Sallen key filter and treble boost filter

[duffy]

not recomending any particular form of connections.

?

Your circuit is starting to look more like what Mr. Sallen and Mr. Key had in mind. Looks like you changed the cutoff frequency and damping, though.

C5 is a DC blocking cap, nothing irregular or unusual about it in the real world. It DOES assume the next stage is biased correctly, however.

 

[audioguru]

Hi Mr DEB,
C5 was a supply bypass capacitor and has nothing to do with the filter.

You completely messed up the "classic" Sallen and Key lowpass filter because you replaced its important resistor with a capacitor and then the opamp does not have an input bias/reference DC voltage and WILL NOT WORK!

 

[MrDEB]

I must have misinterpeted this schematic?

It is on the first page of this thread.

 

[MrDEB]

Take out one battery, not both??

looking at schematic I may have taken both offset voltages out??

 

[audioguru]

The 130k resistor R1 is part of the 2nd-order Sallen and Key highpass filter and biases the input of opamp U2 at half the supply voltage.
U2 has a gain of 1 so its DC output is also at half the supply voltage.

The two 220k resistors R5 and R6 in the lowpass filter bias the input of the second opamp at half the supply voltage because they are connected directly to the output of opamp U2.