we were doing the first ARRL RFI Book, Bob Schetgen, KU7G (SK), the RFI Book ediror, poo-poo'd the warning I wanted to put in the book. He had a small radio receiver in his office suffering from interference from W1AW, so to prove me wrong, he installed a capacitor across the speaker leads. It fixed the RFI problem.

He was rather smug until he smelled something funny in his office. The radio was very hot. Apparently, it had broken into an ultrasonic oscillation with that weird capacitive load and the poor audio-output stage was getting rather overworked. He stopped being smug and added the warning to the RFI Book. :-)

As noted, more info about what problem you are trying to solve may get some better advice.

Ed Hare, W1RFI
Technical forums moderator

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