Soft Knee Duplex Distortion

[WGTP]

http://aronnelson.com/gallery/WGTP/Soft_Knee

Latest on breadboard with the cool stuff.  An assembly of other peoples great ideas.  CMOS op amp, dual MOSFET clipping, variable low frequency rolloff/gain, clipper switching, etc.  Thanks to John Greene, R.G., Mark Hammer, Transmogrifox, Aron, et al.   

[lovekraft0]

In the schematic, you've got the "guide" diodes' (1N5819s) polarity backwards for soft-knee MOSFET clipping - all you're getting as drawn is body diode clipping (not that that's a bad thing, it might sound great, but it's not what you're apparently  trying to set up).

[ragtime8922]

Interesting. Can you explain what is going on in there a bit. What is the swt drive and is that some sort of output on the diodes of the second opamp section?

[WGTP]

I "think" the diodes are correct.  It has been checked by others, although it has been a source of confusion and will change depending on N or P channel MOSFETS.  See here

http://www.elixant.com/~stompbox/smfforum/index.php?topic=38581.0

The Drive switch changes the size of the cap to ground off the first feedback loop.  This controlls both the bass content, and also the gain.  Less bass makes for a smoother distortion.  I'm currently using .15uf there.  The MOSFET clippers are sort of dark and don't need as much bass content.

The switch on the second op amp reverses the diodes so that you can get the MOSFET, the body diode which is more like an  SI, or no clipping for a big boost with some op amp distortion.

Since this is on a breadboard, I don't actually have switches, I just change the parts around.

Lowering the resistance between op amps will increase the drive of the second stage, if even more distortion is needed.

If your using a single coil, the gain pot might need to be increased to 500K.   

[Transmogrifox]

I "think" the diodes are correct.  It has been checked by others, although it has been a source of confusion and will change depending on N or P channel MOSFETS.

You have P channel MOSFETs shown schematically, so this would be the reverse of the desired diode polarity. 

I suspect that you meant to show an N-channel MOSFET, so on your breadboard the diodes are in the correct polarity.  The two different types of symbols for N channel MOSFETs are opposite.  The type you're using is more accurate to reflecting the internal operation of the MOSFET (and the arrow for the N-channel should be pointing in toward the gate), but the other kind of symbol is a more intuitive symbol that represents its "black box" operation, likening it to npn BJT's.

[WGTP]

Sorry about that.  I flipped them over.  To make things more confusing, the 2N7000 and BS170 are reversed, even though both are N channel.

[WGTP]

OK, I flipped the MOSFETs to N channel and the diodes back.  This is too much for my pea brain.  Thanks for helping me and see if I got it right yet.  The idea is for the internal diode and the external to point in opposite directions forcing the signal thru the MOSFET.  With the switch, the diode is reveresed and the internal and external are are pointing the same direction resulting in a different, more converntional diode clipping.  I also added a .1uF cap to the DRIVE switch.  I think the .1uF & .22uF caps may be the more useful.  Here are the roll off points. 

.1uF = 1600Hz
.22uF = 700Hz
.33uF = 500Hz
.47uF = 330Hz
1uF = 160Hz

[Transmogrifox]

looks good

[Kipper4]

Necro post WARNING

Anybody know what op amp (cmos) would be good for this please
Thanks again fellas
Rich

[Kipper4]

For reference here's the schemo

http://www.aronnelson.com/gallery/main.php/v/WGTP/Soft_Knee.jpg.html

[Mark Hammer]

Ah, the good old days.

I don't know that specific  op-amp matters.  What does matter, though, is that the second stage shown in the resuscitated drawing is missing a Vref connection to the non-inverting pin in that op-amp.

Since the circuit uses an inverting op-amp for the second stage, consider using the same trick that the Marshall Bluesbreaker and Analogman King of Tone do.  The same 500k pot can have the wiper tied to the output of Stage 1, one leg serving as the feedback resistance of stage 1, and the other leg placed in series with the 10k interstage resistor.  Turn it in one direction and the Stage 1 feedback resistance goes low as the input resistance to Stage 2 goes high, lowering gain in both stages.  Turn it the other way , and the gain of both stages increases.  This makes it easier to get cleaner or mildly dirty tones.

[PRR]

TLV2372 is $2, low current, R2R.

None of this is too critical here. TL072 will have lower hiss.

As the Hammer says, the drawing is incomplete, and there's other ways to do things.

[Kipper4]

Thanks guys brilliant.

[Kipper4]

I think I might try that King of tone mod. The blues breaker does it in a differant way but i guess the results are similar.
I'll add in a 50-100pf in the NFL to tame some of the oscillation too.
what I have on the breadboard is a variation of the Moster by WGTP so not quite this.
thanks
Also thanks Paul I wasnt sure that the lettering refered to the op amp. Cheers for the tip.