This is a low frequency shelving EQ I want to use for a keyboard amp I'm making. I cascaded two sections to make a 2-pole slope, but I'd like to combine them to use a single opamp if possible. I've been trying to relate the circuit to a Sallen-Key filter, but I can't make the connection in my mind.

Does anyone have any suggestions?

 

You are looking for something like the following?
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

 

or you could use an online tool like that
http://www.ti.com/lsds/ti/analog/we...en&HQS=sva-web-filter-filterdesignervanity-en

 

If you want the filter to be adjustable with a pot, I don't know of any single-pole, 2-pole configuration that will do that.

 

You are looking for something like the following?
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

No, I only mentioned the Sallen-Key because it's an example of a 2-pole filter that uses a single opamp. I was trying to figure out how to use that concept for the shelving EQ.

 

I had not heard the term "Shelving EQ", but it seems it is just another name for lowpass filter or highpass filter. The audio websites imply that such filters have a brickwall characteristic, which is just not the usual case. The proper way to specify a filter is to define:

1. The corner frequency, where the response is 3 dB down
2. The maximum attenuation in the passband
3. The width of the transition band
4. The minimum attenuation in the stopband
5. Allowed ripple in the passband
6. Allowed ripple in the stopband

With all of this known one can select a filter topology and determine the proper order for the filter. Getting component values is then mostly plug and chug. In most case you cannot find a design that simultaneously meets all of the specifications exactly. In this case you replace equality constraints with inequality constraints.

The Sallen-Key filter topology is not particularly difficult to understand.

https://en.wikipedia.org/wiki/Sallen–Key_topology
http://www.ti.com/lit/an/sloa024b/sloa024b.pdf