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June 1, 2016 / consort3

Sallen & Key slope

The humble unity gain Sallen and Key filter can be versatile. Don Lancaster’s Active filter cookbook showed me some of the filter shapes you could get. By varying the damping you can get upward and downward slopes to the frequency response . A nice article by Kenneth Kuhn showed me how to calculate the component values given the damping and the frequency.

At step 4, here is where I deviate from the above by using this website

Enter the capacitor values and it will compute R1 and R2 as 6.2k and 18K respectively, as per Kuhn’s analysis. Here is a Ltspice analysis of the circuit showing the 6dB peak of the underdamped case, even though the op-amp is configured as unity gain! By swapping the capacitors we get an overdamped case.


Good article on the above:

Or you could use a Quad style tilt equaliser, if your overall system has a problem.

Very nice article by the late great Chu Moy on headphone equalisers including the tilt type


Archive of Chu Moys Headwize site:

Good website on headphone amplifiers

Talking of headphones, Bose Qc35

Back to Sallen and Key. an interesting analysis:

The damping factor on the High pass Sallen and Key is perhaps more interesting in that the ratio of the 2 resistors determines the damping. Von Recklinghausen of KLH observed that the family of curves you get by varying one of the resistors could be used in a small vented box loudspeaker to get good bass by varying the bass boost. It will not handle large bass signals, but by varying the boost the drive unit is not overloaded. This is called dynamic equalisation and was used on the KLH 3, they called it their Analog Bass Computer.

The Analog Bass Computer concept is comprised of three basic elements: A variable gain equalizer with equalization slopes that are also dynamically variable, a threshold circuit to determine the levels at which gain and slopes are altered, and a transducer analog circuit that examines the power amplifier signal returning from the loudspeaker for evidence of thermal overload or mechanical fatigue. The analog bass computer is in essence, therefore, an equalizer that changes its equalization curve continuously over a wide range of levels to protect the loudspeaker from damage that might result if a simple bass-boost circuit were used.

Interesting powerpoint slide show on active filters:

Filter program including transistional types

🆕Summary of active filter circuits:

🆕Kenneth Kuhn has an amazing collction of HP instruments:

🆕Sallen & Keys original 1955 paper:


The OP176 op-amp although obsolete had an interesting data sheet with some interesting observations by Walt Jung on S&K filters including a slightly simpler way of calculating values than the above.

Filter design by pole location:

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