A Notch Filter Development Board
Designing audio notch filters can, at the best of times, be difficult.
For example, we have to concern ourselves with notch frequency, depth of notch, filter Q and the shape of the notch. There is also concern with temperature variation and stability of the components. Also, we must have the proper equipment to test the final product and not depend on software simulation alone.
So, what to do?
The first thing, of course, is to choose a circuit. In this case we'll be using the popular Twin-T circuit. Next, after choosing the circuit configuration we run it through a simulator such as LTspice to be sure that we're on the right track.
Assuming that the spice simulation looks good, the next step is to use a breadboard, or, in this case, a development board. We'll be using a development board so that we don't need to concern ourselves with stray capacitance or loose connections.
The schematic below shows the circuit we use and the picture shows the actual board.
What is immediately apparent is that some components are missing and some have been added. For example, there is no dual pot to tune the frequency and the frequency determining capacitors and resistors are missing. What has been added, however, are a number of terminal blocks that allow one to connect and disconnect critical components without soldering or having to worry about loose connections.
For example, terminal blocks TB2, TB7 and TB4 allow easy and secure connection of frequency determining resistors. Likewise, terminal blocks TB3, TB5 and TB6 allow connection of various capacitors. Terminal blocks TB8 and TB9 have been added to control feedback voltage to the filter and thus adjust the filter Q and notch depth.
In practice, a dual pot will be connected to TB2 and TB7 and a single pot will be connected to TB8 and TB9. This makes for easier changes in frequency, Q and notch depth.
Further down are the Bode plots showing how the notch depth is changed with a resulting change in circuit Q. The Bode plots are obtained through the use of a network analyzer and we've found this to be more accurate than using a scope and a waveform generator.