vendredi 23 mars 2012

Overdrive distortion progress.

Still working on my Overdrive distortion and it is not easy as i was thinking.
Here are the spec i planned for this circuit.

- Soft Clipping (on op amp feedback) and hard clipping (after opamp) selectable
- Hard clipping with Led (to add some blinking stuff)
- not adding too much noize
- true bypass with 4066
- unipolar supply using mc-09 supply
- get a multi purpose distortion PCB (modify diodes, modify gain with trimmers, etc)

You can find a lot of great distortion on the net .. but almost all are guitar distortion, meaning the input signal is not the same, in my case input signal is about 1 to 2 Vpp which i much more that a guitar signal.
This couldn't be a problem but i can't set a high gain on the op amp because the op amp will clip (due to the limited headroom using a 9V single supply) ans the op amp clipping doesn't sounds good.
The good thing about that is the limited gain is also limited noise, but limited gain is limited distortion.
Soft clipping is done by adding diodes (1n4148 for ex.)  in the feedback path of the 1st opamp.

And for the Hard clipping. i planned to use LEDs, so they will blink when distortion happen, but a Typical Vf of a Led is 2V so after the first soft clipping op amp the signal peak is not very high (due to soft clipping) and the Hard clipping effect of the Leds is not very efficient.

You can see here the a soft clippping but peak voltage is 2 volt so the hard clipping after that will not work well..

Adding the Led after for Hard Clipping we get these results  (in RED) :

 as you see, the hard clipping is not very effective. .
So the solution i was thinking is to add a second opamp and do hard clipping after this second op amp.
(also added Led with a higher Vf to stop hard clipping when gain is  low at the second op amp)

 With R4 and R8 will be potentiometers to control the amount of soft and hard clipping ..
see the results here

need to test it in a real situation, because the mc-09 signal is not a perfect sin wave (of course) and adding accent will boost the signal and add clipping on opamp.

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