[sdiy] FETs for distortion

[Scott Gravenhorst]

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.

>Since I will breadboard my ideas first, I will be 
>able to play around with biasing and driving levels to get a sound I 
>like, then use the same actual FETs when I build.  The temperature drift 
>is perhaps more tricky, so I will have to read the datasheet and see if 
>I can minimise the effects of temperature (or add some compensation).
>
>So far (from the various schematics I have seen) I'm going to buy some 
>J201 and 2N5457.  Now you have said they may vary a lot, I will buy a 
>few so I can pick and choose.
>
>I think these two are JFETs.  I wonder if I would get a different sound 
>using MOSFETs?  I haven't seen them used in any schematics, and I've 
>heard they are damaged by static very easily, but someone on this list 
>said recently that they give a nice soft clipping (I'm not sure if this 
>comment also applied to JFETs, or was specific about MOSFETs).

MOSFET devices have built in diodes that help to minimize damage from
static.  To be quite honest, I'm a bad boy when it comes to handling
MOSFET ICs like 4007 or 4069UB, I will casually ground myself
occasionally, but I don't obey all the rules all the time and I've never
had a problem.  Atmospheric conditions play a part in this where the more
moisture in the air, the less this is a problem.


>Seb
>
>
>
>James Patchell wrote:
>
>> One thing you should note about fets is that they have a tendency to 
>> "vary" a lot from unit to unit.  Depending on the circuit topology, 
>> you can pick a set of fets that will work perfectly, and if you then 
>> replace them with randomly selected parts...it will not work the way 
>> you expect.  Plus, fet parameters drift with temperature in ways that 
>> you will definitely notice.  By comparison, BJT's are extremely well 
>> behaved.  I am not suggesting you use BJT's, just letting you know 
>> that you are going to be tackling.  Most people are pretty used to 
>> using BJTs.
>>
>>
>>
>> At 02:10 AM 12/31/2005 +0000, Seb Francis wrote:
>>
>>> Hi,
>>>
>>> I'm just playing around making a few distortion circuits (to be used 
>>> for electronic drums + synths, rather than guitars).
>>>
>>> I want to try out some distortions with MOSFETs and/or JFETs and I 
>>> was wondering if anyone had any recommendations of some part numbers 
>>> to buy.  Preferably they need to be available in UK from Rapid or 
>>> Farnell, but if there's something particularly good I don't mind 
>>> hunting around.
>>>
>>> I guess I'm asking whether one FET is going to be pretty much the 
>>> same sound as another (just a case of biasing and driving 
>>> appropriately), or are there some favourites for this type of app?  
>>> Or are there in fact some particular specifications/attributes I 
>>> should look for when choosing a FET?
>>>
>>> I hope this is not too vague a question ;)
>>>
>>> Seb
>>>
>>>
>>
>>         -Jim
>> ***************************************************************
>> http://www.oldcrows.net/~patchell
>>
>> ***************************************************************
>>
>>
>>
>>
>

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