Active Audio Filter Supplemental Page
Discussion
To follow is a collection of images related to the text presented on the Discrete Component Audio-Frequency RC Filters web page.
Supplement to Figure 3b
SPICE runs were made on the figure 3b schematic by Wes Hayward, W7ZOI. Initial simulation with a single 1K/22uF RC VCC decoupling network for the Q1 and Q2 stages had a double-humped response and a small amount of 60 Hertz hum. Giving each stage its own 1K/22uF RC VCC decoupling network as shown in the schematic resulted in a smooth filter response as shown with a VCC of 8, 10, 12 and 13.8 volts. The hum problem also cleared up. The filter pretty much works as designed.
Supplement to Figure 5
A SPICE run of the exact schematic shown in Figure 5. This filter has outstanding lowpass skirt response.
An interesting idea was put forth from N6WG; is it possible to build a peaked highpass filter? Asking W7ZOI brought the following reply: Sure, why not. The basic low pass without peaking has two resistors and two caps. The R values are the same and the C values are the same, and the relationship of the R to the Cs determine the cutoff. If you want to peak it, you start making the capacitors un-equal. A high pass is built by starting with a low pass, and then exchanging the position of the Rs and the Cs. It would seem that a peaked high pass would then result from divergent values for the resistors. And, sure enough, that's what happens. See the attached diagram. Many thanks to Wes Hayward, W7ZOI for his generous support and PSPICE diagrams.
Supplement to Figure 5 - Peaked Highpass Filter as discussed above.
Supplement to Figure 6a
This is my favorite CE filter from the Figure 6 collection. Increasing the 0.01uF caps shifts the center frequency lower. I plan to build and test one using 0.015uF caps after my next parts order. Below is a Electronics Workbench simulation of the 6a amplifier.
