Want to build a simple audio filter

In summary, the speaker is experiencing high frequency noise and the best solution is to build a hi-frequency bandstop filter. Before designing the filter, the source and load impedances, as well as the available voltage, need to be determined. There are many experts available to help with filter design, and options include using passive inductors or an op-amp design. Another option is to use a series capacitor to create a single pole filter, which can be calculated after determining the impedance and voltage.f
  • #1
Hi, I have an old system that outputs a stereo audio signal (using Left and Right RCA audio cables). However for some reason this system is also outputting a high-frequency noise on both channels regardless of its operation. At this point I think it would be simpler to build a simple audio filter (like a hi-frequency bandstop filter) than to troubleshoot the source of the high frequency noise in the system. Also, this sounds like a fun project.
Can anyone give me tips on a good place to start to learn about how to build a hi-frequency bandstop filter than can input a stereo audio signal then filter and output a stereo audio signal? I've built simple amplifiers and circuits before and can solder and stuff but have not worked with audio signals before.
  • #2
The first thing with any filter is to find out the source and load impedances. If just "very high" you need to shunt a resistor acros each to obtain a reasonable value.
You also need to check the available voltage from the source, and what does your device on the output side require?
You cannot design a filter until you have done this.
Then you can look at filter design, and there are hundreds of experts who are going to help you. If you want to use passive inductors then remember that a low filter impedance will allow you to have small inductors. If you want to use an op-amp design, remember that you are trying to remove high frequency noise, not add it!
  • #3
I once had a table radio that had too much bass, and no tone control. So I wired a capacitor in series with the speaker and cured that problem.

A series capacitor is a high pass filter, and a capacitor across the output terminals is a low pass filter. In either case, it is a single pole filter, so has a gradual roll off. But it's simple, easy, and cheap. A starting value for a capacitor can be calculated after finding the impedance and voltage as mentioned by @tech99 above.
  • #4
Thanks for the tips!

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