CircuitLab.com is a cool web-based “workbench” where you can create circuits and simulate it. You can do DC analysis, Time Domain, Frequency Domain analysis. I think it’s a great learning tool to “experiment” with circuits and see what will happen. No special software need to be installed, you can run it on your web browser. I’m using Chrome browser in this example.
For example, shown below is a simplified circuit of the mid-frequencies of an LC based equalizer. To keep things simple, we’re using 10K resistors, 10K potentiometer (represented by R4/R5) and a 0.1uf, 100mH and 1K resistors for the RLC component of our filter circuit. Ignore the LM358 part#, I just used one of the ready-made devices in the simulation software.
In this configuration, I represented R4 with 1-ohm, and R5 with 10K… so the potentiometer’s arm is all the way to the top (if you imagine it). In the Frequency Domain panel, let’s tell it to monitor the Vout and Vin voltage, starting from 1Hz, all the way to 50Khz.
When we’re satisfied, click “Run Simulation.”
Here’s the resulting simulation of that circuit…. as you can see, it’s boosting the mid-band frequencies (with a center frequency of around 1.5Khz)
If we swap the values of R4/R5, (to simulate the potentiometer arm going extreme in the other direction), we get this instead. Now, we’re cutting the mid-band frequencies.
Our EQ filter works great! Now, you can start playing around and changing values of Resistors, Capacitors and Inductors. For example, if we change the Inductor value to be larger (going from 100mH to 1Henry), here’s the resulting EQ curve. (I’m just going to show the cut curve, as the boost curve will just be opposite of it.) 
Now, the entire curve shifted down and has a new center frequency. Great! Our mid-frequency EQ is now behaving as a low-frequency EQ filter.
But as you can see, the EQ curve is too steep, and doesn’t look smooth. If we want a wide, smooth, “musical” curve, we also need to make adjustments to the capacitor value… let’s change our capacitor value from 0.1uf to 1.0uf.
Now that looks more like it! If we superimpose our low-band and mid-band frequencies, we get this result. Both the low and mid-frequencies have the same “curve” based on the component values we’ve chosen. 
Now you can extend this exercise and play with picking the appropriate values for the high-frequencies curve. We need to have at least 3-bands for our EQ circuit, at the minimum.
Or, how about creating a 12-band EQ… can you come up with the values required for a complete 12-band EQ? What about an “AIR BAND” EQ? What RLC values are needed to boost frequencies above 22Khz? You can use the above circuit as a starting point in your circuit simulation. Just head on to CircuitLab.com and have fun playing!
You can see a real-world application of this in our PEQ-503 Equalizer.
