Voltage limitation circuit for analog signal
Seeing that you have to deal with very low-frequency signals (&lt;3 kHz), the easiest approach here would be something like abusing the voltage clipping properties of a real Opamp:

By doing something like

--&gt; Amp1 (gain = 10) --&gt; Zener-based voltage limiter --&gt; attenuator (gain = 1/10) --&gt;


you could have something that works like that without disturbing the signal all too much.

To explain: 

to limit voltages, and, for example, generate a stable reference voltage or cut away voltage spikes in power supplies, you'd simply do







simulate this circuit &ndash; Schematic created using CircuitLab

The Zener diode used is a peculiar semiconductor: It works pretty well as a diode, i.e. it inhibits flowing of current from the positive side to the negative side, up to a breakdown voltage, from which on it becomes very conductive (i.e. low resistance).

Now, since this is not adjustable, you can't use it in your "user-defined level" scenario.

Hence, you'll need adjustable gain on the input, and the inverse gain at the output:







simulate this circuit

Note that you get adjustable overall gain as "bonus".

This is the rare case where it makes sense to chose a high gain for the  opamp stage – the higher the gain here, the higher breakdown Zener voltage you can select, and the more "exact" and less distorted your output becomes. However, if you select a Zener diode (or diode cascade) with a high voltage, you will also need to make sure your Opamp has a supply voltage that can generate that at all.

By the way, you might want to read up on Opamp circuits and active and passive filters – it would make potentially a lot of sense to integrate a low-pass filter in the output stage (which really isn't more than adding a capacitor), since the clipping operation probably has much higher bandwidth than your 3 kHz signal, and I don't know what that does to your signal chain; but that's something that I can't consider without knowing what you want to do with the signal in the end.