[sdiy] FETs for distortion

[Seb Francis]

Hi Jim,

Thanks for your reply.

I have heard it said that FETs give a softer clipping than BJTs, hence 
wanting to try them.  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).

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
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