A Development Board for a Universal Active Audio Filter
By now, anyone who can fog a mirror knows of the popular UAF42 that has long been manufactured by TI. Unfortunately, the '42 is not especially cheap with the unit price at reliable parts houses being over $20.00. Of course, you can get them considerably cheaper at various web sites, but they must be cheaper for a reason. For example, are they pickings from the reject bin; are they used; or even counterfeit? Who knows?
What I would like to propose is to use an existing circuit, the State Variable Filter, that can be configured to respond as sort of a UAF42 clone. So, what would be the best way to do this? Rather than making a standard PCB with everything pre-wired, why not make a flexible "development board" that can be quickly reconfigured with different values for critical components?
What we've done is take the traditional state variable filter and added a number of terminal blocks that make it easier to change out frequency determining components. For example, TB1 and TB3 make it easy to change out resistance values to obtain different frequencies. (In practice, a dual pot of around 50K would be connected to TB1 and TB3). Likewise, TB2 and TB4 are used to connect different capacitance values. In addition, TB7 through TB10 enable the unused op amp to be configured as a buffer or amplifier or simply left unused (with proper termination so it won't "take off").
The Q of the filter may be controlled with a pot between TB5 pin 4 and ground. Also, TB5 allows an input to the non-inverting input as well as to the inverting input. (Normally, the input would go to the non-verting input.)
The output of the filter is taken from TB6 and allows low-pass, high-pass and bandpass outputs.
The filter operates on voltages from 15 to 24 VDC and with a low current draw of less than 12ma is suitable for battery operation, such as two standard 9 volt batteries in series.
So, what chip did we use in the filter? After looking at a number of data sheets we came to the conclusion that the old and reliable TL074 would do nicely. For example it has a reasonably fast slew rate and a fairly low THD, among other reasons. Also, it is realistically priced (keeping in mind that it should be purchased from a reliable parts house).
Take the schematic shown below. As you study it, you'll see it's no more than your run-of-the-mill state variable filter, except that it has a few additions that make it useful for product development and testing.