[sdiy] FETs for distortion

[Scott Gravenhorst]

Seb Francis <seb at burnit.co.uk> wrote:
>Scott Gravenhorst wrote:
>
>>Seb Francis <seb at burnit.co.uk> wrote:
>>  
>>
>>>Hi Jim,
>>>
>>>Thanks for your reply.
>>>
>>>I have heard it said that FETs give a softer clipping than BJTs, hence 
>>>wanting to try them.  
>>>    
>>>
>>
>>I used a 4069UB (or a 4007, can't recall) MOSFET pair as a soft clipper to
>>turn a triangle waveform into a sine-like waveform.  I thought it worked
>>quite well, it's very simple to do this, just run it in "linear" mode.  I
>>still need to play with this for other audio sounds, like guitar.  I've
>>read that MOSFETs used like this can yield a vacuum tube like distortion.
>>  
>>
>
>Do you mean 4049UB (actually it's not very obvious from the datasheets 
>what the difference is between 4049 and 4069)?  This is another thing 
>that I've been intending to try out (I've seen it used in a few 
>distortion circuits, but until now I didn't think of it as a MOSFET).

Actually, I meant the 4069UB.  The difference is that the 4069UB has
"symmetrical" MOSFETs, whereas the 4049UB does not.  I believe the 4049 will
sink more current than it can source.  For a 4069UB, sink and source are
equal.  To me, this fact makes the 4069UB more appropriate for this
particular kind of thing.  However, if you are looking for asymmetrical
distortion, then the 4049 is something worth trying.  In my circuit the
clipping action of negative and positive peaks is essentially equal.








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