Problem
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I have designed an active low-pass filter using Sallen-Key architecture. It is a 4th order filter (two 2nd order stages cascaded). Here is the circuit diagram:

![schematic](https://i.stack.imgur.com/Nl5th.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fNl5th.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

`+-12v_byp` are +- 12V power supplies bypassed by 2.2uF and 0.33uF tantalum capacitors.

LM6171B ([datasheet](https://www.ti.com/lit/ds/symlink/lm6171.pdf?ts=1676368045037&ref_url=https%253A%252F%252Fwww.google.es%252F)) has GBW of 100MHz. I arrived at the component values using a combination of equations, such as
$$F\_c=4\text{ MHz (goal)}=\frac{1}{2\pi RC}$$
for the simplification where resistor and capacitor values are equivalent.

LTSpice frequency analysis
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LTSpice AC analysis is nominal and has a cutoff at 4 MHz:

[![enter image description here](https://i.stack.imgur.com/JqmGN.png)](https://i.stack.imgur.com/JqmGN.png)

Real-life result
----------------

I built the circuit on a solderless breadboard, taking care to choose capacitance values large enough to render parasitic capacitances negligible. Still, the experimental behaviour of the filter deviates from the simulation:

[![Scope shot (Frequency response analysis)](https://i.stack.imgur.com/a1csU.png)](https://i.stack.imgur.com/a1csU.png)

The filter behaves more like band-pass than low-pass and **strongly** attenuates very low frequencies below 50kHz.

What are some possible reasons for this behaviour? The only thing I can think of are parasitic capacitances on the breadboard, but I have tried to take these into account through component choices. Also, shouldn't these parasitic capacitances affect *high-frequency signals* more than low-frequency ones?