Sallen Key Filters for Audio EQ

[ConorK2567]

Hi Guys,

I'm going to try make some sallen key filters for a university project with the hopes of making an EQ unit. I'm studying music technology and am a complete beginner with electronics so I have a few (stupid) questions.

1) Has anyone got any decent sources about the construction of/working with Sallen key filters that they would recommend?

2) I have chosen some component values in the link below to get a desired cutoff and Q. In your experience will these values work or should I aim for different values?
https://docs.google.com/spreadsheets/d/1lN3tLxwtOUL513ik1I3b4O2mQuzJMyJDHgwNR45Xji4/edit?usp=sharing

3) If I want to add a control for the frequency is that as simple as using a potentiometer for the resistors in the circuit?

4) When I have been looking at components I have seen things like 'power rating' (on pots) and "voltage rating" (on capacitors). What values should I be looking at for those in an audio circuit and how do I pick the correct one.

Thank you to anyone who reads this and a big thank you to anyone who replies

C

[schmitt trigger]

Analog Devices has many excellent opamp filter design tutorials.

https://wiki.analog.com/university/courses/tutorials/index#active_filters

To your question #3, you want to make a tunable bandpass filter? That is not as simple, and I don't think that Sallen Key is the correct topology.
Perhaps the better option would be a clock tunable (switched capacitor) device like the classic MF10

[oPossum]

Yes, you can adjust the frequency of a Sallen Key filter with a pot. This is common in active crossovers. One of the problems is that the ideal pot has reverse log taper. Those are usually a build to order part. Another problem is tracking between the gangs.

Most graphic equalizers use gyrators. That is a much simpler approach than using active filters.
A random circuit example... http://www.hobby-circuits.com/files/352/6-band-graphic-equalizer_orig.gif

[David Hess]

What kind of equalizer did you have in mind?  Graphic equalizers generally use a gyrator to synthesize the inductance for a bandpass filter for each section.

Small Signal Audio Design by Douglas Self has a chapter devoted to tone controls and the various types of equalizers.

[schmitt trigger]

Yes, you can adjust the frequency of a Sallen Key filter with a pot. This is common in active crossovers. One of the problems is that the ideal pot has reverse log taper. Those are usually a build to order part. Another problem is tracking between the gangs.

Most graphic equalizers use gyrators. That is a much simpler approach than using active filters.
A random circuit example... http://www.hobby-circuits.com/files/352/6-band-graphic-equalizer_orig.gif

Thanks for reminding me. I vaguely remembered a complication, but could not recall what it was.

Of course, it was the specialty pots.

[ConorK2567]

Thanks so much to everyone who replied!

I have had a look at the sources you guys have mentioned briefly and they look like great resources - really helpful.

What a shame about the BPF - perhaps I will have to say that this is a limitation of it and show a better solution. I always thought it was just a matter of cascading a high and lowpass filter together though?

Am I being really stupid if I say that I could just wire a traditional logarithmic pot backwards ?
Is there a place where I can get some pots like that ? I have been using farnell and RS online so far.

My plan is to make a parametric EQ unit with the following filters:
High pass (Sallen Key)
Low Shelf (Baxendall style)
Band Pass (?)
High Shelf (Baxendall Style)
Low Pass (Sallen Key)


Thanks again