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Smoothing out the diodes.. rounding the edges,,,

10/9/1999 9:07 AM
David Manson
Smoothing out the diodes.. rounding the edges,,,
I've been reading a little more on the subject and  
wanted to know a little more about capacitor  
bounding -- so far the only way to really round  
out the blocky signal is to use an fet booster  
afterwards (from what i read) the only other  
2 are using a resistor in series or a cap in  
parallel with the diodes -- Orman states that the  
resistor only knocks down high order harmonics and  
doesnt really round out the signal -- nothing on  
capacitor bound yet on his page.. Anyone have any  
any experiences with this? i think i remember  
O'connot saying that the screamer uses cap bounding  
in TUT.. but most pedal companies use the resistor  
because its easier/cheaper.. otherwise has anyone  
looked at the signal of the shaka 3 through a  
10/9/1999 1:44 PM

Before you discount using a resistor in series with clipping diodes, try it. You might be surprised. It sounds much better than Jack's comments might indicate. I have modified many distortion plus pedals with a series pot. It lets you vary the distortion from the Plus's slightly dry distortion down to something like only a slight rounding of peaks.  
Note also that Jack's proposed cure for opamp harshness in Tube Screamers is - yep, a resistor in series with the clipping diodes.  
There is another thing you might do. I'm working on a pedal based on this one right now. A diode's voltage is the logarithm of the current through it. We commonly impress a voltage source (zero impedance, all the current needed to drive the load to the desired voltage) across clipping diodes to get clipping. This is what is largely responsible for the sudden onset of clipping and square edges - we drive the current hard.  
What if you drove a pair of clipping diodes with a signal that was a current source, and then took out the voltage across the clipping diodes? The voltage looks like the instantaneous logarithm of the signal, and there in no "corner" anywhere, only progressive compression of the signal. This is the logical conclusion of impedance in series with clipping diodes. Asymmetry can be had by switching in one diode different from the other.  
I'm messing with a pair of diodes at the output of an OTA, driven by the current-source signal of the OTA and with the diode voltage being the signal out. A CA3094 gives you both the OTA and a darlington buffer after the diodes in one 8 pin package. Treble rolloff is very straightforward with a cap across the diodes. We'll see.
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10/13/1999 1:33 PM

If you have an inverting op-amp config, input to resistor to pin (-), diodes from output to pin (-), pin(+) grounded. Then the current through the diodes will be equal to the current through the input resistor, proportional to the input, and the output will be the voltage on the diodes.
10/13/1999 2:56 PM

Yeah, I posted that tidbit a ways back. The feedback resistors tends to be what people change to change gain, but the resistor attached to the inverting input sets the current level in the clipping diodes. This just limits the current.  
Driving the diodes with a constant current source version of the signal should be smoother - I think...  
10/13/1999 6:05 PM

"The feedback resistors tends to be what people change to change gain"
I meant no feedback resistors at all, just feedback diodes. Just like that log amplifiers in opamp cookbooks but with two diodes so it can also handle negative inputs.  
"This just limits the current. "
The input resistor does not limit the current - it just sets the ratio (diode current)/(input voltage). The only limits to the current are the opamp fan-out and the supply voltage.  
"Driving the diodes with a constant current source version of the signal should be smoother - I think... "
This IS driving the diodes with a current source, Idiodes=Vin/Rin, the transconductance is 1/Rin.  
10/9/1999 1:46 PM

Oh, yeah, forgot. Don't just read about what makes sound good. The only thing that really counts is sound on your personal eardrums. Go do the experimentation. Build it and listen. That is the only really valid test.
10/9/1999 7:11 PM
David Manson

yeah.. o'connor suggested using a pot in series -  
I looked at the book again and he says that the  
resistor does a better job than using the cap..  
the cap supposedly only cuts the harshness at  
the highest frequencies.. maybe i should try both  
in combo and see what happens.. I'd probably like  
to see a comparison of rounding between resistor,  
cap and the jfet... your idea sounds cool.. no  
edges at all.. it might end up sounding like a  
fuzz pedal.. or a variation..
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