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2nd Order and 3rd Order Harmonic Generators

Discussion in 'Electronic Repair Modifications DIY' started by Link555, Mar 11, 2010.

  1. Link555

    Link555 Well-Known Member

    For sometime now I have looking all over the net for a thread about the various ways to spice up your mic pre or compressor or well anything…. by adding a little subtle distortion.

    Boxes like SSL alpha and UA twin infinity pre add a 2nd or 3rD harmonic generator that allows the user to dial in a bit of distortion.

    There are a lot of ways to create a circuit such as this, but I wanted to pick the RO communitycollective brain see what methods we use, like and hate.

    Please don’t think this is limited to circuit designers , user input is most welcome.

    To get the ball rolling I have used a class JFET circuit with a control to adjust the level saturation to create mostly 2nd order harmonic distortion. JFETs can have similar transfers curves to triode vacuum tubes, so that when they are driven into saturation they can exhibit more second order harmonics.

    The circuit I worked up adds a variable DC offset to the source a simple class A circuit.

    This DC offset helps shifts the Transfer curve over the Quiescent operating point….

    Anyway would love to hear more ideas...
  2. MrEase

    MrEase Active Member

    OK I have never really tried doing this but it occurs to me that this is not necessarily trivial if you want to try and dial in a particular sound - hence what follows is just really a ramble of initial thoughts. Certainly pure even and pure odd harmonics sound very different with the general wisdom being that odd's are more pleasant (or warmer) than evens - which is why I think that everyone likes the sound of an overdriven push-pull valve stage.

    A simple stage as you described is going to be very limited in the sound palette you could get and very subject to circuit variations. Really what you are getting there is an asymmetric waveform (due to the square law of the device) where negative going output compresses (odd's) and positive swings expand (even's). Although you are generating what you want, there is not much control.

    If you want to dial in variable amounts of either odd's (to warm it up) or even's (to spice it up!) I think it would be an idea to drive three amps., one linear, one odd generator and one even generator and sum their outputs together. This should give you the most flexibility in dialing in a "sound". Although it could also generate so many controls only you will have an idea of how to dial in a sound!

    For generating odds I would tend to stick with the usual Op Amp overdrive circuit (diode pair in the feedback) but "soften" the limiting with a series resistor. I may even use a double diode pair too for higher overdrive with a harder characteristic. It's a question of how far you want to go with it. This would generate symmetrical output but you could switch out one of the diodes to make the output asymmetric if you wanted.

    Getting symmetrical even's is not quite as straightforward but could be implemented by using a difference technique with the existing amps to yield the evens, or perhaps just use an odd's generator in a feedback loop. The latter will need more care with your pole's and zero's though.

    Oh well, just some initial thoughts - if I have any inspirational thoughts I'll pop back!

    EDIT: Ok that's really strange - at the end of the first line I have "sound" which repeats at the start of the second. I went to edit the double "sound" and the word sound only appears once in the edit box????

    EDIT2: Now I did a "full" edit rather than "quick" edit the double "sound" has gone!

    EDIT3: Oh forget it! When I do a refresh I gain an ad to the right and the whole post reformats and gets it right....
  3. Link555

    Link555 Well-Known Member

    LOL thats great stuff Mr ease, Please do pop back. The odd harmonics can be tricky for sure, I would love to know how Dave hill did his in the hedd192...
  4. MrEase

    MrEase Active Member

    Nothing to do with the thread really!

    OK This forum formatting is weird! Now I've lost the ad on the right I've got the double "sound" back. I'm sure I didn't apply any FX to this track!
  5. Link555

    Link555 Well-Known Member

    Found this link I forgot I had....
    Guitar Distortion 101
  6. Link555

    Link555 Well-Known Member

    Saw this on Youtube,
    YouTube - AllAmericanFiveRadio's Channel

    basic ideas, but well done I thought.
  7. Boswell

    Boswell Moderator Distinguished Member

    I agree about the basic ideas, but disagree about it being well done. The valve (tube) circuit examples were particularly poorly chosen, and the voiceover consistently referred to the grid resistors as bias resistors.
  8. Link555

    Link555 Well-Known Member

    True, However I did like the graphic that he showed as he changed to offset at the base (or grid in the tube examples), I thought that was a good way to see the concept, I have never seen it shown like that before.

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