[sdiy] Clipping Amplifier questions...

[Bret Truchan]

Hello!

Does anyone happen to have the book "Practical Electronics for Inventors"?  
In it, there's a very nice looking circuit that they call the "Noninverting 
Clipper Amplifier" (page 245).  If you don't, I made a copy by hand 
(http://www.voxglitch.com/images/clip_amp.jpg)

I can't, for the life of me, get this wee-little circuit to work.  The 
circuit is a normal op-amp based amplifier, but an addition of two zener 
diodes accross the feedback loop was supposed to limit the voltage (both 
positive and negative) to the cutoff voltages of the zener diodes.

I intended to use this amplifier circuit for one of the inputs to a module.  
The module has an imbeded effects board, which has a 4V maximum rating on 
it's audio input.

I had _thought_ that I had it working at one point and I created a PCB with 
the circuit on it.  However, it doesn't seem to be doing the job.  The zener 
diodes are 3.3V, but I can measure a full 12V on the output line of the 
op-amp.  I wired up the op-amp circuit again on a breadboard and got the 
same results.  Something is afoul.  I'm using TL082 op-amps.

If anyone can take a moment to look at the circuit in question, I would be 
very happy.  Or, if I'm approaching this in the wrong way, let me know.  My 
intention was to have an input that I could control the level on (via 
potentiometer), but also have the input voltage clipped below 4 volts to 
protect the imbedded effects unit.

Thanks!
- Bret

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