Suppose you’ve created a digital sound project, but the sound is really harsh … too bright (too many high frequencies). A typical problem – many digital circuits create square waves, which are ‘rich’ in highs. Easily solved with a simple circuit addition. And it’s all analog.

The page Low-Pass and High-Pass RC Filter Frequency Calculator shows how simple these two kinds of filter can be. Two components are needed: a resistor and capacitor.

Here’s a low-pass. The signal comes IN passing through the resistor. Then the capacitor (symbolized by the parallel lines) ‘short circuits’ much of the energy of the highs to ground while the lower freqs (below the cut-off) pass on OUT.

Suppose you want to roll off frequencies above a cut-off frequency of 12kHz using an RC low-pass filter. What values of resistor and capacitor can you use? Follow the link to this calculator. Enter 12000 into the frequency box. The value of the resistor will decide how much current will flow in the circuit. Since this is a signal-level circuit (before the amplifier) the resistance can be quite high. Let’s try 10,000 ohms. Then the capacitor value needed is 0.001uF.

Playing with the calculator a bit, you’ll see that the larger you make the resistance value (without changing the capacitor), the lower the cut-off frequency gets. So you could use a pot (variable resistor) with a maximum value of 1 million ohms to get a minimum cut-off of 159Hz!

If you need more power to pass through the filter (and aren’t worried about shorts), you can get about the same 12kHz roll-off using a 130-ohm resistor and a 0.1uF capacitor. Or if you’ve already got an 0.022uF cap in your junkbox, you’ll need about 620 ohms.

Standard resistor values * Standard capacitor values

Digital gear can do lots of neat stuff, but analog tricks are often the quickest and least expensive solutions.

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