[sdiy] Overdriving 4069UB

[Harry Bissell Jr]

Just saw the schematic.
  
  I would AC couple into the first opamp, bias the non-inverting input
  at 1/2Vcc and run the opamp on 0-15V (like a guitar stompbox). That
  would cure all your worries about exceeding the CMOS limits
  completely.
  
  H^) harry

Seb Francis <seb at burnit.co.uk> wrote:  Funny you should ask, I've just this minute uploaded a gif of the 
schematic ...
http://burnit.co.uk/sdiy/stuff/mosfet_distortion.gif

It really is a lovely sounding distortion, and it's useful to be able to 
set the number of stages to get anything from 'subtle' to 'insane'.

I've been testing it with 3 things: Oakley 3031 with some filter 
resonance, Waldorf Pulse with a fat saw wave patch and no resonance, and 
a 909 bass drum clone.  This seemed like a good cross section of sources 
(these are the kind of things I like to distort anyway).

The circuit differs from others I've seen with the 4049/4069 as it has 
no high frequency roll off (I tried it, but I preferred to let all 
frequencies through then EQ later, or indeed before, if it was wanted).  
For guitar distortion you might want to add some caps across some or all 
of the 4069 stages to roll off the high frequencies.

Remember to tie any unused inputs of the 4069 to ground (or make a 2nd 
channel - that's what I'm doing).

Seb

P.S. The current circuit takes the risk of exceeding the 4069 input 
voltage specification if the input is hot and the drive level is set 
high - it gives a nice hard clip sound when you drive it real hard like 
this.  Having just read Harry's email, I might change this a bit to save 
damaging the IC.


Scott Gravenhorst wrote:

>Very interesting, do you have a schematic up that you can share?
>
>What sort of signal(s) did you test with?
>
>Seb Francis  wrote:
>  
>
>>Well, I've been getting some really nice results with using between 1 
>>and 3 stages of a 4069UB for distortion.  The MOSFET clipping has a 
>>really warm sound to it.  (Incidently the 4049 really wasn't so good - 
>>the clipping was way too asymmetrical.)  Anyway, more on this later .. 
>>I'll publish the final circuit in due course.
>>
>>What I'm concerned about in my circuit design is whether it is safe to 
>>drive the 4069 beyond its absolute maximum input voltage.  The quoted 
>>absolute maximum range is -0.5V to VDD+0.5V (VDD=15 in my circuit).
>>
>>The input current is limited by a 10K resistor (with a DC blocking cap 
>>in series before it), so although the opamp driving it can kick out up 
>>to 28V ptp, the voltage at the 4069 input gets clipped to approx 17V 
>>ptp.  This is a bit beyond the quoted absolute maximum, but I guess as 
>>long as the current through the internal protection diodes is limited by 
>>the 10K resistor then this is ok??  Is this a safe assumption?
>>
>>Seb
>>
>>
>>    
>>
>
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