[sdiy] Negative content on half wave rectifier

[Tom Wiltshire]

I'm surprised it's that bad. I've used these circuits for 'octave up' designs and I don't remember seeing anything like that much negative voltage on the scope. That's what you'd expect *without* the op-amp, isn't it?
I remember using germanium diodes to reduce the diode drops as much as I could. It was for guitar signals, so the levels were low. I think I had it producing a reasonable rectification down to a few tens of mV. My trouble was that a guitar signal isn't actually symmetrical around zero, so rectifying it doesn't actually produce a signal with twice the frequency - it might have a bit more second harmonic, but the fundamental remains a significant part of the sound, which rather spoils things if you're after an octave effect. I spent ages trying to pre-process the guitar to get it more symmetrical when rectified.</sidetrack>

T.


On 22 Jul 2011, at 14:43, Justin Owen wrote:

> Hello,
> 
> I was recently warned off using a single 1N4001 as a half wave rectifier so I started playing with op-amp based versions. I settled on the 'Improved Precision Half Wave Rectifier' in Fig. 3 here: http://sound.westhost.com/appnotes/an001.htm
> 
> This also shows up on P191 of Jung's op-amp cookbook.
> 
> Jung quotes that, on an AC signal, you'll see a negative swing on the output of approx. -0.6V (which I guess is an inverted diode forward voltage drop?) - in reality I'm seeing between 0.8V and 1V depending on the diodes I use (1N914s were best).
> 
> Is there a way to completely get rid of the -V content or at least improve these figures - so you're swinging between 0 and V?
> 
> I have tried biasing the +v terminal but, for input summing of AC and DC signals it's not a solution. To compensate for the +1V bias I made the 0-5V CV swing between -1 and 4V but obviously that -1V portion of the CV is now getting rectified and lower voltages don't smooth up and down like they do without any bias (but with the -V content) - they 'bump' on and off, I guess as the diodes turn on/off - so it's causing more problems than it's solving.
> 
> For the record, I can live with the negative content - but it is bugging me.
> 
> Thanks,
> 
> Justin
> 
> 
> 
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