[sdiy] Negative content on half wave rectifier

[Harry Bissell]

as someone else pointed out, it seems you might have a circuit error somewhere.

I use circuit "6A" from that appnote you posted, its the best absolute value circuit I've seen
in terms of audio range performance. You can tap a half wave out of that as well. If you study my
"Muffy" guitar synthesizer over at wiseguysynth.com (Larry Hendry - gone but not forgotten) you can
see I used it there.

If you need to absolutely get rid of a voltage below some point, you can use active clamping instead
of rectification.  This uses the same opamp and diode, but you run your signal through a resistor and
use the opamp/rectifier to shunt the circuit to ground or whatever voltage you want.

I'll try and find you an example circuit (or draw one...)

h^) HARRY


----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 22 Jul 2011 09:43:26 -0400 (EDT)
Subject: [sdiy] Negative content on half wave rectifier

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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-- 
Harry Bissell & Nora Abdullah 4eva