Ideas to Clip the negative going pulse.

[Link555]

Hi I am new to this forum. I have been designing electronics for a while now, as well as recording bands. I own a small studio and am constantly making toys for it. My latest project is a console ready preamp. I want to add 2nd and 3rd harmonic distortion via a pot adjustment. Something similar to the cranesong Hedd192, at least from the user’s perspective. I was hoping to create an analog circuit to do this. I have tried ideas of lowering the negative rail, and messing with the DC offset as well zener diodes in the feedback path. I was just wondering if anyone else had built a very subtle harmonic generator circuit like this before. How did you do it?
Thanks in advance!

[Richard]

Are you referring to one half of the sine (or whatever shape your signal is) wave? Running it through an overbiased tube should do the trick.

[Link555]

Are you referring to one half of the sine (or whatever shape your signal is) wave? Running it through an overbiased tube should do the trick.

Thanks for the response! Yes, but I really want to just use solid state for this, as tube power supplies kill my pocket book. Do you think monkeying with the DC offest would have the same effect?

[Don Moose]

Do you think monkeying with the DC ofsfet would have the same effect?

Technically, yes ... but it would very likely sound like poot.

Splitting an op-amp's feedback path, and putting two resistor-and-diode series assemblies in antiparallel with different resistor values, and/or different diode types (1n914/1n34a) might be worth a fiddle.

[Link555]

Thanks!
Yep I was toying with that idea too, but then it will only add the flattening on the large signals. This may not be a bad thing. But I was wondering if there was a way to add the clipping to all signals regardless of amplitude.

[Don Moose]

clipping to all signals regardless of amplitude.

The problem there is that you wind up clipping all signals TO the same amplitude - that's what clipping means.

It sounds like you hope to do some kind of percentage clip where you knock the to 20% or so off of any given wave. That's definitely a DSP project.

If you wanted to do it with either DSP or analog, you'd have to:
- Take an envelope to measure the amplitude as a fraction of 1
- pump everything up to the same amplitude of 1
- clip it
- and reduce the gain back to the envelope's fraction

Trying to think of the how of that gives me a headache.

[Link555]

COOL! yes thats a good idea! But yes in the analog world....thats beyond me. Ok so time to suck it up and buy a DSP eval kit.
Thanks again, I really like the concept.

You could take the envolpe input into an ADC of a very fast micro, take the reading and use it to look up a gain value in a look up table. Then load that gain value into a digital pot in a feedback path of an op-amp. Then run the uniform amplitute through the opamp with a zener diode in the feedback path. Clip the waveform. Then get the micro to tell another digitally controlled amplifer to atteunate the signal to where it was previously. Timing timing timing.... DSP for sure....

[Richard]

Would a single diode only clip one half of the wave? If you were able to clip it close enough to the crossing point, I think everything would be pretty much clipped the same. This is basically half wave rectification so if you look that up, you may get some ideas. Rectification is usually applied to a single frequency. This is getting to be an interesting concept. Another idea would be to use capacitors feeding a diode. With single diode the clipping would occur only on the lowest frequency present, because of their smaller amplitude, when the entire signal goes negative (for our conversation I'm thinking of clipping the negative side but I guess it wouldn't matter), the entire signal would be clipped, not the individual superimposed waveforms. Using different sizes of capacitors feeding individual diodes could apply clipping to more frequencies.

I'm envisionizing (new word!):
signal comes in, splits between a small capacitor in series with a diode connected to output and a resistor in the input line.

this repeats with larger capacitors until the final stage doesn't need one.

you could even use pots to vary the different stages, Instead of a tone stack, you have a clip stack.

If I'm imagining this correctly, each diode would feed a clipped signal over to output side, each stage feeds progressively lower frequencies to the output.

[Link555]

Thank again Richard. But I don't think I have been exactly clear Iam not looking to completely clip the negative pulse, only add a small clip to the very peak of its amplitute.
http://http://www.geofex.com/effxfaq/distn101.htm
Like the second picture but only on the negative pulse, and maybe not quite this much clipping.
Thanks again guys! great stuff so far!

[Arthur B.]

Thanks!
Yep I was toying with that idea too, but then it will only add the flattening on the large signals. This may not be a bad thing. But I was wondering if there was a way to add the clipping to all signals regardless of amplitude.

Crossover distortion will fit the bill.

[Link555]

Thanks Arthur!
I am curious does crossover distortion add 2nd and 3rd order harmonics tot he signal too?

[Arthur B.]

Thanks Arthur!
I am curious does crossover distortion add 2nd and 3rd order harmonics tot he signal too?

Mostly second harmonic. A PP class B stage will have lots of crossover distortion, at pretty muchh signal levels.

[Link555]

So to add 2nd order harmonic distortion I could send the desired percentage of signal through a push pull amp. I like that, that I should be able do! Thanks very much Arthur! Any ideas on how to get the 3rd order harmonic?

[CRU JONES]

Try companding.

If you compress the signal before you clip it, it will have a more constant amplitude of which you can get more consistent harmonic generation from. Then you expand it with the inverse control signal that the compressor side of it uses. The result is harmonics that are a closer percentage of the signals amplitude.

You can modify most compressor circuits to be a compander.

DSP is sweet. check these for some fun.

http://www.reaper.fm/sdk/
http://www.synthmaker.co.uk/download.php
http://www.cycling74.com/products/max5

[Link555]

Brilliant thank you, this I will have to try! Thanks so much!