Clip signal with very low threshold
Here's the active clamp circuit I came up with. I entered it into Circuitlab so that I could simulate it and verify its performance.

Whenever the input tries to swing beyond either clamping threshold, which are determined by the &plusmn;Vclamp sources in conjunction with the resistors, the corresponding precision rectifier produces a signal that offsets the overvoltage, holding the output constant at the clamping threshold. With the resistor values shown, the value of &plusmn;Vclamp needs to be 2/3 of the desired clamping level.

Note that if the thresholds are not symmetrical, the output will have a DC offset.

Note that the output is inverted relative to the input. This is required because the inverting input of OA3 needs to be a "virtual ground" for the mixing to work correctly. An inverting buffer can be added at the output if needed.

Note that R1, R2, R3 and R4 need to be the same value. Also, R5, R6, R7 and R8 need to be the same value, but not necessarily the same value as the first group. However, keep in mind that R5 and R6 affect the relationship between &plusmn;Vclamp and the actual clamping threshold. The 2/3 relationship only holds if all 8 resistors have the same value.



The following graph shows three input sinewaves of 200, 300 and 400 mV (blue, brown, gray, respectively) and the corresponding inverted output waves that are clamped to &plusmn;250 mV (red, blue and purple, respectively). I also show the waveforms for V1 and V2 for the 400 mV case (the two funky waveforms running across the middle).