[sdiy] Clipping Amplifier questions...

[Ryan]

Someone correct me if i'm wrong...

I've got that book. I see the circuit but I don't think that will work. 
  It seems to me that the output would be about 4V higher than the input 
when the input is positive, 4V less with negative input. Is that what 
you are measuring?

The opamp will keep its output at just the right voltage so that both of 
the inputs are at the same voltage, and the zeners cause a 4V drop so, 
in this configuration the output would have to be 4V higher/lower than 
the input (depending on the sign) at all times.

You can make it work by changing it to an inverting amplifier, just 
switch the ground with the input you have and it should work, but you'll 
need another opamp to invert it again.

..Ryan

> 
> 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.