[sdiy] Overdriving 4069UB

[Seb Francis]

Hi Harry,

This is the current circuit:
http://burnit.co.uk/sdiy/stuff/mosfet_distortion.gif

Harry Bissell Jr wrote:

> Hi Seb
>
> A good shottky might be the 1N5817
>
Ok, I see it has a relatively low turn on voltage so should protect the 
internal diodes.

> I have done input overdrive with 100K series resistor. Two notes...
> I don't intentionally overdrive at that level... and in your linear case
> (I assume you have a feebback resistor) making the input much higher
> will make the feeback value much higher as well...

Yep, I've got a 100K feedback resistor.  So I would change the input to 
100K and the feedback to 1M.  I don't expect this will make very much 
difference to the sound.  But you are saying that even then it's not 
guaranteed not to damage the IC with extended use.

>
> why not a low impedance voltage divider to reduce the opamp swing first
> to below the 15V limit ?  Knock the gain down first, then go through the
> 22K input resistor.
>
The thing is that I like the option of the harder clipping sound when 
the input drive level is turned up high.  I'm currently leaning towards 
the schottky diode option - this seems the safest way to protect the IC 
without compromising the sound.

> 28V p-pk sounds like the opamp is +/-15V supply. Then you go to a 0-15V
> on the CMOS ???
>
> Are you AC coupling into the CMOS ??
>
Yep, that's right - with a 1uF cap before the 10K resistor.  I 
experimented with also adding DC bias to the CMOS input to make the 
distortion more symmetrical, but I found that it didn't make much 
difference.

Thanks for your help.
Seb