[sdiy] Clipping summing opamp issue..

[Ian Fritz]

Hi J-P (inline)

At 12:26 PM 1/27/2010, Jean-Pierre Desrochers wrote:
>Suppose the following setup:
>6 VCO's wave outputs wired to summing amp inputs.
>Each VCO has +/-5vpeak waveforms. (10vpp)
>At some points all VCO's will allign their
>peak levels all together.
>That means 6 X 5vpeak = 30vpeak (instantaneous)
>positive or negative.. doesn't matter here.
>This means clipping to me for normal summing
>opamps supplied to +/- 15vdc.. ??

I call these "superpeaks".  I thought I had invented the concept until I 
found out that Lord Rayleigh described them (and the associated statistics) 
in the 1880's.  I believe it's because of these occassional superpeaks that 
0dB on my soundcard's level display is at about 18dB below digital 
clipping.  I'm told this is normal.

>How do you guys manage this overvoltage situation?
>By reducing the summing gain of the receiving opamp?

Yes.  My version of the Electronotes Saw Animator has six phase shifters 
and each signal is attenuated by 1/2 in the final mix.

>If so let's reduce the summing opamp to 50%.
>Then if sometime we receive only one wave form
>to this opamp it's gonna be attenuated at 50%...
>bad thing to me.

Perhaps.  But remember that at some times the waves also cancel 
completely.  So there is a large variation in amplitude anyway.

>Conclusion, to me a autovariable gain (compressor)
>summing amp would be neat here keeping a strong level
>but never clipping..

But the superpeaks are usually of short duration.  I'm not sure a 
compressor could catch them.  I'd be interested in hearing how this works 
if you try it.

   Ian